Air is to be cooled and dehumidified in a twostep process Th

Air is to be cooled and dehumidified in a two-step process. The air enters the process at a rate of 20 m^3/min at 32 degree C and 90% relative humidity. The air is cooled in a condenser to a temperature of 11 degree C and some of the water in the air condenses to liquid at 11 degree C. The air leaving the condenser is heated in a heat exchanger to 21 degree C and fed to a building air distribution system. Use the psychrometric chart for your calculations. Calculate the rate at which liquid water leaves the condenser in kg/min. Calculate the rate at which heat is removed from the condenser in kW. Calculate the rate at which heat is added to the heat exchanger in kW. Find the relative humidity of the air leaving the heat exchanger. Show the points 1, 2 and 3 in the process on a psychrometric chart. Circle and label each of the points on the diagram.

Solution

from defn sp humidity = m(water)/mass (air) = X

RH = XP/(0.622+X)Pg, where Pg is sat vp of water at that temp

from tables sat pressure for 32C is about 4200 Pa

atm pressure 100000 Pa

RH .9 = X*100000/[(.622+X)*4200]

solve for X = = .0224

For Saturated air at 11 C, sat pressure is 1330 Pa, and formula gives .0884 as the humidity ratio

if all of this condenses, then from 20 m³ air a minute, which is .03*20 kg/min =.6 kg/min= .01 kg/sec.,

.0884*.01 water should condense every second or .884 gm/sec

Latent heat removal plus sp.heat*(temp difff) of air. use charts of calculate

Heat addition same: temp diff 21-11 =10 deg, velocity known sp heat, rate of heat found Cp(m\') DT