Page 218 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

The Use of Water in HVAC Systems 215

then be 75 20 for a total of 95 ft. If the equipment efficiencies were
83 percent for the pumps and 90 percent for the wire-to-shaft efficiency
of the motor and variable-speed drive, the input energy K would be
i
1000 95
K 24.0 kW
i 5308 0.83 0.90
The efficiency of the system WS would be

14.1
WS 100 58.8 percent

24.0
The preceding calculations are for the system at full flow and with
no overpressure generated by the pumps. It is apparent that for this
equation to be meaningful, similar calculations must be made under
part-load conditions.
For example, assume that the system is operating at 50 percent
load, where the system flow is 500 gal/min and the useful system
head has dropped to 40 ft. The useful kilowatts, therefore, are

500 40
K 3.8 kW
e
5308
Also assume that a constant-speed pump is used on the preceding
application at 50 percent load. At this point, the pump has moved up
its curve and is operating at 500 gal/min, 105 ft of head, and an effi-
ciency of 78 percent. Since no variable-speed drive is involved, E is

the efficiency of the electric motor, which would be near 91 percent.
K , therefore, becomes
i
500 105
K 13.9 kW
i 5308 0.78 0.91

The overall efficiency of the water system becomes

3.8 100
WS 27.3 percent

13.9
This demonstrates that only slightly more than one-fourth of the
energy applied to the pump motors is being used for efficient trans-
portation of the water through the system. As dramatic as this may
seem, there are water systems utilizing constant-speed pumps and
mechanical devices to overcome the pump overpressure where less
than 10 percent of the energy applied is used to move the needed
water through the system at moderate and low loads on the system.

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