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
Select materials of construction
Tower size
Economics
Select and design model

6. DESIGN OPTIONS Increase Tower Size Reduce air velocity
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

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 TYPES CROSSFLOW 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 TYPES COUNTER 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 PROBLEMS 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
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
Circulation rate (CR)
Cooling tower capacity tons x 0.189
litre/sec
Evaporation rate (ER)
Circulation rate x 0.008
Drift rate (DR)
Circulation rate x 0.002
Average bleed-off rate (ABF)
Circulation rate x 0.006
Make-up water
(BR + ER + ABF) * 15,768
m3/yr
Make-up water cost
Make up water in m3/yr x $4.58 $
Bleed-off cost
ABF x 15,768 x $1.2/m3
Annual chemical treatment cost
Circulation rate x 400
$/yr