Humidification Operation and Cooling Tower

Introduction, Performance, and Design

Content Basic Concept of Humidification

Humidity Chart

Cooling TowerBasic Terms and ParametersTypes

Design Equation

OutcomesBasic knowledge of humidification

Reading of Humidity chart

Basic concept of Cooling tower

Energy Balance of Cooling Tower

Hydraulic and operational design

Analyze the performance of cooling Tower

Basic Concept of HumidificationHumidificationAddition of vapor of a pure liquid into gas.

DehumidificationReverse process of humidification in which gas added into the liquid stream.

In Humidification heat and mass transfer Simultaneously.

HumidityMass of vapor carried by a unit mass of vapor free gas

H = humidity (lb of vapor/lb of dry air)M = Molecular Weight of vaporM = Molecular weight of gasP = Partial pressure of Vapor (psi)P = Total Pressure (Psi)

Saturation HumiditySaturated gas in which vapor in equilibrium with the liquid at gas temperature.Humidity at this point is saturation humidity

P= Vapor pressure of the liquidHs = Saturation Humidity

Relative HumidityRatio of Partial pressure and vapor pressure of liquid vapor

Where:HR = Relative HumidityP = Vapor pressure (Psi)

Percentage of HumidityRatio of humidity and saturation humidity

Humid Heat

Cs= Humid Heat Cpb= Specific Heat of gas (Btu/lb.F)Cpa = Specific Heat of Vapor (Btu/lb.F)

Dry Bulb TemperatureTemperature of the humid gas.Most of the time is the atmospheric temperature

Wet Bulb TemperatureSteady state, non equilibrium temperature reached by a small mass of liquid exposed under adiabatic condition to continues stream of gasCs= Cpb + Cpa H

Wet Bulb TemperatureDry Bulb Temperature

COOLING TOWERIntroduction, Design and Performance

IntroductionEquipment which evaporate a part of the liquid (water) and cool down temperature of rest of the liquid(water).

Water Air system in which water vapors are added in the main Air stream.

Remove heat from the system by evaporating vapor in air/gas stream.

Simultaneous mass and heat transfer in this operation

Types Natural DraftAir/Gas flow under the natural convection of the tower which is produced by height.Use in huge capacity operation like power plantsInduced DraftAir/Gas flow under the induced pressure of the fan which is placed at the top of the tower.Use in small capacity operationForced DraftAir/Gas flows under the forced pressure of the fan which is placed at The bottom of the towerUse in small capacity operation

Forced Draft Cooling TowerCounter Current Induced Draft (Common Design)

Natural Draft Cooling Tower

Basic terms and ParametersApproachDifference between the water supply temperature and the entering wet bulb temperature.

RangeTemperature difference of water inlet and outlet streams.

CapacityTotal amount of heat that the cooling tower can reject at a given flow rate, approach, and ambient wet bulb temperature.

Evaporation RateMass of the liquid vaporize per unit time

DriftUnevaporated water droplets that are lost from the cooling tower.

Evaporation (Loss)Water that undergoes a phase change from liquid to vapor and exits the tower as part of an airvapor mixture.

Make-Up (Water)Water added to overcome water lost to evaporation, drift and blow down.

Blow downWater discharged from the system to control concentration of solids or other impurities in the water.

Design Of Cooling Tower

Hydraulic Design

Operational Design

Mechanical Design

Circulation Water InletGxa = Mass Flux of water lb/Sq.ft. h.C = Circulation Rate ft/hTwi = Water inlet Temperature (F) Circulation Water OutletGxb = Mass Flux of water lb/Sq.ft. h.C = Circulation Rate ft/hTwo = Water out Temperature (F) Air/Gas EnteringGyb = Mass Flux of Air/Gas lb/Sq.ft. h.Hb = Enthalpy of gas entering (BTU/lb)TD = Dry Bulb Temperature (F) Air/Gas EnteringGyb = Mass Flux of Air/Gas lb/Sq.ft. h.Ha = Enthalpy of gas Leaving (BTU/lb)T = Temperature of gas leaving (F) ZTLeaving Gas contain vapor with high energy

Hydraulic Design

Enthalpy of Entering Gas

HB = Cs(TD TR) + 0 H

Where:HA = Enthalpy of entering gas (Btu/lb)TD = Dry bulb temperature (oF)TR = Reference Temperature (oF)0 = Heat of vaporization of liquid at TD (Btu/lb)

Overall Energy Balance Equation of cooling Tower

Gy(HA - HB) = GxCL(TWi - Two)

Where:Gy = Gas mass velocity per unit area (lb/h.m2)HA = Enthalpy of gas leaving (Btu/lb)Gx = Liquid mass velocity per unit area (lb/h.m2)CL = Specific heat of liquid (Btu/lb.oF)TWi = Liquid inlet TemperatureTwo = Liquid outlet Temperature

Total height of Contact sectionZT = Noy x HoyWhere:ZT = Total height of contact section (ft)Noy = Total number of transfer height or number of Ideal stagesHoy = height of one transfer unit or one ideal stage (ft)

Where:Ka = Volumetric mass Transfer Coefficient (lb/ft3.s)H = Enthalpy of gas at leaving (Btu/lb)H =Enthalpy of gas in equilibrium with liquid (Btu/lb)

Thermal Capacity

TC = mw CL(TWi - Two)

TC = Thermal Capacity (Btu/s)mw = Mass Flow rate (lb/s)CL = Specific Heat of water (btu/lb.oF)TWi = Temperature water inlet stream (oF)Two = Temperature Water outlet stream (oF)

Operational DesignCirculating Cooling WaterEvaporation RateDrift LossesBlow DownCycle of concentration

Design EquationEvaporation Rate

E = TC/ L

E = Evaporation rate ft/sTC = Thermal capacity (Btu/s)L = Latent heat of vaporization

Total Water Loss (ft/h )

TWL = E + BD + DL + L

Drift losses 0.05 0.2% of RR

Mackup Water (ft/h)

M = TWL = E + BD + DL+ L

Cycle of ConcentrationRepresents the accumulation of dissolved minerals in the recirculation cooling water. Blow Down is used to control the buildup of these minerals.

COC = XC / XMBlow Down

BD = (M/COC) - D

Where:

M = Make-up water in ft/h

C = Circulating water in ft/h

BD = Blow Down ft/h

E = Evaporated water in ft/h

DL = Drift loss of water in ft/h

COC = Cycles of concentration (dimensionless)

XM = Concentration of chloride in make-up water (M), in ppmw

XC = Concentration of chlorides in circulating water (C), in ppmw

ConclusionMass transfer from vapor phase to gas phase is HumidificationCooling tower is industrial application of humidificationCooling below the wet bulb temperature is impossibleApproach is the actual parameter to analyze the performance of cooling tower

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