Handbook of air conditioning and refrigeration / Shan K

20.68 CHAPTER TWENTY
Winter outdoor design conditions
Air temperature 32°F (0°C)
Relative humidity 30 percent
Winter indoor design conditions
Air temperature 72°F (22.2°C)
Space sensible cooling load 800,000 Btu/h (234,400 W)
Space latent load 600,000 Btu/h (175,800 W)
Temperature rise due to supply system heat gain 4°F (2.22°C)
Temperature rise due to return system heat gain 2°F (1.11°C)
Supply volume flow rate 100,000 cfm (47,190 L/s)
Minimum outdoor air requirement 26,250 cfm (12,387 L/s)
At winter design condition, a mixture of outdoor air and recirculating air forms the cold air supply
(cooling mode operation) to offset the space cooling load.
Determine the space relative humidity at winter design conditions and the actual amount of outdoor
air intake.
Solution. Assume that the moist volume of the supply air vs � 13.3 ft3 /lb. From Eq. (20.65)
and the given data, the humidity ratio difference between the space and supply air can be calculated
as
w r � w s �
600,000 � 13.3
�
� 0.00125 lb / lb (kg / kg)
60 � 100,000 � 1075
From Eq. (20.31) and the given data, the supply air temperature difference Tr � Ts can be calculated
as
T r � T s �
Q rl
60V˙ s� sh fg,32
Q rs
60V˙ s� sc pa
800,000 � 13.3
�
� 7.30 �F (4.1�C)
60 � 100,000 � 0.243
From the given data
Tru � Tr � 2 � 72 � 2 � 74°F
and Tm � Tr � (Tr � Ts) � 4 � 72 � 7.3 � 4 � 60.7°F (15.9°C)
Because wru � wr and wm � ws, from Eq. (20.67)
w r � w s
w ru � w o
�
� T ru � T m
T ru � T o
74 � 60.7
74 � 32
� 0.317
Then
wr � wo �
From the psychrometric chart, at a temperature of 32°F and a relative humidity of 30 percent, wo �
0.0012 lb/lb, so
wr � wo � 0.00394 � 0.0012 � 0.00394 � 0.00514 lb/lb
wr � ws � 0.00394 lb / lb (kg / kg)
0.317