Handbook of air conditioning and refrigeration / Shan K

where w s � humidity ratio of fan-coil supply air, lb/lb (kg/kg). As shown in Fig. 28.1b, w s � w m
� w fc, and from the psychrometric chart, the humidity ratio of outdoor air w o � 0.0017 lb/lb.
Therefore,
So
and w s � w m � w r � 0.00097 � 0.0046 � 0.00097 � 0.0036 lb/lb (0.0036 kg/kg).
(28.21)
12. Because at heating mode operation T r � 72°F (22.2°C), from the psychrometric chart, the
relative humidity at winter design condition � r � 28 percent.
13. If air leaves the preheating coil of the outdoor air AHU at T ph � 83°F and T pf � T ph � fan
temperature rise � 83 � 2 � 85°F, because w o � w pf � 0.0017 lb/lb, from the psychrometric
chart, air density at the fan supply outlet in the outdoor-air AHU � pf � 1/13.75 � 0.073 lb/ft 3 .
The preheating coil load Q cph, in Btu/h (W), can be calculated as
Q cph � 60V˙ p� pf c pa(T ph � T o)
(28.22)
14. If the heat loss of outdoor air supply duct is 0.5°F, T ps � 85 � 0.5 � 84.5°F, and w ps � w pf
� 0.0017 lb/lb, then point ps can be determined. Draw line ps-r. Because w m � 0.0036 lb/lb
(0.0036 kg/kg), point m can be plotted on the psychrometric chart. From the chart, T m � 76.2°F
(24.5°C).
15. The temperature of the fan-coil supply air at winter design load T s, in °F (°C), can be calculated
as
T s � T r �
� 72 �
Q rh
60V˙ fc� cf c pa
(28.23)
As w s � w m � 0.0036 lb/lb (0.0036 kg/kg), point s can be determined, and lines r-s and m-s can
be drawn. The temperature difference T s � T r � 86.9 � 72 � 14.9°F (8.3°C), which is smaller
than the supply air temperature difference for warm air during heating mode 15°F (8.3°C)
specified by ASHRAE handbooks to prevent an excessive buoyancy effect, as discussed in
Sec. 18.6.
16. The air temperature at the fan outlet in the fan coil T cf � T m � 0.5 � 76.2 � 0.5 � 76.7°F.
The heating coil load in the fan-coil unit Q ch, in Btu/h (W), can be calculated as
Q ch � 60V˙ fc� cf c pa(T s � T cf)
AIR CONDITIONING SYSTEMS: SPACE CONDITIONING SYSTEMS 28.19
w r � w s
w r � w o
� 60 � 200 � 0.073 � 0.243(83 � 20) � 13,411 Btu/h (3929 W)
9500
� V˙ p
V˙ fc
w r � w o � w r � w s
0.33
� 0.0046 lb/lb (0.0046 kg/kg)
60 � 2 � 300 � 0.073 � 0.243
� 200
600
� 0.33
� 0.0017 � 0.00097
0.33
� 72 � 14.9 � 86.9�F (30.5�C)
(28.24)
� 60 � 2 � 300 � 0.073 � 0.243(86.9 � 76.7) � 7152 Btu/h (2095 W)
17. At winter heating mode part-load operation, the warm supply air from the fan coil, point sp,
moves along the horizontal line m-sp and is supplied at a lower temperature Tsp in order to maintain
the required zone temperature Trp.