Page 169 - HVAC Pump Handbook

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HVAC Pump Performance

166 HVAC Pumps and Their Performance

6.6.2 Pump brake horsepower
The energy required to operate a pump is determined in brake horse-
power (bhp); the difference between water horsepower and pump
brake horsepower is the energy lost in the pump. The pump brake
horsepower equation is therefore
whp
bhp (6.12)
P

where P is the pump efficiency as a decimal.

For example, if the pump in the preceding example is operating at
an efficiency of 85 percent,

25.25
bhp 29.7 hp
0.85

6.6.3 Pump motor power in kilowatts
The electrical energy, pump kilowatts, for a motor-driven pump must
take in consideration the efficiency of the motor on constant-speed
pumps and the wire-to-shaft efficiency of the motor and variable-
speed drive on variable-speed pumps. Again, the difference between
the pump brake horsepower and electric power input to the motor or
variable-speed drive and motor is the efficiency of these devices. The
power must be converted from brake horsepower to kilowatts, so the
pump brake horsepower must be multiplied by 0.746. The equation is

bhp 0.746
Pump kW (6.13)
E

where E is the efficiency of the electric motor or the motor and vari-

able-speed drive as a decimal.
For example, if the preceding pump is variable speed with a wire-
to-shaft efficiency of 89 percent for the variable-speed drive and high-
efficiency motor, then

29.7 0.746
Pump kW 24.9 kW
0.89
Summarizing Eqs. 6.11 through 6.13,
Q h s
Pump kW (6.14)
5308 P E

As will be explained in Chap. 7, the combined efficiency of an elec-
tric motor and a variable-speed drive cannot be just the multiplication

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