Page 324 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 321

at the remote differential pressure transmitter and at the pumping
system. This procedure allows the use of ordinary telephone cables for
transmission of the control signals.
Care should be taken to avoid routing the control signals through
building management systems that reduce the rate of response from
the transmitter; the signal and data load on these central manage-
ment systems may be such that the transmitter signal for pump
speed control cannot be processed faster than 2 to 4 seconds. This is
too slow for accurate pump speed control. This must not occur rou-
tinely or when the central management system is responding to emer-
gency conditions such as fire or building power failure.

10.12 Effects of Water Systems
on Pump Performance
The pump is the heart of every water system; when it fails, the sys-
tem fails. Unfortunately, many times pump failure is due to the water
system’s effects on the pump’s performance and physical condition.
The problems with cavitation and entrained air on pump performance
and damage have already been discussed in Chap. 6.
Other deleterious effects on pumps are control systems that force
pumps to run in the high-thrust areas, namely, at very low or high
flow rates. The pump wears rapidly, and it is not the pump’s fault.
One of the most disastrous practices in pump application is the
installation of a relief valve on a pump discharge that returns the
water to the pump suction, as shown in Fig. 10.24a. It should be
remembered that the thermal equivalent of a brake horsepower is
2544 Btu/h. All the energy destroyed by the relief valve is returned to
the pump suction. If the system is operating at low loads, where the
flow through the pump is low, it is very apparent that heat will build
until the pump can become very hot. Also, hot water will surge
through the system when the flow does increase.
Relief valves should be avoided wherever possible on chilled water
systems because every brake horsepower destroyed by the relief valve
adds one-fifth of a ton load on the chillers. If there is no other way to
control the water through the system than by a relief valve, it should
be connected as shown in Fig. 10.24b. The heat is returned to the
chiller, and the pump continues to receive and deliver chilled water.
Heating and cooling coils that are dirty on the air or water side or
are operating with laminar flow increase the system flow beyond the
design flow rates. Often pumps are blamed for their inability to provide
these greater flows; good system control is the answer for many system
problems that cause unnecessary pump wear and maintenance.

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