Page 318 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 315

accommodate the system when it is operating at 100 percent load. If
the transmitter is located at the far end of the loop, the distribution
loss of 80 ft is eliminated from the control signal. The transmitter can
be set at just 20 ft, the loss across the largest coil and its piping. The
pump speed can be varied to match the load on the system, thus sav-
ing appreciable energy by operating the pump at lower speeds. The
pump will operate with less radial thrust and have less repair.
The water system just described is of the direct return type; if it
were of the reverse return type, differential pressure transmitters
would be required at both ends of the distribution loop. Both the
direct and reverse return systems and their transmitter locations are
shown in Fig. 10.21a. With variable loads in a reverse return loop, if
all the loads are located at one end of the loop, the differential pres-
sure transmitter located at the other end will not provide adequate
control. One transmitter can be located at the center of the loop, but
one-half the distribution loss must be set into the control signal, thus
forcing the pump to run at a higher and less efficient speed. This may
be acceptable on small systems without much distribution friction,
but it would be totally unacceptable on the preceding system with 80 ft
of distribution friction. In this case, the differential pressure trans-
mitter would be required to be set at 60 ft, causing the pumps to run
at a much higher speed than when set at 20 ft with the differential
pressure transmitters at each end of the loop.
Along with the proper location of the differential pressure transmit-
ters, the transmitters must be interrogated properly by the control
system. As indicated earlier, the response time of the controller must
be fast enough to prevent pump hunting, but it must hold the differ-
ential pressure to an error signal of 1 foot or less. Any variation
greater than this for the controlling transmitter is unacceptable; if a
greater variation occurs, it indicates that the pumps are hunting, and
energy is being lost.
Following are several special applications of differential pressure
transmitters on hot or chilled water systems. All these applications
with 2 to 10 transmitters will utilize a low signal selection procedure
that will select the controller that has deviated the farthest from it
set point.
Figure 10.21b. A two-wing building usually requires a differential
pressure transmitter at the end of each wing.
Figure 10.22a. When the chillers, boilers, and pumps are located on
top of a high-rise building, the differential pressure transmitters
should be located at the bottom of the building to eliminate the dis-
tribution friction.

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