Prof. Osama El Masry

1. Prof. Osama El Masry

2. COOLING TOWER DESIGN COOLING TOWER DESIGN IS A COMPROMISE Specification • Water flow rate • Hot water temperature • Cold water temperature • Location • Material of construction • Water quality Target • Low capital cost • Low operating cost • Meeting specifications

3. PITFALLS - SPECIFICATION Water Flow Rate • Increased flow provides design margin • Too much margin causes loss of efficiency • Tower should be testable at real flow Wet Bulb Temperature • Unrealistic value makes testing difficult • Possibility of unfair design Range • Increased range makes testing difficult • Tower size increase not linear with range

4. PITFALLS - SPECIFICATION Cold Water Temperature • Low specification increases tower cost • Increases fan power • Provides design margin • Cost increase not linear

5. DESIGN PROCESS Select fill type • Water quality • Application Select tower type • Fill type • Specification • Application Select materials of construction • Tower size • Specification • Economics Select and design model

6. DESIGN OPTIONS LOW FAN POWER Increase Tower Size • Reduce air velocity • Reduce air pressure drop and fan power Increase Fill Volume • Increase water residence time • Reduce air required • Reduce fan power Increase Fan Diameter • Reduce exit velocity • Reduce fan power

7. COOLING TOWER TYPES Flow Pattern • Crossflow cooling towers • Counterflow cooling towers Structure • Steel/frp structure • Timber structure • Rcc structure Fill Type • Splash fill • Film flow fill

8. TOWER TYPESCROSSFLOW Water and air in Crossflow Commonly used with Splash fill Low fan power Higher pump head Higher capital cost Than counterflOW

9. WHY CROSSFLOW • The Dominant tower type for 30 years • Very efficient utilization of splash fill • Low fan power consumption with splash Fill • Resistant to poor water quality • Low plan area PROBLEMS • High pump head (up to 12 m) • Not efficient with film fills • Higher tower size-higher civil costs • Higher capital cost than counterflow with film fills

10. CROSS FLOW FILLS SPLASH FILLS • Rectangular timber • Triangular timber • PVC Veebar • Triangular PVC • Pre stressed RCC

11. TOWER TYPESCOUNTER FLOW Water and air in Counterflow Usually with film fill Low pump head Low fan power Low capital cost Economical civil design Default option for power Plants

12. WHY COUNTER FLOW • The original cooling tower type • Very efficient utilization of film fill • Competitive fan power with film fill • Low pump head • Easy construction, low civil costs • Lowest capital cost with film fills PROBLEMS 􀂾 High fan power with splash fill 􀂾 Film fill is sensitive to water quality

13. COUNTERFLOW FILLS FILM FLOW FILLS • High efficiency cross corrugated fills • Range of flute sizes • Special vertical fluted fills for poor Water quality

14. COMPONENTS • DRIFT ELIMINATORS • Timber and PVC herringbone • Cellular PVC • PVC extruded profile • Low drift losses • Low pressure drop

15. OPERATING COSTS FAN POWER • Can be reduced by design • Operating cost reduced with speed control PUMP POWER • Usually overlooked in analysis • Lower for film filled towers • Operating cost is fixed WATER TREATMENT COST • Biological control • Corrosion control • Scaling control

16. Cooling Towers Definitions • Cooling towers are evaporative coolers used for cooling water or other working medium to near the ambient wet-bulb air temperature. • Cooling towers use evaporation of water to reject heat from processes such as cooling the circulating water used in oil refineries, chemical plants, power plants and building cooling. • The towers vary in size from small roof-top units to very large hyperboloid structures that can be up to 200 m tall and 100 m in diameter, or rectangular structures , that can be over 40 m tall and 80 m long. Smaller towers are normally factory-built, while larger ones are constructed on site.

17. Definitions • Range: Difference between entering and leaving water temperature • Approach: Difference between leaving water temperature and entering air wet bulb • Evaporation: Method by which cooling towers cool the water • Drift: Entrained water droplets carried off by the cooling tower • Blow down: Water intentionally discharged from cooling tower to maintain water quality • Plume: Hot moist air discharged from the cooling tower forming a dense fog

18. Natural Draft Cooling Tower • Natural Draft Cooling Tower : This type depends upon the natural driving pressure caused by the difference in density between the cool outside air and the hot , humid air inside . The driving pressure ΔPD is given by : • Δ PD = (ρo – ρi) H gc Where: • ρo is density of outside air kg/m3. • H height of the tower in m • ρi is density of inside air kg/m3 • gc is gravitatinal acc. =9.81 m/s2

19. Psychrometric Chart • Dry bulb temp. • Wet bulb temp. • Relative Humidity • Dew point • Moisture content • Enthalpy • Sp. volume

20. Psychrometric Chart • used to determine wet-bulb air temperature

21. A Psychrometer The psychrometer is a device composed of two thermometers mounted on a sling. One thermometer is fitted with a wet gauze and reads the wet-bulb temperature. The other thermometer reads thedry-bulb, or ordinary, temperature. Sling Psychrometer

23. Energy and Mass Balance

24. Cooling Tower as steady-state steady-flow System

25. Energy Balance for a unit mass of dry air ha1+whv1+WAhwA= ha2+whv2+WBhwB (1) where • ha = Enthalpy of dry air J/kg • w = water vapor/dry air (humidity ratio) • hv= Enthalpy of water vapor J/kg • W= mass of water/mass of dry air • hw= Enthalpy of water J/kg

26. hv = hg &hw =hf from steam tables ha2 - ha1 =Cp (T2-T1) Mass Balance w2 –w1= WA – WB Eq. (1) can be written as: whg1+WAhfA= Cp (T2-T1) +[WA –(w2 –w1)] hfB (2)

27. Air Density • Δ PD = (ρo – ρi) H gc where: ρo =m/V= P1 /Ra T1 ρi =m/V= P2 /Ra T2

32. Useful formulas in design of system of cooling tower