Handbook of air conditioning and refrigeration / Shan K

21.54 CHAPTER TWENTY-ONE
Percentage of zone peak (design) supply flow
80
60
40
20
0
Heating
Cooling
Dead
band
FIGURE 21.16 Supply volume flow rate of a control zone in the perimeter zone served by a fan-powered VAV system.
Fan Energy Use
.
Vsn, r
.
V sn, c
T s�n
. .
�
V sn, c
In a fan-powered VAV system, there are two kinds of fan energy use: the supply fan energy use in
an AHU or a PU and the energy use of the small centrifugal fans in the fan-powered boxes. Typical
supply fan efficiency is about 60 percent, and the motor efficiency is around 90 percent, which
gives a combined fan and motor efficiency of about 55 percent.
According to Elleson (1993), the efficiency of fans in fan-powered boxes from four manufacturers
ranged from 16 to 35 percent, and the motor efficiency ranged from 45 to 55 percent. Assuming
a fan efficiency of 30 percent and a motor efficiency of 50 percent, the combined fan and motor efficiency
for the fan in the fan-powered box is 15 percent.
Elleson (1993) has collected detailed data for the 1988 cooling season from the fourth floor of a
12-story building in southern California using a fan-powered VAV system (parallel fan-powered
boxes). The results are as follows:
55°F (12.8°C) Supply air 45°F (7.2°C) Supply air
Fan-powered Fan-powered
Supply fan boxes Total Supply fan boxes Total
Continuous operation, kWh 11,720 7,000 18,720 7,794 7,000 14,794
Intermittent operation, kWh 11,720 1,861 13,581 7,794 2,140 9,934
No fan-powered boxes, kWh 11,720 0 11,720 8,417 0 8,417
Intermittent operation indicates that fans in parallel fan-powered boxes operated only when the cold
primary airflow fell below 50 percent of design flow.
V sn, r
80
60
40
Supply temp T sxn , �F