Page 303 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

300 The HVAC World

TABLE 10.7 Wire-to-Water Efficiencies from an Actual Test Program

No. of System flow, System Pump Wire-to-water
pumps gal/min head, ft rev/min Input, kW efficiency, %
1 2475 28.3 1170 34.6 47.4
2 2475 28.3 852 30.7 53.4*
2 3960 42.7 1123 62.6 58.4
3 3960 42.7 977 62.4 58.6*
3 4950 55.4 1124 91.7 62.5
3 7425 97.0 1518 211 67.0
3 7425 97.0 1394 210 67.3*
4 9900 153.6 1759 404 71.7
*Transition points from one to two, two to three, and three to four pumps for peak wire-
to-water efficiency.

10.10 Kilowatt Input for a Pumping System
Wire-to-water efficiency can be calculated using Eq. 10.2 and it can be
displayed on the pump controller for the pumping system. However,
wire-to-water efficiency instrumentation requires a differential pressure
transmitter located across the pumping system headers and a system
flow meter, as shown in Fig. 10.13a. Three instruments, and therefore,
three instrument errors are involved. There are many applications of
this instrumentation that have proved it to be an effective method of
evaluating the overall operating efficiency of a pumping system.
Field experience has proved that total kW input for a pumping sys-
tem provides a better indicator and means of programming multiple
pump systems, particularly variable-speed pumps. The only instru-
mentation required for this is the watt transmitter and flow meter as
shown in Fig 10.14. This simplifies the equations for evaluating
energy input to a pumping system and results in one equation for all
types of systems.
Q h
kW input (10.3)
5308 or
P E WW
where Q Water system flow, gal/min
h Water system head, ft
Pump efficiency
P
Motor efficiency for constant-speed pumps
E
Wire-to-shaft efficiency for variable-speed pumps
WW
Kilowatt input can be calculated at all loads and with different
numbers of pumps in operation. There is no requirement for deter-
mining the friction loss through the pump fittings or dividing the
total system flow by the number of pumps in operation. The operator

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