Handbook of air conditioning and refrigeration / Shan K

21.42 CHAPTER TWENTY-ONE
where Q rsxn � control zone maximum sensible cooling load, Btu/h (W). The control zone peak
supply volume flow rate for cold air in the perimeter zone at summer design conditions is
(21.13)
For any control zone in the perimeter zone, the peak supply volume flow rate delivered to the
conditioned space from the mixing VAV box, including 5 percent air leakage from the warm air
damper at summer design conditions, in cfm (L/s), is therefore
V˙
sxn � (21.14)
Because only cold air from the VAV box is supplied to the interior zone, for any control zone in the interior
zone, the peak supply volume flow rate V˙ sin at summer design conditions, in cfm (L/s), is then
V˙
sin � V˙
rin
(21.15)
V˙ cxn
0.95
Peak supply volume flow rate for control zone n, , can be calculated by Eq. (21.12).
Case Study: A Dual-Fan Dual-Duct VAV System
Consider a dual-fan dual-duct VAV system serving a typical 20,000-ft 2 floor in an office building
with the same operating parameters as the VAV reheat system described in Sec. 21.3, except the
winter outdoor design temperature is 30°F (� 1.1°C) and the outdoor humidity ratio is 0.003 lb/lb
(kg/kg). As in the VAV reheat system, the maximum supply volume flow rate for the interior zone
at summer design conditions based on block load is
For a control zone in the perimeter zone, if the temperature of the mixture of outdoor and recirculating
air Tmx � 80.5°F (26.9°C), humidity ratio wmx � 0.010 lb/lb (kg/kg), Tsx � Ts,c � 55°F
(12.8°C), and �sx � �s,c � 80 percent, draw line sx-mx as shown in Fig. 21.9b.
For a warm air leakage of 0.05 from the inlet damper, the state point of the mixture of cold
air supply and warm air leakage sxn can be determined. From the psychrometric chart, T sxn �
56.5°F (13.6°C), and w sxn � 0.0083 lb/lb (kg/kg).
The volume flow rate of cold air supply to the perimeter zone at summer design conditions, based
on the block load and including 5 percent air leakage from the warm air inlet damper, is therefore
The total cold air supply volume flow rate to both the perimeter zone and the interior zone at summer
design conditions is
(21.16)
The volume flow rate of warm air supply in the perimeter zone is usually expressed as a percentage
of peak supply volume flow rate of cold air in the perimeter zone, usually between 0.5V˙ scx and
1.0 . It depends mainly on the space heating load linear density qh,ft, in Btu/h�ft (W/m), and the
V˙ scx
V˙ si �
V˙ scx �
�
Q rs
60� sc pa(T r � T s,c)
135,000
60 � 0.075 � 0.243(75 � 55)
V˙ sxn
285,500
60 � 0.075 � 0.243(75 � 56.5)
V˙ s,c � V˙ scx � V˙ sci
V˙ cxn � V˙ rxn � V˙ lk
V˙ sin
� 6173 cfm (2913 L / s)
� 14,113 cfm (6660 L / s)
� 14,113 � 6713 � 20,826 cfm (9828 L / s)
V˙ sxn