Prof. Osama El Masry

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

Psychrometric Chart used to determine wet-bulb air temperature

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

Energy and Mass Balance

Cooling Tower as steady-state steady-flow System

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

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)

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

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