Summer Training Report On,: Cooling Tower Fundamentals

SUMMER TRAINING REPORT ON, COOLING TOWER FUNDAMENTALS

MEJIA THERMAL POWER STATION (DV.C) NPTI(ER), DURGAPUR TRAINING PROJECT (27THJUNE, 2018 – 17THJULY, 2018) SUBMITTED BY,

SOUMYADIP MONDAL (15702615044, 151570110045) UNDER THE GUIDENCE OF: P.K. DUBEY & P. MUKHERJEE (DY. DIRECTOR, D.V.C, M.T.P.S)

THIS PROJECT HAS BEEN SUBMITTED IN PARTIAL FULFILLMENT FOR THE AWARD OF DEGREE OF

BACHELOR OF TECHNOLOGY IN POWER ENGINEERING UNDER M.A.K.A.U.T, W.B

ACKNOWLEDGEMENTS In this era of extreme global development, we need a great balance between the theoretical and practical knowledge. For this reason, the vocational training is a great boon for the to-be engineers. It gives us a great chance to come in line with the actual problems going on the industries and getting a chance to work with engineers and learn how to tackle every situation. The successful completion of any task would be incomplete without mentioning the name of person who helped us to make it possible. We take this opportunity to express my gratitude in few words to all those who helped me in the completion of this project. First and foremost, I would like to express my sincere gratitude towards Mr. P.K. Dubey for his guidance. I am also thankful to persons for their cooperation during this period. Mr. P. Mukherjee, DVC, MTPS. and my friend, Aditya Anand.

Certificate Format

To Whom it May Concern

This is to certify that Mr./Mrs. Soumyadip Mondal of National Power Training Institute (E.R) has undertaken a project work entitled “COOLING TOWER FUNDAMENTALS” at MTPS (MEJIA THERMAL POWER STATION), DVC, BANKURA, WESTBENGAL from 27.06.2018 to 17.07.2018 in partial fulfilment of B.tech (Power Engineering).

INDEX 1. INTRODUCTION 1.1 POWER GENERATION IN INDIA

2. THERMAL POWER STATIONS 3. HISTORY OF ORGANISATION A. TECHNICAL SPECIFICATIONS OF MTPS

4. COOLING TOWER 4.1 COMPONENTS 4.2 MATERIALS 4.3 TYPES OF COOLOING TOWER A. B. C. D.

NATURAL DRAFT, MECHANICAL DRAFT, OPEN VS. CLOSED CKT TOWER HYBRID TOWER

4.4 PERFORMANCE 4.5 ASSESSMENT 4.6 FACTORS AFFECTING PERFORMANCE A. DESIGN, B. FILL MEDIA EFFECTS, C. WATER DISTRIBUTION

4.7 GENERAL IMPROVEMENT PROCEDURES

5. COOLING WATER CHEMISTRY 5.1 CORROSION A. CORROSION CONTROL, B. CORROSION INHIBITORS, C. INHIBITORS SELECTION

5.2 SCALING

A. TYPES OF SCALING, B. DEPOSIT CONTROL METHODS

5.3 MICROBIAL GROWTH A. PROBLEMS OF MICROBIAL GROWTH, B. SELECTION OF MICRO BIOCIDES, I. II.

OXIDIZING TOXICANTS, NON OXIDIZING BIOCIDES

6. PERFORMANCE IMPROVEMENT 6.1 WATER USE A. B. C. D.

REDUCE WATER LOSE, REDUCE BLOW DOWN, USE ALTERNATIVE WATER SUPPLIES, REUSE BLOW DOWN

6.2 WATER TREATMENT A. B. C. D. E. F. G. H. I. J.

SULPHURIC ACID TREATMENT, SIDE STREAM FILTRATION, OZONE MAGNETS, SONIFICATION, ELECTRO COAGULATION, ACTIVATED CARBON, U.V RADIATION, HYDROCAVITATION, RADIO FREQUENCY

7. RECOMMENDATION 8. CONCLUSION 9. REFERENCES

1. INTRODUCTION 1.1 Power generation in India India is the world’s fifth largest electricity generator with a total installed capacity of 2,28,722 MW. Out of this, 90,062 MW is from state owned utilities, 72,927 MW is from privately owned utilities and 65,733 MW is from central owned utilities. (as on Dec. 23 2013) A power station is also referred to as generating station, power plant, powerhouse, or generating plant, is an industrial facility for the generation of electric power. Most power station contains one or more generators, ( A rotating electrical machine that converts mechanical power into electrical power). The relative motion between a magnetic field and conductor induces electromotive force (EMF). The energy source harnessed to turn the generator varies widely. Most power plants in the word burn fossil fuels such as coal, oil, and natural gas to generate electricity. Others use nuclear power, but there is an increasing use of cleaner Renewable sources such as wind, solar, wave and hydroelectric.  TYPES OF RENEWABLE ENERGY SOURCES:

1. 2. 3. 4.

Hydroelectric power Solar energy Wind energy Marine energy

 TYPES OF NON-RENEWABLE ENERGY SOURCES:

1. THERMAL POWER PLANTS  Coal fired  Oil fired  Gas fired  Biomass fired

2. NUCLEAR POWER PLANTS

2. THERMAL POWER STATIONS A thermal power station is a power plant in which heat energy is converted into electrical energy. Fuel is burnt, chemical energy stored in it transformed into heat energy using which, and water is heated. Then water turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated (according to Rankine cycle).

Regenerative rankine cycle

3. HISTORY OF ORGANIGASION Damoder Valley Corporation was established on 7thjuly 1948. It is the most reputed company in the eastern zone of India. DVC in established on the Damoder river. It also consists of the Durgapur thermal power plant in Durgapur. The MTPS under the DVC is the second largest thermal plant in west Bengal. It has the capacity of 2340MW with 4 units of 210MW each, 2 units of 250MW each & 2 units of 500MW each. With the introduction of another two units of 500MW that is in construction it will be the largest in West Bengal. Mejia Thermal Power Station also known as MTPS is located in the outskirts of raniganj in Bankura district. It is the one of the 5 thermal power stations of Damodar Valley Corporation in the state of west Bengal. The total power plant campus area is surrounded by boundary walls and is basically divided into two major parts, first the power plant itself and second is the colony area for the residence and other facilities for MTPSs employees.

GLIMPSE OF MEJIA THERMAL POWER STATION

A. Technical specifications of MTPS: Installed capacity: Total number of Units: 4 X 210 MW (unit 1 to 4) with brush type generators, 2X 250 MW (unit 5 to 6) with brushless type generators, and 2 X 500 MW (unit 7 to 8) generators.  Total Energy Generation: 2340 MW.  Source of water: Damodar river.  Sources of coal: B.C.C.L and E.C.L, also imported from Indonesia. Station Mejia TPS Mejia TPS Ph-ii

Unit no. 1,2,3& 4 5&6 7&8

Capacity(MW) Boiler Make 210 BHEL 250 BHEL 500 BHEL

SCHEMETIC DIAGRAM OF THERMAL POWER STATION

Turbine Make BHEL BHEL BHEL

4. Cooling Tower Cooling towers are a very important part of many chemical plants. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling. Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and are therefore more cost-effective and energy efficient.

Schematic of an Induced Draft Cooling Tower

4.1 Components The basic components of a cooling tower include the frame and casing, fill, cold-water basin, drift eliminators, air inlet, louvers, nozzles and fans. These are described below.

a) Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fibre units, the casing may essentially be the frame. b) Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. There are two types of fill:  Splash fill: Waterfalls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fills promote better heat transfer than wood splash fills.  Film fill: consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill. c) Cold-water basin: The cold-water basin is located at or near the bottom of the tower, and it receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold-water discharge connection. In many tower designs, the coldwater basin is beneath the entire fill. In some forced draft counter flow design, however, the water at the bottom of the fill is channelled to a perimeter trough that functions as the coldwater basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors.

d) Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere.

e) Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower (cross-flow design) or be located low on the side or the bottom of the tower (counter-flow design). f) Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers.

g) Nozzles: These spray water to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed and spray in a round or square patterns, or they can be part of a rotating assembly as found in some circular cross-section towers. h) Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, the type of propeller fans used is either fixed or variable pitch. A fan with non-automatic adjustable pitch blades can be used over a wide kW range because the fan can be adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions.

4.2 Materials: Originally, cooling towers were constructed primarily with wood, including the frame, casing, louvers, fill and cold-water basin. Sometimes the cold-water basin was made of concrete. Today, manufacturers use a variety of materials to construct cooling towers. Materials are chosen to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fibre, and concrete are widely used in tower construction, as well as aluminium and plastics for some components. a) Frame and casing: Wooden towers are still available, but many components are made of different materials, such as the casing around the wooden framework of glass fibre, the inlet air louvers of glass fibre, the fill of plastic and the cold-water basin of steel. Many towers (casings and basins) are constructed of galvanized steel or, where a corrosive atmosphere is a problem, the tower and/or the basis are made of stainless steel. Larger towers sometimes are made of concrete. Glass fibre is also widely used for cooling tower casings and basins, because they extend the life of the cooling tower and provide protection against harmful chemicals.

b) Fill: Plastics are widely used for fill, including PVC, polypropylene, and other polymers. When water conditions require the use of splash fill, treated wood splash fill is still used in wooden towers, but plastic splash fill is also widely used. Because of greater heat transfer efficiency, film fill is chosen for applications where the circulating water is generally free of debris that could block the fill passageways . c) Nozzles: Plastics are also widely used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon. d) Fans: Aluminium, glass fibre and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are made from galvanized steel, aluminium, or moulded glass fibre reinforced plastic.

4.3 Types of Cooling tower: A. Natural draft cooling tower: The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. As hot air moves upwards through the tower (because hot air rises), fresh cool air is drawn into the tower through an air inlet at the bottom. Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive. There are two main types of natural draft towers: 

 Cross flow tower: air is drawn across the falling water and the fill is located outside the tower.  Counter flow tower: air is drawn up through the falling water and the fill is therefore located inside the tower, although design depends on specific site conditions.

B. Mechanical draft cooling tower:

Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc.

C. Open vs. Closed-Circuit Towers: One of the primary differentiations between cooling towers is whether it is an open or closed-circuit tower. In open towers, the cooling water is pumped through the equipment where it picks up thermal energy and then flows directly to the cooling tower where it is dispersed through spray nozzles over the fill, where heat transfer occurs. Then, this same water is collected in the tower sump and is sent back to the equipment to begin the process again. In an open tower any contaminants in the water are circulated through the equipment being cooled. In a closed-circuit tower, sometimes referred to as a fluid cooler, the cooling water flows through the equipment as in the open tower. The difference is when the water is pumped to the cooling tower, it is pumped through a closed loop heat exchanger that is internal to the cooling tower, and then returned to the equipment. In this application, water in the closed loop is not in direct contact with the evaporative water in the tower, which means contaminants are not circulated through the equipment. In a closed-circuit tower, a small pump, known as a “spray pump” circulates a separate body of evaporative water from the tower sump, through the spray nozzles and over the internal heat exchanger piping. This “open” evaporative body of water is contained within the tower and needs to be regularly made up to replenish evaporative and other losses. However, once water treatment in the closed cooling loop is stabilized, the only time it needs to be made up or adjusted is if there is a leak.

D. Hybrid Towers: Hybrid towers are closed towers which can operate either in the sensible heat transfer mode only (without evaporation) or a combination of sensible and latent heat transfer (with evaporation). During periods of low load and/or low ambient temperature, the spray of water is stopped and heat is sensibly transferred to the flow of air across the fins of the coils containing the cooling

fluid. During periods when this is not enough, a latent heat transfer system is activated by switching on an evaporative cooler or water is sprayed across the dry coils to allow for increased heat transfer through evaporation. These processes offer substantial savings in water.

Types of Cooling tower

 Mechanical draft towers are available in a large range of capacities. Towers can be either factory built or field erected – for example concrete towers are only field erected.  Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., an eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells. Table-1: Types of Cooling tower

Type  Forced draft: Air is blown through the tower by a fan located in the air inlet.

 Induced draft cross flow:  Water enters at top and passes over fill  Air enters on one side (single-flow tower) or opposite sides (double-flow tower)  An induced draft fan draws air across fill towards exit at top of tower.    

Advantages

Disadvantages

 Suited for high air resistance due to centrifugal blower fans  Fans are relatively quiet.

 Recirculation due to high air-entry and low air-exit velocities, which can be solved by locating towers in plant rooms combined with discharge ducts.

 Less recirculation than forced draft towers because the speed of exit air is 34 times higher than entering air.

 Fans and the motor drive mechanism require weatherproofing against moisture and corrosion because they are in the path of humid exit air.

Induced draft counter flow: Hot water enters at the top. Air enters bottom and exits at the top. Uses forced and induced draft fans.

4.4 Performance These measured parameters and then used to determine the cooling tower performance in several ways. a) Range: This is the difference between the cooling tower water inlet and outlet temperature. A high CT Range means that the cooling tower has been able to reduce the water temperature effectively, and is thus performing well. The formula is: Equation 1: CT Range: 𝑪𝑻 𝑹𝒂𝒏𝒈𝒆 (°𝑪) =𝑪𝑾 𝒊𝒏𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (°𝑪)−𝑪𝑾 𝒐𝒖𝒕𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (°𝑪) b) Approach: This is the difference between the cooling tower outlet coldwater temperature and ambient wet bulb temperature. The lower the approach the better the cooling tower performance; although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance. Equation 2: CT Approach: 𝑪𝑻 𝑨𝒑𝒑𝒓𝒐𝒂𝒄𝒉 (°𝑪) =𝑪𝑾 𝒐𝒖𝒕𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (°𝑪)−𝑾𝒆𝒕 𝒃𝒖𝒍𝒃 𝒕𝒆𝒎𝒑 (°𝑪) c) Effectiveness.: This is the ratio between the range and the ideal range (in percentage), i.e. difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach). The higher this ratio, the higher the cooling tower effectiveness. Equation 3: CT Effectiveness: 𝑪𝑻 𝑬𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆𝒏𝒆𝒔𝒔 (%)=(𝑪𝑾 𝒕𝒆𝒎𝒑 – 𝑪𝑾 𝒐𝒖𝒕 𝒕𝒆𝒎𝒑)/ (𝑪𝑾 𝒊𝒏 𝒕𝒆𝒎𝒑 – 𝑾𝑩 𝒕𝒆𝒎𝒑) × 𝟏𝟎𝟎 d) Cooling capacity. This is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.

e) Evaporation loss. This is the water quantity evaporated for cooling duty. Theoretically the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected. The following formula can be used. Equation 4: Evaporation Loss: 𝑬𝒗𝒂𝒑𝒐𝒓𝒂𝒕𝒊𝒐𝒏 𝒍𝒐𝒔𝒔 (𝒎𝟑/𝒉𝒓 )= 𝟎.𝟎𝟎𝟎𝟖𝟓 × 𝟏.𝟖 𝒙 𝒄𝒊𝒓𝒄𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝒓𝒂𝒕𝒆 (𝒎𝟑/𝒉𝒓 ) × (𝑻𝟏−𝑻𝟐) T1 - T2 = temperature difference between inlet and outlet water. f) Cycles of concentration (C.O.C). This is the ratio of dissolved solids in circulating water to the dissolved solids in makeup water. g) Blow down losses depend upon cycles of concentration and the evaporation losses and is given by formula: Equation 5: Blow down: 𝑩𝒍𝒐𝒘 𝒅𝒐𝒘𝒏 = 𝑬𝒗𝒂𝒑𝒐𝒓𝒂𝒕𝒊𝒐𝒏 𝑳𝒐𝒔𝒔/(𝑪.𝑶.𝑪.− 𝟏). h) Liquid/Gas (L/G) ratio: The L/G ratio of a cooling tower is the ratio between the water and the air mass flow rates. Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynamic rules also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used: 𝐿 (𝑻𝟏−𝑻 𝟐) = 𝑮 (𝒉𝟐−𝒉𝟏) Equation 6: Liquid/Gas ratio: 𝑳/𝑮=(𝒉𝟐−𝒉𝟏)/(𝑻𝟏−𝑻 𝟐) Where: L/G = liquid to gas mass flow ratio (kg/kg); T1 = hot water temperature (°C); T2 = cold-water temperature (°C); h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature; h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature.

4.5 Assessment The performance of cooling towers is evaluated to assess present levels of approach and range against their design values, identify areas of energy wastage and to suggest improvements. During the performance evaluation, portable monitoring instruments are used to measure the following parameters: 1. Wet bulb temperature of air 2. Dry bulb temperature of air 3. Cooling tower inlet water temperature 4. Cooling tower outlet water temperature 5. Exhaust air temperature 6. Electrical readings of pump and fan motors 7. Water flow rate 8. Air flow rate

4.6 Factors Affecting Performance A. Design:  Capacity : Heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are not sufficient to understand cooling tower performance. Other factors, which we will see, must be stated along with flow rate m3/hr. For example, a cooling tower sized to cool 4540 m3/hr through a 13.9°C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5°C range.  Range : Range is determined not by the cooling tower, but by the process it is serving. The range at the exchanger is determined entirely by the heat load

and the water circulation rate through the exchanger and on to the cooling water. Equation 7: CT Range: 𝑹𝒂𝒏𝒈𝒆 °𝑪 = 𝑯𝒆𝒂𝒕 𝑳𝒐𝒂𝒅 (𝒌𝑪𝒂𝒍/𝒉𝒓)/𝑾𝒂𝒕𝒆𝒓 𝑪𝒊𝒓𝒄𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝑹𝒂𝒕𝒆 (𝑳𝑷𝑯) Thus, Range is a function of the heat load and the flow circulated through the system. Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 48000 m3/hr from 44°C to 34°C at 26.7°C wet bulb temperature. 𝑪𝑻 𝑨𝒑𝒑𝒓𝒐𝒂𝒄𝒉 (𝟓°𝑪) =𝑪𝑾 𝒐𝒖𝒕𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (𝟑𝟒°𝑪) −𝑾𝒆𝒕 𝒃𝒖𝒍𝒃 𝒕𝒆𝒎𝒑 (𝟐𝟗°𝑪) As a generalization, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.8°C approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. If flow rate, range, approach and wet bulb had to be ranked in the order of their importance in sizing a tower, approach would be first with flow rate closely following the range and wet bulb would be of lesser importance. The range increases when the quantity of circulated water and heat load increase. This means that increasing the range as a result of added heat load requires a larger tower. There are two possible causes for the increased range: 

 The inlet water temperature is increased (and the cold-water temperature at the exit remains the same). In this case it is economical to invest in removing the additional heat.  The exit water temperature is decreased (and the hot water temperature at the inlet remains the same). In this case the tower size would have to be increased considerably because the approach is also reduced, and this is not always economical.

 Heat Load : The heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired operating temperature level of the process. In most cases, a low operating temperature is desirable to increase process efficiency or to improve the quality or quantity of the product. In some applications (e.g. internal combustion engines), however, high operating temperatures are desirable. The size and cost of the cooling tower is proportional to the heat load. If heat load calculations are low undersized equipment will be purchased. If the calculated load is high, oversize and more costly, equipment will result. Process heat loads may vary considerably depending upon the process involved. Determination of accurate process heat loads can become very complex but proper consideration can produce satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy.  Wet Bulb Temperature : Wet bulb temperature is an important factor in performance of evaporative water cooling equipment. It is a controlling factor from the aspect of minimum cold water temperature to which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the air entering the cooling tower determines operating temperature levels throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load. However, a thermal potential is required to reject heat, so it is not possible to cool water to the entering air wet bulb temperature, when a heat load is applied. The approach obtained is a function of thermal conditions and tower capability. Initial selection of towers with respect to design wet bulb temperature must be made on the basis of conditions existing at the tower site. The temperature selected is generally close to the average maximum wet bulb for the summer months. An important aspect of wet bulb selection is whether it is specified as ambient or inlet. The ambient wet bulb is the temperature, which exists generally in the cooling tower area, whereas inlet wet bulb is the wet bulb temperature of the air entering the tower. The later can be, and often is, affected by discharge vapours being re-circulated into the tower. Recirculation

raises the effective wet bulb temperature of the air entering the tower with corresponding increase in the cold water temperature. Since there is no initial knowledge or control over the recirculation factor, the ambient wet bulb should be specified. The cooling tower supplier is required to furnish a tower of sufficient capability to absorb the effects of the increased wet bulb temperature peculiar to his own equipment. It is very important to have the cold water temperature low enough to exchange heat or to condense vapours at the optimum temperature level. By evaluating the cost and size of heat exchangers versus the cost and size of the cooling tower, the quantity and temperature of the cooling tower water can be selected to get the maximum economy for the particular process. The design wet bulb temperature is determined by the geographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, the smaller the tower required. For example, a 4540 m3/hr cooling tower selected for a16.67°C range and a 4.45°C approach to 21.11°C wet bulb would be larger than the same tower to a 26.67°C wet bulb. The reason is that air at the higher wet bulb temperature is capable of picking up more heat.  Tower Size: If heat load, range, approach and wet-bulb temperature are held constant, changing the fourth will affect the tower size as follows: a) Tower size varies inversely with approach. A longer approach requires a smaller tower. Conversely, a smaller approach requires an increasingly larger tower and, at 5°F approach, the effect upon tower size begins to become asymptotic. For that reason, it is not customary in the cooling tower industry to guarantee any approach of less than 5°F.

Tower size v/s approach

b) Tower size varies inversely with wet bulb temperature. When heat load, range, and approach values are fixed, reducing the design wet-bulb temperature increases the size of the tower. This is because most of the heat transfer in a cooling tower occurs by virtue of evaporation (which extracts approximately 1000 Btu’s for every pound of water evaporated), and air’s ability to absorb moisture reduces with temperature.

Tower size v/s wet-bulb

c) Tower size varies directly and linearly with heat load.

Tower size v/s head load

d) Tower size varies inversely with range. Two primary factors account for this. First; increasing the range—also increases the ITD (driving force) between the incoming hot water temperature and the entering wet-bulb temperature.

Second, increasing the range (at a constant heat load) requires that the water flow rate be decreased—which reduces the static pressure opposing the flow of air.

Tower size v/s range variance

B. Fill media effects: In a cooling tower, hot water is distributed above fill media and is cooled down through evaporation as it flows down the tower and gets in contact with air. The fill media impacts energy consumption in two ways:  Electricity is used for pumping above the fill and for fans that create the air draft. An efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore lead to lower electricity consumption.  Heat exchange between air and water is influenced by surface area of heat exchange, duration of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. The fill media determines all of these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower becomes. There are three types of fills:

a) Splash fill media: Splash fill media generates the required heat exchange area by splashing water over the fill media into smaller water droplets. The surface area of the water droplets is the surface area for heat exchange with the air. b) Film fill media: In a film fill, water forms a thin film on either side of fill sheets. The surface area of the fill sheets is the area for heat exchange with the surrounding air. Film fill can result in significant electricity savings due to fewer air and pumping head requirements. c) Low-clog film fills: Low-clog film fills with higher flute sizes were recently developed to handle high turbid waters. Low clog film fills are considered as the best choice for sea water in terms of power savings and performance compared to conventional splash type fills.

C. Water Distribution:  Optimize cooling water treatment: Cooling water treatment (e.g. to control suspended solids, algae growth) is mandatory for any cooling tower independent of what fill media is used. With increasing costs of water, efforts to increase Cycles of Concentration (COC), by cooling water treatment would help to reduce make up water requirements significantly. In large industries and power plants improving the COC is often considered a key area for water conservation.  Install drift eliminators : It is very difficult to ignore drift problems in cooling towers. Nowadays most of the end user specifications assume a 0.02% drift loss. But thanks to technological developments and the production of PVC, manufacturers have improved drift eliminator designs. As a result drift losses can now be as low as 0.003 –0.001%.  Fans : The purpose of a cooling tower fan is to move a specified quantity of air through the system. The fan has to overcome the system resistance, which is defined as the pressure loss, to move the air. The fan output or work done by

the fan is the product of air flow and the pressure loss. The fan output and kW input determines the fan efficiency. The fan efficiency in turn is greatly dependent on the profile of the blade. Blades include: a) Metallic blades, which are manufactured by extrusion or casting processes and therefore it is difficult to produce ideal aerodynamic profiles b) Fibre reinforced plastic (FRP) blades, are normally hand moulded which makes it easier to produce an optimum aerodynamic profile tailored to specific duty conditions. Because FRP fans are light, they need a low starting torque requiring a lower HP motor, the lives of the gear box, motor and bearing is increased, and maintenance is easier. 85-92% efficiency can be achieved with blades with an aerodynamic profile, optimum twist, taper and a high coefficient of lift to coefficient of drop ratio. However, this efficiency is drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, etc. Cases reported where metallic or glass fibber reinforced plastic fan blades have been replaced by efficient hollow FRP blades. The resulting fan energy savings were in the order of 20-30%and with simple payback period of 6 to 7 months (NPC).

4.7 General Improvement Procedures The following could be fruitful options to improve energy efficiency of cooling towers: i. Follow manufacturer’s recommended clearances around cooling towers and relocate or modify structures that interfere with the air intake or exhaust. ii. Optimize cooling tower fan blade angle on a seasonal and/or load basis. iii. Correct excessive and/or uneven fan blade tip clearance and poor fan balance. iv. In old counter-flow cooling towers, replace old spray type nozzles with new square spray nozzles that do not clog. v. Replace splash bars with self-extinguishing PVC cellular film fill. vi. Install nozzles that spray in a more uniform water pattern.

vii. Clean plugged cooling tower distribution nozzles regularly. viii. Balance flow to cooling tower hot water basins. ix. Cover hot water basins to minimize algae growth that contributes to fouling. x. Optimize the blow down flow rate, taking into account the cycles of concentration (COC) limit. xi. Replace slat type drift eliminators with low-pressure drop, self-extinguishing PVC cellular units. xii. Restrict flows through large loads to design values. xiii. Keep the cooling water temperature to a minimum level by (a) segregating high heat loads like furnaces, air compressors, DG sets and (b) isolating cooling towers from sensitive applications like A/C plants, condensers of captive power plant etc. Note: A 1°Ccooling water temperature increase may increase the A/C compressor electricity consumption by 2.7%. A 1oC drop in cooling water temperature can give a heat rate saving of 5 kCal/kWh in a thermal power plant. xiv. Monitor approach, effectiveness and cooling capacity to continuously optimize the cooling tower performance, but consider seasonal variations and side variations. xv. Monitor liquid to gas ratio and cooling water flow rates and amend these depending on the design values and seasonal variations. For example: increase water loads during summer and times when approach is high and increase air flow during monsoon times and when approach is low. xvi. Consider COC improvement measures for water savings. xvii. Consider energy efficient fibre reinforced plastic blade adoption for fan energy savings. xviii. Control cooling tower fans based on exit water temperatures especially in small units. xix. Check cooling water pumps regularly to maximize their efficiency.

5. Cooling Water Chemistry Cooling towers are dynamic systems because of the nature of their operation and the environment they function within. Tower systems sit outside, open to the elements, which makes them susceptible to dirt and debris carried by the wind. Their structure is also popular for birds and bugs to live in or around, because of the warm, wet environment. These factors present a wide range of operational concerns that must be understood and managed to ensure optimal thermal performance and asset reliability. Below is a brief discussion on the four primary cooling system treatment concerns encountered in most open recirculating cooling systems.

5.1 Corrosion Corrosion is an electrochemical or chemical process that leads to the destruction of the system metallurgy. Figure illustrates the nature of a corrosion cell that may be encountered throughout the cooling system metallurgy. Metal is lost at the anode and deposited at the cathode. The process is enhanced by elevated dissolved mineral content in the water and the presence of oxygen, both of which are typical of most cooling tower systems. There are different types of corrosion encountered in cooling tower systems including pitting, galvanic, microbiologically influenced and erosion corrosion Loss of system metallurgy, if pervasive enough, can result in failed heat exchangers, piping, or portions of the cooling tower itself.

A. Corrosion Control:  Cathodic Polarization : Process of changing the anodic or cathodic potential or both to reduce the driving force of the corrosion reaction is called “polarization”. Polarization reduces the driving force of the corrosion reaction and minimizes metal loss by

changing the potential of either the anode or the cathode or both so that the difference in potential between them is reduced to a minimum. If the amount of oxygen diffusion to the metal surface can be controlled, the corrosion reaction can be polarized. This is achieved by cathodic corrosion inhibitors. They form a film, which prevents the diffusion of oxygen to the cathode side.  Anodic Polarization : Anodic surfaces can be polarized by formation of an oxide layer. This film formation is accomplished by a mechanism known as chemisorption. Stainless steel naturally forms such films. This unfortunately is not always the case with all metals. Most metals must be aided by the addition of such anodic corrosion inhibitors as chromate, nitrite, etc.  Passivation: When corrosion reactions are completely polarized, the metal is said to be at “passive state” At this point there is no difference in potential between the anode and cathode areas, and corrosion ceases. When polarization is disrupted in a passive metal at a given point, a very active anodic site is set up, with resultant accelerated local corrosion, particularly if the metal was strongly anodically polarized.

B. Corrosion Inhibitors : The principal method of controlling corrosion in cooling water system is by means of chemical corrosion inhibitors. Their function in preventing corrosion lies in their ability to insulate the electric current between the cathode and anode. If the insulation effect occurs at the anodic site, then the inhibitor is classified as an anodic inhibitor and if the cathodic site is insulated then the inhibitor is classified as a cathodic inhibitor. Corrosion inhibitors are classified as anodic, cathodic or both depending upon the corrosion reaction each controls. Inhibition usually results from one or more of three general mechanisms. In the first, the inhibitor molecule is adsorbed on the metal surface by the process of chemisorptions, forming a thin protective film either by itself or in conjunction with metallic ions. In second mechanism inhibitors however merely cause a metal to form its own

protective film of metal oxides, by increasing its resistance. In the third type inhibitor reacts with a potentially corrosive substance in the water. Anodic inhibitors build a thin protective film along the anode increasing the potential at the anode and slowing the corrosion reaction, the film is initiated at the anode although it may eventually cover the entire metal surface. Because this film is not visible to the naked eye so the appearance of the metal will be left unchanged. Cathodic inhibitors are generally less effective than the anodic type. But they often form a visible film along the cathode surface, which polarizes the metal by restricting the access of dissolved oxygen to the metal substrate. The film also acts to block hydrogen evolution sites and prevent the resultant depolarizing effect. Examples include:  Chromates  Orthophosphates  Zinc  Polyphosphates Synergic Blends like: o zinc-chromates o chromate-polyphosphates o chromate-orthophosphate

C. Inhibitor Selection: It is often difficult to make a proper choice between the many cooling water corrosion inhibitors unless there is some understanding of their properties. Choice of the proper inhibitor is determined by:  Design parameters  Water composition 

   

Metals in the system Stress conditions Treatment level required pH Dissolved oxygen content

 Salts and SS composition

5.2 Scaling Scaling is the precipitation of dissolved minerals components that have become saturated in solution. Factors that contribute to scaling tendencies include water quality, pH, and temperature. Scale formation reduces the heat exchange ability of the system because of the insulating properties of scale, making the entire system work harder to meet the cooling demand. Deposits typically consist of mineral scales (i.e.CaCO3. CaSO4, Ca3(PO4)2, CaF2, etc), corrosion products (i.e. Fe2O3, Fe3O4, CuO etc), particular matter (i.e. clay, slit), and microbiological mass.

A. Types of Scaling:  Waterborne salts: Precipitated salts of calcium and magnesium often form dense scales and sludge’s which are usually quite adherent and therefore difficult to remove. In addition they are effective heat insulators, which reduce process efficiency. Calcium carbonate, calcium sulphate, calcium and magnesium silicates and calcium phosphate are some of the more prevalent compounds found in cooling water systems.  Waterborne foulants: A variety of such materials as suspended mud, sand, silt, clay, biological matter or even oil may enter a cooling water system through its make up supply. They usually accumulate in low flow areas, or in locations at which an abrupt change in flow velocity occurs. Therefore the most sedimentation is found in such places as cooling tower basins and heat exchangers. To control sedimentation it is necessary to control the suspended particulate matter. The control of particle size and density is accomplished by use of modern deposit control materials. To a certain degree mud, sand, slit, dirt and clay are

suspended in most make up supplies. However the amount of these constituents is usually much greater for surface waters. Microbiological growth may be a particularly troublesome foulant in the makeup supply. The microbiological population in a towers make up supply often approaches or exceeds the control limit for proper tower operation. Oil often adheres to metal; surfaces and acts as a deposit binder. Oil films serve as insulators and can seriously retard heat transfer. In addition oil acts as a nutrient for microbes, therefore increasing microbiological activity, fouling and slime binding. Also oil films prevent corrosion inhibitors from reaching and passivating metal surfaces.  Airborne foulants : The air in contact with open cooling water systems contains many of the same suspended materials found in the makeup water. Sand, slit, clay, dirt, bacteria etc. entering with the air add to the overall fouling of the system. Airborne contamination by gases also helps in deposition. Oxygen and carbon dioxide accelerate corrosion, leading to deposition and further corrosion by the under-deposit mechanism. Since pick up of both gases occur continuously, near saturation levels of these dissolved gasses are present in the water. Gaseous contaminants such as sulphur dioxide, hydrogen sulphide and ammonia may also be absorbed from the air. The first two reduce oxidizing corrosion inhibitors (e.g. chromates) to insoluble foulants. Hydrogen sulphide is very corrosive and quickly forms iron sulphide deposits, which lead to further corrosion. Ammonia selectively corrodes copper and its alloys leading to the deposition of copper corrosion products.

B. Deposit Control Methods:  Conventional treatments:  Softening (sodium or hydrogen zeolite exchange, lime softening and demineralization all remove the ions that cause scale formation)  Acid feed (acid neutralizes alkalinity in the water, thereby preventing carbonate formation)

 Side stream filtration (Side stream filters are used in some cooling tower applications, with 1 to 5 % of the cooling water flow passing through the filter. Several types of media are used but sand is the most common, operating at a 10 % to 20 % efficiency level. For greater efficiency, anthracite or mixed media can be substituted. If the suspended solids are in the range of 10 to 30 ppm, 50~75 % removal can be achieved, and in highly turbid waters, 90 % removal is possible. In general a side stream filter allows cooling water turbidity to approach the turbidity of the filter effluent. With oil contamination side stream filters are impractical because of rapid fouling of the filter medium.)  Use of Polymeric Deposit Control Agents: A polymer is defined as macromolecule consisting of a number of repeating units of “building blocks”. These units are referred to as monomers. Modern technology has made it possible to build chains of various lengths and compositions by varying the polymerization conditions and the monomer groups incorporated into the structure. The behaviour of a polymer results primarily from two factors: its chain length or molecular weight and its functional group. These polymeric deposit control agents include, Scale inhibitors, Dispersants, Flocculants.  Scale Inhibitors: Scale inhibitors are important to the performance of many treatment programs. Scale inhibitors function by adsorbing on to suspended solids/scaling particles and adsorbing on to solids/ surfaces in the system, thereby acting to prevent growth of scale/deposits and enhancing performance of corrosion inhibitors. These polymers have the ability of adsorbing on active sites of the crystal to prevent any further growth of crystal. Some of the functional groups of the scale inhibitor adsorbed on the crystals but the rest of them are free from the adsorption and give electrical charge to the crystals. Thus, the static electrical repelling force of the crystals is increased and the crystals are kept in a dispersed condition.  Dispersants:

“The principal role of a dispersant is to reduce the tendency for small particles to agglomerate”. Dispersants are polymers, which control particles by increasing charge on the particle surface, thereby keeping the particles repelled and suspended. A polymer can be adsorbed on foulant surface imparting a like charge to them and thereby causing the particles to remain in suspension because of charge repulsion. Dispersant polymer is a common component of cooling water treatment programs. These polymers prevent deposit because they keep suspended particles from adhering to pipes, tubes, or other surfaces in the cooling systems and are removed with the water by blow down.  Flocculants: A high molecular weight polymer can attach itself to many foulant particles creating a low density floc. With an increase in the overall size of suspended material, there is a corresponding decrease in the surface area available for attachment, which reduces the extent of deposition possible. Much of suspended matter found in cooling water has a negative surface charge. This charge keeps the suspended matter separated. If the surface charge of the particles can be reduced, the particle will agglomerate into light, fluffy flocs with little tendency to adhere to metal surfaces. This can be accomplished by adding a long chain oppositely charged (cationic) polymer to the cooling water, which neutralizes the negative charge of the suspended material.

5.3 Microbial Growth Microbiological activity is microorganisms that live and grow in the cooling tower and cooling system. Cooling towers present the perfect environment for biological activity due to the warm, moist environment. There are two distinct categories of biological activity in the tower system. The first being plank tonic, which is bioactivity suspended, or floating in solution. The other is sessile biogrowth, which is the category given to all biological activity, bio films, or biofouling that stick to a surface in the cooling system. Bio films are problematic

for multiple reasons. They have strong insulating properties, they contribute to fouling and corrosion, and the bi-products they create that contribute to further micro-biological activity. They can be found in and around the tower structure, or they can be found in chiller bundles, on heat exchangers surfaces, and in the system piping. Additionally, bio films and algae mats are problematic because they are difficult to kill. Careful monitoring of biocide treatments, along with routine measurements of biological activity are important to ensure bio-activity is controlled and limited throughout the cooling system. Cooling water microorganisms include: Algae, Fungi, and Bacteria etc.

A. Problems of Microbial growth: Continued accumulation and growth of microorganisms in a cooling water system causes a number of problems. Good corrosion and deposit control programs are incumbent upon a successful microbial control program. A plant unable to control microbial growth will experience increased difficulty in controlling corrosion and deposition. Another problem associated with microbial growth is the deterioration of cooling tower lumber this reduces the efficiency of the cooling tower operation and increases operating cost of the plant. Microbiological growth also causes environmental pollution.

 Microbiological Induced Corrosion, (MIC): Any corrosion initiated or propagated by the action of microorganisms either directly or indirectly is called MIC. Many microorganisms found in cooling water utilize hydrogen in their metabolic processes, which often results in the cathodic depolarization of the corrosion reaction. Many microbial species present special corrosion problems, in addition to those inherent in the basic nature of their actions. Sulphate reducing bacteria produce extremely dangerous hydrogen sulphide gas, which corrodes metals by low pH attack and by the formation of ferrous sulphide.

Sulphate oxidizing bacteria produce sulphuric acid and produce localized low pH areas in the system. Corrosion proceeds very rapidly in these low pH areas. Nitrifying bacteria nullify the effectiveness of nitrite corrosion inhibitors by oxidizing nitrite to nitrate. This is the most serious in closed re-circulating systems which commonly use nitrite as a corrosion inhibitor in the systems where NH3 is present in water.  Deposit Problems: Deposit of microbial matter may lead to physical problems in the system, culminating in loss of efficiency, heat transfer and production. The accumulation of bio matter on the internal sections of cooling towers can seriously reduce the units efficiency e.g. deposition on splash plates will increase the water droplet size and will reduce the effective surface area. Algae can plug the holes in the distribution deck of a cooling tower producing uneven distribution of water over the tower packing, resulting in a serious loss in efficiency.

B. Selection of Micro Biocides: A number of factors will determine the proper choice of micro biocide or combination of micro biocides, oxidizing and non-oxidizing micro biocide. The selection of a micro biocide involves several factors. First it must be effective in inhibiting almost all -microbial activity. Second, it must be economical in a treatment programme. This is often accomplished by combining a small amount of an expensive but highly effective, micro biocide with another less expensive one resulting in broad spectrum control at reasonable cost. Environmental discharge and disposal considerations constitute another factor, which determines the choice of micro biocides. Disposal problems caused by toxicity have limited the use of certain micro biocides in many areas. The micro biocide chosen must be easily detoxified before cooling system bleed off reaches receiving streams. The operating parameters of the cooling water system will also affect the choice of a micro biocide. Temperature, pH and system design are fundamental considerations in a decision involving oxidizing or non oxidizing toxicants.

I.

Oxidizing Toxicants:  Chlorination:

The most commonly used oxidizing micro biocide is Chlorine. It is the most effective of all halogens. Chlorine is an excellent algaecide and sporicide. It is also an excellent bactericide in most circumstances. Free residual chlorine at levels of 0.5 ppm and slightly above are usually enough to control most microbial growth. A number of factors determine the amount of chlorine required in an open cooling water system. These include chlorine demand, contact time, pH, and temperature of the water. When chlorine gas is fed to water, it hydrolyzes to form two acids, hypochlorous acid (biocide) and hydrochloric acid, respectively. Cl2 + H2O = HOCl + HCl Hypochlorous acid is very weak acid but an extremely powerful oxidizing agent. It easily diffuses through the cell walls of microorganisms, and reacts with the cytoplasm to produce chemically stable nitrogen chlorine bonds with the cell proteins. The PH of the cooling water is directly responsible for the extent of ionization of hypochlorous acid. The acid state is favoured by low pH .At pH 7.5 there will be approximately equal amounts of acid and hypochlorite ion. Chlorine becomes ineffective as a micro biocide at pH 9.5 or greater as a result of total ionization. A, pH range of 6.5~7 is considered practical for chlorine based microbial control programme. The amount of chlorine added to the system is directly proportional to the alkalinity reduction. Many plants find it necessary to suspend acid feed during chlorination periods in order to avoid low PH excursions. Chlorine is destroyed by sunlight and by aeration so, its dosing is preferred at night to prolong its effect.

Other oxidizing biocides include ozone, chlorine dioxide and hypochlorites.

 Bromination: Target bromination is one of the most effective oxidizing biocide treatments for cooling water systems. For systems, operating at above 7.0 pH i.e. alkaline media like Phosphate treatment system, bromine is more efficient than chlorine as a biocide. Because 50 % of hypochlorous acid, HOCl (biocide) formed due to chlorination, ionize into hypochlorite ions (OCl-) at pH 7.5. Hypochlorite ions as a biocide are twenty times less effective than HOCl. At pH 8.0, Chlorination will yield only 20 % HOCl& 80 % OCl ions. But at this pH bromination will yield 80 % HOBr (micro biocide) & 20 % OBr ions-. That is why at alkaline pH bromination is more effective than chlorination in the control of microbiological growth. At pH (8~9.3), only a small percentage of chlorine is available as the active toxicant, hypochlorous acid. In the presence of NH3 bromamines are formed which are more effective than chloramines in the control of bacteria. Also bromamines breakdown more quickly than chloramines in the environment and has lower long- term environmental toxicity.

II.

Non Oxidizing Biocides: Non-oxidizing biocides can be more effective than oxidizing biocide because of their overall control of algae, fungi, and bacteria. They have also greater persistence, as many of them are PH independent. They are used in conjunction with oxidizing micro biocides for broad control. Most of plants chlorinate intermittently and add a non -oxidizer once or twice a week or as per requirement. Their mode of activity is ‘to inhibit cell growth by preventing the transfer of energy or life sustaining chemical reactions occurring within the cell’. Organic sulphur

compounds include a wide variety of different biocides, Methylene bisthiocynate` (MTB) is most common, which is effective in controlling algae, Fungi and bacteria.

6. Performance Improvement 6.1 Water Use The hierarchy of opportunities approach can be used to identify and prioritise water efficiency opportunities.

   

1. Reduce water loss 2. Reduce blow down 3. Use alternative water supplies 4. Reuse blow down

A. Reduce water loss: Reducing water losses reduces the quantity of make-up water required for the system. Potential opportunities to reduce water loss include: • Fixing leaks • Reducing splash • Optimising overflow • Eliminating drift – drift losses should be maintained at less than 0.002% of cooling water circulation rate. Repair or install new systems to achieve best practice. Equation 8: Water losses 𝐖𝐚𝐭𝐞𝐫 𝐋𝐨𝐬𝐬𝐞𝐬 = (𝐐𝐮𝐚𝐧𝐭𝐢𝐭𝐲 𝐨𝐟 𝐦𝐚𝐤𝐞−𝐮𝐩 𝐰𝐚𝐭𝐞𝐫/𝐂𝐲𝐜𝐥𝐞 𝐨𝐟 𝐜𝐨𝐧𝐜𝐞𝐧𝐭𝐫𝐚𝐭𝐢𝐨𝐧) − 𝐐𝐮𝐚𝐧𝐭𝐢𝐭𝐲 𝐨𝐟 𝐛𝐥𝐨𝐰𝐝𝐨𝐰𝐧

B. Reduce blow down:  Increase cycles of concentration: As water evaporates from cooling towers the contaminants, salts and minerals measured as total dissolved solids (TDS) that accumulate can cause biological growth, corrosion and scale resulting in tower damage, poor heat transfer and possibly the growth of harmful bacteria such as Legionella. The sources of contaminants include: • Salts and minerals already in the make-up water • Chemicals added to reduce corrosion, scale and biological growth • Pollutants entering the water during the evaporation phase from the surrounding air such as dust. To reduce the build up of these contaminants, a portion of the water in the tower is bled off (blow down). This water loss from the tower is then replaced with fresh incoming make-up water. A conductivity probe or sensor in the tower basin initiates blow down when the levels of dissolved solids exceed a set value. ‘Cycles of concentration’ (C.O.C.) compare the level of dissolved solids in the tower’s make-up water to the level of dissolved solids in the tower’s bleed water. Equation 9: Cycle of Concentration (C.O.C) 𝐂𝐲𝐜𝐥𝐞𝐬 𝐨𝐟 𝐂𝐨𝐧𝐜𝐞𝐧𝐭𝐫𝐚𝐭𝐢𝐨𝐧 𝐂.𝐎.𝐂 = (𝐓𝐨𝐭𝐚𝐥 𝐝𝐢𝐬𝐬𝐨𝐥𝐯𝐞𝐝 𝐬𝐨𝐥𝐢𝐝𝐬 𝐢𝐧 𝐭𝐡𝐞 𝐦𝐚𝐤𝐞−𝐮𝐩 𝐰𝐚𝐭𝐞𝐫)/𝐓𝐨𝐭𝐚𝐥 𝐝𝐢𝐬𝐨𝐥𝐯𝐞𝐝 𝐬𝐨𝐥𝐢𝐝𝐬 𝐢𝐧 𝐭𝐡𝐞 𝐛𝐥𝐞𝐞𝐝𝐞𝐝 𝐰𝐚𝐭𝐞𝐫 Increasing the number of C.O.C. will reduce the volume of blow down and consequently the volume of make-up water required by the tower. The maximum C.O.C. for a tower will depend on the quality of the make-up water and the corrosion resistance of the tower’s basin and condenser. C.O.C. over 5 is considered to be efficient but this is not always achievable. Scale forming ions such as calcium and magnesium can often be precipitated out (by water softeners) or kept in solution (by acids) through effective water treatment enabling the tower to operate at higher cycles of concentration. According to the Queensland Water Commission, a cooling tower is considered inefficient if:

• The system is operating at less than 5 COC or 1850 mg/L TDS/2750 μs/cm conductivity (allowed only in documented instances of high-TDS make-up water); and/or • System losses are greater than 8% of the make-up water.

C. Use alternative water supplies: Alternative water supplies have the potential to reduce potable water requirements in cooling towers, through direct substitution and by reducing the cycles of concentration. Alternative water supply options include recycled water, process or rainwater. Note that health risks need to be considered when assessing the viability of alternate water supplies. Additional water treatment may also be required depending on the quality of water available.

D. Reuse blow down: Potential opportunities to reuse cooling tower blow down include: • Toilet and urinal flushing (treatment may be required) • Landscape irrigation (may require dilution with potable or rainwater due to salt content or treatment) • Cleaning (health risk assessment may be required and the impacts of corrosion should be considered).

6.2 Water treatment Almost all well-managed cooling towers use a water treatment program. The goal of a water treatment program is to maintain a clean heat transfer surface and preserve capital while minimizing water consumption and meeting discharge limits. Critical water chemistry parameters that require review and control include pH, alkalinity, conductivity, hardness, microbial growth, biocide and corrosion inhibitor levels. Depending on the quality of the make-up water, treatment programs may include corrosion and scaling inhibitors, such as organo-phosphate types, along with biological fouling inhibitors. Historically, chemicals have been fed into the system by automatic feeders on timers or actuated by conductivity meters. Automatic chemical feeding tends to decrease chemical dosing requirements. Current technology allows chemicals

to be monitored and controlled online 24-7 in proportion to demand. This ensures results and can allow cycles to be increased. Where overfeed is prevalent, it can reduce chemical feed, too. Water treatment is required in cooling towers to prevent corrosion of the system, build up of scale and for microbiological control. Typically this is carried out through one of the following: • Direct chemical dosing (to prevent scale and prohibit corrosion) • Acid dosing (to control ph and scale) • Ozone dosing (or other microbial treatment to prevent microbial growth) • Pre-treatment of make-up water (e.g. Water softening, reverse osmosis) • Side stream filtration (to prevent solid build up) • Cover exposed areas of cooling towers (to reduce algal growth).

A. Sulphuric “Acid” Treatment: Sulphuric acid can be used in cooling tower water to help control scale build-up. When properly applied, sulphuric acid will lower the water’s pH and help convert the calcium bicarbonate scale to a more soluble calcium sulphate form. In central North Carolina, most plants will be able to operate six to 10 cycles of concentration without acid feed. Along our coasts, acid can be used to increase cycles as water tends to be harder and higher in alkalinity. The same can be said if hard alkaline well water is used as tower make-up. Important precautions need to be taken when using sulphuric acid treatment. Because sulphuric acid is an aggressive acid that will corrode metal, it must be carefully dosed into the system and must be used in conjunction with an appropriate corrosion inhibitor. Workers handling sulphuric acid must exercise caution to prevent contact with eyes or skin. All personnel should receive training on proper handling, management and accident response for sulphuric acid used at the facility.

B. Side Stream Filtration: In cooling towers that use make-up water with high suspended solids, or in cases where airborne contaminants such as dust can enter cooling tower water, side stream filtration can be used to reduce solids build up in the system. Typically, five to 20 per-cent of the circulating flow can be filtered using a rapid sand filter or a cartridge filter system.

Rapid sand filters can remove solids as small as 15 microns in diameter while cartridges are effective to remove solids to 10 microns or less. High efficiency filters can remove particles down to 0.5 microns. Neither of these filters are effective at removing dissolved solids, but can remove mobile mineral scale precipitants and other solid contaminants in the water. The advantages of side stream filtration systems are reduced particle loading on the tower. This ensures heat transfer efficiency and may reduce biocide or dispersant demands.

C. Ozone: Ozone can be a very effective agent to treat nuisance organics in the cooling water. Ozone treatment also is reported to control the scale by forming mineral oxides that will precipitate out to the water in the form of sludge. This sludge collects on the cooling tower basin, in a separation tank or other lowflow areas. Ozone treatment consists of an air compressor, an ozone generator, a diffuser or contactor and a control system. The initial capital costs of such systems are high but have been reported to provide payback in 18months.

D. Magnets: Some vendors offer special water-treating magnets that are reported to alter the surface charge of suspended particles in cooling tower water. The particles help disrupt and break loose deposits on surfaces in the cooling tower system. The particles settle in a low-velocity area of the cooling tower -- such as sumps --where they can be mechanically removed. Suppliers of these magnetic treatment systems claim that magnets will remove scale without conventional chemicals. Also, a similar novel treatment technology, called an electrostatic field generator, is also reported.

E. Sonication: An emerging technology is sonication or ultrasound which uses vibration to remove fats. This technology can be used in wastewater systems to emulsify fats making them easier to remove by methods such as DAF. Sonication has also been trialled in conjunction with anaerobic treatment as a means of disrupting sludge production to yield a larger quantity of biogas.

F. Electro coagulation: Electro coagulation can be used to remove suspended and colloidal solids, fats, oils and grease and complex organics. The process involves passing an electrical current through water to initiate a range of electrochemical reactions which destabilise, suspend, emulsify or dissolve contaminants in the wastewater which forces them to precipitate.

G. Activated carbon: Activated carbon is generally used after biological or physical-chemical treatment to polish waste water for reuse. The carbon absorbs both organic and inorganic compounds including heavy metals. Activated carbon is formed by heating carbon containing substances such as coal or charcoal in the presence of steam to form highly porous carbon providing a large surface area for contaminants to adsorb onto. Activated carbon can be regenerated on site by heating carbon to a high temperature. Using activated carbon prior to a disinfection phase can reduce the disinfection requirement. The use of activated carbon as part of the cooling tower or boiler water treatment can lead to better water efficiencies through reduced bleed.

H. Ultraviolet radiation (UV): This chemical-free method of disinfecting water inactivates microorganisms such as protozoa, bacteria, moulds and yeasts through the use of ultraviolet radiation. The effectiveness of the system can be increased with the simultaneous use of ozone. However, water quality characteristics such as high turbidity, organic components and flow rate can reduce efficacy. Like ozone, UV radiation does not provide any residual sanitisation compared with chlorine .

I. Hydrocavitation: Hydrocavitation is a chemical free system of water treatment. Two streams of water are accelerated to high velocities and collide which results in hydrodynamic cavitation and mechanical shear forces, which are believed to kill bacteria and reduce corrosion activity. It removes the need for chemicals and can increase the ability to reuse water. It is generally applied to cooling tower water (refer to case study below) as it can control corrosion and kill legionella. However, new studies are investigating the efficiency of removing

heavy metals, phosphorous and trichloroethylene (TCE) from wastewater with additional reductions in BOD.

J. Radio frequencies: Radio frequencies alter the water’s scaling tendencies by creating a “seeding” mechanism that agglomerates scale-forming minerals in the water. This technology removes minerals before they can be deposited on heat exchange surfaces.

7. Recommendation Based on the study on the assigned project, it is recommended to reduce the water leakages in the tower by overcoming the construction flaws of the project. Further it also recommended to pursue the options for water and chemical conservation opportunities in cooling tower operation. The field will unleash the wide spectrum of cost effective and environmental friendly operating practices which would be next to the international eco-efficiency standards. Water conservation will not only reduce the load on environment and natural resources, but would also enable the organization to claim for ecoefficiency indicator points – a new brand image perspective. The adoption of chemical free platforms completely or partially will reduce the cost of chemical purchases, dependence of service provider and most important – regional leadership in emerging the cooling water treatment technologies.

Cooling towers of MTPS, DVC.

8. Conclusion Industrial training being an integral part of engineering curriculum provides not only easier understanding but also helps acquaint an individual with technologies. It exposes an individual to practical aspect of all things which differ considerably from theoretical models. During my training, I gained a lot of practical knowledge which otherwise could have been exclusive to me. The practical exposure required here will pay dividends to me when I will set my foot as an Engineer. The training at MTPS was altogether an exotic experience, since work, culture and mutual cooperation was excellent here. Moreover, fruitful result of adherence to quality control awareness of safety and employees were fare which is much evident here.

Glimpse of Mejia Thermal Power Station

9. References 1. Power Plant Engineering by P.K Nag. 2. Technical hard copy received from MTPS, DVC. 3. Cooling Tower (chemistry and performance improvement) for Engro Fertilizers Limited (EFERT) Daharki, District Ghotki, Sindh, prepared by Osama Hasan Operations (URUT III), Intern School of Chemical and Materials Engineering (SCME), National University of Sciences and Technology (NUST). 4. Photos from www.slideshare.net. 5. Short notes and photos from MTPS premises.

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