3340-lectures-7-to-12-s1-2015

Refrigeration and AirConditioning(MEng 5212) Part –I, Refrigeration

By Yisfalign Demissie(MSc.) Wolkite University Nov. 2011 EC

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PART-ONE 1. Refrigeration

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1.1 Basic Concepts in Refrigeration  Objective:- To introduce students about the basics of refrigeration

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Introduction I) Refrigeration • Refrigeration - is a process used to cool the air and

also for removing moisture content from the air to specific levels. • Refrigeration - is the transfer of heat from a lower

temperature region to a higher temperature region. • Refrigeration - is concerned with the production of cool

confinement absorbing heat from the space where cooling is required

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 Refrigeration may be defined as the process of achieving and maintaining a temperature below that of the surroundings,  The aim being to cool some product or space to the required temperature.  One of the most important applications of refrigeration has been the preservation of perishable food products by storing them at low temperatures.

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 The branch of science which deals with the

process of reducing and maintaining the temperature of a space or material below the temperature of the surrounding  The heat is then rejected to some natural sink such as:   

The atmospheric air Surface water Any external body lower in temperature compared to the space

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II) A Refrigeration System  A refrigeration system is a combination of components, equipment and piping connected

in a sequence to produce the refrigeration effect  Refrigeration system is also used extensively for providing thermal comfort to human beings by means of air conditioning.  The rate at which heat is absorbed from space to be cooled is termed as the refrigeration effect

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III) Refrigerator • Device that produce refrigeration effect are called refrigerators, and the cycle on which they operate are called refrigeration cycle. • We know from experience that heat flows spontaneously or by itself from a hightemperature region to a low-temperature region.

• But, the transfer of heat from a low-temperature region to a high-temperature region is possible only with the help of external work. Hence, this requires a special type of device called a refrigerator or a heat pump. 8

 Refrigerators

and heat pumps are essentially the same devices; they differ in their objectives only.  The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it.  Whereas the objective of a heat pump is to maintain a heated space at a high temperature.  But, air conditioners are basically refrigerators whose refrigerated space is a room or a building instead of the food compartment. 9

IV) Refrigeration Cycle  Refrigeration Cycle: when a refrigerant undergoes a series of processes like evaporation, compression, condensation, throttling and expansion, it is said

to have undergone a refrigeration cycle.  A cycle on which refrigerators operate.  Thermodynamically closed cycle not open cycle.

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Closed cycle

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Open cycle

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The most frequently used refrigeration cycles are:• Vapor-compression refrigeration cycle - in which the refrigerant is vaporized and condensed

alternately and is compressed in the vapor phase. • Gas-refrigeration cycle - in which the refrigerant remains in the gaseous phase throughout the cycle. •Vapor-absorption refrigeration - where the refrigerant is dissolved in a liquid before it is compressed. •Thermoelectric refrigeration - where refrigeration is produced by the passage of electric current through two dissimilar materials. 13

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Principles of Refrigeration REFRIGERATION SYSTEM COMPONENTS The basic components of a refrigeration system are: - Evaporator - Compressor - Condenser - Expansion Valve - Refrigerant; to conduct the heat from the product

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Evaporator  The purpose of the evaporator is to remove unwanted heat from the product

 Refrigerant contained within the evaporator is boiling at a low-pressure. The level of this pressure is determined by two factors: - The rate at which the heat is absorbed from the product to the liquid refrigerant in the evaporator - The rate at which the low-pressure vapor is removed from the evaporator by the compressor

 When leaving the evaporator coil the liquid refrigerant is in vapor form. 17

Compressor  The purpose of the compressor is to draw the low-

temperature, low-pressure vapor from the evaporator via the suction line.  When vapor is compressed it rises in temperature.  The compressor transforms the vapor from a lowtemperature vapor to a high-temperature vapor, in turn increasing the pressure.

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Condenser  The purpose of the condenser is to extract heat from the   

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refrigerant to the outside air. May be air cooled or water cooled Fans mounted above the condenser unit are used to draw air through the condenser coils. The temperature of the high-pressure vapor determines the temperature at which the condensation begins. As heat has to flow from the condenser to the air, the condensation temperature must be higher than that of the air. The high-pressure vapor within the condenser is then cooled to the point where it becomes a liquid refrigerant once more, whilst retaining some heat. 19

Expansion Valve  The expansion valve is located at the end of the liquid line, before the evaporator. The high-

pressure liquid reaches the expansion valve, having come from the condenser.  The valve then reduces the pressure of the refrigerant as it passes through the orifice, which is located inside the valve.  On reducing the pressure, the temperature of the refrigerant also decreases to a level below the surrounding air.  This low-pressure, low-temperature liquid is then pumped in to the evaporator.  May be capillary tube/orifice 20

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Need for Refrigeration  The growth of microorganisms is temperaturedependent, that growth declines as temperature falls, and that growth becomes very slow at temperatures below +10 °C

 Use of refrigeration to conserve foodstuffs and natural ice came into use for this purpose

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Application of Refrigeration  Foodstuff production, conservation and preservation  Chemical processing industry  Industrial and comfort air conditioning plants  Drying plants, etc

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Requirements of Refrigerant  There are certain desirable characteristics which a fluid

used as a refrigerant should posses:  non-poisonous  Non-explosive  Non-corrosive  Non-inflammable  Leaks should be easily detected  Leaks should be easy to locate  Should operate under low pressure  Stable gas

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Continued…….. ◦ Parts moving in the fluid should be easy to lubricate ◦ Non- toxic ◦ Well balanced enthalpy of evaporation per unit mass

◦ Small relative displacement to obtain a certain

refrigerating effect ◦ A minimum difference between the vaporizing and condensing pressure is desirable  The standard comparison of refrigerants as used in refrigeration industry is based on an evaporator temperature of -15oC and condensing temperature of 30oC

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1.2 Refrigerants

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The specific objectives of this lecture are to: 1. Discuss the importance of selection of suitable refrigerant in a refrigeration system. 2. Classify refrigerants into primary and secondary, and discuss the important differences between primary and secondary refrigerants. 3. Discuss refrigerant selection criteria based on thermodynamic, thermo physical, environmental and economic properties. 4. Describe the numbering system used for designating refrigerants. 5. Present a comparison between different refrigerants.

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Introduction  Refrigerant : is the primary working fluid used

for absorbing and transmitting heat in a refrigeration system  Refrigerants absorb heat at low temperature and low pressure and release heat at a higher temperature and pressure  Most refrigerants undergo phase changes during heat absorption (evaporation) and heat releasing (condensation)

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 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, 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. 29

 Replacement of an existing refrigerant by a completely new refrigerant, for whatever reason, is an expensive proposition as it may call for several changes in the design and manufacturing of refrigeration systems.  Hence it is very important to understand the issues related to the selection and use of refrigerants. In principle, any fluid can be used as a refrigerant.  Air used in an air cycle refrigeration system can also be considered as a refrigerant.  However, in this lecture the attention is mainly focused on those fluids that can be used as refrigerants in vapor compression refrigeration systems only. 30

Classification of Refrigerants Primary and Secondary Refrigerants  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 vapor compression and vapor absorption refrigeration systems.  When used in compression or absorption systems, these fluids provide refrigeration by undergoing a phase change process in the evaporator.

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 As the name implies, 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.  Of course, if the operating temperatures are above 0oC, then pure water can also be used as secondary refrigerant, for example in large air conditioning systems.  Antifreezes or brines are used when refrigeration is required at sub-zero temperatures.  Unlike primary refrigerants, the secondary refrigerants do not undergo phase change as they transport energy from one location to other. 32

 Cooling Medium: is a working fluid cooled by the

refrigerant to transport the cooling effect between a central plant and remot cooling units and terminals  Chilled water, brine, and glycol are used as cooling media in many refrigeration systems.  The cooling medium is often called a secondary refrigerant, because it reduces the extensive circulation of the primary refrigerant

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 An important property of a secondary refrigerant is its freezing point. Generally, the freezing point of a brine will be lower than the freezing point of its constituents.  The temperature at which freezing of a brine takes place depends on its concentration. The concentration at which a lowest temperature can be reached without solidification is called as eutectic point.  The commonly used secondary refrigerants are the solutions of water and ethylene glycol, propylene glycol or calcium chloride. These solutions are known under the general name of brines.

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Essential Properties of Refrigerants  Chemical stability under conditions of use is the

most important characteristics  Safety codes may require a nonflammable refrigerant of low toxicity for most applications  Cost, availability, efficiency, and compatibility with compressor lubricants and materials with which the equipment is constructed are other concerns  Latent heat of vaporization is another important property

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Physical Properties  The refrigerants are arranged in increasing order of

atmospheric boiling point, from air at -194.3 oC to water at 100 oC  The boiling point is most important because it is a direct indicator of the temperature level at which a refrigerant can be used

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Physical Properties of Selected Refrigerants

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CAUTION!!! Some topics related to Refrigerants are missing, like, Properties of Refrigerants, Refrigeration effect and unit of refrigeration, Refrigerant selection criteria, Thermodynamic and Thermo-physical Properties, Environmental and Safety Properties, Economic properties, Designation of refrigerants, Types of refrigerants…(…are to be covered by assignment one).

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1.3 Applications of Refrigeration and Air-Conditioning

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1.3.1 Objectives of the lesson: The objectives of this lesson are to introduce the students to:  Applications of refrigeration in: a. b. c. d.

Food processing, preservation and distribution Chemical and process industries Special Applications such as cold treatment of metals, medical, construction, ice skating etc. Comfort air-conditioning

 Applications of air conditioning, namely: a. Industrial, such as in textiles, printing, manufacturing, photographic, computer rooms, power plants, vehicular etc. b. Comfort – commercial, residential etc. 40

1.3.2 Introduction  Refrigeration deals with cooling of bodies or fluids to temperatures lower than those of surroundings. This involves absorption of heat at a lower temperature and rejection to higher temperature of the surroundings.

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 In olden days,  The main purpose of refrigeration was to produce ice, which was used for cooling beverages, food preservation and refrigerated transport etc.  Now-a-days refrigeration and air conditioning find so many applications that they have become very essential for mankind, and without refrigeration and air conditioning the basic fabric of the society will be adversely affected.  Refrigeration and air conditioning are generally treated in a single subject due to the fact that one of the most important applications of refrigeration is in cooling and dehumidification as required for summer air conditioning.  Of course, refrigeration is required for many applications other than air conditioning, and air conditioning also involves processes other than cooling and dehumidification. Figure above shows the relation between refrigeration and air conditioning in a pictorial form. 42

 The temperature range of interest in refrigeration extends down to about –100oC.  Now-a-days refrigeration has become an essential part of food chain- from post harvest heat removal to processing, distribution and storage.  Refrigeration has become essential for many chemical and processing industries to improve the standard, quality, precision and efficiency of many manufacturing processes.

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 As mentioned before, air-conditioning is one of the major applications of refrigeration.  Air-conditioning has made the living conditions more comfortable, hygienic and healthy in offices, work places and homes.  It involves control of temperature, humidity, cleanliness of air and its distribution to meet the comfort requirements of human beings and/or some industrial requirements.  Air-conditioning involves cooling and dehumidification in summer months; this is essentially done by refrigeration.  It also involves heating and humidification in cold climates, which is conventionally done by a boiler unless a heat pump is used. 44

1.3.3 Applications of Refrigeration The major applications of refrigeration can be grouped into following four major equally important areas. 1. Food processing, preservation and distribution 2. Chemical and process industries 3. Special Applications 4. Comfort air-conditioning

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I) Application of refrigeration in Food processing, preservation and distribution  Storage of Raw Fruits and Vegetables: It is well-known that some bacteria are responsible for degradation of food, and enzymatic processing cause ripening of the fruits and vegetables.  The growth of bacteria and the rate of enzymatic processes are reduced at low temperature.  This helps in reducing the spoilage and improving the shelf life of the food.

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Effect of storage temperature on useful storage life of food products

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 In case of fruits and vegetables, the use of refrigeration starts right after harvesting to remove the post-harvest heat, transport in refrigerated transport to the cold storage or the processing plant.  A part of it may be stored in cold storage to maintain its sensory qualities and a part may be distributed to retail shops, where again refrigeration is used for short time storage.  Depending upon the size, the required capacity of refrigeration plants for cold storages can be very high.  Ammonia is one of the common refrigerants used in cold storages.

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Photograph of a typical Cold storage Ammonia based refrigeration cold storage plant for a large cold storage

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Recommended storage conditions for fruits and vegetables

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 Meat and poultry: These items also require refrigeration right after slaughter during processing, packaging. Short-term storage is done at 0oC. Longterm storage requires freezing and storage at -25oC.  Dairy Products: The important dairy products are milk, butter, buttermilk and ice cream. To maintain good quality, the milk is cooled in bulk milk coolers immediately after being taken from cow. Bulk milk cooler is a large refrigerated tank that cools it between 10 to 15oC. Then it is transported to dairy farms, where it is pasteurized. Pasteurization involves heating it to 73oC and holding it at this temperature for 20 seconds. Thereafter, it is cooled to 3 to 4oC. The dairies have to have a very large cooling capacity, since a large quantity of milk has to be immediately cooled after arrival. 51

 Beverages: Production of beer, wine and concentrated fruit juices require refrigeration. The taste of many drinks can be improved by serving them cold or by adding ice to them. Juices can be preserved for a longer period of time than the fruits. Also, Fruit juice concentrates when frozen can be more easily shipped and transported by road. To preserve the taste and flavor of juice, the water is driven out of it by boiling it at low temperature under reduced pressure. The concentrate is frozen and transported at –20oC.

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Processing and distribution of frozen food: Many vegetables, meat, fish and poultry are frozen to sustain the taste, which nearly duplicates that of the fresh product. Freezing retains the sensory qualities of color, texture and taste apart from nutritional qualities. The refrigeration systems for frozen food applications are very liberally designed, since the food items are frozen in shortest period of time. Ready-to-eat frozen foods, packed dinners and bakery items are frozen and stored at temperatures of –25 to 20 oC for distribution to retail stores during peak demands or off-season demands. 53

II) Applications of refrigeration in chemical and process industries  The industries like petroleum refineries, petrochemical plants and paper pulp industries etc. require very large cooling capacities.  The main applications of refrigeration in chemical and process industries involve the following categories. 1. Separation of gases: In petrochemical plant, temperatures as low as –150oC with refrigeration capacities as high as 10,000 Tons of Refrigeration (TR) are used for separation of gases by fractional distillation. Some gases condense readily at lower temperatures from the mixtures of hydrocarbon. Propane is used as refrigerant in many of these plants. 54

2. Condensation of Gases: some gases that are produced synthetically, are condensed to liquid state by cooling, so that these can be easily stored and transported in liquid state. For example, in synthetic ammonia plant, ammonia is condensed at –10 to 10oC before filling in the cylinders, storage and shipment. This low temperature requires refrigeration. 3. Dehumidification of Air: Low humidity air is required in many pharmaceutical industries. It is also required for air liquefaction plants.  This is also required to prevent static electricity and prevents short circuits in places where high voltages are used.  The air is cooled below its dew point temperature, so that some water vapor condenses out and the air gets dehumidified. 55

III) Special applications of refrigeration  In this category we consider applications other than chemical uses. These are in manufacturing processes, applications in medicine, construction units etc. 1. Cold Treatment of Metals: The dimensions of precision parts and gauge blocks can be stabilized by soaking the product at temperature around – 90oC. The hardness and wear resistance of carburized steel can be increased by this process.  Keeping the cutting tool at –100oC for 15 minutes can also increase the life of cutting tool. In deep drawing process the ductility of metal increases at low temperature. Mercury patterns frozen by refrigeration can be used for precision casting. 56

2. Medical: Blood plasma and antibiotics are manufactured by freeze-drying process where water is made to sublime at low pressure and low temperature.  This does not affect the tissues of blood.  Centrifuges refrigerated at –10oC, are used in the manufacture of drugs.  Localized refrigeration by liquid nitrogen can be used as anesthesia also.

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3. Construction: Setting of concrete is an exothermic process. If the heat of setting is not removed the concrete will expand and produce cracks in the structure. Concrete may be cooled by cooling sand, gravel and water before mixing them or by passing chilled water through the pipes embedded in the concrete. Another application is to freeze the wet soil by refrigeration to facilitate its excavation. 4. Desalination of Water: In some countries fresh water is scarce and seawater is desalinated to obtain fresh water. Solar energy is used in some cases for desalination. An alternative is to freeze the seawater. The ice thus formed will be relatively free of salt. The ice can be separated and thawed to obtain fresh water. 58

5. Ice Manufacture: This was the classical application of refrigeration. Ice was manufactured in plants by dipping water containers in chilled brine and it used to take about 36 hours to freeze all the water in cans into ice. The ice thus formed was stored in ice warehouses. Now that small freezers and icemakers are available.  Hotels and restaurants make their own ice, in a hygienic manner. Household refrigerators also have the facility to make ice in small quantities. The use of ice warehouses is dwindling because of this reason. Coastal areas still have ice plants where it is used for transport of iced fish. 59

Refrigeration systems are also required in remote and rural areas for a wide variety of applications such as storage of milk, vegetables, fruits, food grains etc., and also for storage of vaccines etc. in health centers.  One typical problem with many of the rural and remote areas is the continuous availability of electricity. Since space is not constraint, and most of these areas in tropical countries are blessed with alternate energy sources such as solar energy, biomass etc., it is preferable to use these clean and renewable energy sources in these areas. Thermal energy driven absorption systems have been used in some instances. Vapour compression systems that run on photovoltaic (PV) cells have also been developed for small applications. Figure shows the schematic of solar PV cell driven vapor compression refrigeration system for 60 vaccine storage.

Solar energy driven refrigeration system for vaccine storage 61

1.3.4 Application of AirConditioning:

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I) Industrial Air-Conditioning- the main purpose of industrial air conditioning system is to provide conducive conditions so that the required processes can be carried out and required products can ne produced. The following are the applications to name a few

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II) Comfort Air-Conditioning:  Energy of food is converted into chemical energy for functioning of brain, lungs, heart and other organs and this energy is ultimately rejected to the surroundings.  Also the internal organs require a temperature close to 35oC for their efficient operation, and regulatory mechanisms of human body maintain this temperature by rejecting appropriate amount of heat.  Human beings do not feel comfortable if some extra effort is required by the body to reject this energy.  The air temperature, humidity and velocity at which human body does not have to take any extra action, is called comfort condition.

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 Restaurants, theatres and other places of amusement require air-conditioning for the comfort of patrons.  All places where, a large number of people assemble should have sufficient supply of fresh air to dilute CO2 and body odors emitted by persons.  In addition, people dissipate large quantities of heat that has to be removed by air-conditioning for the comfort of persons.  These places have wide variation in air-conditioning load throughout the day.  These have large number of persons, which add a lot of water vapor by respiration and perspiration.  The food cooked and consumed also adds water vapor. This vapor has to be removed by air-conditioning plant. Hence, these buildings have large latent heat loads. Infiltration of warm outdoor is also large since the large number of persons enter and leave the building leading to entry of outdoor air with every door opening.  Ventilation requirement is also very large. 69

Exercise 1. What do you understand by a cold chain for food

products? 2. Explain the importance of cold storage 3. What are important issues to be considered in the design of refrigeration systems? 4. What is the relation between refrigeration and air conditioning?

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1.4 VapourCompression Refrigeration Cycle 71

The Specific Objectives of the Lesson: This lesson discusses the most commonly used refrigeration system, i.e. Vapour compression refrigeration system. The following things are emphasized in detail: 1. The Carnot Refrigeration Cycle & its Practical Limitations. 2. The Ideal(Standard) Vapour Compression Refrigeration

System. 3. The Actual Vapor-Compression Refrigeration Cycle 4. Analysis of Standard Vapour Compression Refrigeration System. 72

1.4.1 Application Areas of Thermodynamics The main application areas of thermodynamics are presented as follows:

Main application areas of Thermodynamics

Power cycles

Vapor power cycles • Steam power plants • Nuclear power plants

Refrigeration cycles • Refrigerators • Heat pumps • Air Conditioners

Gas power cycles • Gasoline (Otto cycle) engines • Diesel (Diesel cycle) engines • Gas turbines (Brayton cycle)

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1.4.2 Comparison Between Gas Cycles and Vapor Cycles  Thermodynamic cycles can be categorized into gas cycles and vapour cycles.  In a typical gas cycle, the working fluid (a gas) does not undergo phase change.  In gas cycles, heat rejection and refrigeration take place as the gas undergoes sensible cooling and heating.  In a vapour cycle the working fluid undergoes phase change and refrigeration effect is due to the vaporization of refrigerant liquid.  Since the refrigeration effect is produced during phase change, large amount of heat (latent heat) can be transferred per kilogram of refrigerant at a near constant temperature.  Vapour cycles can be subdivided into vapour compression systems, vapour absorption systems, vapour jet systems etc. Among these the vapour compression refrigeration systems are predominant. 74

1.4.3 Vapour Compression Refrigeration Systems  As mentioned, vapour compression refrigeration systems    

are the most commonly used among all refrigeration systems. In a vapour compression refrigeration system, refrigeration is obtained as the refrigerant evaporates at low temperatures. The input to the system is in the form of mechanical energy required to run the compressor. Hence these systems are also called as mechanical refrigeration systems. The actual vapour compression cycle is based on EvansPerkins cycle, which is also called as reverse Rankine cycle. Before the actual cycle is discussed and analyzed, it is essential to find the upper limit of performance of vapour compression cycles. This limit is set by a completely reversible cycle(Carnot cycle). 75

The vapor-compression refrigeration cycle Main application areas of the Vapor-Compression Refrigeration Cycle is in the design and operation of vapor-compression Refrigerators and Heat pumps.

Vapor-Compression Refrigerator

Vapor-Compression Heat pump 76

Refrigerators and Heat pumps…

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The vapor-compression refrigeration cycle… We know from experience that heat flows spontaneously or by itself from a high-temperature region to a low-temperature region. But, the transfer of heat from a low-temperature region to a high-temperature region is possible only with the help of external work. Hence, this requires a special type of device called a refrigerator or a heat pump. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only. The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it, whereas the objective of a heat pump is to maintain a heated space at a high temperature. The performance of refrigerators and heat pumps is expressed in terms of the Coefficient Of Performance (COP), defined as;

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Refrigerators and Heat pumps

Note:- Air conditioners are basically refrigerators whose refrigerated space is a room or a building instead of the food compartment.

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Refrigerators and Heat pumps…

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Refrigerators and Heat pumps…

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Refrigerators and Heat pumps…

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Refrigerators and Heat pumps…

Class work:Show that

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1.4.4 The Carnot Vapor Cycle and The Carnot Refrigeration Cycle I) The Carnot Vapor Cycle  The Carnot cycle is the most efficient cycle operating between

two specified temperature limits. Thus it is natural to look at the Carnot cycle first as a prospective ideal cycle for vapor power plants.  Consider a steady-flow Carnot cycle shown on a T-s diagram below. The fluid is heated reversibly and isothermally in a boiler (process 1-2), expanded isentropically in a turbine (process 2-3), condensed reversibly and isothermally in a condenser (process 3-4), and compressed isentropically by a compressor to the initial state (process 4-1). 84

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The Carnot efficiency  The thermal efficiency of the Carnot cycle is a function of the sink and source temperatures only and is given by,

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II) The Carnot Refrigeration Cycle  It is also called the Reversed Carnot Vapour Cycle  The Carnot cycle is a totally reversible cycle that consists of two reversible

isothermal and two isentropic processes. It has the maximum thermal efficiency for the given temperature limits, and serves as a standard cycle against which actual power cycles can be compared. If the Carnot vapor power cycle is reversed, then the cycle becomes a Carnot refrigeration or heat pump cycle.

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The Reversed Carnot Vapour Cycle… The coefficients of performance of Carnot refrigerators and heat pumps are expressed in terms of temperatures as,

From these relations, it is observed that the COP can be increased by increasing TL and/or by decreasing TH. But, the reversed Carnot cycle cannot represent actual cycles. Why ? Explain. However, the reversed Carnot cycle can serve as a standard cycle against which actual refrigeration cycles are compared.

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As shown in Fig.10.1(a), the basic Carnot refrigeration system for pure vapour consists of four components: compressor, condenser, turbine and evaporator. The cycle involves two isothermal heat transfer processes (processes 4-1 and 2-3) and two isentropic work transfer processes (processes 1-2 and 3-4). Heat is extracted isothermally at evaporator temperature Te during process 4-1, heat is rejected isothermally at condenser temperature Tc during process 2-3. Work is supplied to the compressor during the isentropic compression (1-2) of refrigerant vapour from evaporator pressure Pe to condenser pressure Pc, and Work is produced by the system as refrigerant liquid expands isentropically in the turbine from condenser pressure Pc to evaporator pressure Pe. 89

 All the processes are both internally as well as

externally reversible, i.e., net entropy generation for the system and environment is zero. Applying first and second laws of thermodynamics to the Carnot refrigeration cycle,

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 Thus the COP of Carnot refrigeration cycle is a

function of evaporator and condenser temperatures only and is independent of the nature of the working substance.  The Carnot COP sets an upper limit for refrigeration systems operating between two constant temperature thermal reservoirs (heat source and sink).  From Carnot’s theorems, for the same heat source and sink temperatures, no irreversible cycle can have COP higher than that of Carnot COP.

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Practical Difficulties With Carnot Refrigeration System: It is difficult to build and operate a Carnot refrigeration system due to the following practical difficulties:

i. During process 1-2, a mixture consisting of liquid and vapour have to be compressed isentropically in the compressor.  Such a compression is known as wet compression due to the presence of liquid.  In practice, wet compression is very difficult especially with reciprocating compressors.  This problem is particularly severe in case of high speed reciprocating compressors, which get damaged due to the presence of liquid droplets in the vapour. 95

 Since reciprocating compressors are most widely used in

refrigeration, traditionally dry compression (compression of vapour only) is preferred to wet compression.

ii. During process 3-4, the second practical difficulty with Carnot cycle is that using a turbine and extracting work from the system during the isentropic expansion of liquid refrigerant is not economically feasible, particularly in case of small capacity systems.  This is due to the fact that the specific work output (per kilogram of refrigerant) from the turbine is given by:

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 Since the specific volume of liquid is much smaller compared

to the specific volume of a vapour/gas, the work output from the turbine in case of the liquid will be small.  In addition, if one considers the inefficiencies of the turbine, then the net output will be further reduced.  As a result using a turbine for extracting the work from the high pressure liquid is not economically justified in most of the cases.  One way of achieving dry compression in Carnot refrigeration cycle is to have two compressors – one isentropic and one isothermal as shown in Fig.10.4.

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 As shown in Fig.10.4, the Carnot refrigeration system with dry

compression consists of one isentropic compression process (12) from evaporator pressure Pe to an intermediate pressure Pi and temperature Tc, followed by an isothermal compression process (2-3) from the intermediate pressure Pi to the condenser pressure Pc.  Though with this modification the problem of wet compression can be avoided, still this modified system is not practical due to the difficulty in achieving true isothermal compression using high speed compressors.  In addition, use of two compressors in place of one is not economically justified.

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 From the above discussion, it is clear that from practical

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considerations, the Carnot refrigeration system need to be modified. Dry compression with a single compressor is possible if the isothermal heat rejection process is replaced by isobaric heat rejection process. Similarly, the isentropic expansion process can be replaced by an isenthalpic throttling process. A refrigeration system, which incorporates these two changes is known as Evans-Perkins or reverse Rankine cycle. This is the theoretical cycle on which the actual vapour compression refrigeration systems are based.

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1.4.5 The Rankine Cycle and The Ideal(Standard) Vapor-Compression Refrigeration Cycle I) The Rankine Cycle The Rankine cycle is the ideal cycle for vapor power cycles. Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser, as shown in the figure below. The ideal Rankine cycle consists of four processes: isentropic compression in pump (1-2), constant pressure heat addition in a boiler (2-3), isentropic expansion in a turbine (3-4), and constant pressure heat rejection in a condenser (4-1).

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II)The Ideal(Standard) Vapor-Compression Refrigeration Cycle(VCRS) • It is also called Reversed Rankine Cycle. • Many of the impracticalities associated with the reversed Carnot cycles can be eliminated by vaporizing the refrigerant completely before it is compressed and by replacing the turbine with a throttling device, such as an expansion valve or capillary tube. The cycle so obtained is called the ideal vapor-compression cycle, and it is shown in the figure. The vaporcompression cycle is a modified reversed Rankine cycle. • The vapor-compression refrigeration cycle is the most widely used cycle for refrigerators, heat pumps, air-conditioning systems. It consists of four processes:

• Process (1-2), Isentropic compression of saturated vapour in compressor • Process (2-3), Isobaric heat rejection in condenser • Process (3-4), Isenthalpic expansion of saturated liquid in expansion device • Process (4-1), Isobaric heat extraction in the evaporator 103

 By comparing with Carnot Refrigeration Cycle, it can be seen

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that the standard vapour compression refrigeration cycle introduces two irreversibilities: 1) Irreversibility due to non-isothermal heat rejection (process 2-3) and 2) Irreversibility due to isenthalpic throttling (process 3- 4). As a result, one would expect the theoretical COP of standard cycle to be smaller than that of a Carnot system for the same heat source and sink temperatures. Due to these irreversibilities, the cooling effect reduces and work input increases, thus reducing the system COP. This can be explained easily with the help of the cycle diagrams on T s charts. Figure 10.6(a) shows comparison between Carnot and standard VCRS in terms of refrigeration effect. 104

 The heat extraction (evaporation) process is reversible for

both the Carnot cycle and VCRS cycle. Hence the refrigeration effect is given by: For Carnot refrigeration cycle (1-2’’-3-4’):

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 For VCRS cycle (1-2-3-4):

 Thus there is a reduction in refrigeration effect when the isentropic expansion process of Carnot cycle is replaced by isenthalpic throttling process of VCRS

cycle, this reduction is equal to the area d-4-4’-c-d (area A2) and is known as throttling loss.  The throttling loss is equal to the enthalpy difference between state points 3 and 4’, i.e,

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 It is easy to show that the loss in refrigeration effect increases

as the evaporator temperature decreases and/or condenser temperature increases. A practical consequence of this is a requirement of higher refrigerant mass flow rate. The heat rejection in case of VCRS cycle also increases when compared to Carnot cycle.

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 As shown in Fig.10.6(b), the heat rejection in case of Carnot

cycle (1-2’’-3-4’) is given by:

 Hence the increase in heat rejection rate of VCRS compared to

Carnot cycle is equal to the area 2’’-2-2’ (area A1). This region is known as superheat horn, and is due to the replacement of isothermal heat rejection process of Carnot cycle by isobaric heat rejection in case of VCRS.

 Since the heat rejection increases and refrigeration effect

reduces when the Carnot cycle is modified to standard VCRS cycle, the net work input to the VCRS increases compared to Carnot cycle. The net work input in case of Carnot and VCRS cycles are given by: 108

As shown in Fig.10.6(c), the increase in net work input in VCRS cycle is given by:

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 To summarize the refrigeration effect and net work input of

VCRS cycle are given by:

 The COP of VCRS cycle is given by:

 If we define the cycle efficiency, ηR as the ratio of COP of VCRS

cycle to the COP of Carnot cycle, then:

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The Ideal(Standard) Vapor-Compression Refrigeration Cycle… Another diagram frequently used in the analysis of vapor-compression refrigeration cycles is the p-h diagram shown in the figure. All four components of associated with the vaporcompression refrigeration cycle are steady-flow devices, and all four processes that make up the Cycle can be analyzed as steady-flow processes. The kinetic and potential energy changes of the refrigerant are usually small relative to the work and heat transfer terms, and hence can be neglected. H = U + pV The stead-flow equation on a unit-mass basis is given by,

The condenser and the evaporator do not involve any work, and the compressor can be assumed as adiabatic. Then COP can be expressed as, 112

1.4.6 The Actual Vapor-Compression Refrigeration Cycle • An actual vapor-compression refrigeration cycle differs from the

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ideal one in several ways, owing mostly to the irreversibilities that occur in various components. Two common sources of irreversibilities are fluid friction (causes pressure drops) and heat transfer to or from the surroundings. The T-s diagram of an actual vapor-compression refrigeration cycle is shown in Fig. 11–7. In the ideal cycle, the refrigerant leaves the evaporator and enters the compressor as saturated vapor. In practice, however, it may not be possible to control the state of the refrigerant so precisely. Instead, it is easier to design the system so that the refrigerant is slightly superheated at the compressor inlet. 113

 This slight overdesign ensures that the refrigerant is completely

vaporized when it enters the compressor.  Also, the line connecting the evaporator to the compressor is usually very long; thus the pressure drop caused by fluid friction and heat transfer from the surroundings to the refrigerant can be very significant.  The result of superheating, heat gain in the connecting line, and pressure drops in the evaporator and the connecting line is an increase in the specific volume, thus an increase in the power input requirements to the compressor since steady-flow work is proportional to the specific volume.  The compression process in the ideal cycle is internally reversible and adiabatic, and thus isentropic. The actual compression process, however, involves frictional effects, which increase the entropy, and heat transfer, which may increase or decrease the entropy, depending on the direction. 114

 Therefore, the entropy of the refrigerant may increase (process

1-2) or decrease (process 1-2’) during an actual compression process, depending on which effects dominate.  The compression process 1-2’ may be even more desirable than the isentropic compression process since the specific volume of the refrigerant and thus the work input requirement are smaller in this case.  Therefore, the refrigerant should be cooled during the compression process whenever it is practical and economical to do so. In the ideal case, the refrigerant is assumed to leave the condenser as saturated liquid at the compressor exit pressure.  In reality, however, it is unavoidable to have some pressure drop in the condenser as well as in the lines connecting the condenser to the compressor and to the throttling valve. 115

 Also, it is not easy to execute the condensation process with

such precision that the refrigerant is a saturated liquid at the end, and it is undesirable to route the refrigerant to the throttling valve before the refrigerant is completely condensed.  Therefore, the refrigerant is subcooled somewhat before it enters the throttling valve. We do not mind this at all, however, since the refrigerant in this case enters the evaporator with a lower enthalpy and thus can absorb more heat from the refrigerated space.  The throttling valve and the evaporator are usually located very close to each other, so the pressure drop in the connecting line is small.

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1.4.7 Analysis of Standard Vapour Compression Refrigeration System  A simple analysis of standard vapour compression refrigeration

system can be carried out by assuming a) Steady flow; b) negligible kinetic and potential energy changes across each component, and c) no heat transfer in connecting pipe lines. The steady flow energy equation is applied to each of the four components.

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