Page 323 - HVAC Pump Handbook

Rishel_CH10.qxd 21/4/06 6:20 PM Page 320

Basics of Pump Application for HVAC Systems

320 The HVAC World

water temperature has no relationship to system load on variable-
volume systems; it should be maintained as closely as possible to a
constant value. For example, a chilled water system operating with
44°F supply temperature and 56°F return temperature should hold
these temperatures whether the load is heavy or light on the system. There-
fore, return temperature cannot possibly be used as a control parameter for
variable-speed pumping. Often there has been confusion of variable
return water temperature that results from the use of three-way
valves; these systems were, of course, constant-volume systems and
not used with variable-speed pumps for most applications.

10.11.6 Communication from
remote transmitters
The method of communication with remote transmitters is particu-
larly important to ensure that a noninterrupted signal is received at
the pump controller at the rate of response specified by the pump con-
trol manufacturer. The route that the signal takes is of importance to
the water system designer.
Most signals from remote transmitters are direct current and 4 to
20 mA in strength. These signals must be protected from electromag-
netic and radiofrequency interference. Therefore, the interconnecting
cabling must be shielded and grounded at the pump controller end.
Table 10.11 provides typical distances between the transmitter and
the pump controller that can be used with various sizes of wire.
These cables can be installed in conduit or cable trays that carry
other instrument, data, or telephone signals. They must be shielded
for electromagnetic and radio frequency interference, and the shield
must be grounded at the end of the cable near the pumping system
controller.
On some large installations, the cable installation for pump control
can become expensive; an alternate procedure is to install modems

TABLE 10.11 Maximum Linear
Distances for Transmitters from Pump
Controllers
Wire size, AWG Linear distance, ft
16 20,000
18 11,000
20 8000
22 5000
24 3500
26 2000

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