Energy Conservation Opportunities in Cooling Tower.pdf
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A cooling tower works by evaporating a portion of circulating water to reject process heat into the atmosphere. It has components like fill media, drift eliminators, nozzles, and fans. Key performance parameters include range, approach, effectiveness, cooling capacity, and cycles of concentration. Cooling tower performance depends on factors like heat load, flow rate, wet bulb temperature, and approach. Proper sizing considers these factors and energy efficiency can be improved through control strategies and opportunities like optimized fan operation.
Energy Conservation Opportunities in Cooling Tower.pdf
- 1. © Confederation of Indian Industry COOLING TOWER
- 2. © Confederation of Indian Industry © UNEP 2006 Introduction Main Features of Cooling Towers (Pacific Northwest National Library, 2001)
- 3. © Confederation of Indian Industry © UNEP 2006 Introduction Frame and casing: support exterior enclosures facilitate heat transfer by maximizing water / air contact ◼ Splash fill ◼ Film fill Cold water basin: receives water at bottom of tower Components of a cooling tower
- 4. © Confederation of Indian Industry © UNEP 2006 Components of a cooling tower Drift eliminators: capture droplets in air stream Air inlet: entry point of air Louvers: equalize air flow into the fill and retain water within tower Nozzles: spray water to wet the fill Fans: deliver air flow in the tower Introduction
- 5. © Confederation of Indian Industry COOLING TOWER Primary task of a cooling tower is to reject heat into the atmosphere Evaporation consumption of water Relatively inexpensive and dependable means of removing low- grade heat from cooling water
- 6. © Confederation of Indian Industry Cooling Water System
- 7. © Confederation of Indian Industry Cooling Tower Types Cooling towers fall into two main categories: Natural draft ◼ Generally used for water flow rates above 45,000 m3/hr ◼ Used only by utility power stations. Mechanical draft ◼ more widely used
- 8. © Confederation of Indian Industry Mechanical draft towers Mechanical draft towers are available in the following airflow arrangements: Counter flows induced draft. Counter flow forced draft. Cross flow induced draft. Normal capacities range from approximately 10 tons, 2.5 m3/hr flow to several thousand tons and m3/hr
- 9. © Confederation of Indian Industry Components of Cooling Tower The basic components of an evaporative tower are: Frame and casing Fill Cold water basin, Drift eliminators Air inlet Louvers Nozzles Fans.
- 10. © Confederation of Indian Industry Cooling Tower Types
- 11. © Confederation of Indian Industry Performance Parameters 1. Range 2. Approach 3. Effectiveness 4. Cooling capacity 5. Evaporation loss 6. Cycles of concentration 7. Blow down losses 8. Liquid / Gas ratio Assessment of Cooling Towers
- 12. © Confederation of Indian Industry 1. Range Difference between cooling water inlet and outlet temperature: Range (°C) = CW inlet temp – CW outlet temp High range = good performance Range Approach Hot Water Temperature (In) Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) (In) to the Tower (Out) from the Tower Assessment of Cooling Towers
- 13. © Confederation of Indian Industry 2. Approach Difference between cooling tower outlet cold water temperature and ambient wet bulb temperature: Approach (°C) = CW outlet temp – Wet bulb temp Low approach = good performance Range Approach Hot Water Temperature (In) Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) (In) to the Tower (Out) from the Tower Assessment of Cooling Towers
- 14. © Confederation of Indian Industry 3. Effectiveness Effectiveness in % = Range / (Range + Approach) = 100 x (CW temp – CW out temp) / (CW in temp – Wet bulb temp) High effectiveness = good performance Range Approach Hot Water Temperature (In) Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) (In) to the Tower (Out) from the Tower Assessment of Cooling Towers
- 15. © Confederation of Indian Industry 4. Cooling Capacity Heat rejected in kCal/hr or tons of refrigeration (TR) = mass flow rate of water X specific heat X temperature difference High cooling capacity = good performance Range Approach Hot Water Temperature (In) Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) (In) to the Tower (Out) from the Tower Assessment of Cooling Towers
- 16. © Confederation of Indian Industry 5. Evaporation Loss Water quantity (m3/hr) evaporated for cooling duty = theoretically, 1.8 m3 for every 10,000,000 kCal heat rejected = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1- T2) T1-T2 = Temp. difference between inlet and outlet water Range Approach Hot Water Temperature (In) Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) (In) to the Tower (Out) from the Tower Assessment of Cooling Towers
- 17. © Confederation of Indian Industry 6. Cycles of concentration (C.O.C.) Ratio of dissolved solids in circulating water to the dissolved solids in make up water Depend on cycles of concentration and the evaporation losses Blow Down = Evaporation Loss / (C.O.C. – 1) 7. Cycles of concentration (C.O.C.) Assessment of Cooling Towers
- 18. © Confederation of Indian Industry © UNEP 2006 8. Liquid Gas (L/G) Ratio Ratio between water and air mass flow rates Heat removed from the water must be equal to the heat absorbed by the surrounding air L(T1 – T2) = G(h2 – h1) L/G = (h2 – h1) / (T1 – T2) T1 = hot water temp (oC) T2 = cold water temp (oC) Enthalpy of air water vapor mixture at inlet wet bulb temp (h1) and outlet wet bulb temp (h2) Assessment of Cooling Towers
- 19. © Confederation of Indian Industry Factors Affecting Cooling Tower Performance Capacity Heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are not sufficient to understand cooling tower performance ◼ 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 Range °C = Heat Load in kcals/hour / Water Circulation Rate in LPH
- 20. © Confederation of Indian Industry Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature ◼ Cold Water Temperature 32.2°C – Wet Bulb Temperature (26.7°C) = Approach (5.5°C) ◼ 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 Factors Affecting Cooling Tower Performance
- 21. © Confederation of Indian Industry Order of importance in sizing a tower Approach Flow rate Range Wet bulb Factors Affecting Cooling Tower Performance
- 22. © Confederation of Indian Industry Heat Load Size and cost of the cooling tower is proportional to the heat load Determination of accurate process heat loads can become very complex but proper consideration can produce satisfactory results Factors Affecting Cooling Tower Performance
- 23. © Confederation of Indian Industry Heat rejection requirements of various types of power equipment A sample list is as follows: Air Compressor - Single-stage - 129 kCal/kW/hr - Single-stage with after cooler - 862 kCal/kW/hr - Two-stage with intercooler - 518 kCal/kW/hr - Two-stage with intercooler and after cooler - 862kCal/kW/hr Refrigeration, Compression - 63 kCal/min/TR Refrigeration, Absorption - 127 kCal/min/TR Steam Turbine Condenser - 555 kCal/kg of steam Diesel Engine, Four-Cycle, Supercharged - 880 kCal/kW/hr Natural Gas Engine, Four-cycle - 1523 kCal/kW/hr (18 kg/cm2 compression)
- 24. © Confederation of Indian Industry Wet Bulb Temperature Wet bulb temperature important factor in performance of evaporating-cooling controlling factor for minimum cold water temperature Theoretically, a cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load wet bulb: whether specified as ambient or inlet ◼ The ambient wet bulb is the temperature, which exists generally in the cooling tower area ◼ Inlet wet bulb is the wet bulb temperature of the air entering the tower ◼ Affected by discharge vapours being recirculated into the tower
- 25. © Confederation of Indian Industry Wet Bulb Temperature
- 26. © Confederation of Indian Industry Approach and Flow
- 27. © Confederation of Indian Industry Range, Flow and Heat Load Range is a direct function of the quantity of water circulated and the heat load Increasing the range as a result of added heat load does require an increase in the tower size. If the cold water temperature is not changed and the range is increased with higher hot water temperature, the driving force between the wet bulb temperature of the air entering the tower and the hot water temperature is increased, the higher level heat is economical to dissipate
- 28. © Confederation of Indian Industry Range, Flow and Heat Load If the hot water temperature is left constant and the range is increased by specifying a lower cold water temperature, the tower size would have to be increased considerably. Not only would the range be increased, but the lower cold water temperature would lower the approach. The resulting change in both range and approach would require a much larger cooling tower.
- 29. © Confederation of Indian Industry Fill Media Function of Fill media in a Cooling Tower Heat exchange between air and water is influenced by surface area of heat exchange, time of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing
- 30. © Confederation of Indian Industry Choosing a Cooling Tower
- 31. © Confederation of Indian Industry Efficient System Operation Cooling Water Treatment COC Drift Loss in the Cooling Towers As low as 0.003 – 0.001% Cooling Tower Fans Fan efficiency – profile of blade ◼ As high as 85 – 92% FRP blades of optimum aerodynamic profile ◼ fan energy savings of the order of 20–30% ◼ Increased life of the gear box, motor and bearing
- 32. © Confederation of Indian Industry Performance assessment Observation / measurements required Cooling tower design data and curves to be referred to as the basis. Intake air WBT and DBT at each cell at ground level Exhaust air WBT and DBT at each cell CW inlet temperature at risers or top of tower CW outlet temperature at full bottom Process data on heat exchangers, loads on line CW flow measurements, either direct or inferred from pump characteristics. CT fan motor amps, volts, kW and blade angle settings TDS of cooling water. Rated cycles of concentration at the site conditions. Observations on nozzle flows, drift eliminators, condition of fills, splash bars
- 33. © Confederation of Indian Industry Control stratergies Various methods starting and stopping (On-off) of fans ◼ Less effective use of two- or three-speed fan motors ◼ Better than On/Off use of automatically adjustable pitch fans or use of variable speed fans ◼ Best methods of control
- 34. © Confederation of Indian Industry Energy Saving Opportunities Avoid interference with the air intake or exhaust Optimise cooling tower fan blade angle on a seasonal and/or load basis Correct excessive and/or uneven fan blade tip clearance and poor fan balance replace old spray type nozzles with new square spray ABS practically non-clogging nozzles Replace splash bars with self-extinguishing PVC cellular film fill New nozzles for uniform water pattern Periodically clean plugged cooling tower distribution nozzles Balance flow to cooling tower hot water basins Cover hot water basins to minimise algae growth that contributes to fouling Optimise blow down flow rate, as per COC limit Replace slat type drift eliminators with low pressure drop, self extinguishing, PVC cellular units. Restrict flows through large loads to design values.
- 35. © Confederation of Indian Industry Energy Saving Opportunities Segregate high heat loads and sensitive applications 1°C cooling water temperature increase may increase A/C compressor kW by 2.7% 1°C drop in cooling water temperature can give a heat rate saving of 5 kCal/kWh in a thermal power plant Monitor L/G ratio, CW flow rates w.r.t. design as well as seasonal variations increase water load during summer when approach is high and increase air flow during monsoon when approach is narrow. Monitor approach, effectiveness and cooling capacity for continuous optimisation Consider COC improvement measures for water savings. Consider energy efficient FRP blade adoption for fan energy savings. Consider possible improvements on CW pumps w.r.t. efficiency improvement. Control cooling tower fans based on leaving water temperatures especially in case of small units. Optimise process CW flow requirements, to save on pumping energy