Handbook of air conditioning and refrigeration / Shan K

Operating Parameters and Calculations
1. The zone supply air condition should be determined so that the space temperature and relative
humidity in each zone or room can be maintained at design load operation by using the reheating
processes.
2. The zone n supply mass flow rate m˙ an,
in lb/min [kg/(60 s)], and volume flow rate V˙
sn,
in cfm
[kg/(60 s)], at summer design condition can be calculated as
where Qrsn � sensible cooling load for zone n, Btu/h (W)
Trn � temperature of zone n,°F (°C)
Ts � supply temperature of zone n,°F (°C)
vs � 1/�s, moist volume of supply air, ft3 /lb (m3 V˙
sn � vsm˙ an
/kg)
AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.77
m˙ an �
(20.89)
(20.90)
3. The zone n supply temperature at minimum part-load operation T snp, in°F (°C), can be calculated
as
Tsnp � Trnp � (20.91)
where Trnp � temperature of zone at minimum part-load, operation °F (°C).
4. The zone n reheating coil load Qhn, in Btu/h (W), at minimum part-load operation can be calculated
as
Qrsn 60m˙ ancpa Q hnp � 60m˙ anc pa(T snp � T s)
(20.92)
System supply temperature T s varies along the supply main duct. For simplicity, T s can be taken as
an average value.
5. The weighted mean value of zone temperature T rm, in °F (°C), and mean humidity ratio w rm,
in lb/lb (kg/kg), can be calculated as
T rm � m˙ a1T r1 � m˙ a2T r2 �� � ��m˙ anT rn
m˙ a1 � m˙ a2 �� � ��m˙ an
(20.93)
and wrm � (20.94)
where m˙ a1, m˙ a2, � � �, m˙ an � mass flow rate of supply air for zones 1, 2, ...,n, lb/min (kg/s)
Tr1, Tr2, ...,Trn � air temperatures in zones 1, 2, ...,n,°F (°C)
wr1, wr2, ...,wrn � air humidity ratio in zones 1, 2, ...,n, lb/lb (kg/kg)
m˙ a1wr1 � m˙ a2wr2 �� � ��m˙ anwrn m˙ a1 � m˙ a2 �� � ��m˙ an
6. The system supply volume flow rate , in cfm (m 3 /s), of the AHU or PU is calculated as
V˙ s
Q rsn
60c pa(T rn � T s)
(20.95)
where supply volume flow rate for zones 1, 2, ...,n, cfm (m3 V˙ s � V˙ s1 � V˙ s2 �� � ��V˙ sn
V˙ s1, V˙ s2, . . . , V˙ sn �
/s).
7. The condition of recirculating air entering the AHU or PU, point ru, and the mixture of outdoor
and recirculating air m can be determined graphically on the psychrometric chart.The cooling
coil load Qcc, in Btu/h (W), can therefore be calculated as
Q cc � 60(m˙ a1 � m˙ a2 �� � ��m˙ an)(h m � h cc)
(20.96)