The air conditioning by evaporative cooling in Problem 11101

The air conditioning by evaporative cooling in Problem 11.101 is modified by adding a dehumidification proccess beore the water spray cooling process. The dehumidification is achieved, as shown in Fig. P11.144, by using a desiccant material, which absorbs water on one side of a rotating drum heat exchanger. The desiccant is regenerated by heating on the other side of the drum to drive the water out. The pressure is 100kPa everywhere, and other properties are indicated on the diagram. Calculate the relative humidity of the cool air supplied to the room at state 4 and the heat transfer per unit mass of air that needs to be supplied to the heater unit.

Problem 11.101

Solution

solution:

1) here at point 1 air is at DBT=35 and RH=30%

and DBT=35,Pvs=.05622 bar

RH=Pv/Pvs

Pv=.016866 bar

where specific humidity is

w=.622*(Pv/Pa)

Pa=1 bar

w1=.01049 kg/kg of dry air

in this process latent heat loss is given to sensible heat rise of air

2) at point 2

T2=60

w2=.5*w1=5.24*10^-3 kg/kg of dry air

3) heat loss in heat exchanger is only sensible heat as

Q23=cpm*(T3-T2)=1.022*(60-25)=35.77 kj/kg

w2=w3

4) at point 4 in evaporative cooler whatever sensible heat loss is occure is converted to latent heat rise

cpa(T3-T4)=(w4-w3)*Hf at1 bar

on putting value we get

1.005*(25-20)=(w4-5.24*10^-3)*417.5

w4=w5=.0172812 kg/kg of dry air

where relative humidity is

RH=1.6*w4*(Pa/Pvs)

at 20 degree Pvs=.02337 bar

RH=1.183 or100 %

5)here again in evaporative cooler there is rise specific humidity with fall in temperature

1.005(25-20)=(w6-w5)*417.5

as we know w5=.0172812

we get

w6=.02931 kg/kg of dry air

6) again in heat exchanger total heat loss between 23 is lead to rise the temperature in heat exchanger

Q23=Cpm(T7-T6)

we get

T7=55 degree

8) where total heat added in heater to rise temperature to take care of moisture in duhumidifier is

Qh=Cpm(T8-T7)

Cpm=1.022 kj/kg k

we get

Qh=1.022(80-55)=55.55 kj/kg of moist air

9) this heated air will take care of moisture in dehumidifier by losing sensibe heat and make w equal to atmosphere