Page 296 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 293

8. Discharge shutoff valve
9. Discharge piping
10. Exit loss into discharge header

Wire-to-water efficiency is an old term that has been used for many
years in the pumping industry to determine the overall efficiency of a
pump and motor combination. This efficiency was used first for constant-
speed pumps of considerable size, where large motors were involved.
When these pumps were equipped with variable-speed drives, the
wire-to-water efficiency represented the overall efficiency of drives,
motors, and pumps. Wire-to-water efficiency has now been applied to
smaller pumping systems because the computer capability is now
available to accomplish the tedious calculations.
The wire-to-water efficiency of a proposed pumping system can be
estimated using the following procedure:
1. A computer program can be developed to calculate the overall
wire-to-water efficiency of a pumping system from minimum to
maximum system flow. This program utilizes a standard curve-
tracing technique through the calculation of binomial equations for
the following curves:
a. Pump head-flow curve
b. Pump efficiency curve
c. Motor efficiency or wire-to-shaft efficiency curve for the variable-
speed drive and motor
d. System head curve
e. Pump fitting loss curve
2. These equations are then used to compute system performance
using the pump energy equations of Chap. 6; values computed are:
a. System flow and head
b. Water horsepower
c. Pump gallons per minute
d. Pump fitting loss
e. Pump brake horsepower
f. Pump revolutions per minute
g. Pump efficiency
h. System brake horsepower
i. Wire-to-shaft efficiency (variable-speed drive and motor effi-
ciencies combined)
j. Input kilowatts
k. Wire-to-water efficiency
For variable-speed pumping systems using variable-frequency drives,
the wire-to-shaft efficiency for the motor and variable-speed drive
listed in the previous paragraph, is a single efficiency developed by

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