Page 301 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

298 The HVAC World

wire-to-water efficiency is shown in Fig. 10.13a and includes a system
flowmeter, a differential pressure transmitter across the pumping
system headers, and a watt transmitter.
Wire-to-water efficiency is computed by dividing the energy applied
to the water (water horsepower) by the power input to motor or variable-
speed drive. Using the equations of Chap. 6 for pumping energy, the
equation for wire-to-water efficiency WWE is
water horsepower (whp) 0.746 100
WWE (%)
input kW
Q (gal/min) H (ft)
(10.2)
53.08 input kW

where Q flow through the system in U.S. gallons per minute
H net pumping system head in ft measured by the differen-
tial pressure transmitter connected across the headers of
the pumping system
input kW power input to the pumping system (The watt transmitter
must be located ahead of any variable-frequency drive.)

Equation 10.2 demonstrates that the acquisition of the three values
of flow, head, and kilowatts can produce continuously the wire-to-
water efficiency of a pumping system. No allowance for changes in
specific gravity or viscosity need be made, since the kilowatt input
changes automatically with these conditions. Actual wire-to-water
efficiencies from a test program on a four-pump system are listed in
Table 10.7. This test is typical of those used to search for peak wire-
to-water efficiency for a four pump system.
Wire-to-water efficiency can provide a record of pumping perfor-
mance that enables the operator to compare the present operation
with that in the past when the system was new and properly
adjusted. This procedure allows the operator to determine if a partic-
ular pump is operating inefficiently; the operator can cycle various
pumps and record the differences in efficiency. The pump that is oper-
ating inefficiently can be taken out of service and repaired or cleaned.
Wire-to-water efficiency indication is a must for large or medium-
sized pumps in areas that have high electrical rates.
Wire-to-water efficiencies for a pumping system can be plotted
against system flow and used for system evaluation as well as control,
which will be described later in this chapter. Figure 10.13b describes
the wire-to-water efficiency curves for the five-pump system that was
described in Fig. 9.8c. This type of information can be secured directly
from a computer capable of curve plotting.

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