Handbook of air conditioning and refrigeration / Shan K

20.70 CHAPTER TWENTY
5. Sensible cooling process sf-s due to the heat loss from the warm air inside the supply duct
through the duct wall to the ambient air qsd. 6. Space conditioning process sr. During heating mode operation in winter, heat is supplied to the
space to compensate for the transmission loss, and moisture is absorbed by the supply air due to
the space latent load. Supply air at point s is then changed to r.
The heat supplied to the conditioned space to offset the space heating load Qrh, in Btu/h (W),
can be calculated as
Qrh � 60 V˙ s�scpa(Ts � Tr) (20.86)
And the latent heat absorbed by the supply air Qrl, in Btu/h (W), is
(20.87)
Example 20.6. For the same constant-volume single-zone system as that in Example 20.4, shown
in Fig. 20.22, the following winter design conditions are given:
Winter outdoor design conditions:
Dry-bulb temperature 15°F (9.4°C)
Relative humidity 30 percent
Winter indoor design temperature 72°F (22.2°C)
Outdoor air requirement 3200 cfm (1510 L/s)
Temperature rise:
Due to heat release from light fixtures 2°F (1.11°C)
Return fan power heat gain 1°F (0.56°C)
Supply fan 2°F (1.11°C)
Temperature drop of the supply duct 1°F (0.56°C)
Space heating load 225,000 Btu/h (65,925 W)
Space latent load 60,000 Btu/h (17,580 W)
Supply volume flow rate 21,140 cfm (9976 L/s)
Determine
1. The space relative humidity at winter design conditions
2. The temperature of supply air
3. The heating coil load
Solution
Q rl � 60V˙ s� s(w r � w s)h fg,32
1. To develop a winter heating mode air conditioning cycle for a constant-volume single-zone
system, the space humidity ratio wr must be determined. Because the winter indoor temperature is
given, the space relative humidity can then be found from the psychrometric chart.
Assume that the moist volume of the warm supply air vs � 13.75 ft3 /lb (0.86 m3 /kg). Because
�s � 1/vs, from Eq. (20.87)
w r � w s �
Q rl
60V˙ s � s h fg,32
60,000 � 13.75
�
60 � 21,140 � 1075
� 0.000605 lb / lb