An adiabatic swampcooler installed in a greenhouse in Phoeni

An adiabatic “swamp-cooler” installed in a green-house in Phoenix Arizona uses the evaporation of water (humidification of moist-air) to cool hot relatively dry air. Specifically, moist-air at 36C and 20%RH is humidified via injection of saturated liquid water available at the inlet air temperature of 36C. The water completely evaporates within the cooler resulting in a cooled moist-air exit RH of 90%. The inlet moist-air flow is 10m3/min. Determine the mass-flow rate of the dry-air component, the outlet moist-air flow temperature and mass-flow rate of the liquid cooling water. Hint: This problem, baring the use of software such as EES, must be solved iteratively. Start by assuming an outlet moist-air temperature and then solve the H₂0 mass-balance for the water injection mass-flow rate. Then, check for satisfaction of the energy-balance (the cooler is adiabatic). Iterate the outlet temperature as necessary. Ans. 20.8C, 0.073kg/min (water)

Solution

Given data:

Inlet Moist air flow = 10 m³/min

Intial Dry bulb temperature t1=36⁰C

RH1=20%

RH2=90% (Relative humidity)

From the Pyschrometric chart

At 36⁰C & RH1=20%

tw1= 19.50⁰c ,w1= 0.0074kgH20/kg of dry air, V1=0.887 m3/kg of dry air

w1= Specific humidity ratio tw1=wet bulb temperature

Assuming that liquid water is supplied at a temperature not much different than exit temperature of tha saturated air stream, the swamp cooler follows a line of constant web bulb temperaturet tw1=tw2

= 19.5 ⁰C

At the above wetbulb temperature and RH2=90%

then t2= 20.5⁰C

w2=0.0137kgH20 /kg of dry air

Thus air will be cooled to 20.5⁰C in this swamp cooler

Mass flow rate of dry air   

= Inlet moisture air flow /V1

=(10 m³/min)/(0.887 m³/kg of dry air)

=11.27 kg /min

Then the required supply to the swamp cooler

= mw2-mw1= mass flow rate of dry air x( w2-w1)

11.27 x(0.0137-0.0074)

0.07100 kg/min water