TITLE.PM5

PSYCHROMETRICS 473

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(a)

18 Cº 38 Cº

1

2

W 1

W 2

W

D B T

85%

75%

h 2

h 1

(b)
Fig. 10.16
Heat transfer rate,
q = ()Wh 21 g 21 −Whg + cp ()ttdb 21 − db + (W 1 – W 2 ) hf 2
= (0.01108 × 2534.4 – 0.0325 × 2570.7) + 1.005 (18 – 38) + (0.0325 – 0.01108) × 75.6
= – 55.46 – 20.1 + 1.62 = – 73.94 kJ/kg of dry air. (Ans.)
Example 10.12. Evaporative Cooler : Atmospheric air at 38ºC and 25% relative humid-
ity passes through an evaporator cooler. If the final temperature of air is 18ºC, how much water
is added per kg of dry air and what is the final relative humidity?

Solution. At 38ºC :
pvs = 0.0663 bar, hg 1 = 2570.7 kJ/kg
and pv = φ × pvs = 0.25 × 0.0663 = 0.01657 bar