Handbook of air conditioning and refrigeration / Shan K

(11.1°C), as shown in Fig. 21.5b. If the temperature rise from supply duct heat gain is 2°F (1.1°C),
the temperature of supply air entering the reheating coil in a reheating VAV box T en is 54°F
(12.2°C).
One of the primary considerations in the design of warm air supply VAV systems (including
VAV reheat system) at winter design conditions is the control of stratification when warm air is supplied
from overhead slot and ceiling diffusers in the perimeter zone. As discussed in Sec. 18.6, it is
recommended that the supply air temperature differential T r � T s not exceed 15°F (8.3°C), to prevent
excessive buoyancy effects. A terminal velocity of 150 fpm (0.75 m/s) is required at the
5-ft (1.5-m) level to offset the cold draft from the external window. An even distribution of the volume
flow rate of supply air between the warm airstream blowing downward toward the window
glass and the warm airstream horizontally discharged toward the partition wall across from the window
is preferable.
At a warm air supply volume flow rate equal to 30 percent of the summer peak volume flow rate
of 1.5 cfm/ft 2 (9.14 L/s�m 2 ), the space heating load per ft 2 of floor area that can be offset by warm
air supply in the perimeter zone during winter design conditions is
q rh
A � 0.3 � 1.5 � 60 � 0.075 � 0.243 � 15 � 7.4 Btu / h�ft2 (23.3 W / m 2 )
If the depth of the perimeter zone is 15 ft (4.6 m), the heating load linear density q h,ft is
q h,ft � 7.4 � 15 � 111 Btu/h�ft (107 W/m)
The maximum reheating coil load for any control zone in the perimeter zone to offset the zone heating
load at winter design conditions Q chxn, in Btu/h (W), can be calculated as
(21.10)
where Qrhxn � space heating load for control zone n in perimeter zone, Btu/h (W)
Qvenxn � ventilation load due to a 30 percent of peak volume flow of cold supply
air, Btu/h (W)
V˙ s,cxn � volume flow rate of cold primary air discharged to control zone from AHU of VAV
reheating system, cfm [m3/(60 s)]
Ten,n, Trxn � cold primary air temperature and zone temperature of control zone n, °F (°C)
For a summer peak volume flow rate of 1.5 cfm/ft 2 (9.14 L/s�m 2 ) in the perimeter zone, at 30 percent
of peak volume flow that means 0.3 � 1.5 � 0.45 cfm/ft 2 (2.3 L/s�m 2 ) of cold supply air.
If the zone heating load linear density q h,ft is 111 Btu/h�ft (107 W/m) and the zone heating load
density is 7.4 Btu/h�ft 2 , the reheating coil load for 1 ft 2 (0.0929 m 2 ) of floor area at winter design
conditions can be calculated as
Q chn
A
Q chxn � Q rhxn � Q venxn
� Q rhxn � 60V˙ s, cxn� sc pa (T rxn � T en, n)
� 7.4 � 60 � 0.045 � 0.075 � 0.243(72 � 54) � 7.4 � 8.9
� 16.3 Btu / h�ft 2 (51.4 W / m 2 )
If the AHU that serves the perimeter zone in a VAV reheat system is separated from the interior
zone, and a minimum outdoor air intake of 15 percent of the peak supply volume flow rate can be
set up for the perimeter zone during the winter heating mode operation, as shown in the air conditioning
cycle in Fig. 21.1b, the reheating coil load is then
Q chn � 7.4 � 60 � 0.45 � 0.075 � 0.243(72 � 64) � 7.4 � 3.9
� 11.3 Btu / h�ft 2 (35.6 W / m 2 )
A saving of about 8.9 � 3.9 � 5.0 Btu/h� ft 2 (15.7 W/m 2 ) is possible.
AIR SYSTEMS: VARIABLE-AIR-VOLUME SYSTEMS 21.31