Page 283 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

280 The HVAC World

high as 1210 or 1840 rev/min provides an additional load factor.
Reiterating, in all this work, the maximum pump horsepower must be
calculated to ensure that the motor and the variable-speed drive are
not overloaded. Likewise, it is wise to verify that the pump manufac-
turer approves the operation of the pump at the proposed speed.

10.3.1 Variable-speed pump operation
Most variable-speed pump operations in HVAC water systems track a
system head curve as was defined by Eq. 9.3 in the previous chapter.
Both the flow and head vary with this curve. As will be seen in the
chapters on chilled and hot water distribution, a remote differential
pressure transmitter is utilized to cause the pump head and flow to
follow the system head curve or the desirable point in the system
head area.
There are other useful variable-speed operations in the HVAC field,
such as constant flow, variable head. In this case, the pump flow
remains constant while the pump head and speed change with system
conditions. Typical of this is constant flow through a chiller condenser.
The head increases as more chillers are placed in operation as shown
in Fig. 10.4. The flow is a constant 2000 gal/min as the system head
increases and decreases. Such an operation can result in relatively
high pump efficiency, as shown in Fig. 10.5.
This figure describes the pump efficiency, as it cuts across the
curves of constant efficiency. In all variable-speed operations, this
should be observed to ensure that the pumps are operating at the best

80 1175 RPM
70% 75% 80% 83%

60 1000 RPM 86% 87% Bep curve
Pump head—ft. 40 800 RPM 83%
86%

Individual
efficiency
20 lines of

0
0 400 800 1200 1600 2000 2400 2800 3200
U.S. Gallons per minute
Figure 10.4 Constant flow, variable head process.

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