Handbook of air conditioning and refrigeration / Shan K

21.32 CHAPTER TWENTY-ONE
For the winter heating mode operation in the perimeter zone of a VAV reheat system, the following
is recommended:
● If there is only one AHU or PU to serve both the perimeter and interior zones in a VAV reheat system
and if the zone heating load linear density q h,ft � 120 Btu/h�ft (115 W/m), then the warm air
supply volume flow rate from the reheating box in heating mode operation (at cold primary air
minimum setting) should be proportionally increased to prevent excessive buoyancy effects.
● It is more energy-efficient to use a perimeter heating VAV system; a dual-fan, dual-duct VAV
system; or a fan-powered VAV system; or else the AHU or PU that serves the perimeter zone is
separated from another AHU or PU serving the interior zone.
Winter Cooling Mode Operation in Interior Zone
At winter design conditions, the outdoor humidity ratio wo � 0.0017 lb/lb (kg/kg), and the space
sensible cooling load in the interior zone may still equal that of summer design conditions. Therefore,
the supply temperature differential for the interior zone is Tr � Ts � 20°F, or Ts � 72 �
20 � 52°F (11.1°C). The supply volume flow rate for the interior zone is still V˙ si � 6173 cfm
(2913 L/s).
If the same AHU or PU is used for both perimeter and interior zones, a minimum supply volume
flow rate of cold primary air that equals 0.3 of the peak supply volume flow rate (including the
required outdoor ventilation) is supplied to the perimeter zone during winter heating mode and can
be calculated as
The difference in humidity ratio between supply and zone air is
where w r, w s � humidity ratio of zone and supply air, lb/lb (kg/kg). The temperature rise from the
return system heat gain at winter design conditions is then
and
If the temperature rise from the supply fan power is still about 2°F (1.1°C) and the temperature rise
from supply duct heat gain is 3°F (1.7°C) for the interior zone, then
and
�
V˙ sx,c
V˙ sx,c � 0.3 � 13,054 � 3916 cfm (1848 L / s)
�T ret,w �
w r � w s �
13,300 � 13,300
60 � (6173 � 3916) � 0.075 � 1061
3
(6173 � 3916) / 19,227
T ru � T r ��T ret,w � 72 � 5.7 � 2 � 79.7�F (26.5�C)
T m � T s � (2 � 3) � 52 � (2 � 3) � 47°F (7.2°C)
T ru � T m
T ru � T o
�
� V˙ o
V˙ s
79.7 � 47
79.7 � 20
� 0.55
� 0.00055 lb / lb (kg / kg)
� 5.7�F (3.2�C)
From the psychrometric chart, at a temperature T r � 72°F (22.2°C) and � r � 30 percent,
w r � 0.005 lb/lb (kg/kg). Draw line r-s from point r with SHR s � 0.84, which intersects the
Q rl
60V˙ s� sh fg, o