Page 315 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

312 The HVAC World

10.11.4 Pump speed control
All the preceding discussion has been on programming the pumps
efficiently. The second part of operating variable-speed pumps is con-
trol of the pump speed itself. Development of the variable-frequency
drive has greatly changed the design, selection, and operation of
pumps. The resulting variable-speed pump offers many benefits such
as reduced power consumption and lower radial thrusts on the pump
shafts. The variable-speed drives themselves have been reviewed in
Chap. 7. Contemporary speed control of a pumping system has
evolved over the past 25 years.
The basic control arrangement for these variable-speed pumps con-
sists of a water system sensor, a pump controller, and a variable-speed
drive (Fig. 10.20a). In most cases, the water system sensor can be for
(1) temperature, (2) differential temperature, (3) pressure, (4) differen-
tial pressure, (5) water level, (6) flow, and (7) kilowatts. Special sys-
tems may use other physical or calculated values for varying the speed
of the pumps.
The pump controller can be of many different configurations
depending on the response time required to adequately control the
pump speed without continual speed changes or hunting. With initial
use of variable-speed drives on HVAC systems, it was assumed that
since the loads did not change rapidly on these water systems, there
was no need for rapid speed regulation of the pumps. It was learned
during actual operation of these pumps on HVAC systems that rapid
speed regulation was required, particularly on systems where pres-
sure or differential pressure sensors were used to regulate the speed
of the pumps. Although the load on these systems was not changing
rapidly, pressure waves within the water systems created changes at
the sensors that caused speed changes. Also, with digital control,
after computation of the Propotional-Integral-Derivative error, an
output signal is given the variable-speed drive to increase or
decrease the speed of the pump. PID error for proportional-integral-
derivative control is the difference between the setpoint and the
process value. This increase or decrease continues until the next sig-
nal change. Attempts to reduce the sensitivity of the pump control
only caused increased variation in the actual pressure or differential
pressure at the point where the sensor was installed. Currently, 500 ms
has proved to be an adequate response time for most HVAC variable-
speed pumping systems. If HVAC variable-speed pumps continuously
change their speed, the response time is inadequate, or another control
problem exists! Properly controlled variable-speed pumps in this
industry should not change their speed so that the changes are
either visible or audible.

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