Handbook of air conditioning and refrigeration / Shan K

supply temperature differential Ts � Tr). If the volume flow rate of the warm air supply in the
perimeter zone at winter design condition , in cfm (L/s), is 0.7 , then
Other parameters such as cooling and heating coil loads can be calculated as for VAV reheat systems.
Winter Heating and Winter Cooling Mode Operation
Part-Load Operation
In a dual-fan dual-duct VAV system, the cold deck and the hot deck have their own outdoor and recirculating
air supply. In the hot deck during winter heating mode operation, it is possible to have
an outdoor air ratio of only about 22 percent and a mixing temperature of outdoor and recirculating
air T mx � 62°F (16.7°C) for the perimeter zone, so that only necessary heating energy is provided
for a hot deck discharge air temperature of 87°F (30.6°C), as shown in Fig. 21.9b. In the cold deck
winter cooling mode operation, nearly 60 percent outdoor air is required to form a mixture of outdoor
and recirculating air with a temperature T mi of about 50°F (10.0°C) and a cold deck discharge
air temperature of 52°F (11.1°C), so that outdoor air free cooling can be fully utilized to offset the
zone cooling load in the interior zone in winter.
Consider a typical control zone in the perimeter zone of this dual-fan, dual-duct VAV system whose
operating parameters at summer design condition and summer part-load operation are as follows:
If the supply air temperature discharged from the mixing VAV box Tsxn � Tm2 � 56.5°F (13.6°C),
the volume flow rate of conditioning air supplied to this control zone V˙
sxn,
in cfm (L/s), including
5 percent warm air leakage from the damper at summer design conditions, is therefore
V˙ sxn �
During part-load operation, as the zone sensible cooling load is reduced to 30 percent of the design
load, T sxnp � 50 � 10 � 60°F (15.6°C). Assuming that the percentage of warm air leakage and the
temperature rise due to this warm air leakage are the same at design condition, the supply temperature
of the mixture of cold and warm air of control zone xn in the perimeter zone at part-load T sxnp is
now equal to 50 � 10 � 1.5 � 61.5°F (16.4°C).
The supply volume flow rate from the mixing VAV box in the perimeter zone at part-load can be
calculated as
V˙ sxnp �
�
14,200
60 � 0.075 � 0.243(75 � 56.5)
Q rsxnp
60�c pa (T rxn � T sxnp)
AIR SYSTEMS: VARIABLE-AIR-VOLUME SYSTEMS 21.43
V˙ shx
Full load Part load
Zone sensible cooling load, Btu/h 14,200 4,260
Leaving cooling-coil temperature, °F 50 50
Relative humidity, percent 95 95
Supply system heat gain, °F (°C) 5 (2.8) 10 (5.6)
Warm air supply temperature, °F (°C) 87 (30.6) 80 (26.7)
4260
60 � 0.075 � 0.243(75 � 61.5)
V˙ scx
V˙ shx � 0.7 � 14,113 � 9879 cfm (4661 L / s)
� 702 cfm (331 L / s)
� 289 cfm (136 L / s)
(21.17)