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LECTURE NOTES COURSE: REFRIGERATION & AIR CONDITIONING
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Methods of Air Refrigeration system • • • • • •
Simple air cooling system Simple air evaporative cooling system Boot strap air cooling system Boot strap air evaporative cooling system Reduced ambient air cooling system Regenerative air cooling system
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Simple air evaporative cooling system
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Refrigeration Systems
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COP
COP = coefficient of performance Air conditioners, refrigerators: COP=QL/Wnet Heat pumps: COP=QH/Wnet Energy balance: Wnet+QL=QH
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From Cengel, Thermodynamics: An Engineering Approach, 6th ed.
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Reversed Carnot cycle -- ideal COPR,Carnot
1 TH
TL 1
1 COPHP,Carnot 1 TL
Why isn’t this cycle possible in real life?
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TH
From Cengel, Thermodynamics: An Specworld.in Engineering Approach, 6th ed.
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Ideal Refrigeration Cycle 1)
2)
3)
4)
x=1 (saturated vapor), P=Plow or T=Tlow P=Phigh, s2=s1 (constant entropy) P=Phigh, x=0 (saturated liquid) h3=h4 (constant enthalpy), P=Plow
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From Cengel, Thermodynamics: An Specworld.in Engineering Approach, 6th ed.
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Example
An ideal vapor-compression cycle has a mass flow rate of R-134a of 0.05 kg/s. The low and high system pressures are 0.12 MPa and 0.70 MPa. Find – – – –
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The rate of power input The rate of heat transfer out of the refrigerated space The rate of heat transfer to the surroundings COP
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Cycle efficiency
To increase the COP of the cycle, increase the evaporation temperature or decrease the condensing temperature. –
–
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However, you can’t achieve as cold of a temperature now, and your heat exchanger will need to be larger since DT is smaller. 2-4% increase in COP per degree temperature change Specworld.in
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Sources of Inefficiencies *Compressor efficiency < 100% Pressure drop in piping Heat transfer to/from lines *Superheating of fluid entering compressor to prevent liquid from entering *Subcooling of fluid entering expansion valve to prevent vapor from entering * We will only look at these in class.
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Expansion Valve Operation
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
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Example
A vapor-compression refrigeration cycle operates using R-134a with low and high system pressures of 0.10 MPa and 1.20 MPa. The fluid leaves the evaporator superheated by 6.37°C and leaves the condenser subcooled by 4.29°C. Calculate the COP if the compressor efficiency is a) 100% and b) 84%.
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Compressor Performance
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Compressor Basics
As with ideal pumps, dh=vdP. However, v is not a constant, making calculation of h2-h1 more complicated. For a polytropic process, Pvn=C 2
2
p
h2 h1 vdP C 1
1
1
n
dP
(1)
For an isentropic process and an ideal gas, n=k (where k=cp/cv), and for an isothermal process, n=1.
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Compressor Efficiency
The adiabatic compressor efficiency:
a
Wisentropic
h2 h1 s
Wactual h2 h1 actual Total compressor efficiency:
compressor amotor drivemechanical
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(2)
(3)
Typical efficiencies are 90% for the motor drive at peak load, 90% for the mechanical efficiency, and 76% to 97% for the adiabatic (isentropic) efficiency. Typically, as the compressor size increases, so does the adiabatic efficiency.
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Exit temperature
A maximum recommended fluid temperature is given based on compressor and fluid type. Air compressors typically shouldn’t have an air exit temperature greater than 300-375ºF to prevent carbonizing, combustion of oil vapor, or weakening of parts over time. Air can be modeled as a perfect gas where and R
Pv RT / M
c p cv
M
These can be substituted into Equ. (1), and using Equ. (2) or (3) as well, the exit air temperature T2 can be found as a function of pressure ratio r, n, k, and efficiency. Once T2 is known, the compressor work can be found using
Wactual h2 h1 mc p T2 T1 jntuworldupdates.org
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Decreasing Compressor Work
Two possible methods – –
Minimize irreversibilities due to friction, turbulence, and nonquasi-equilibrium compression Make the specific volume of the gas as low as possible by cooling the compressor since in the ideal case 2
Wcompressor h2 h1 vdP
One cooling possibility
1
is to use multi-stage compression with intercooling. From Cengel, Thermodynamics: An Engineering Approach, 4th ed.
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Compressor Types
Five most common types –
Reciprocating
–
Screw
–
Uses a roller to compress gas Used in most domestic refrigeration and ac systems
Scroll
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Uses centrifugal force to compress gas Common in large refrigeration systems (200 to 10000 kW of refrigeration capacity)
Vane
–
Lobes of two rotating screws trap and compress gas
Centrifugal
–
Uses a piston-cylinder and valves Most common type of compressor
Two inter-fitting spiral-shaped scrolls compress the gas Used in 1-15 ton (3.5 to 53 kW) range AC applications Specworld.in
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Compressor pressure ranges
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From Burmeister, Elements of Thermal-Fluid Design.
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Terminology
Open-type compressor – –
Hermetically sealed – –
Motor and compressor are combined in the same housing Used for small domestic air conditioning systems
Semi-hermetic –
Crankshaft extends through housing to connect with the motor Seals are used to limit leakage
Cylinder heads are removable for serviceability. Good for AC systems larger than domestic.
Condensing unit –
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Motor, compressor, and condenser are combined in Specworld.in one unit and sold together
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Reciprocating Compressors
Gas in the clearance volume must expand to V1 before the pressure is low enough to open the suction valves and draw more gas in.
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
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Reciprocating Compressors, cont.
Actual volumetric efficiency va
3
Clearance volumetric efficiency va –
s x100 displacement rate of compressor m s
3 volume flow rate entering compressor m
volume of gas drawn into cylinder V V x100 3 1 x100 useable volume of cylinder V3 Vc
The clearance volumetric efficiency tells us what percent of the clearance volume is used to bring new gas in.
Percent clearance
m
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Vc x100 V3 Vc
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Reciprocating Compressors, cont.
After some algebra V1 vc 100 m 1 Vc
where
V1 vsuc Vc vdis
vsuc=specific volume of vapor entering compressor vdis=specific volume of vapor after isentropic compression jntuworldupdates.org
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Reciprocating Compressors, cont.
To find mass flow rate (kg/s) m displacement rate x
vc 100 vsuc
The displacement rate is a volumetric flow rate; vsuc converts that to a mass flow rate As the suction pressure (and evaporating temperature) drops, what happens to the mass flow rate? On a cold winter day, the evaporating temperature will be very low for a room AC unit. What problems could this cause?
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Reciprocating Compressor Performance
Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill. jntuworldupdates.org
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Reciprocating Compressor Performance
Most refrigeration systems operate on the left side of the power curve. During startup, the power requirement may pass the peak and demand more motor power.
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
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Reciprocating Compressors, cont.
Adiabatic compression (isentropic) efficiency (use this to find the actual enthalpy at the compressor exit)
a
wisentropic
x100
wactual Losses are due mainly to friction of rubbing surfaces and pressure drop across valves Watch your exit conditions. If the exit temperature is too hot, the oil will break down and reduce the life of your valves. The maximum recommended oil temperature varies with the oil type. This can be a problem especially with ammonia, which tends to have high discharge temperatures. Ammonia compressors often are equipped with external water jntuworldupdates.org Specworld.in cooling.
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Rotary Screw Compressors
Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
Good efficiency (60-80%) for pressure ratios above approx. 2.5.
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Vane Compressors
No suction valve needed. Minimum gas pulsation
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill. Specworld.in
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Dynamic Compressors -- Centrifugal
Commonly used for large systems, including chillers Gas enters a spinning impeller and is thrown to the outside of the impeller through centrifugal force Impeller provides the gas with a high velocity (kinetic energy) which is converted to pressure (internal energy); remember Bernoulli’s Law! 70-80% isentropic efficiencies Axial compressors are a somewhat less common form of dynamic compressors
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Scroll Compressors
Need close machining tolerances Low noise, high efficiency Incompatible with solid contaminants and poor performance at low suction pressures
From McQuiston, Parker, and Spitler, Heating, Ventilating, and Air Conditioning Analysis and Design jntuworldupdates.org
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Expansion Valves
Figures are from Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed., McGraw Hill
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Expansion Devices
Two purposes – –
Reduce pressure of refrigerant at approx. constant enthalpy, resulting in a large temperature drop Regulate refrigerant flow to the evaporator
Main types – – – –
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Capillary tubes – used up for refrigerating capacities of approx. 10 kW or less; common in domestic refrigerators Constant-pressure expansion valve – for systems with refrigerating capacity of 30 kW or less Float valves – used in large industrial applications Thermostatic expansion valve - the most popular type of valve, capable of providing a wide range of evaporator temperatures
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Capillary Tubes
1 to 6 m long, 0.5 to 2 mm inside diameter Pressure drops through the tube due to friction and fluid acceleration Cap tubes and cheap and reliable, but they can’t adjust to changes in parameters such as added load, suction pressure, etc. You’d need to install a new tube to get different system performance. They also can be clogged. Mass flow rate is determined by a balance point between cap tube and compressor performance. If there is too much or too little heat transfer in the evaporator for the given balance point, the evaporator will be starved or overfed.
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Capillary Tubes, cont.
Fig. 13-1
Starved evaporator– not enough refrigerant to provide enough cooling capacity Overfed evaporator – too much refrigerant for the amount of cooling needed, resulting in slugging of the compressor (liquid drops enter the compressor)
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Capillary Tubes, cont.
As a result, refrigerant charge must be within close limits. Therefore, cap tubes are usually used only with hermetically sealed compressors since they don’t leak. Usually only liquid enters the tube. As the pressure and temperature drop, more and more of the liquid flashes to vapor. Vapor has a larger specific volume than liquid, so the fluid must speed up. If the pressure drops low enough, choked flow will result. Further decreases in pressure will have no effect on the flow rate through the nozzle. In this case, sonic velocity occurs at the end of the tube!
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Capillary Tubes, cont. Table 13-1 quality
Choked flow jntuworldupdates.org
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Note 70 m/s=157 mi/hr=252 km/hr
Constant-Pressure Expansion and Float Valves
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Constant-pressure expansion valves maintain constant pressure in the evaporator by opening or closing –
Used a lot when a very precise evaporator temperature is needed, such as in water coolers (to prevent freezing) or rooms where humidity control is very important (such as banana-curing rooms)
Float valves maintain the liquid level in the evaporator at a constant level by opening or closing –
– –
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Can react easily to changes in load Used in large installations In smaller installations where continuous-tube evaporators are used, they can’t be used since it’s nearly impossible to establish a liquid level. Specworld.in
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Thermostatic (Superheat-Controlled) Expansion Valve
Fig 13-12
Feeler bulb is filled with same refrigerant as in system and is clamped to the outlet of the evaporator. If too little refrigerant is in the evaporator, it will be very superheated at the exit. This will make the refrigerant in the feeler bulb evaporate, increasing the pressure on the diaphragm. This will open the valve further, letting more refrigerant in, decreasing the temperature at the evaporator exit.
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Electric Expansion Valve
Like a thermostatic expansion valve, except a thermister is used to sense the evaporator exit temperature. Used for a lot for systems that can be run as either heat pumps or ac units since it’s OK to run fluid through them backwards
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Multi-Pressure Refrigeration Systems
Figures from Refrigeration and Air Conditioning, 2.5 edition, by Stoecker and Jones and Thermodynamics:An Engineering Approach by Çengel and Boles
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Cascade Refrigeration Systems
Used in industrial applications where quite low temperatures are required The large temp difference requires a large pressure difference Compressors have low efficiencies for large pressure differences; this results in low system efficiency Refrigeration cycle is performed in stages The refrigerant in the two stages doesn’t mix Higher efficiency results but also a higher first cost
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From Cengel and Boles, Thermodynamics: An Engineering Approach
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Multistage Compression Refrigeration
Similar to a cascade system except the same fluid is used for both stages Compression is done in two stages with a mixing chamber in between. Expansion is also done in two stages. After the first expansion, a liquid/vapor mix is present. In the flash chamber, the saturated vapor is removed and sent to the mixing chamber while the liquid goes through the second expansion valve. This ensures that sufficient cooling capacity and mass flow rate through the valve and is achieved. Watch your mass flow rates! They’re different in different parts of the cycle
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From Cengel and Boles, Thermodynamics: An Engineering Approach
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Benefits of Flash Gas Removal
Here ammonia is best
Here R-134a is best For re-compression of flash gas
From Stoecker and Jones, Refrigeration and Air Conditioning jntuworldupdates.org
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Flash Gas Removal Plus Intercooling
From Stoecker and Jones, Refrigeration and Air Conditioning
•This is a similar process, but the vapor at 2 is also cooled to the saturation temperature by bubbling it through the liquid in the flash tank. Vapor velocity must be less than 1 m/s for this setup to work well. jntuworldupdates.org
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Flash Gas Removal Plus Intercooling • Intercooling alone usually doesn’t result in a power reduction for R-134a, but it does for some refrigerants like ammonia (~4%). • Intercooling may also be done with an external liquid such as water. • When intercooling and flash gas removal are combined, the savings is similar for most refrigerants. • A rough estimate of the optimum intermediate pressure can be found from
Pintermediate Ps uction Pdischarge
From Stoecker and Jones, Refrigeration and Air Conditioning
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One Compressor & 2 Levels of Evap. Temp
Often two evaporating temps are required – one for a freezer, and one for a refrigerator Why not use one evaporator with a really cold refrigerant temperature for both cases? – – –
If you’re using the evaporator to chill liquid, the liquid could freeze on the surface of the coils In an air-cooling coil, excessive frost may form If the air-cooling coil cools food, food near the coil could freeze
Use of two compressors instead of one is more efficient but results in a greater first cost
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From Cengel and Boles, Thermodynamics: An Engineering Approach
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A more common form of this system
From Stoecker and Jones, Refrigeration and Air Conditioning
Pressure regulator (sometimes called a back-pressure valve) maintains the higher evaporating temperature in the first evaporator. This results in a loss of efficiency but is easier to control than the previous configuration. The pressure regulator may be simply modeled as an expansion valve.
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2 Compressors & 2 Evap. Temps
More efficient but greater first cost than using one compressor Used often in a plant storing both frozen & unfrozen foods where required refrigeration capacity is high (well over 100 kW) Approximation of optimum intermediate pressure:
Pintermediate Psuction Pdischarge
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From Stoecker and Jones, Refrigeration and Air Conditioning
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Introduction The mechanism used for lowering or producing low temp. in a body or a space, whose temp. is already below the temp. of its surrounding, is called the refrigeration system. Here the heat is being generally pumped from low level to the higher one & is rejected at high temp. jntuworldupdates.org
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Refrigeration The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions. It also includes the process of reducing heat & maintaining the temp. of a body below the general temp. of its surroundings.
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Contd…. In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temp. of its surroundings.
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Refrigerator & Refrigerant A refrigerator is a reversed heat engine or a heat pump which takes out heat from a cold body & delivers it to a hot body. The refrigerant is a heat carrying medium which during their cycle in a refrigeration system absorbs heat from a low temp. system & delivers it to a higher temp. system. jntuworldupdates.org
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Refrigeration Cycle In refrigeration system the heat is being generally pumped from low level to higher one & rejected at that temp. This rejection of heat from low level to higher level of temp. can only be performed with the help of external work according to second law of thermodynamics. jntuworldupdates.org
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Contd…. The total amount of heat being rejected to the outside body consist of two parts:- the heat extracted from the body to be cooled . - the heat equivalent to the mechanical work required for extracting it. jntuworldupdates.org
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Contd…..
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Contd…. A refrigerator is a reverse heat engine run in the reverse direction by means of external aid. Every type of refrigeration system used for producing cold must have the following four basic units:-
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Contd…. • Low temp. thermal sink to which the heat is rejected for cooling the space. • Means of extracting the heat energy from the sink, raising its level of temp. before delivering it to heat receiver. • A receiver is a storage to which the heat is transferred from the high temp., high pressure refrigerant. jntuworldupdates.org
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Contd….. • Means of reducing the pressure & temp. of the refrigerant before it return to the sink. The processes of the cycle are evaporation, compression, condensation & expansion. By reversing the heat engine cycle completely & by changing the working agent, a refrigeration cycle is obtained. jntuworldupdates.org
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Refrigeration Systems • Vapour compression refrigeration system
• Vapour absorption refrigeration system • Thermo electric refrigeration system
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Vapour Compression Refrigeration • This is the most important system from the point of commercial & domestic utility & most practical form of refrigeration. • The working fluid refrigerant used in this refrigeration system readily evaporates & condenses or changes alternatively between the vapour & liquid phases without leaving the refrigerating plant jntuworldupdates.org
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Contd…. • During evaporation it absorbs heat from the cold body or in condensing or cooling it rejects heat to the external hot body . • The heat absorbed from cold body during evaporation is used as its latent heat for converting it from liquid to vapour. • Thus a cooling effect is created in working fluid. jntuworldupdates.org
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Contd…. • This system of refrigeration thus act as latent heat pump since its pump its latent heat from the cold body or brine & rejects it or deliver it to the external hot body or the cooling medium. • According to the law of thermodynamics , this can be done only on the expenditure of energy which is supplied to the system in the form of electrical energy driving the compressor.
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Contd…. • The vapour compression cycle is used in most of the modern refrigeration systems in large industrial plants. • The vapour in this cycle is circulated through the various components of the system, where it undergoes a number of changes in its state or condition. jntuworldupdates.org
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Contd…. • Each cycle of operation consists of the four fundamental changes of state or processes: Expansion Vaporisation Compression Condensation
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Components of Vapour Compression Systems
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Compressor The low pressure & temp. refrigerant from evaporator is drawn into the compressor through the inlet or suction valve , where it is compressed to a high pressure & temp.
The high pressure & temp vapour refrigerant is discharged into the condenser through the delivery or discharge valve. jntuworldupdates.org
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Condenser The condenser or the cooler consists of coils of pipe in which the high pressure & temp. vapour refrigerant is cooled & condensed. The refrigerant while passing through the condenser, rejects its latent heat to surrounding condensing medium which is normally air or water. Thus hot refrigerant vapour received from compressor is converted into liquid form in condenser.
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Receiver The condensed liquid refrigerant from the condenser is stored in a vessel, known as receiver, from where it is supplied to the expansion valve or refrigerant control valve.
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Expansion Valve The function of this valve is to allow the liquid refrigerant under high pressure & temp. to pass at a controlled rate after reducing its pressure & temp. some of liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator at the low pressure & temp. jntuworldupdates.org
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Evaporator An evaporator consists of coils of pipes in which the liquid vapour refrigerant at low pressure & temp. is evaporated & changed into vapour refrigerant at low pressure & temp. During evaporation process, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium which is to be cooled.
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Advantages • Smaller size for a given refrigerating capacity • Higher coeff. of performance • Lower power requirements for a given capacity • Less complexity in both design & operation • It can be used over large of temp. jntuworldupdates.org
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Domestic Refrigerator • The application of refrigeration for domestic purposes are mainly in the form of domestic refrigerators & home freezers. • The main purpose of this type of refrigeration is to provide low temp. for storage & distribution of foods & drinks.
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Contd…. • It represents a significant portion of the refrigeration industry due to the use of these units in large number. • For domestic preservation, the storage is generally short term. The domestic refrigerators used for the purposes are usually small in sizes with rating in ranges from 1/20 to ½ tonne. jntuworldupdates.org
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Contd…. • The unit is usually self contained and hermetically sealed. • Due to short term storage the domestic refrigerator load is intermittent.
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Contd…. The requirement of domestic refrigerator is that:• it should be simple in construction • automatic in action • nominal in initial cost jntuworldupdates.org
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Contd…. • dependable and without any necessity of expert inspection & repair. • Non irritant & non toxic refrigerant should be used. • Generally methylene chloride, freon-12, freon -11 are used as refrigerants. jntuworldupdates.org
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Contd… • The common type of domestic refrigerator have a cabinet shaped with compressor motor-fan assembly, the condensed and receiver fitted in their basement. • The expansion valve evaporator coils are exposed in the storage cabinet with the piping, carrying liquid refrigerant passing through the body. jntuworldupdates.org
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Contd…. • The heat of the bodies to be cooled is carried to the evaporator coils by means of air trapped in the cabinet. • Refrigeration is not only provided with double walled cabinet packed with materials having high thermal insulation such as fibre glass or expanded rubber but also all around the inside of door flap soft rubber seal is used which makes rubber air tight.
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Electrical Circuit • Refrigerator is provided with a door push switch, which closes on opening of refrigerator and puts the lamp on. • Capacitor start single phase induction motor is used in open type refrigerators and split phase induction motor is used in sealed unit refrigerators. • Electromagnetic relay is provided to connect auxiliary winding on the start & disconnect it when the motor picks up the speed.
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Circuit
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Contd….. • Thermal overload release is provided to protect the motor from damage against flow of over current. • Thermostat switch is provided to control the temp. inside the refrigerator. • Temp. inside the refrigerator can be adjusted by means of temp. control screw.
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Contd… • To protect the motor against under voltage use of automatic voltage regulator is essential since in case of fall in applied voltage, motor will draw heavy current to develop the required torque and will become hot, thermal overload relay will therefore repeatedly disconnect and connect the motor to supply, eventually burning it out. jntuworldupdates.org
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THANKS…..
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Training Session on Energy Equipment
Refrigeration & Air Conditioning Presentation from the “Energy Efficiency Guide for Industry in Asia”
www.energyefficiencyasia.org
1 jntuworldupdates.org
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
2 jntuworldupdates.org
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Introduction How does it work?
High Temperature Reservoir
Heat Rejected
R
Work Input
Heat Absorbed Low Temperature Reservoir 3 jntuworldupdates.org
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Introduction How does it work? Thermal energy moves from left to right through five loops of heat transfer:
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1)
2)
3)
4)
5)
Indoor air loop
Chilled water loop
Refrigerant loop
Condenser water loop
Cooling water loop
(Bureau of Energy Efficiency, 2004)
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Introduction AC Systems AC options / combinations: • Air Conditioning (for comfort / machine) • Split air conditioners • Fan coil units in a larger system • Air handling units in a larger system 5 jntuworldupdates.org
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Introduction Refrigeration systems for industrial processes • Small capacity modular units of direct expansion type (50 Tons of Refrigeration) • Centralized chilled water plants with chilled water as a secondary coolant
(50
– 250 TR)
• Brine plants with brines as lower temperature, secondary coolant (>250 TR) 6 jntuworldupdates.org
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Introduction Refrigeration at large companies • Bank of units off-site with common • Chilled water pumps • Condenser water pumps • Cooling towers
• More levels of refrigeration/AC, e.g.
• Comfort air conditioning (20-25 oC) • Chilled water system (8 – 10 oC) jntuworldupdates.org
• Brine system (< 0 oC)
7 ©Specworld.in UNEP 2006
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
8 jntuworldupdates.org
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Types of Refrigeration Refrigeration systems • Vapour Compression Refrigeration (VCR): uses mechanical energy • Vapour Absorption Refrigeration (VAR): uses thermal energy
9 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration • Highly compressed fluids tend to get colder when allowed to expand • If pressure high enough
• Compressed air hotter than source of cooling • Expanded gas cooler than desired cold temperature 10 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration Two advantages
• Lot of heat can be removed (lot of thermal energy to change liquid to vapour) • Heat transfer rate remains high (temperature of working fluid much lower than what is being cooled) 11 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration Refrigeration cycle 3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
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Type of Refrigeration Low pressure liquid refrigerant Vapour in evaporator Compression absorbs heat and changes to a gas Refrigeration cycle
Refrigeration 3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
13 ©Specworld.in UNEP 2006
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Type of Refrigeration The superheated vapour enters the compressor Vapour Compression where its pressure is raised Refrigeration cycle
Refrigeration 3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
14 ©Specworld.in UNEP 2006
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Type of Refrigeration The high pressure superheated gas is cooled Vapour Compression in several stages in the condenser
Refrigeration
Refrigeration cycle
3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
15 ©Specworld.in UNEP 2006
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Type of Refrigeration Liquid passes through expansion device, which reduces its pressure Vapour and Compression Refrigeration controls the flow into the evaporator
Refrigeration cycle
3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
16 ©Specworld.in UNEP 2006
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Type of Refrigeration Vapour Compression Refrigeration Type of refrigerant
• Refrigerant determined by the required cooling temperature • Chlorinated fluorocarbons (CFCs) or freons: R-11, R-12, R-21, R-22 and R502 17 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration Choice of compressor, design of condenser, evaporator determined by • Refrigerant • Required cooling
• Load • Ease of maintenance • Physical space requirements • Availability of utilities (water, power) jntuworldupdates.org
18 ©Specworld.in UNEP 2006
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Type of Refrigeration Vapour Absorption Refrigeration Condenser
Generator Hot Side
Evaporator Cold Side
Absorber
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Type of Refrigeration Vapour Absorption Refrigeration Evaporator
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Type of Refrigeration Vapour Absorption Refrigeration Absorber
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Type of Refrigeration Vapour Absorption Refrigeration High pressure generator
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Type of Refrigeration Vapour Absorption Refrigeration Condenser
23 jntuworldupdates.org
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Type of Refrigeration Evaporative Cooling •
Air in contact with water to cool it close to ‘wet bulb temperature’
•
Advantage: efficient cooling at low cost
•
Disadvantage: air is rich in moisture Sprinkling Water
Hot Air
Cold Air
(Adapted from Munters, 2001) 24
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
25 jntuworldupdates.org
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Assessment of Refrigeration and AC Assessment of Refrigeration • Cooling effect: Tons of Refrigeration 1 TR = 3024 kCal/hr heat rejected
• TR is assessed as: TR = Q xCp x (Ti – To) / 3024 Q= Cp = Ti = To =
mass flow rate of coolant in kg/hr is coolant specific heat in kCal /kg deg C inlet, temperature of coolant to evaporator (chiller) in 0C outlet temperature of coolant from evaporator (chiller) in 0C 26
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Assessment of Refrigeration and AC Assessment of Refrigeration Specific Power Consumption (kW/TR)
• Indicator of refrigeration system’s performance • kW/TR of centralized chilled water system is sum of • Compressor kW/TR
• Chilled water pump kW/TR • Condenser water pump kW/TR jntuworldupdates.org
• Cooling tower fan kW/TR
27 ©Specworld.in UNEP 2006
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Assessment of Refrigeration and AC Assessment of Refrigeration Coefficient of Performance (COPCarnot) •
Standard measure of refrigeration efficiency
•
Depends on evaporator temperature Te and condensing temperature Tc: COPCarnot
•
Te / (Tc - Te)
COP in industry calculated for type of compressor: COP =
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=
Cooling effect (kW) Power input to compressor (kW)
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Assessment of Refrigeration and AC Assessment of Refrigeration
COP increases with rising evaporator temperature (Te) jntuworldupdates.org
COP increases with decreasing condensing temperature (Tc)
29
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Assessment of Refrigeration and AC Assessment of Air Conditioning Measure •
Airflow Q (m3/s) at Fan Coil Units (FCU) or Air Handling Units (AHU): anemometer
•
Air density (kg/m3)
•
Dry bulb and wet bulb temperature: psychrometer
•
Enthalpy (kCal/kg) of inlet air (hin) and outlet air (Hout): psychrometric charts
Calculate TR
TR
Q ρ h in h out 3024 30
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Assessment of Refrigeration and AC
Assessment of Air Conditioning Indicative TR load profile • Small office cabins : 0.1 TR/m2
• Medium size office (10 – 30 people occupancy) with central A/C: 0.06 TR/m2 • Large multistoried office complexes with central A/C: 0.04 TR/m2 31 jntuworldupdates.org
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Assessment of Refrigeration and AC Considerations for Assessment • Accuracy of measurements • Inlet/outlet temp of chilled and condenser water
• Flow of chilled and condenser water
• Integrated Part Load Value (IPLV) • kW/TR for 100% load but most equipment operate between 50-75% of full load • IPLV calculates kW/TR with partial loads • Four points in cycle: 100%, 75%, 50%, 25% jntuworldupdates.org
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
33 jntuworldupdates.org
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Energy Efficiency Opportunities 1. Optimize process heat exchange 2. Maintain heat exchanger surfaces
3. Multi-staging systems 4. Matching capacity to system load 5. Capacity control of compressors 6. Multi-level refrigeration for plant needs 7. Chilled water storage 8. System design features jntuworldupdates.org
34 ©Specworld.in UNEP 2006
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Energy Efficiency Opportunities 1. Optimize Process Heat Exchange High compressor safety margins: energy loss 1. Proper sizing heat transfer areas of heat exchangers and evaporators
• Heat transfer coefficient on refrigerant side: 1400 – 2800 Watt/m2K • Heat transfer area refrigerant side: >0.5 m2/TR
2. Optimum driving force (difference Te and Tc): 1oC raise in Te = 3% power savings 35 jntuworldupdates.org
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Energy Efficiency Opportunities 1. Optimize Process Heat Exchange Evaporator Temperature (0C)
Refrigeration Capacity*(tons)
Specific Power Consumption (kW/TR)
Increase kW/TR (%)
5.0
67.58
0.81
-
0.0
56.07
0.94
16.0
-5.0
45.98
1.08
33.0
-10.0
37.20
1.25
54.0
-20.0
23.12
1.67
106.0
Condenser temperature 40◦C
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(National Productivity Council)
Condensing Temperature (0C)
Refrigeration Capacity (tons)
Specific Power Consumption (kW /TR)
Increase kW/TR (%)
26.7
31.5
1.17
-
35.0
21.4
1.27
8.5
40.0
20.0
1.41
20.5
*Reciprocating compressor using R-22 refrigerant. Evaporator temperature.-10◦ C
36 ©Specworld.in UNEP 2006
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Energy Efficiency Opportunities 1. Optimize Process Heat Exchange 3. Selection of condensers • Options: • • •
Air cooled condensers Air-cooled with water spray condensers Shell & tube condensers with water-cooling
• Water-cooled shell & tube condenser • • •
Lower discharge pressure Higher TR Lower power consumption 37
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Energy Efficiency Opportunities 2. Maintain Heat Exchanger Surfaces • Poor maintenance = increased power consumption • Maintain condensers and evaporators • Separation of lubricating oil and refrigerant
• Timely defrosting of coils • Increased velocity of secondary coolant
• Maintain cooling towers • 0.55◦C reduction in returning water from cooling tower = 3.0 % reduced power jntuworldupdates.org
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Energy Efficiency Opportunities 2. Maintain Heat Exchanger Surfaces Effect of poor maintenance on compressor power consumption Te (0C)
Tc (0C)
Refrigeration Capacity* (TR)
Specific Power Consumption (kW/TR)
Normal
7.2
40.5
17.0
0.69
-
Dirty condenser
7.2
46.1
15.6
0.84
20.4
Dirty evaporator
1.7
40.5
13.8
0.82
18.3
Dirty condenser and evaporator
1.7
46.1
12.7
0.96
38.7
Condition
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(National Productivity Council)
Increase kW/TR (%)
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Energy Efficiency Opportunities 3. Multi-Staging Systems • Suited for
• Low temp applications with high compression • Wide temperature range
• Two types for all compressor types • Compound • Cascade 40 jntuworldupdates.org
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Energy Efficiency Opportunities 3. Multi-Stage Systems a. Compound •
Two low compression ratios = 1 high
•
First stage compressor meets cooling load
•
Second stage compressor meets load evaporator and flash gas
•
Single refrigerant
b. Cascade
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•
Preferred for -46 oC to -101oC
•
Two systems with different refrigerants
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Energy Efficiency Opportunities 4. Matching Capacity to Load System • Most applications have varying loads
• Consequence of part-load operation • COP increases • but lower efficiency
• Match refrigeration capacity to load requires knowledge of • Compressor performance • Variations in ambient conditions • Cooling load jntuworldupdates.org
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Energy Efficiency Opportunities 5. Capacity Control of Compressors • Cylinder unloading, vanes, valves • Reciprocating compressors: step-by-step through cylinder unloading:
• Centrifugal compressors: continuous modulation through vane control • Screw compressors: sliding valves
• Speed control • Reciprocating compressors: ensure lubrication system is not affected • Centrifugal compressors: >50% of capacity jntuworldupdates.org
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Energy Efficiency Opportunities 5. Capacity Control of Compressors • Temperature monitoring • Reciprocating compressors: return water (if varying loads), water leaving chiller (constant loads) • Centrifugal compressors: outgoing water temperature • Screw compressors: outgoing water temperature
• Part load applications: screw compressors more efficient jntuworldupdates.org
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Energy Efficiency Opportunities 6. Multi-Level Refrigeration Bank of compressors at central plant
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•
Monitor cooling and chiller load: 1 chiller full load more efficient than 2 chillers at part-load
•
Distribution system: individual chillers feed all branch lines; Isolation valves; Valves to isolate sections
•
Load individual compressors to full capacity before operating second compressor
•
Provide smaller capacity chiller to meet peak demands
45
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Energy Efficiency Opportunities 6. Multi-Level Refrigeration Packaged units (instead of central plant) • Diverse applications with wide temp range and long distance
• Benefits: economical, flexible and reliable • Disadvantage: central plants use less power
Flow control • Reduced flow • Operation at normal flow with shut-off periods 46 jntuworldupdates.org
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Energy Efficiency Opportunities 7. Chilled Water Storage • Chilled water storage facility with insulation • Suited only if temp variations are acceptable • Economical because • Chillers operate during low peak demand hours: reduced peak demand charges
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• Chillers operate at nighttime: reduced tariffs and improved COP
47
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Energy Efficiency Opportunities 8. System Design Features • FRP impellers, film fills, PVC drift eliminators • Softened water for condensers • Economic insulation thickness • Roof coatings and false ceilings • Energy efficient heat recovery devices
• Variable air volume systems • Sun film application for heat reflection jntuworldupdates.org
• Optimizing lighting loads
48 ©Specworld.in UNEP 2006
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Training Session on Energy Equipment
Refrigeration & Air Conditioning Systems THANK YOU FOR YOUR ATTENTION
49 jntuworldupdates.org
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Disclaimer and References • This PowerPoint training session was prepared as part of the project “Greenhouse Gas Emission Reduction from Industry in Asia and the Pacific” (GERIAP). While reasonable efforts have been made to ensure that the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication. © UNEP, 2006.
• The GERIAP project was funded by the Swedish International Development Cooperation Agency (Sida) • Full references are included in the textbook chapter that is 50 available on www.energyefficiencyasia.org jntuworldupdates.org Specworld.in
© UNEP 2006
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Refrigeration & Air Conditioning jntuworldupdates.org
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Objectives • •
• •
Basic operation of refrigeration and AC systems Principle components of refrigeration and AC systems Thermodynamic principles of refrigeration cycle Safety considerations
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Uses of Systems • •
Cooling of food stores and cargo Cooling of electronic spaces and equipment •
• • • •
• •
CIC (computers and consoles) Radio (communications gear) Radars ESGN/RLGN Sonar
Cooling of magazines Air conditioning for crew comfort
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Definition Review •
• •
•
Specific heat (cp): Amount of heat required to raise the temperature of 1 lb of substance 1°F (BTU/lb) – how much for water? Sensible heat vs Latent heat LHV/LHF Second Law of Thermodynamics: must expend energy to get process to work
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Refrigeration Cycle •
•
•
Refrigeration - Cooling of an object and maintenance of its temp below that of surroundings Working substance must alternate b/t colder and hotter regions Most common: vapor compression • •
Reverse of power cycle Heat absorbed in low temp region and released in high temp region
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Generic Refrigeration Cycle
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Thermodynamic Cycle
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Typical Refrigeration Cycle
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Components • • • • • •
Refrigerant Evaporator/Chiller Compressor Condenser Receiver Thermostatic expansion valve (TXV)
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Refrigerant •
Desirable properties: • •
• • • • •
•
High latent heat of vaporization - max cooling Non-toxicity (no health hazard) Desirable saturation temp (for operating pressure) Chemical stability (non-flammable/non-explosive) Ease of leak detection Low cost Readily available
Commonly use FREON (R-12, R-114, etc.)
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Evaporator/Chiller • •
•
Located in space to be refrigerated Cooling coil acts as an indirect heat exchanger Absorbs heat from surroundings and vaporizes • •
Latent Heat of Vaporization Sensible Heat of surroundings
Slightly superheated (10°F) ensures no liquid carryover into compressor •
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Compressor •
Superheated Vapor: • •
•
•
•
Enters as low press, low temp vapor Exits as high press, high temp vapor
Temp: creates differential (DT) promotes heat transfer Press: Tsat allows for condensation at warmer temps Increase in energy provides the driving force to circulate refrigerant through the system
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Condenser •
• •
Refrigerant rejects latent heat to cooling medium Latent heat of condensation (LHC) Indirect heat exchanger: seawater absorbs the heat and discharges it overboard
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Receiver •
•
Temporary storage space & surge volume for the sub-cooled refrigerant Serves as a vapor seal to prevent vapor from entering the expansion valve
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Expansion Device • •
•
Thermostatic Expansion Valve (TXV) Liquid Freon enters the expansion valve at high pressure and leaves as a low pressure wet vapor (vapor forms as refrigerant enters saturation region) Controls: • •
Pressure reduction Amount of refrigerant entering evaporator controls capacity
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Air Conditioning •
• • •
Purpose: maintain the atmosphere of an enclosed space at a required temp, humidity and purity Refrigeration system is at heart of AC system Heaters in ventilation system Types Used: • • •
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Self-contained Refrigerant circulating Chill water circulating Specworld.in
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AC System Types •
Self-Contained System •
•
•
Refrigerant circulating system •
•
Add-on to ships that originally did not have AC plants Not located in ventilation system (window unit) Hot air passed over refrigerant cooling coils directly
Chilled water circulating system •
•
Refrigerant cools chill water Hot air passes over chill water cooling coils
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Basic AC System
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Safety Precautions •
Phosgene gas hazard • •
•
Handling procedures •
•
•
Lethal Created when refrigerant is exposed to high temperatures Wear goggles and gloves to avoid eye irritation and frostbite
Asphyxiation hazard in non-ventilated spaces (bilges since heavier than air) Handling of compressed gas bottles
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Questions?
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Eco-friendly Refrigerants Dr Alka Bani Agrawal Professor,Mechanical Engg UIT,RGPV
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History Of Refrigeration • Refrigeration relates to the cooling of air or liquids, thus providing lower temperature to preserve food, cool beverages, make ice and for many other . • Most evidence indicate that the Chinese were the first to store natural ice and snow to cool wine and other delicacies. • Ancient people of India and Egypt cooled liquids in porous earthen jars. • In 1834, Jacob Perkins, an American, developed a closed refrigeration system using liquid expansion and then compression to produce cooling. He used Ether as refrigerant, in a hand- operated compressor, a water-cooled condenser and an evaporator in liquid cooler.
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Refrigerantion Principle • Modern refrigeration and air-conditioning equipment is dominated by vapour compression refrigeration technology built upon the thermodynamic principles of the reverse Carnot cycle. • Refrigerant Changes phases during cooling and used again and again.
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What is a Refrigerant • Refrigerants are used as working substances in a Refrigeration systems. • Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants. • Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems. • These fluids provide refrigeration by undergoing a phase change process in the evaporator. • Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes
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What is ChloroFloroCarcons • Today’s refrigerants are predominantly from a group of compounds called halocarbons (halogenated hydrocarbons) or specifically fluorocarbons. • Chlorofluorocarbons were first developed by General Motor’s researchers in the 1920’s and commercialized by Dupont as “Freons”.
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Halocarbon Refrigerants • Halocarbon Refrigerant are all synthetically produced and were developed as the Freon family of refrigerants. Examples :
– CFC’s : R11, R12, R113, R114, R115
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Freon Group Refrigerants Application and ODP Values Refrigerant
Areas of Application
ODP
Air-conditioning Systems ranging from 200 to 1.0 2000 tons in capacity. It is used where low freezing point and non-corrosive properties are CFC 12 ( R 12 important. ) It is used for most of the applications. Air- 1.0 conditioning plants, refrigerators, freezers, icecream cabinets, water coolers, window airconditioners, automobile air conditioners. CFC 13 (R 13) For low temp refrigeration up to – 90 C in 1.0 cascade system CFC 11(R11)
CFC113 ( R113 Small to medium air-conditioning system and 1.07 ) industrial cooling 0.8 CFC114 ( R114 In household refrigerators and in large industrial 0.34 ) cooling Blend of R22 Frozen food ice-cream display cases and and R115 warehouses and food freezing plants. An (R502) excellent general low temp refrigerant jntuworldupdates.org
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What is Ozone Layer • Ozone is an isotope of oxygen with three atoms instead of normal two. It is naturally occurring gas which is created by high energy radiation from the Sun. • The greatest concentration of ozone are found from 12 km to 50 km above the earth forming a layer in the stratosphere which is called the ozone layer. • This layer, which forms a semi-permeable blanket, protects the earth by reducing the intensity of harmful ultra-violet (UV) radiation from the sun. jntuworldupdates.org
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Ozone Layer Depletion • In the early70’s,scientists Sherwood Roland and Mario Molina at the University of California at Irvine were the first to discover the loss of ozone in stratosphere while investigating the ozone layer from highflying aircraft and spacecraft. • They postulated the theory that exceptionally stable chlorine containing fluorocarbons could, overtime, migrate to the upper reaches of the atmosphere and be broken by the intense radiation and release chlorine atoms responsible for
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OZONE LAYER DEPLETION • • •
N0RMAL REACTION O2 = O + O O 2 + O = O3
•
But CFC refrigerants leaked during the manufacturing and normal operation or at the time of servicing or repair, mix with surrounding air and rise to troposphere and then into stratosphere due to normal wind or storm. The Ultraviolet rays act on CFC releasing Cl atom, which retards the normal reaction:
• • • • •
RETARDED REACTION O3 = O2 + O CCL2F2 = CCLF2 + CL O3 + CL = CLO + O2 O + CLO = CL + O2
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Harmful consequences of ozone depletion • • • • • • • • • • • • • • • • • • •
For Humans
Increase in
skin cancer snow blindness cataracts Less immunity to infectious diseases malaria herpes
For plants smaller size lower yield increased toxicity altered form
For marine life Reduced plankton juvenile fish larval crabs and shrimps
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MONTREAL PROTOCOL • SIGNED IN 1987 UNDER THE ‘UNEP’, AFTER MUCH DISCUSSIONS • MORE THAN 170 COUNTRIES HAVE RATIFIED • INDIA RATIFIED ON SEPT 17,1992 • ONE OF MOST SUCCESSFUL EXAMPLE OF INTERNATIONAL COOPERATION IN UN HISTORY
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Montreal protocol- Control Schedule
Montréal Protocol- Control Schedule ozone depleting substance
developed countries
developing countries
CFCs
phased out end of 1995
total phase out by 2010
halons
phased out end of 1993
total phase out by 2010
HCFCs
total phase out by 2020
total phase out by 2040
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CFC Phase-out in India • • •
What is to be phased out? CFC-11, CFC-12 & CFC-113a. How much and when?
• • • • •
1999 22,588 MT 2005 11,294 MT 2010 o MT How to achieve the target? Production is controlled through a production quota allocated to each producer every year. The Ozone Cell conducts audits twice a year to monitor the production. How much has been Phaseout? CFC has been completely phased out as on 1st August, 2008
•
Year
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Vapor compression refrigeration System • In 1834 an American inventor named Jacob Perkins obtained the first patent for a vapor-compression refrigeration system, it used ether in a vapor compression cycle. • Joule-Thomson (Kelvin) expansion • Low pressure (1.5 atm) low temperature (10 to +15 ℃) inside • High pressure (7.5 atm) high temperature (+15 to +40 ℃) outside
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Components • • • • • •
Refrigerant Evaporator/Chiller Compressor Condenser Receiver Thermostatic expansion valve (TXV)
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Circulation of Refrigerant •
•
•
•
Compressor cold vapor from the evaporator is compressed, raising it temperature and boiling point adiabatic compression T, b.p. ~ P work done on the gas Condenser hot vapor from the compressor condenses outside the cold box, releasing latent heat isothermal, isobaric condensation (horizontal line on PV diagram) high temperature T (hot) latent heat of vaporization Q (hot) Expansion valve (throttling valve) hot liquid from the condenser is depressurized, lowering its temperature and boiling point adiabatic, isochoric expansion (vertical line on PV diagram) T, b.p. ~ P no work done W = 0 Evaporator cold liquid from the expansion valve boils inside the cold box, absorbing latent heat isothermal, isobaric boiling (horizontal line on PV diagram) low temperature T (cold) latent heat of vaporization Q (cold)
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Importance of Refrigerant • The thermodynamic efficiency of a refrigeration system depends mainly on its operating temperatures. • However, important practical issues such as the system design, size, initial and operating costs, safety, reliability, and serviceability etc. depend very much on the type of refrigerant selected for a given application. • Due to several environmental issues such as ozone layer depletion and global warming and their relation to the various refrigerants used, the selection of suitable refrigerant has become one of the most important issues in recent times. jntuworldupdates.org
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Refrigerant selection criteria • Selection of refrigerant for a particular application is based on the following requirements: – i. Thermodynamic and thermo-physical properties – ii. Environmental and safety properties – Iii. Economics jntuworldupdates.org
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Thermodynamic and thermo-physical properties • The requirements are: • a) Suction pressure: At a given evaporator temperature, the saturation pressure should be above atmospheric for prevention of air or moisture ingress into the system and ease of leak detection. Higher suction pressure is better as it leads to smaller compressor displacement • b) Discharge pressure: At a given condenser temperature, the discharge pressure should be as small as possible to allow lightweight construction of compressor, condenser etc. • c) Pressure ratio: Should be as small as possible for high volumetric efficiency and low power consumption • d) Latent heat of vaporization: Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small
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Thermodynamic and thermo-physical properties • In addition to the above properties; the following properties are also important: • e) Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be small • f) Liquid specific heat: Should be small so that degree of subcooling will be large leading to smaller amount of flash gas at evaporator inlet • g) Vapour specific heat: Should be large so that the degree of superheating will be small • h) Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficients • i) Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops • The thermodynamic properties are interrelated and mainly depend on normal boiling point, critical temperature, molecular weight and structure.
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Environmental and safety properties • At present the environment friendliness of the refrigerant is a major factor in deciding the usefulness of a particular refrigerant. The important environmental and safety properties are: • a) Ozone Depletion Potential (ODP): According to the Montreal protocol, the ODP of refrigerants should be zero, i.e., they should be non-ozone depleting substances. Refrigerants having nonzero ODP have either already been phased-out (e.g. R 11, R 12) or will be phased-out in nearfuture(e.g. R22). Since ODP depends mainly on the presence of chlorine or bromine in the molecules, refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be used under the new regulations jntuworldupdates.org Specworld.in
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Environmental Effects of Refrigerants Global warming :
Refrigerants directly contributing to global warming when released to the atmosphere Indirect contribution based on the energy consumption of among others the compressors ( CO2 produced by power stations ) jntuworldupdates.org
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Environmental and safety properties • b) Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming. Refrigerants with zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future. • c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming. Naturally, refrigerants with as a low a value of TEWI are preferable from global warming point of view. jntuworldupdates.org
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Environmental and safety properties •
d) Toxicity: Ideally, refrigerants used in a refrigeration system should be non-toxic. Toxicity is a relative term, which becomes meaningful only when the degree of concentration and time of exposure required to produce harmful effects are specified. Some fluids are toxic even in small concentrations. Some fluids are mildly toxic, i.e., they are dangerous only when the concentration is large and duration of exposure is long. In general the degree of hazard depends on: – - Amount of refrigerant used vs total space – - Type of occupancy – - Presence of open flames – - Odor of refrigerant, and
– - Maintenance condition jntuworldupdates.org
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Environmental and safety properties • e) Flammability: The refrigerants should preferably be non-flammable and non-explosive. For flammable refrigerants special precautions should be taken to avoid accidents. • f) Chemical stability: The refrigerants should be chemically stable as long as they are inside the refrigeration system. • g) Compatibility with common materials of construction (both metals and non-metals) • h) Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. ammonia). Refrigerants that are completely miscible with oils are easier to handle(R12). jntuworldupdates.org
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Environmental and safety properties • Ease of leak detection: In the event of leakage of refrigerant from the system, it should be easy to detect the leaks. Economic properties: • The refrigerant used should preferably be inexpensive and easily available. jntuworldupdates.org
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ECO-FRIENDLY REFRIGERANTS
CFC ALTERNATIVES. HCFC R22,R124
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HFC R134a,R152a
NATURAL REFRIGERANT NH3, HC'S
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Halocarbon Refrigerants • Halocarbon Refrigerant are all synthetically produced and were developed as the Freon family of refrigerants. Examples :
– CFC’s : R11, R12, R113, R114, R115 – HCFC’s : R22, R123 – HFC’s : R134a, R404a, R407C, R410a
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HFCs • Remain a popular choice – especially for R22 phase out
• Good efforts at improving leakage performance – e.g. Real Zero project
• Interest in R407A to replace R404A – 50% reduction in GWP
Fjntuworldupdates.org Gas Stakeholder Group, 14th October 2009
Specworld.in Slide 30
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Inorganic Refrigerants • • • • •
Carbon Dioxide Water Ammonia Air Sulphur dioxide
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HCFC • Transitional compounds with low ODP • Partially halogenated compounds of hydrocarbon • Remaining hydrogen atom allows Hydrolysis and can be absorbed. • R22, R123
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HCFC • • • •
Production frozen at 1996 level 35% cut by 2005,65% by 2010 90% by 2015,100 % by 2030 10 year grace period for developing countries.
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R22 • ODP-0.05, GWP-1700 • R22 has 40% more refrigerating capacity • Higher pressure and discharge temp and not suitable for low temp application • Extensively used in commercial airconditioning and frozen food storage and display cases
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R123 • ODP-0.02,GWP-90 • As a replacement for R11 as similar thermodynamic properties. • Very short atmospheric life but classified as carcinogen • Retrofit alternative to R11
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HFC • Zero ODP as no chlorine atom contains only Hydrogen and Flurodine • Very small GWP values • No phase out date in Montreal Protocol • R134a and R152 a – Very popular refrigerants • HFC refrigerants are costly refrigerants
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R134a • ODP-0, GWP-1300 • Used as a substitute for R12 and to a limited range for R22 • Good performance in medium and high temp application • Toxicity is very low • Not miscible with mineral oil
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R152a • ODP-0,GWP-140 • R152a is another attractive HFC with similar properties to R12. • GWP is one order less than HFC134a but it is slightly flammable. • Also it has lower energy consumption. Hence the Environmental Protection Agency of Europe prefers HFC152a to HFC134a
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Hydrocarbon • Very promising non-halogenated organic compounds • With no ODP and very small GWP values • Their efficiency is slightly better than other leading alternative refrigerants • They are fully compatible with lubricating oils conventionally used with CFC12.
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Hydrocarbon Refrigerants • Extraordinary reliability- The most convincing argument is the reliability of the hydrocarbon system because of fewer compressor failures. • But most of the hydrocarbons are highly flammable and require additional safety precaution during its use as refrigerants. • Virtually no refrigerant losses • Hydrocarbons have been used since the beginning of the century and now being considered as long term solutions to environmental problems,
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Hydrocarbons • Dominant in domestic market like household refrigerators and freezers • Growing use in very small commercial systems like car air-conditioning system • Examples:
R170, Ethane, C2H6 R290 , Propane C3H3 R600, Butane, C4H10 R600a, Isobutane, C4H10 Blends of the above Gases
Fjntuworldupdates.org Gas Stakeholder Group, 14th October 2009
Specworld.in Slide 42
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R290 • • • •
ODP-0,GWP-3 Compatible with copper.Miscible with mineral oil Highest latent heat and largest vapour density A third of original charge only is required when replacing halocarbons refrigerant in existing equipment • Energy saving : up to 20% due to lower molecular mass and vapour pressure
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R 600a • ODP-0,GWP-3 • Higher boiling point hence lower evaporator pressure • Discharge temp is lowest • Very good compatibility with mineral oil
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Flammability • Approximate auto ignition temperatures • R22 630 ºC • R12 750 ºC • R134a 740 ºC • R290 465 ºC • R600a 470 ºC • jntuworldupdates.org
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Modifications of Electrical Equipment • Replaced with solid state equivalents • Sealed to ensure that any sparks do not come into contact with leaking gas • Relocated to a position where the component would not come into contact with leaking gas jntuworldupdates.org
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Modifications of Electrical Equipment • Faulty components. • Poor, corroded, loose, or dirty electrical connections. • Missing or broken insulation which could cause arcing/sparks. • Friction sparks, like a metal fan blade hitting a metal enclosure. jntuworldupdates.org
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Blends & Mixtures • Limited no of pure refrigerants with low ODP & GWP values • To try a mixture of pure refrigerants to meet specific requirement
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Azeotropic Refrigerants • A stable mixture of two or several refrigerants whose vapour and liquid phases retain identical compositions over a wide range of temperatures. • Examples : R-500 :
73.8% R12 and 26.2%
R152 R-502 : 8.8% R22 and 51.2% R115 R-503 : 40.1% R23 and 59.9% R13 jntuworldupdates.org
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Zeotropic Refrigerants • A zeotropic mixture is one whose composition in liquid phase differs to that in vapour phase. Zeotropic refrigerants therefore do not boil at constant temperatures unlike azeotropic refrigerants. • Examples :R404a : R125/143a/134a
(44%,52%,4%) R407c : R32/125/134a (23%, 25%, 52%) R410a : R32/125 (50%, 50%) R413a : R600a/218/134a (3%, 9%, 88%)
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Inorganic Refrigerants • • • • •
Carbon Dioxide Water Ammonia Air Sulphur dioxide
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Carbon Dioxide • • • •
Zero ODP & GWP Non Flammable, Non toxic Inexpensive and widely available Its high operating pressure provides potential for system size and weight reducing potential.
• Drawbacks: • Operating pressure (high side) : 80 bars • Low efficiency jntuworldupdates.org
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Ammonia –A Natural Refrigerant Ammonia is produced in a natural way by human beings and animals; 17 grams/day for humans. Natural production
3000 million tons/year
Production in factories
120 million tons/year
Used in refrigeration
6 million tons/year
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Ammonia as Refrigerant • ODP = 0 • GWP = 0 • Excellent thermodynamic characteristics: small molecular mass, large latent heat, large vapour density and excellent heat transfer characteristics • High critical temperature (132C) : highly efficient cycles at high condensing temperatures • Its smell causes leaks to be detected and fixed before reaching dangerous concentration • Relatively Low price jntuworldupdates.org
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Some Drawbacks of Ammonia as Refrigerant • • • •
Toxic Flammable ( 16 – 28% concentration ) Not compatible with copper Temperature on discharge side of compressor is higher compared to other refrigerants
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Water • Zero ODP & GWP • Water as refrigerant is used in absorption system .New developing technology has created space for it for use in compression cycles also. • But higher than normal working pressure in the system can be a factor in restricted use of water as refrigerant jntuworldupdates.org
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Application of New Eco-friendly Refrigerants • • • • • • • • • • • •
Application Eco-friendly refrigerant Domestic refrigeration Commercial refrigeration Cold storage ,food processing And industrial refrigeration Unitary air conditioners Centralized AC (chillers) Transport refrigeration Mobile air conditioner Heat pumps
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HFCs used
Possible
R134a,R152a R134a,R404A,R407C
HC600a and blends HC blends,NH3 ,CO2 **
R134a,R404A,R507A R410A,R407C R134a,R410A,R407C R134a,R404A R134a R134a,R152a,R404A R407C,R410A
NH3 ,HCs,CO2 ** CO2 , HC s NH3 ,HCs,CO2, water ** CO 2, CO2 ,HCs NH3 ,HCs,CO2, water **
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Table 3 -Performance Results of R12 and New Proposed Refrigerants using the same compressor as with R12 in Milk Chilling Unit
•
• Refrigerants • Mass flow rate • kg / min • Refrigerating capacity • kW • Compressor power • kW • COP • Volumetric Refrigeration Capacity(kj/ m3) • Discharge • Temp • C • • R 32 • R 290 • R 22 • R 12 • R 134a • R 152a • R 124 • R 600a • R 142b • R 600 • • • 8.81 • 4.45 • 7.45 jntuworldupdates.org • 6.99
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General Safety measures for refrigerating plants • Reduction of refrigerant contents: – Components with reduced contents – Indirect systems with secondary refrigerant: distinction between generation and transport of cold • Scheduled maintenance and leak testing • Governmental surveillance – Refrigerant Audits for systems operating with HFC’s. Recovery, Stock of used refrigerants, Recycling of refrigerants. • For the Netherlands, the combined measures resulted in a leak rate reduction of 35% (1995) to 8% (2001) for R22systems jntuworldupdates.org
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Survey Of Refrigerants Refrigerant
Group
Atmospheri c life
ODP
GWP
R11
CFC
130
1
4000
R12
CFC
130
1
8500
R22
HCFC
15
.05
1500
R134a
HFC
16
0
1300
R404a
HFC
16
0
3260
R410a
HFC
16
0
1720
R507
HFC
130
1
3300
R717
NH3
-
0
0
R744
CO2
-
0
1
R290
HC
<1
0
8
R600a
HC
<1
0
8
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Conclusions • In the aftermath of the Montreal protocole HFC’s have predominantly replaced CFC’s and HCFC’s in RAC equipment. • Due to their high GWP, HFC’s are not a good replacement solution. • The solution are the natural refrigerants : Ammonia, Hydrocarbons and Carbon dioxide • System need to have low TEWI factor • High efficiency with ammonia and lower power consumption with hydrocarbons jntuworldupdates.org
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• •
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Thank You
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Environmental Effects of Refrigerants Global warming :
Refrigerants directly contributing to global warming when released to the atmosphere Indirect contribution based on the energy consumption of among others the compressors ( CO2 produced by power stations ) jntuworldupdates.org
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PowerPoint® Presentation
Chapter 6 Refrigeration Systems Refrigeration • Mechanical Compression Refrigeration • Absorption Systems • Troubleshooting and Maintaining Refrigeration Systems • Refrigerant Regulations • Refrigerant Handling jntuworldupdates.org
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Chapter 6 — Refrigeration Systems
A refrigeration system controls the absorption and rejection of heat by refrigerant to move heat from inside a cooled space to outside the cooled space.
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Chapter 6 — Refrigeration Systems
In a mechanical compression refrigeration system, a compressor is used to produce the refrigeration effect.
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Chapter 6 — Refrigeration Systems
Refrigerant vapor pressure charts list the saturation temperature and pressure of refrigerants.
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Chapter 6 — Refrigeration Systems
Refrigeration compressors include reciprocating, vane, centrifugal, and screw compressors.
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Chapter 6 — Refrigeration Systems
Refrigerant compressors are available in hermetic and semi-hermetic configurations.
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Chapter 6 — Refrigeration Systems
An air-cooled condenser removes heat from highpressure refrigerant vapor by air blown across the condenser coils.
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Chapter 6 — Refrigeration Systems
Water-cooled condensers transfer heat from refrigerant vapor to water.
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Chapter 6 — Refrigeration Systems
Evaporative condensers reject heat through the evaporation of water.
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Chapter 6 — Refrigeration Systems
A fin comb is used for condenser maintenance to straighten damaged or bent fins, which limit airflow and reduce condenser efficiency. Robinair Division, SPX Corporation
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Chapter 6 — Refrigeration Systems
A thermostatic expansion valve uses temperature readings at the evaporator outlet to control the rate of refrigerant flow into the evaporator.
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Chapter 6 — Refrigeration Systems
The opening and closing of a thermostatic expansion valve is controlled by the pressure in the remote bulb.
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Chapter 6 — Refrigeration Systems
An automatic expansion valve controls the temperature of the refrigerant by controlling the pressure in the evaporator.
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Chapter 6 — Refrigeration Systems
As refrigerant is forced through the capillary tube, it loses pressure until it is at the desired evaporator pressure.
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Chapter 6 — Refrigeration Systems
An evaporator vaporizes low-pressure refrigerant liquid into a low-pressure vapor.
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Chapter 6 — Refrigeration Systems
A hot-gas defrost uses hot gas from the compressor to melt frost on the evaporator.
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Chapter 6 — Refrigeration Systems
An evaporator pressure regulating valve allows two evaporators running from the same compressor to maintain different temperatures.
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Chapter 6 — Refrigeration Systems
Accessories are used for maintaining and controlling the flow of refrigerant in a refrigeration system.
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Chapter 6 — Refrigeration Systems
Pressure switches control refrigeration system temperature through changes in system pressure.
Ranco Inc.
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Chapter 6 — Refrigeration Systems
Ammonia systems operate at high temperatures and pressures and must have special controls and fittings to control the release of ammonia gas.
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Chapter 6 — Refrigeration Systems
The direction of refrigerant flow in a heat pump is controlled by a reversing valve.
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Chapter 6 — Refrigeration Systems
Heat pumps may use air or water as the heat source.
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Chapter 6 — Refrigeration Systems
Chillers use chilled water to cool large building spaces.
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Chapter 6 — Refrigeration Systems
A cooling tower cools water from a condenser by the evaporation of water as it cascades through the tower.
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Chapter 6 — Refrigeration Systems
Absorption systems use a generator and absorber in place of the compressor to raise system pressure.
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Chapter 6 — Refrigeration Systems
Gauge manifolds are used to take pressure readings, add or remove refrigerant, and remove air from a system before it is filled with refrigerant.
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Chapter 6 — Refrigeration Systems
Service valves are frontseated for isolating parts of the system, midseated for adding or removing refrigerant or taking system pressures, and back-seated during normal operation.
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Chapter 6 — Refrigeration Systems
Refrigeration system pressure readings are taken by connecting the blue hose to the low-pressure side service valve and the red hose to the high-pressure side service valve with both gauge manifold valves front-seated.
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Chapter 6 — Refrigeration Systems
A head pressure controller prevents the condenser pressure from falling too low and starving the evaporator for refrigerant.
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Chapter 6 — Refrigeration Systems
The EPA has established regulations under Section 608 of the Clean Air Act to regulate the handling of ozone-depleting substances.
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Chapter 6 — Refrigeration Systems
A recovery unit is used to recover refrigerant for reuse.
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Chapter 6 — Refrigeration Systems
A leak detector is a device used to detect refrigerant leaks in air conditioning or refrigeration systems.
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Chapter 6 — Refrigeration Systems
A vacuum pump removes all air from a refrigeration system.
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Chapter 6 — Refrigeration Systems
The vacuum pump is connected to the system service valves using a gauge manifold and hoses.
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Chapter 6 — Refrigeration Systems
Electronic leak detectors are extremely sensitive and indicate the general location of a leak.
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Chapter 6 — Refrigeration Systems
The refrigerant container is placed upside down to charge with liquid and right side up to charge with vapor.
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LECTURE NOTES COURSE: REFRIGERATION & AIR CONDITIONING
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Methods of Air Refrigeration system • • • • • •
Simple air cooling system Simple air evaporative cooling system Boot strap air cooling system Boot strap air evaporative cooling system Reduced ambient air cooling system Regenerative air cooling system
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Simple air evaporative cooling system
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Refrigeration Systems
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COP
COP = coefficient of performance Air conditioners, refrigerators: COP=QL/Wnet Heat pumps: COP=QH/Wnet Energy balance: Wnet+QL=QH
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From Cengel, Thermodynamics: An Engineering Approach, 6th ed.
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Reversed Carnot cycle -- ideal COPR,Carnot
1 TH
TL 1
1 COPHP,Carnot 1 TL
Why isn’t this cycle possible in real life?
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TH
From Cengel, Thermodynamics: An Specworld.in Engineering Approach, 6th ed.
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Ideal Refrigeration Cycle 1)
2)
3)
4)
x=1 (saturated vapor), P=Plow or T=Tlow P=Phigh, s2=s1 (constant entropy) P=Phigh, x=0 (saturated liquid) h3=h4 (constant enthalpy), P=Plow
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From Cengel, Thermodynamics: An Specworld.in Engineering Approach, 6th ed.
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Example
An ideal vapor-compression cycle has a mass flow rate of R-134a of 0.05 kg/s. The low and high system pressures are 0.12 MPa and 0.70 MPa. Find – – – –
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The rate of power input The rate of heat transfer out of the refrigerated space The rate of heat transfer to the surroundings COP
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Cycle efficiency
To increase the COP of the cycle, increase the evaporation temperature or decrease the condensing temperature. –
–
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However, you can’t achieve as cold of a temperature now, and your heat exchanger will need to be larger since DT is smaller. 2-4% increase in COP per degree temperature change Specworld.in
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Sources of Inefficiencies *Compressor efficiency < 100% Pressure drop in piping Heat transfer to/from lines *Superheating of fluid entering compressor to prevent liquid from entering *Subcooling of fluid entering expansion valve to prevent vapor from entering * We will only look at these in class.
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Expansion Valve Operation
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
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Example
A vapor-compression refrigeration cycle operates using R-134a with low and high system pressures of 0.10 MPa and 1.20 MPa. The fluid leaves the evaporator superheated by 6.37°C and leaves the condenser subcooled by 4.29°C. Calculate the COP if the compressor efficiency is a) 100% and b) 84%.
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Compressor Performance
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Compressor Basics
As with ideal pumps, dh=vdP. However, v is not a constant, making calculation of h2-h1 more complicated. For a polytropic process, Pvn=C 2
2
p
h2 h1 vdP C 1
1
1
n
dP
(1)
For an isentropic process and an ideal gas, n=k (where k=cp/cv), and for an isothermal process, n=1.
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Compressor Efficiency
The adiabatic compressor efficiency:
a
Wisentropic
h2 h1 s
Wactual h2 h1 actual Total compressor efficiency:
compressor amotor drivemechanical
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(2)
(3)
Typical efficiencies are 90% for the motor drive at peak load, 90% for the mechanical efficiency, and 76% to 97% for the adiabatic (isentropic) efficiency. Typically, as the compressor size increases, so does the adiabatic efficiency.
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Exit temperature
A maximum recommended fluid temperature is given based on compressor and fluid type. Air compressors typically shouldn’t have an air exit temperature greater than 300-375ºF to prevent carbonizing, combustion of oil vapor, or weakening of parts over time. Air can be modeled as a perfect gas where and R
Pv RT / M
c p cv
M
These can be substituted into Equ. (1), and using Equ. (2) or (3) as well, the exit air temperature T2 can be found as a function of pressure ratio r, n, k, and efficiency. Once T2 is known, the compressor work can be found using
Wactual h2 h1 mc p T2 T1 jntuworldupdates.org
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Decreasing Compressor Work
Two possible methods – –
Minimize irreversibilities due to friction, turbulence, and nonquasi-equilibrium compression Make the specific volume of the gas as low as possible by cooling the compressor since in the ideal case 2
Wcompressor h2 h1 vdP
One cooling possibility
1
is to use multi-stage compression with intercooling. From Cengel, Thermodynamics: An Engineering Approach, 4th ed.
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Compressor Types
Five most common types –
Reciprocating
–
Screw
–
Uses a roller to compress gas Used in most domestic refrigeration and ac systems
Scroll
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Uses centrifugal force to compress gas Common in large refrigeration systems (200 to 10000 kW of refrigeration capacity)
Vane
–
Lobes of two rotating screws trap and compress gas
Centrifugal
–
Uses a piston-cylinder and valves Most common type of compressor
Two inter-fitting spiral-shaped scrolls compress the gas Used in 1-15 ton (3.5 to 53 kW) range AC applications Specworld.in
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Compressor pressure ranges
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From Burmeister, Elements of Thermal-Fluid Design.
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Terminology
Open-type compressor – –
Hermetically sealed – –
Motor and compressor are combined in the same housing Used for small domestic air conditioning systems
Semi-hermetic –
Crankshaft extends through housing to connect with the motor Seals are used to limit leakage
Cylinder heads are removable for serviceability. Good for AC systems larger than domestic.
Condensing unit –
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Motor, compressor, and condenser are combined in Specworld.in one unit and sold together
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Reciprocating Compressors
Gas in the clearance volume must expand to V1 before the pressure is low enough to open the suction valves and draw more gas in.
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
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Reciprocating Compressors, cont.
Actual volumetric efficiency va
3
Clearance volumetric efficiency va –
s x100 displacement rate of compressor m s
3 volume flow rate entering compressor m
volume of gas drawn into cylinder V V x100 3 1 x100 useable volume of cylinder V3 Vc
The clearance volumetric efficiency tells us what percent of the clearance volume is used to bring new gas in.
Percent clearance
m
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Vc x100 V3 Vc
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Reciprocating Compressors, cont.
After some algebra V1 vc 100 m 1 Vc
where
V1 vsuc Vc vdis
vsuc=specific volume of vapor entering compressor vdis=specific volume of vapor after isentropic compression jntuworldupdates.org
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Reciprocating Compressors, cont.
To find mass flow rate (kg/s) m displacement rate x
vc 100 vsuc
The displacement rate is a volumetric flow rate; vsuc converts that to a mass flow rate As the suction pressure (and evaporating temperature) drops, what happens to the mass flow rate? On a cold winter day, the evaporating temperature will be very low for a room AC unit. What problems could this cause?
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Reciprocating Compressor Performance
Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill. jntuworldupdates.org
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Reciprocating Compressor Performance
Most refrigeration systems operate on the left side of the power curve. During startup, the power requirement may pass the peak and demand more motor power.
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
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Reciprocating Compressors, cont.
Adiabatic compression (isentropic) efficiency (use this to find the actual enthalpy at the compressor exit)
a
wisentropic
x100
wactual Losses are due mainly to friction of rubbing surfaces and pressure drop across valves Watch your exit conditions. If the exit temperature is too hot, the oil will break down and reduce the life of your valves. The maximum recommended oil temperature varies with the oil type. This can be a problem especially with ammonia, which tends to have high discharge temperatures. Ammonia compressors often are equipped with external water jntuworldupdates.org Specworld.in cooling.
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Rotary Screw Compressors
Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill.
Good efficiency (60-80%) for pressure ratios above approx. 2.5.
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Vane Compressors
No suction valve needed. Minimum gas pulsation
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Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed, Mc-Graw Hill. Specworld.in
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Dynamic Compressors -- Centrifugal
Commonly used for large systems, including chillers Gas enters a spinning impeller and is thrown to the outside of the impeller through centrifugal force Impeller provides the gas with a high velocity (kinetic energy) which is converted to pressure (internal energy); remember Bernoulli’s Law! 70-80% isentropic efficiencies Axial compressors are a somewhat less common form of dynamic compressors
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Scroll Compressors
Need close machining tolerances Low noise, high efficiency Incompatible with solid contaminants and poor performance at low suction pressures
From McQuiston, Parker, and Spitler, Heating, Ventilating, and Air Conditioning Analysis and Design jntuworldupdates.org
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Expansion Valves
Figures are from Stoecker and Jones, Refrigeration and Air Conditioning, 2nd ed., McGraw Hill
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Expansion Devices
Two purposes – –
Reduce pressure of refrigerant at approx. constant enthalpy, resulting in a large temperature drop Regulate refrigerant flow to the evaporator
Main types – – – –
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Capillary tubes – used up for refrigerating capacities of approx. 10 kW or less; common in domestic refrigerators Constant-pressure expansion valve – for systems with refrigerating capacity of 30 kW or less Float valves – used in large industrial applications Thermostatic expansion valve - the most popular type of valve, capable of providing a wide range of evaporator temperatures
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Capillary Tubes
1 to 6 m long, 0.5 to 2 mm inside diameter Pressure drops through the tube due to friction and fluid acceleration Cap tubes and cheap and reliable, but they can’t adjust to changes in parameters such as added load, suction pressure, etc. You’d need to install a new tube to get different system performance. They also can be clogged. Mass flow rate is determined by a balance point between cap tube and compressor performance. If there is too much or too little heat transfer in the evaporator for the given balance point, the evaporator will be starved or overfed.
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Capillary Tubes, cont.
Fig. 13-1
Starved evaporator– not enough refrigerant to provide enough cooling capacity Overfed evaporator – too much refrigerant for the amount of cooling needed, resulting in slugging of the compressor (liquid drops enter the compressor)
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Capillary Tubes, cont.
As a result, refrigerant charge must be within close limits. Therefore, cap tubes are usually used only with hermetically sealed compressors since they don’t leak. Usually only liquid enters the tube. As the pressure and temperature drop, more and more of the liquid flashes to vapor. Vapor has a larger specific volume than liquid, so the fluid must speed up. If the pressure drops low enough, choked flow will result. Further decreases in pressure will have no effect on the flow rate through the nozzle. In this case, sonic velocity occurs at the end of the tube!
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Capillary Tubes, cont. Table 13-1 quality
Choked flow jntuworldupdates.org
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Note 70 m/s=157 mi/hr=252 km/hr
Constant-Pressure Expansion and Float Valves
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Constant-pressure expansion valves maintain constant pressure in the evaporator by opening or closing –
Used a lot when a very precise evaporator temperature is needed, such as in water coolers (to prevent freezing) or rooms where humidity control is very important (such as banana-curing rooms)
Float valves maintain the liquid level in the evaporator at a constant level by opening or closing –
– –
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Can react easily to changes in load Used in large installations In smaller installations where continuous-tube evaporators are used, they can’t be used since it’s nearly impossible to establish a liquid level. Specworld.in
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Thermostatic (Superheat-Controlled) Expansion Valve
Fig 13-12
Feeler bulb is filled with same refrigerant as in system and is clamped to the outlet of the evaporator. If too little refrigerant is in the evaporator, it will be very superheated at the exit. This will make the refrigerant in the feeler bulb evaporate, increasing the pressure on the diaphragm. This will open the valve further, letting more refrigerant in, decreasing the temperature at the evaporator exit.
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Electric Expansion Valve
Like a thermostatic expansion valve, except a thermister is used to sense the evaporator exit temperature. Used for a lot for systems that can be run as either heat pumps or ac units since it’s OK to run fluid through them backwards
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Multi-Pressure Refrigeration Systems
Figures from Refrigeration and Air Conditioning, 2.5 edition, by Stoecker and Jones and Thermodynamics:An Engineering Approach by Çengel and Boles
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Cascade Refrigeration Systems
Used in industrial applications where quite low temperatures are required The large temp difference requires a large pressure difference Compressors have low efficiencies for large pressure differences; this results in low system efficiency Refrigeration cycle is performed in stages The refrigerant in the two stages doesn’t mix Higher efficiency results but also a higher first cost
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From Cengel and Boles, Thermodynamics: An Engineering Approach
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Multistage Compression Refrigeration
Similar to a cascade system except the same fluid is used for both stages Compression is done in two stages with a mixing chamber in between. Expansion is also done in two stages. After the first expansion, a liquid/vapor mix is present. In the flash chamber, the saturated vapor is removed and sent to the mixing chamber while the liquid goes through the second expansion valve. This ensures that sufficient cooling capacity and mass flow rate through the valve and is achieved. Watch your mass flow rates! They’re different in different parts of the cycle
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From Cengel and Boles, Thermodynamics: An Engineering Approach
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Benefits of Flash Gas Removal
Here ammonia is best
Here R-134a is best For re-compression of flash gas
From Stoecker and Jones, Refrigeration and Air Conditioning jntuworldupdates.org
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Flash Gas Removal Plus Intercooling
From Stoecker and Jones, Refrigeration and Air Conditioning
•This is a similar process, but the vapor at 2 is also cooled to the saturation temperature by bubbling it through the liquid in the flash tank. Vapor velocity must be less than 1 m/s for this setup to work well. jntuworldupdates.org
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Flash Gas Removal Plus Intercooling • Intercooling alone usually doesn’t result in a power reduction for R-134a, but it does for some refrigerants like ammonia (~4%). • Intercooling may also be done with an external liquid such as water. • When intercooling and flash gas removal are combined, the savings is similar for most refrigerants. • A rough estimate of the optimum intermediate pressure can be found from
Pintermediate Ps uction Pdischarge
From Stoecker and Jones, Refrigeration and Air Conditioning
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One Compressor & 2 Levels of Evap. Temp
Often two evaporating temps are required – one for a freezer, and one for a refrigerator Why not use one evaporator with a really cold refrigerant temperature for both cases? – – –
If you’re using the evaporator to chill liquid, the liquid could freeze on the surface of the coils In an air-cooling coil, excessive frost may form If the air-cooling coil cools food, food near the coil could freeze
Use of two compressors instead of one is more efficient but results in a greater first cost
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From Cengel and Boles, Thermodynamics: An Engineering Approach
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A more common form of this system
From Stoecker and Jones, Refrigeration and Air Conditioning
Pressure regulator (sometimes called a back-pressure valve) maintains the higher evaporating temperature in the first evaporator. This results in a loss of efficiency but is easier to control than the previous configuration. The pressure regulator may be simply modeled as an expansion valve.
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2 Compressors & 2 Evap. Temps
More efficient but greater first cost than using one compressor Used often in a plant storing both frozen & unfrozen foods where required refrigeration capacity is high (well over 100 kW) Approximation of optimum intermediate pressure:
Pintermediate Psuction Pdischarge
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From Stoecker and Jones, Refrigeration and Air Conditioning
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Introduction The mechanism used for lowering or producing low temp. in a body or a space, whose temp. is already below the temp. of its surrounding, is called the refrigeration system. Here the heat is being generally pumped from low level to the higher one & is rejected at high temp. jntuworldupdates.org
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Refrigeration The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions. It also includes the process of reducing heat & maintaining the temp. of a body below the general temp. of its surroundings.
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Contd…. In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temp. of its surroundings.
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Refrigerator & Refrigerant A refrigerator is a reversed heat engine or a heat pump which takes out heat from a cold body & delivers it to a hot body. The refrigerant is a heat carrying medium which during their cycle in a refrigeration system absorbs heat from a low temp. system & delivers it to a higher temp. system. jntuworldupdates.org
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Refrigeration Cycle In refrigeration system the heat is being generally pumped from low level to higher one & rejected at that temp. This rejection of heat from low level to higher level of temp. can only be performed with the help of external work according to second law of thermodynamics. jntuworldupdates.org
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Contd…. The total amount of heat being rejected to the outside body consist of two parts:- the heat extracted from the body to be cooled . - the heat equivalent to the mechanical work required for extracting it. jntuworldupdates.org
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Contd…..
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Contd…. A refrigerator is a reverse heat engine run in the reverse direction by means of external aid. Every type of refrigeration system used for producing cold must have the following four basic units:-
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Contd…. • Low temp. thermal sink to which the heat is rejected for cooling the space. • Means of extracting the heat energy from the sink, raising its level of temp. before delivering it to heat receiver. • A receiver is a storage to which the heat is transferred from the high temp., high pressure refrigerant. jntuworldupdates.org
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Contd….. • Means of reducing the pressure & temp. of the refrigerant before it return to the sink. The processes of the cycle are evaporation, compression, condensation & expansion. By reversing the heat engine cycle completely & by changing the working agent, a refrigeration cycle is obtained. jntuworldupdates.org
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Refrigeration Systems • Vapour compression refrigeration system
• Vapour absorption refrigeration system • Thermo electric refrigeration system
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Vapour Compression Refrigeration • This is the most important system from the point of commercial & domestic utility & most practical form of refrigeration. • The working fluid refrigerant used in this refrigeration system readily evaporates & condenses or changes alternatively between the vapour & liquid phases without leaving the refrigerating plant jntuworldupdates.org
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Contd…. • During evaporation it absorbs heat from the cold body or in condensing or cooling it rejects heat to the external hot body . • The heat absorbed from cold body during evaporation is used as its latent heat for converting it from liquid to vapour. • Thus a cooling effect is created in working fluid. jntuworldupdates.org
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Contd…. • This system of refrigeration thus act as latent heat pump since its pump its latent heat from the cold body or brine & rejects it or deliver it to the external hot body or the cooling medium. • According to the law of thermodynamics , this can be done only on the expenditure of energy which is supplied to the system in the form of electrical energy driving the compressor.
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Contd…. • The vapour compression cycle is used in most of the modern refrigeration systems in large industrial plants. • The vapour in this cycle is circulated through the various components of the system, where it undergoes a number of changes in its state or condition. jntuworldupdates.org
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Contd…. • Each cycle of operation consists of the four fundamental changes of state or processes: Expansion Vaporisation Compression Condensation
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Components of Vapour Compression Systems
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Compressor The low pressure & temp. refrigerant from evaporator is drawn into the compressor through the inlet or suction valve , where it is compressed to a high pressure & temp.
The high pressure & temp vapour refrigerant is discharged into the condenser through the delivery or discharge valve. jntuworldupdates.org
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Condenser The condenser or the cooler consists of coils of pipe in which the high pressure & temp. vapour refrigerant is cooled & condensed. The refrigerant while passing through the condenser, rejects its latent heat to surrounding condensing medium which is normally air or water. Thus hot refrigerant vapour received from compressor is converted into liquid form in condenser.
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Receiver The condensed liquid refrigerant from the condenser is stored in a vessel, known as receiver, from where it is supplied to the expansion valve or refrigerant control valve.
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Expansion Valve The function of this valve is to allow the liquid refrigerant under high pressure & temp. to pass at a controlled rate after reducing its pressure & temp. some of liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator at the low pressure & temp. jntuworldupdates.org
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Evaporator An evaporator consists of coils of pipes in which the liquid vapour refrigerant at low pressure & temp. is evaporated & changed into vapour refrigerant at low pressure & temp. During evaporation process, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium which is to be cooled.
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Advantages • Smaller size for a given refrigerating capacity • Higher coeff. of performance • Lower power requirements for a given capacity • Less complexity in both design & operation • It can be used over large of temp. jntuworldupdates.org
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Domestic Refrigerator • The application of refrigeration for domestic purposes are mainly in the form of domestic refrigerators & home freezers. • The main purpose of this type of refrigeration is to provide low temp. for storage & distribution of foods & drinks.
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Contd…. • It represents a significant portion of the refrigeration industry due to the use of these units in large number. • For domestic preservation, the storage is generally short term. The domestic refrigerators used for the purposes are usually small in sizes with rating in ranges from 1/20 to ½ tonne. jntuworldupdates.org
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Contd…. • The unit is usually self contained and hermetically sealed. • Due to short term storage the domestic refrigerator load is intermittent.
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Contd…. The requirement of domestic refrigerator is that:• it should be simple in construction • automatic in action • nominal in initial cost jntuworldupdates.org
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Contd…. • dependable and without any necessity of expert inspection & repair. • Non irritant & non toxic refrigerant should be used. • Generally methylene chloride, freon-12, freon -11 are used as refrigerants. jntuworldupdates.org
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Contd… • The common type of domestic refrigerator have a cabinet shaped with compressor motor-fan assembly, the condensed and receiver fitted in their basement. • The expansion valve evaporator coils are exposed in the storage cabinet with the piping, carrying liquid refrigerant passing through the body. jntuworldupdates.org
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Contd…. • The heat of the bodies to be cooled is carried to the evaporator coils by means of air trapped in the cabinet. • Refrigeration is not only provided with double walled cabinet packed with materials having high thermal insulation such as fibre glass or expanded rubber but also all around the inside of door flap soft rubber seal is used which makes rubber air tight.
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Electrical Circuit • Refrigerator is provided with a door push switch, which closes on opening of refrigerator and puts the lamp on. • Capacitor start single phase induction motor is used in open type refrigerators and split phase induction motor is used in sealed unit refrigerators. • Electromagnetic relay is provided to connect auxiliary winding on the start & disconnect it when the motor picks up the speed.
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Circuit
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Contd….. • Thermal overload release is provided to protect the motor from damage against flow of over current. • Thermostat switch is provided to control the temp. inside the refrigerator. • Temp. inside the refrigerator can be adjusted by means of temp. control screw.
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Contd… • To protect the motor against under voltage use of automatic voltage regulator is essential since in case of fall in applied voltage, motor will draw heavy current to develop the required torque and will become hot, thermal overload relay will therefore repeatedly disconnect and connect the motor to supply, eventually burning it out. jntuworldupdates.org
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THANKS…..
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Training Session on Energy Equipment
Refrigeration & Air Conditioning Presentation from the “Energy Efficiency Guide for Industry in Asia”
www.energyefficiencyasia.org
1 jntuworldupdates.org
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
2 jntuworldupdates.org
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Introduction How does it work?
High Temperature Reservoir
Heat Rejected
R
Work Input
Heat Absorbed Low Temperature Reservoir 3 jntuworldupdates.org
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Introduction How does it work? Thermal energy moves from left to right through five loops of heat transfer:
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1)
2)
3)
4)
5)
Indoor air loop
Chilled water loop
Refrigerant loop
Condenser water loop
Cooling water loop
(Bureau of Energy Efficiency, 2004)
4 ©Specworld.in UNEP 2006
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Introduction AC Systems AC options / combinations: • Air Conditioning (for comfort / machine) • Split air conditioners • Fan coil units in a larger system • Air handling units in a larger system 5 jntuworldupdates.org
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Introduction Refrigeration systems for industrial processes • Small capacity modular units of direct expansion type (50 Tons of Refrigeration) • Centralized chilled water plants with chilled water as a secondary coolant
(50
– 250 TR)
• Brine plants with brines as lower temperature, secondary coolant (>250 TR) 6 jntuworldupdates.org
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Introduction Refrigeration at large companies • Bank of units off-site with common • Chilled water pumps • Condenser water pumps • Cooling towers
• More levels of refrigeration/AC, e.g.
• Comfort air conditioning (20-25 oC) • Chilled water system (8 – 10 oC) jntuworldupdates.org
• Brine system (< 0 oC)
7 ©Specworld.in UNEP 2006
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
8 jntuworldupdates.org
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Types of Refrigeration Refrigeration systems • Vapour Compression Refrigeration (VCR): uses mechanical energy • Vapour Absorption Refrigeration (VAR): uses thermal energy
9 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration • Highly compressed fluids tend to get colder when allowed to expand • If pressure high enough
• Compressed air hotter than source of cooling • Expanded gas cooler than desired cold temperature 10 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration Two advantages
• Lot of heat can be removed (lot of thermal energy to change liquid to vapour) • Heat transfer rate remains high (temperature of working fluid much lower than what is being cooled) 11 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration Refrigeration cycle 3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
12 ©Specworld.in UNEP 2006
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Type of Refrigeration Low pressure liquid refrigerant Vapour in evaporator Compression absorbs heat and changes to a gas Refrigeration cycle
Refrigeration 3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
13 ©Specworld.in UNEP 2006
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Type of Refrigeration The superheated vapour enters the compressor Vapour Compression where its pressure is raised Refrigeration cycle
Refrigeration 3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
14 ©Specworld.in UNEP 2006
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Type of Refrigeration The high pressure superheated gas is cooled Vapour Compression in several stages in the condenser
Refrigeration
Refrigeration cycle
3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
15 ©Specworld.in UNEP 2006
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Type of Refrigeration Liquid passes through expansion device, which reduces its pressure Vapour and Compression Refrigeration controls the flow into the evaporator
Refrigeration cycle
3
Condenser
High Pressure Side
4 Expansion Device
Compressor
2
1 Evaporator jntuworldupdates.org
Low Pressure Side
16 ©Specworld.in UNEP 2006
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Type of Refrigeration Vapour Compression Refrigeration Type of refrigerant
• Refrigerant determined by the required cooling temperature • Chlorinated fluorocarbons (CFCs) or freons: R-11, R-12, R-21, R-22 and R502 17 jntuworldupdates.org
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Type of Refrigeration Vapour Compression Refrigeration Choice of compressor, design of condenser, evaporator determined by • Refrigerant • Required cooling
• Load • Ease of maintenance • Physical space requirements • Availability of utilities (water, power) jntuworldupdates.org
18 ©Specworld.in UNEP 2006
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Type of Refrigeration Vapour Absorption Refrigeration Condenser
Generator Hot Side
Evaporator Cold Side
Absorber
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Type of Refrigeration Vapour Absorption Refrigeration Evaporator
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Type of Refrigeration Vapour Absorption Refrigeration Absorber
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Type of Refrigeration Vapour Absorption Refrigeration High pressure generator
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Type of Refrigeration Vapour Absorption Refrigeration Condenser
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Type of Refrigeration Evaporative Cooling •
Air in contact with water to cool it close to ‘wet bulb temperature’
•
Advantage: efficient cooling at low cost
•
Disadvantage: air is rich in moisture Sprinkling Water
Hot Air
Cold Air
(Adapted from Munters, 2001) 24
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
25 jntuworldupdates.org
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Assessment of Refrigeration and AC Assessment of Refrigeration • Cooling effect: Tons of Refrigeration 1 TR = 3024 kCal/hr heat rejected
• TR is assessed as: TR = Q xCp x (Ti – To) / 3024 Q= Cp = Ti = To =
mass flow rate of coolant in kg/hr is coolant specific heat in kCal /kg deg C inlet, temperature of coolant to evaporator (chiller) in 0C outlet temperature of coolant from evaporator (chiller) in 0C 26
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Assessment of Refrigeration and AC Assessment of Refrigeration Specific Power Consumption (kW/TR)
• Indicator of refrigeration system’s performance • kW/TR of centralized chilled water system is sum of • Compressor kW/TR
• Chilled water pump kW/TR • Condenser water pump kW/TR jntuworldupdates.org
• Cooling tower fan kW/TR
27 ©Specworld.in UNEP 2006
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Assessment of Refrigeration and AC Assessment of Refrigeration Coefficient of Performance (COPCarnot) •
Standard measure of refrigeration efficiency
•
Depends on evaporator temperature Te and condensing temperature Tc: COPCarnot
•
Te / (Tc - Te)
COP in industry calculated for type of compressor: COP =
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=
Cooling effect (kW) Power input to compressor (kW)
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Assessment of Refrigeration and AC Assessment of Refrigeration
COP increases with rising evaporator temperature (Te) jntuworldupdates.org
COP increases with decreasing condensing temperature (Tc)
29
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Assessment of Refrigeration and AC Assessment of Air Conditioning Measure •
Airflow Q (m3/s) at Fan Coil Units (FCU) or Air Handling Units (AHU): anemometer
•
Air density (kg/m3)
•
Dry bulb and wet bulb temperature: psychrometer
•
Enthalpy (kCal/kg) of inlet air (hin) and outlet air (Hout): psychrometric charts
Calculate TR
TR
Q ρ h in h out 3024 30
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Assessment of Refrigeration and AC
Assessment of Air Conditioning Indicative TR load profile • Small office cabins : 0.1 TR/m2
• Medium size office (10 – 30 people occupancy) with central A/C: 0.06 TR/m2 • Large multistoried office complexes with central A/C: 0.04 TR/m2 31 jntuworldupdates.org
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Assessment of Refrigeration and AC Considerations for Assessment • Accuracy of measurements • Inlet/outlet temp of chilled and condenser water
• Flow of chilled and condenser water
• Integrated Part Load Value (IPLV) • kW/TR for 100% load but most equipment operate between 50-75% of full load • IPLV calculates kW/TR with partial loads • Four points in cycle: 100%, 75%, 50%, 25% jntuworldupdates.org
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Training Agenda: Refrigeration & Air Conditioning Introduction
Type of refrigeration Assessment of refrigeration and AC Energy efficiency opportunities
33 jntuworldupdates.org
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Energy Efficiency Opportunities 1. Optimize process heat exchange 2. Maintain heat exchanger surfaces
3. Multi-staging systems 4. Matching capacity to system load 5. Capacity control of compressors 6. Multi-level refrigeration for plant needs 7. Chilled water storage 8. System design features jntuworldupdates.org
34 ©Specworld.in UNEP 2006
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Energy Efficiency Opportunities 1. Optimize Process Heat Exchange High compressor safety margins: energy loss 1. Proper sizing heat transfer areas of heat exchangers and evaporators
• Heat transfer coefficient on refrigerant side: 1400 – 2800 Watt/m2K • Heat transfer area refrigerant side: >0.5 m2/TR
2. Optimum driving force (difference Te and Tc): 1oC raise in Te = 3% power savings 35 jntuworldupdates.org
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Energy Efficiency Opportunities 1. Optimize Process Heat Exchange Evaporator Temperature (0C)
Refrigeration Capacity*(tons)
Specific Power Consumption (kW/TR)
Increase kW/TR (%)
5.0
67.58
0.81
-
0.0
56.07
0.94
16.0
-5.0
45.98
1.08
33.0
-10.0
37.20
1.25
54.0
-20.0
23.12
1.67
106.0
Condenser temperature 40◦C
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(National Productivity Council)
Condensing Temperature (0C)
Refrigeration Capacity (tons)
Specific Power Consumption (kW /TR)
Increase kW/TR (%)
26.7
31.5
1.17
-
35.0
21.4
1.27
8.5
40.0
20.0
1.41
20.5
*Reciprocating compressor using R-22 refrigerant. Evaporator temperature.-10◦ C
36 ©Specworld.in UNEP 2006
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Energy Efficiency Opportunities 1. Optimize Process Heat Exchange 3. Selection of condensers • Options: • • •
Air cooled condensers Air-cooled with water spray condensers Shell & tube condensers with water-cooling
• Water-cooled shell & tube condenser • • •
Lower discharge pressure Higher TR Lower power consumption 37
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Energy Efficiency Opportunities 2. Maintain Heat Exchanger Surfaces • Poor maintenance = increased power consumption • Maintain condensers and evaporators • Separation of lubricating oil and refrigerant
• Timely defrosting of coils • Increased velocity of secondary coolant
• Maintain cooling towers • 0.55◦C reduction in returning water from cooling tower = 3.0 % reduced power jntuworldupdates.org
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Energy Efficiency Opportunities 2. Maintain Heat Exchanger Surfaces Effect of poor maintenance on compressor power consumption Te (0C)
Tc (0C)
Refrigeration Capacity* (TR)
Specific Power Consumption (kW/TR)
Normal
7.2
40.5
17.0
0.69
-
Dirty condenser
7.2
46.1
15.6
0.84
20.4
Dirty evaporator
1.7
40.5
13.8
0.82
18.3
Dirty condenser and evaporator
1.7
46.1
12.7
0.96
38.7
Condition
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(National Productivity Council)
Increase kW/TR (%)
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Energy Efficiency Opportunities 3. Multi-Staging Systems • Suited for
• Low temp applications with high compression • Wide temperature range
• Two types for all compressor types • Compound • Cascade 40 jntuworldupdates.org
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Energy Efficiency Opportunities 3. Multi-Stage Systems a. Compound •
Two low compression ratios = 1 high
•
First stage compressor meets cooling load
•
Second stage compressor meets load evaporator and flash gas
•
Single refrigerant
b. Cascade
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•
Preferred for -46 oC to -101oC
•
Two systems with different refrigerants
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Energy Efficiency Opportunities 4. Matching Capacity to Load System • Most applications have varying loads
• Consequence of part-load operation • COP increases • but lower efficiency
• Match refrigeration capacity to load requires knowledge of • Compressor performance • Variations in ambient conditions • Cooling load jntuworldupdates.org
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Energy Efficiency Opportunities 5. Capacity Control of Compressors • Cylinder unloading, vanes, valves • Reciprocating compressors: step-by-step through cylinder unloading:
• Centrifugal compressors: continuous modulation through vane control • Screw compressors: sliding valves
• Speed control • Reciprocating compressors: ensure lubrication system is not affected • Centrifugal compressors: >50% of capacity jntuworldupdates.org
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Energy Efficiency Opportunities 5. Capacity Control of Compressors • Temperature monitoring • Reciprocating compressors: return water (if varying loads), water leaving chiller (constant loads) • Centrifugal compressors: outgoing water temperature • Screw compressors: outgoing water temperature
• Part load applications: screw compressors more efficient jntuworldupdates.org
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Energy Efficiency Opportunities 6. Multi-Level Refrigeration Bank of compressors at central plant
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•
Monitor cooling and chiller load: 1 chiller full load more efficient than 2 chillers at part-load
•
Distribution system: individual chillers feed all branch lines; Isolation valves; Valves to isolate sections
•
Load individual compressors to full capacity before operating second compressor
•
Provide smaller capacity chiller to meet peak demands
45
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Energy Efficiency Opportunities 6. Multi-Level Refrigeration Packaged units (instead of central plant) • Diverse applications with wide temp range and long distance
• Benefits: economical, flexible and reliable • Disadvantage: central plants use less power
Flow control • Reduced flow • Operation at normal flow with shut-off periods 46 jntuworldupdates.org
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Energy Efficiency Opportunities 7. Chilled Water Storage • Chilled water storage facility with insulation • Suited only if temp variations are acceptable • Economical because • Chillers operate during low peak demand hours: reduced peak demand charges
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• Chillers operate at nighttime: reduced tariffs and improved COP
47
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Energy Efficiency Opportunities 8. System Design Features • FRP impellers, film fills, PVC drift eliminators • Softened water for condensers • Economic insulation thickness • Roof coatings and false ceilings • Energy efficient heat recovery devices
• Variable air volume systems • Sun film application for heat reflection jntuworldupdates.org
• Optimizing lighting loads
48 ©Specworld.in UNEP 2006
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Training Session on Energy Equipment
Refrigeration & Air Conditioning Systems THANK YOU FOR YOUR ATTENTION
49 jntuworldupdates.org
Specworld.in © UNEP 2006
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Disclaimer and References • This PowerPoint training session was prepared as part of the project “Greenhouse Gas Emission Reduction from Industry in Asia and the Pacific” (GERIAP). While reasonable efforts have been made to ensure that the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication. © UNEP, 2006.
• The GERIAP project was funded by the Swedish International Development Cooperation Agency (Sida) • Full references are included in the textbook chapter that is 50 available on www.energyefficiencyasia.org jntuworldupdates.org Specworld.in
© UNEP 2006
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Refrigeration & Air Conditioning jntuworldupdates.org
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Objectives • •
• •
Basic operation of refrigeration and AC systems Principle components of refrigeration and AC systems Thermodynamic principles of refrigeration cycle Safety considerations
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Uses of Systems • •
Cooling of food stores and cargo Cooling of electronic spaces and equipment •
• • • •
• •
CIC (computers and consoles) Radio (communications gear) Radars ESGN/RLGN Sonar
Cooling of magazines Air conditioning for crew comfort
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Definition Review •
• •
•
Specific heat (cp): Amount of heat required to raise the temperature of 1 lb of substance 1°F (BTU/lb) – how much for water? Sensible heat vs Latent heat LHV/LHF Second Law of Thermodynamics: must expend energy to get process to work
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Refrigeration Cycle •
•
•
Refrigeration - Cooling of an object and maintenance of its temp below that of surroundings Working substance must alternate b/t colder and hotter regions Most common: vapor compression • •
Reverse of power cycle Heat absorbed in low temp region and released in high temp region
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Generic Refrigeration Cycle
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Thermodynamic Cycle
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Typical Refrigeration Cycle
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Components • • • • • •
Refrigerant Evaporator/Chiller Compressor Condenser Receiver Thermostatic expansion valve (TXV)
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Refrigerant •
Desirable properties: • •
• • • • •
•
High latent heat of vaporization - max cooling Non-toxicity (no health hazard) Desirable saturation temp (for operating pressure) Chemical stability (non-flammable/non-explosive) Ease of leak detection Low cost Readily available
Commonly use FREON (R-12, R-114, etc.)
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Evaporator/Chiller • •
•
Located in space to be refrigerated Cooling coil acts as an indirect heat exchanger Absorbs heat from surroundings and vaporizes • •
Latent Heat of Vaporization Sensible Heat of surroundings
Slightly superheated (10°F) ensures no liquid carryover into compressor •
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Compressor •
Superheated Vapor: • •
•
•
•
Enters as low press, low temp vapor Exits as high press, high temp vapor
Temp: creates differential (DT) promotes heat transfer Press: Tsat allows for condensation at warmer temps Increase in energy provides the driving force to circulate refrigerant through the system
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Condenser •
• •
Refrigerant rejects latent heat to cooling medium Latent heat of condensation (LHC) Indirect heat exchanger: seawater absorbs the heat and discharges it overboard
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Receiver •
•
Temporary storage space & surge volume for the sub-cooled refrigerant Serves as a vapor seal to prevent vapor from entering the expansion valve
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Expansion Device • •
•
Thermostatic Expansion Valve (TXV) Liquid Freon enters the expansion valve at high pressure and leaves as a low pressure wet vapor (vapor forms as refrigerant enters saturation region) Controls: • •
Pressure reduction Amount of refrigerant entering evaporator controls capacity
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Air Conditioning •
• • •
Purpose: maintain the atmosphere of an enclosed space at a required temp, humidity and purity Refrigeration system is at heart of AC system Heaters in ventilation system Types Used: • • •
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Self-contained Refrigerant circulating Chill water circulating Specworld.in
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AC System Types •
Self-Contained System •
•
•
Refrigerant circulating system •
•
Add-on to ships that originally did not have AC plants Not located in ventilation system (window unit) Hot air passed over refrigerant cooling coils directly
Chilled water circulating system •
•
Refrigerant cools chill water Hot air passes over chill water cooling coils
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Basic AC System
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Safety Precautions •
Phosgene gas hazard • •
•
Handling procedures •
•
•
Lethal Created when refrigerant is exposed to high temperatures Wear goggles and gloves to avoid eye irritation and frostbite
Asphyxiation hazard in non-ventilated spaces (bilges since heavier than air) Handling of compressed gas bottles
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Questions?
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Eco-friendly Refrigerants Dr Alka Bani Agrawal Professor,Mechanical Engg UIT,RGPV
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History Of Refrigeration • Refrigeration relates to the cooling of air or liquids, thus providing lower temperature to preserve food, cool beverages, make ice and for many other . • Most evidence indicate that the Chinese were the first to store natural ice and snow to cool wine and other delicacies. • Ancient people of India and Egypt cooled liquids in porous earthen jars. • In 1834, Jacob Perkins, an American, developed a closed refrigeration system using liquid expansion and then compression to produce cooling. He used Ether as refrigerant, in a hand- operated compressor, a water-cooled condenser and an evaporator in liquid cooler.
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Refrigerantion Principle • Modern refrigeration and air-conditioning equipment is dominated by vapour compression refrigeration technology built upon the thermodynamic principles of the reverse Carnot cycle. • Refrigerant Changes phases during cooling and used again and again.
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What is a Refrigerant • Refrigerants are used as working substances in a Refrigeration systems. • Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants. • Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems. • These fluids provide refrigeration by undergoing a phase change process in the evaporator. • Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes
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What is ChloroFloroCarcons • Today’s refrigerants are predominantly from a group of compounds called halocarbons (halogenated hydrocarbons) or specifically fluorocarbons. • Chlorofluorocarbons were first developed by General Motor’s researchers in the 1920’s and commercialized by Dupont as “Freons”.
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Halocarbon Refrigerants • Halocarbon Refrigerant are all synthetically produced and were developed as the Freon family of refrigerants. Examples :
– CFC’s : R11, R12, R113, R114, R115
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Freon Group Refrigerants Application and ODP Values Refrigerant
Areas of Application
ODP
Air-conditioning Systems ranging from 200 to 1.0 2000 tons in capacity. It is used where low freezing point and non-corrosive properties are CFC 12 ( R 12 important. ) It is used for most of the applications. Air- 1.0 conditioning plants, refrigerators, freezers, icecream cabinets, water coolers, window airconditioners, automobile air conditioners. CFC 13 (R 13) For low temp refrigeration up to – 90 C in 1.0 cascade system CFC 11(R11)
CFC113 ( R113 Small to medium air-conditioning system and 1.07 ) industrial cooling 0.8 CFC114 ( R114 In household refrigerators and in large industrial 0.34 ) cooling Blend of R22 Frozen food ice-cream display cases and and R115 warehouses and food freezing plants. An (R502) excellent general low temp refrigerant jntuworldupdates.org
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What is Ozone Layer • Ozone is an isotope of oxygen with three atoms instead of normal two. It is naturally occurring gas which is created by high energy radiation from the Sun. • The greatest concentration of ozone are found from 12 km to 50 km above the earth forming a layer in the stratosphere which is called the ozone layer. • This layer, which forms a semi-permeable blanket, protects the earth by reducing the intensity of harmful ultra-violet (UV) radiation from the sun. jntuworldupdates.org
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Ozone Layer Depletion • In the early70’s,scientists Sherwood Roland and Mario Molina at the University of California at Irvine were the first to discover the loss of ozone in stratosphere while investigating the ozone layer from highflying aircraft and spacecraft. • They postulated the theory that exceptionally stable chlorine containing fluorocarbons could, overtime, migrate to the upper reaches of the atmosphere and be broken by the intense radiation and release chlorine atoms responsible for
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OZONE LAYER DEPLETION • • •
N0RMAL REACTION O2 = O + O O 2 + O = O3
•
But CFC refrigerants leaked during the manufacturing and normal operation or at the time of servicing or repair, mix with surrounding air and rise to troposphere and then into stratosphere due to normal wind or storm. The Ultraviolet rays act on CFC releasing Cl atom, which retards the normal reaction:
• • • • •
RETARDED REACTION O3 = O2 + O CCL2F2 = CCLF2 + CL O3 + CL = CLO + O2 O + CLO = CL + O2
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Harmful consequences of ozone depletion • • • • • • • • • • • • • • • • • • •
For Humans
Increase in
skin cancer snow blindness cataracts Less immunity to infectious diseases malaria herpes
For plants smaller size lower yield increased toxicity altered form
For marine life Reduced plankton juvenile fish larval crabs and shrimps
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MONTREAL PROTOCOL • SIGNED IN 1987 UNDER THE ‘UNEP’, AFTER MUCH DISCUSSIONS • MORE THAN 170 COUNTRIES HAVE RATIFIED • INDIA RATIFIED ON SEPT 17,1992 • ONE OF MOST SUCCESSFUL EXAMPLE OF INTERNATIONAL COOPERATION IN UN HISTORY
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Montreal protocol- Control Schedule
Montréal Protocol- Control Schedule ozone depleting substance
developed countries
developing countries
CFCs
phased out end of 1995
total phase out by 2010
halons
phased out end of 1993
total phase out by 2010
HCFCs
total phase out by 2020
total phase out by 2040
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CFC Phase-out in India • • •
What is to be phased out? CFC-11, CFC-12 & CFC-113a. How much and when?
• • • • •
1999 22,588 MT 2005 11,294 MT 2010 o MT How to achieve the target? Production is controlled through a production quota allocated to each producer every year. The Ozone Cell conducts audits twice a year to monitor the production. How much has been Phaseout? CFC has been completely phased out as on 1st August, 2008
•
Year
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Vapor compression refrigeration System • In 1834 an American inventor named Jacob Perkins obtained the first patent for a vapor-compression refrigeration system, it used ether in a vapor compression cycle. • Joule-Thomson (Kelvin) expansion • Low pressure (1.5 atm) low temperature (10 to +15 ℃) inside • High pressure (7.5 atm) high temperature (+15 to +40 ℃) outside
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Components • • • • • •
Refrigerant Evaporator/Chiller Compressor Condenser Receiver Thermostatic expansion valve (TXV)
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Circulation of Refrigerant •
•
•
•
Compressor cold vapor from the evaporator is compressed, raising it temperature and boiling point adiabatic compression T, b.p. ~ P work done on the gas Condenser hot vapor from the compressor condenses outside the cold box, releasing latent heat isothermal, isobaric condensation (horizontal line on PV diagram) high temperature T (hot) latent heat of vaporization Q (hot) Expansion valve (throttling valve) hot liquid from the condenser is depressurized, lowering its temperature and boiling point adiabatic, isochoric expansion (vertical line on PV diagram) T, b.p. ~ P no work done W = 0 Evaporator cold liquid from the expansion valve boils inside the cold box, absorbing latent heat isothermal, isobaric boiling (horizontal line on PV diagram) low temperature T (cold) latent heat of vaporization Q (cold)
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Importance of Refrigerant • The thermodynamic efficiency of a refrigeration system depends mainly on its operating temperatures. • However, important practical issues such as the system design, size, initial and operating costs, safety, reliability, and serviceability etc. depend very much on the type of refrigerant selected for a given application. • Due to several environmental issues such as ozone layer depletion and global warming and their relation to the various refrigerants used, the selection of suitable refrigerant has become one of the most important issues in recent times. jntuworldupdates.org
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Refrigerant selection criteria • Selection of refrigerant for a particular application is based on the following requirements: – i. Thermodynamic and thermo-physical properties – ii. Environmental and safety properties – Iii. Economics jntuworldupdates.org
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Thermodynamic and thermo-physical properties • The requirements are: • a) Suction pressure: At a given evaporator temperature, the saturation pressure should be above atmospheric for prevention of air or moisture ingress into the system and ease of leak detection. Higher suction pressure is better as it leads to smaller compressor displacement • b) Discharge pressure: At a given condenser temperature, the discharge pressure should be as small as possible to allow lightweight construction of compressor, condenser etc. • c) Pressure ratio: Should be as small as possible for high volumetric efficiency and low power consumption • d) Latent heat of vaporization: Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small
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Thermodynamic and thermo-physical properties • In addition to the above properties; the following properties are also important: • e) Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be small • f) Liquid specific heat: Should be small so that degree of subcooling will be large leading to smaller amount of flash gas at evaporator inlet • g) Vapour specific heat: Should be large so that the degree of superheating will be small • h) Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficients • i) Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops • The thermodynamic properties are interrelated and mainly depend on normal boiling point, critical temperature, molecular weight and structure.
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Environmental and safety properties • At present the environment friendliness of the refrigerant is a major factor in deciding the usefulness of a particular refrigerant. The important environmental and safety properties are: • a) Ozone Depletion Potential (ODP): According to the Montreal protocol, the ODP of refrigerants should be zero, i.e., they should be non-ozone depleting substances. Refrigerants having nonzero ODP have either already been phased-out (e.g. R 11, R 12) or will be phased-out in nearfuture(e.g. R22). Since ODP depends mainly on the presence of chlorine or bromine in the molecules, refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be used under the new regulations jntuworldupdates.org Specworld.in
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Environmental Effects of Refrigerants Global warming :
Refrigerants directly contributing to global warming when released to the atmosphere Indirect contribution based on the energy consumption of among others the compressors ( CO2 produced by power stations ) jntuworldupdates.org
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Environmental and safety properties • b) Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming. Refrigerants with zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future. • c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming. Naturally, refrigerants with as a low a value of TEWI are preferable from global warming point of view. jntuworldupdates.org
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Environmental and safety properties •
d) Toxicity: Ideally, refrigerants used in a refrigeration system should be non-toxic. Toxicity is a relative term, which becomes meaningful only when the degree of concentration and time of exposure required to produce harmful effects are specified. Some fluids are toxic even in small concentrations. Some fluids are mildly toxic, i.e., they are dangerous only when the concentration is large and duration of exposure is long. In general the degree of hazard depends on: – - Amount of refrigerant used vs total space – - Type of occupancy – - Presence of open flames – - Odor of refrigerant, and
– - Maintenance condition jntuworldupdates.org
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Environmental and safety properties • e) Flammability: The refrigerants should preferably be non-flammable and non-explosive. For flammable refrigerants special precautions should be taken to avoid accidents. • f) Chemical stability: The refrigerants should be chemically stable as long as they are inside the refrigeration system. • g) Compatibility with common materials of construction (both metals and non-metals) • h) Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. ammonia). Refrigerants that are completely miscible with oils are easier to handle(R12). jntuworldupdates.org
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Environmental and safety properties • Ease of leak detection: In the event of leakage of refrigerant from the system, it should be easy to detect the leaks. Economic properties: • The refrigerant used should preferably be inexpensive and easily available. jntuworldupdates.org
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ECO-FRIENDLY REFRIGERANTS
CFC ALTERNATIVES. HCFC R22,R124
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HFC R134a,R152a
NATURAL REFRIGERANT NH3, HC'S
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Halocarbon Refrigerants • Halocarbon Refrigerant are all synthetically produced and were developed as the Freon family of refrigerants. Examples :
– CFC’s : R11, R12, R113, R114, R115 – HCFC’s : R22, R123 – HFC’s : R134a, R404a, R407C, R410a
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HFCs • Remain a popular choice – especially for R22 phase out
• Good efforts at improving leakage performance – e.g. Real Zero project
• Interest in R407A to replace R404A – 50% reduction in GWP
Fjntuworldupdates.org Gas Stakeholder Group, 14th October 2009
Specworld.in Slide 30
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Inorganic Refrigerants • • • • •
Carbon Dioxide Water Ammonia Air Sulphur dioxide
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HCFC • Transitional compounds with low ODP • Partially halogenated compounds of hydrocarbon • Remaining hydrogen atom allows Hydrolysis and can be absorbed. • R22, R123
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HCFC • • • •
Production frozen at 1996 level 35% cut by 2005,65% by 2010 90% by 2015,100 % by 2030 10 year grace period for developing countries.
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R22 • ODP-0.05, GWP-1700 • R22 has 40% more refrigerating capacity • Higher pressure and discharge temp and not suitable for low temp application • Extensively used in commercial airconditioning and frozen food storage and display cases
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R123 • ODP-0.02,GWP-90 • As a replacement for R11 as similar thermodynamic properties. • Very short atmospheric life but classified as carcinogen • Retrofit alternative to R11
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HFC • Zero ODP as no chlorine atom contains only Hydrogen and Flurodine • Very small GWP values • No phase out date in Montreal Protocol • R134a and R152 a – Very popular refrigerants • HFC refrigerants are costly refrigerants
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R134a • ODP-0, GWP-1300 • Used as a substitute for R12 and to a limited range for R22 • Good performance in medium and high temp application • Toxicity is very low • Not miscible with mineral oil
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R152a • ODP-0,GWP-140 • R152a is another attractive HFC with similar properties to R12. • GWP is one order less than HFC134a but it is slightly flammable. • Also it has lower energy consumption. Hence the Environmental Protection Agency of Europe prefers HFC152a to HFC134a
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Hydrocarbon • Very promising non-halogenated organic compounds • With no ODP and very small GWP values • Their efficiency is slightly better than other leading alternative refrigerants • They are fully compatible with lubricating oils conventionally used with CFC12.
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Hydrocarbon Refrigerants • Extraordinary reliability- The most convincing argument is the reliability of the hydrocarbon system because of fewer compressor failures. • But most of the hydrocarbons are highly flammable and require additional safety precaution during its use as refrigerants. • Virtually no refrigerant losses • Hydrocarbons have been used since the beginning of the century and now being considered as long term solutions to environmental problems,
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Hydrocarbons • Dominant in domestic market like household refrigerators and freezers • Growing use in very small commercial systems like car air-conditioning system • Examples:
R170, Ethane, C2H6 R290 , Propane C3H3 R600, Butane, C4H10 R600a, Isobutane, C4H10 Blends of the above Gases
Fjntuworldupdates.org Gas Stakeholder Group, 14th October 2009
Specworld.in Slide 42
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R290 • • • •
ODP-0,GWP-3 Compatible with copper.Miscible with mineral oil Highest latent heat and largest vapour density A third of original charge only is required when replacing halocarbons refrigerant in existing equipment • Energy saving : up to 20% due to lower molecular mass and vapour pressure
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R 600a • ODP-0,GWP-3 • Higher boiling point hence lower evaporator pressure • Discharge temp is lowest • Very good compatibility with mineral oil
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Flammability • Approximate auto ignition temperatures • R22 630 ºC • R12 750 ºC • R134a 740 ºC • R290 465 ºC • R600a 470 ºC • jntuworldupdates.org
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Modifications of Electrical Equipment • Replaced with solid state equivalents • Sealed to ensure that any sparks do not come into contact with leaking gas • Relocated to a position where the component would not come into contact with leaking gas jntuworldupdates.org
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Modifications of Electrical Equipment • Faulty components. • Poor, corroded, loose, or dirty electrical connections. • Missing or broken insulation which could cause arcing/sparks. • Friction sparks, like a metal fan blade hitting a metal enclosure. jntuworldupdates.org
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Blends & Mixtures • Limited no of pure refrigerants with low ODP & GWP values • To try a mixture of pure refrigerants to meet specific requirement
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Azeotropic Refrigerants • A stable mixture of two or several refrigerants whose vapour and liquid phases retain identical compositions over a wide range of temperatures. • Examples : R-500 :
73.8% R12 and 26.2%
R152 R-502 : 8.8% R22 and 51.2% R115 R-503 : 40.1% R23 and 59.9% R13 jntuworldupdates.org
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Zeotropic Refrigerants • A zeotropic mixture is one whose composition in liquid phase differs to that in vapour phase. Zeotropic refrigerants therefore do not boil at constant temperatures unlike azeotropic refrigerants. • Examples :R404a : R125/143a/134a
(44%,52%,4%) R407c : R32/125/134a (23%, 25%, 52%) R410a : R32/125 (50%, 50%) R413a : R600a/218/134a (3%, 9%, 88%)
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Inorganic Refrigerants • • • • •
Carbon Dioxide Water Ammonia Air Sulphur dioxide
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Carbon Dioxide • • • •
Zero ODP & GWP Non Flammable, Non toxic Inexpensive and widely available Its high operating pressure provides potential for system size and weight reducing potential.
• Drawbacks: • Operating pressure (high side) : 80 bars • Low efficiency jntuworldupdates.org
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Ammonia –A Natural Refrigerant Ammonia is produced in a natural way by human beings and animals; 17 grams/day for humans. Natural production
3000 million tons/year
Production in factories
120 million tons/year
Used in refrigeration
6 million tons/year
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Ammonia as Refrigerant • ODP = 0 • GWP = 0 • Excellent thermodynamic characteristics: small molecular mass, large latent heat, large vapour density and excellent heat transfer characteristics • High critical temperature (132C) : highly efficient cycles at high condensing temperatures • Its smell causes leaks to be detected and fixed before reaching dangerous concentration • Relatively Low price jntuworldupdates.org
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Some Drawbacks of Ammonia as Refrigerant • • • •
Toxic Flammable ( 16 – 28% concentration ) Not compatible with copper Temperature on discharge side of compressor is higher compared to other refrigerants
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Water • Zero ODP & GWP • Water as refrigerant is used in absorption system .New developing technology has created space for it for use in compression cycles also. • But higher than normal working pressure in the system can be a factor in restricted use of water as refrigerant jntuworldupdates.org
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Application of New Eco-friendly Refrigerants • • • • • • • • • • • •
Application Eco-friendly refrigerant Domestic refrigeration Commercial refrigeration Cold storage ,food processing And industrial refrigeration Unitary air conditioners Centralized AC (chillers) Transport refrigeration Mobile air conditioner Heat pumps
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HFCs used
Possible
R134a,R152a R134a,R404A,R407C
HC600a and blends HC blends,NH3 ,CO2 **
R134a,R404A,R507A R410A,R407C R134a,R410A,R407C R134a,R404A R134a R134a,R152a,R404A R407C,R410A
NH3 ,HCs,CO2 ** CO2 , HC s NH3 ,HCs,CO2, water ** CO 2, CO2 ,HCs NH3 ,HCs,CO2, water **
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Table 3 -Performance Results of R12 and New Proposed Refrigerants using the same compressor as with R12 in Milk Chilling Unit
•
• Refrigerants • Mass flow rate • kg / min • Refrigerating capacity • kW • Compressor power • kW • COP • Volumetric Refrigeration Capacity(kj/ m3) • Discharge • Temp • C • • R 32 • R 290 • R 22 • R 12 • R 134a • R 152a • R 124 • R 600a • R 142b • R 600 • • • 8.81 • 4.45 • 7.45 jntuworldupdates.org • 6.99
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General Safety measures for refrigerating plants • Reduction of refrigerant contents: – Components with reduced contents – Indirect systems with secondary refrigerant: distinction between generation and transport of cold • Scheduled maintenance and leak testing • Governmental surveillance – Refrigerant Audits for systems operating with HFC’s. Recovery, Stock of used refrigerants, Recycling of refrigerants. • For the Netherlands, the combined measures resulted in a leak rate reduction of 35% (1995) to 8% (2001) for R22systems jntuworldupdates.org
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Survey Of Refrigerants Refrigerant
Group
Atmospheri c life
ODP
GWP
R11
CFC
130
1
4000
R12
CFC
130
1
8500
R22
HCFC
15
.05
1500
R134a
HFC
16
0
1300
R404a
HFC
16
0
3260
R410a
HFC
16
0
1720
R507
HFC
130
1
3300
R717
NH3
-
0
0
R744
CO2
-
0
1
R290
HC
<1
0
8
R600a
HC
<1
0
8
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Conclusions • In the aftermath of the Montreal protocole HFC’s have predominantly replaced CFC’s and HCFC’s in RAC equipment. • Due to their high GWP, HFC’s are not a good replacement solution. • The solution are the natural refrigerants : Ammonia, Hydrocarbons and Carbon dioxide • System need to have low TEWI factor • High efficiency with ammonia and lower power consumption with hydrocarbons jntuworldupdates.org
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• •
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Thank You
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Environmental Effects of Refrigerants Global warming :
Refrigerants directly contributing to global warming when released to the atmosphere Indirect contribution based on the energy consumption of among others the compressors ( CO2 produced by power stations ) jntuworldupdates.org
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PowerPoint® Presentation
Chapter 6 Refrigeration Systems Refrigeration • Mechanical Compression Refrigeration • Absorption Systems • Troubleshooting and Maintaining Refrigeration Systems • Refrigerant Regulations • Refrigerant Handling jntuworldupdates.org
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Chapter 6 — Refrigeration Systems
A refrigeration system controls the absorption and rejection of heat by refrigerant to move heat from inside a cooled space to outside the cooled space.
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Chapter 6 — Refrigeration Systems
In a mechanical compression refrigeration system, a compressor is used to produce the refrigeration effect.
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Chapter 6 — Refrigeration Systems
Refrigerant vapor pressure charts list the saturation temperature and pressure of refrigerants.
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Chapter 6 — Refrigeration Systems
Refrigeration compressors include reciprocating, vane, centrifugal, and screw compressors.
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Chapter 6 — Refrigeration Systems
Refrigerant compressors are available in hermetic and semi-hermetic configurations.
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Chapter 6 — Refrigeration Systems
An air-cooled condenser removes heat from highpressure refrigerant vapor by air blown across the condenser coils.
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Chapter 6 — Refrigeration Systems
Water-cooled condensers transfer heat from refrigerant vapor to water.
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Chapter 6 — Refrigeration Systems
Evaporative condensers reject heat through the evaporation of water.
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Chapter 6 — Refrigeration Systems
A fin comb is used for condenser maintenance to straighten damaged or bent fins, which limit airflow and reduce condenser efficiency. Robinair Division, SPX Corporation
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Chapter 6 — Refrigeration Systems
A thermostatic expansion valve uses temperature readings at the evaporator outlet to control the rate of refrigerant flow into the evaporator.
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Chapter 6 — Refrigeration Systems
The opening and closing of a thermostatic expansion valve is controlled by the pressure in the remote bulb.
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Chapter 6 — Refrigeration Systems
An automatic expansion valve controls the temperature of the refrigerant by controlling the pressure in the evaporator.
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Chapter 6 — Refrigeration Systems
As refrigerant is forced through the capillary tube, it loses pressure until it is at the desired evaporator pressure.
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Chapter 6 — Refrigeration Systems
An evaporator vaporizes low-pressure refrigerant liquid into a low-pressure vapor.
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Chapter 6 — Refrigeration Systems
A hot-gas defrost uses hot gas from the compressor to melt frost on the evaporator.
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Chapter 6 — Refrigeration Systems
An evaporator pressure regulating valve allows two evaporators running from the same compressor to maintain different temperatures.
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Chapter 6 — Refrigeration Systems
Accessories are used for maintaining and controlling the flow of refrigerant in a refrigeration system.
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Chapter 6 — Refrigeration Systems
Pressure switches control refrigeration system temperature through changes in system pressure.
Ranco Inc.
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Chapter 6 — Refrigeration Systems
Ammonia systems operate at high temperatures and pressures and must have special controls and fittings to control the release of ammonia gas.
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Chapter 6 — Refrigeration Systems
The direction of refrigerant flow in a heat pump is controlled by a reversing valve.
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Chapter 6 — Refrigeration Systems
Heat pumps may use air or water as the heat source.
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Chapter 6 — Refrigeration Systems
Chillers use chilled water to cool large building spaces.
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Chapter 6 — Refrigeration Systems
A cooling tower cools water from a condenser by the evaporation of water as it cascades through the tower.
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Chapter 6 — Refrigeration Systems
Absorption systems use a generator and absorber in place of the compressor to raise system pressure.
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Chapter 6 — Refrigeration Systems
Gauge manifolds are used to take pressure readings, add or remove refrigerant, and remove air from a system before it is filled with refrigerant.
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Chapter 6 — Refrigeration Systems
Service valves are frontseated for isolating parts of the system, midseated for adding or removing refrigerant or taking system pressures, and back-seated during normal operation.
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Chapter 6 — Refrigeration Systems
Refrigeration system pressure readings are taken by connecting the blue hose to the low-pressure side service valve and the red hose to the high-pressure side service valve with both gauge manifold valves front-seated.
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Chapter 6 — Refrigeration Systems
A head pressure controller prevents the condenser pressure from falling too low and starving the evaporator for refrigerant.
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Chapter 6 — Refrigeration Systems
The EPA has established regulations under Section 608 of the Clean Air Act to regulate the handling of ozone-depleting substances.
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Chapter 6 — Refrigeration Systems
A recovery unit is used to recover refrigerant for reuse.
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Chapter 6 — Refrigeration Systems
A leak detector is a device used to detect refrigerant leaks in air conditioning or refrigeration systems.
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Chapter 6 — Refrigeration Systems
A vacuum pump removes all air from a refrigeration system.
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Chapter 6 — Refrigeration Systems
The vacuum pump is connected to the system service valves using a gauge manifold and hoses.
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Chapter 6 — Refrigeration Systems
Electronic leak detectors are extremely sensitive and indicate the general location of a leak.
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Chapter 6 — Refrigeration Systems
The refrigerant container is placed upside down to charge with liquid and right side up to charge with vapor.
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