Handbook of air conditioning and refrigeration / Shan K

28.18 CHAPTER TWENTY-EIGHT
calculated as
T fc,p � T rp �
� 78 �
where Q rsp � zone sensible cooling load at part load, Btu/h (W)
T rp � zone air temperature at part load, °F (°C)
(28.16)
At part-load operation, the relative humidity of supply air leaving the fan coil � fc,p may vary between
80 and 90 percent. If � fc,p � 88 percent, point fcp can be plotted on the psychrometric chart
as shown in Fig. 28.1b. From the chart, h fcp � 27.2 Btu/lb (63.3 kJ/kg).
8. During cooling mode part-load operation, the sensible heat ratio of space conditioning line
SHR s � 13,100/15,900 � 0.82. Draw line fcp-rp from fcp with a sensible heat ratio of the space
conditioning line at part load SHR sp � 0.82. Line fcp-rp intersects the zone temperature line T rp at
rp. Therefore, point rp can be determined. From the psychrometric chart, � rp � 57.5 percent.
9. When the space cooling load is reduced to 50 percent of design load, if the temperature of
chilled water supplied to fan coils T w,f and that to the outdoor air AHU T w,p are reset to 50°F (10°C)
at part-load operation, and the chilled water temperature increase in the cooling coil of the outdoorair
AHU is still 10°F, then the condition of supply air from the outdoor air AHU at part-load point
psp can be determined as in full-load operation, T psp � 60 � 2 � 4 � 66°F (18.9°C), and w psp �
0.0110 lb/lb (0.0110 kg/kg).
Draw line psp-rp. The condition of the mixture at part load, point mp, can be determined from
the following relationship:
(28.17)
From the psychrometric chart, T mp � 75.3°F (24.1°C), and w mp � 0.0116 lb/lb (0.0116 kg/kg).
Because T cfp � T mp � 0.5°F � 75.3 � 0.5 � 75.8°F, and w cfp � w mp � 0.0116 lb/lb, from the
psychrometric chart, h cfp � 31.0 Btu/lb (72.1 kJ/kg).
10. Fan-coil cooling coil load at part load Q cfcp and sensible coil load at part load Q sfcp, both in
Btu/h (W), can be calculated as
� 60(4)(300)(0.7)(0.073)(31.0 � 27.2) � 13,980 Btu/h (4096 W)
� 60(4)(300)(0.7)(0.073)(0.243)(75.8 � 63.3) � 11,176 Btu/h (3275 W)
(28.18)
(28.19)
11. At winter heating mode design load operation, if only two fan-coil units are operated and
h fg,o � 1061 Btu/lb (2468 kJ/kg), the humidity ratio difference between zone air and the fan-coil
supply air is
w r � w s �
�
Q rl
Q rsp
60 � 0.7V˙ fc� cf c pa
Q cfcp � 60V˙ fc� cf (h cfp � h fcp)
Q sfcp � 60V˙ fc� cf c pa(T cfp � T fcp)
60V˙ fc� hfh fg,o
rp-mp
rp-ps � V˙ p
V˙ fc
0.5(26,200)
60 � 4 � 300 � 0.7 � 0.073 � 0.243
2700
�
60 � 2 � 300 � 0.073 � 1061
200
4 � 300 � 0.7
� 63.3�F(17.4�C)
� 0.24
� 0.00097 lb/lb (0.00097 kg/kg)
(28.20)