Page 317 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

314 The HVAC World

Most pump controllers are now based on digital electronics and are
of the proportional-integral-derivative or digital type, which sense a
signal, compute the error, and output the speed signal to the variable-
speed drive. Since the speed of the pump is held at a discrete point by
such a controller until it is updated, there can be continuous speed
fluctuation when rapid control response is not provided. Standard
commercial PID computer chips are available with a rate of response
as fast as 10 to 500 ms. Rate of response should not be a problem on
variable-speed HVAC pumps due to the digital technology that is now
available for the control of such pumps. Any variable-speed pump that
has continuous speed change is not being controlled properly. It may
have too slow a speed response from the transmitters that are con-
trolling the pump speed.
Although standard commercial computer chips are designed for
proportional-integral-derivative control, most HVAC applications do
not require any derivative value inserted in the actual control algo-
rithm. Also, proportional-integral controllers of the analog type are
seldom used in this industry.

10.11.5 Sensors for pump speed control
The preceding description of basic variable-speed control listed the
types of sensors used for pump operation. Some HVAC systems
require more than one sensor to maintain the proper speed of the
pumps. This is due to shifting heating or cooling loads on the water
system. The standard signal-selection technique is utilized to accom-
modate more than one transmitter (Fig. 10.20b). The controller selects
the transmitter at which the signal has deviated the farthest from its
set point. Following is information on the selection and location of
sensors or transmitters.
The location of the sensors is important to ensure that the pumps
can be operated at the lowest possible speed from minimum to maxi-
mum system load. See Chap. 10 for proper sensor location for specific
types of water systems.
When pressure or differential pressure is used to control pumps,
these transmitters must be located so that the distribution friction
pressure loss of the system is eliminated from the signal. For exam-
ple, assume that a distribution system for a hot or chilled water sys-
tem consists of differential pressure with direct return piping and has
a 20-ft loss across the largest coil, its control valve, and piping. Also
assume that the distribution friction loss out to and back from the
farthest coil is 80 ft. The total pump head is therefore 100 ft. If the
pressure transmitter is located at the pump, it must be set at 100 ft to

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