Page 321 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

318 The HVAC World

Figure 10.22b. This building is typical of reverse return systems
requiring a differential pressure transmitter at each end of the loop.
One differential pressure transmitter can be located at the center
point of the reverse return loop if the distribution friction of the sup-
ply and return loops is less than 20 ft.

Buildings that have a great amount of glass and a pronounced sun
load may require a differential pressure transmitter on both the east
and west sides of the building, regardless of the configuration of the
piping.
A number of control procedures are being developed to control
variable-speed pumps; some have been successful, and some have not.
Following are control techniques for loop-type variable-speed pump-
ing systems that have not been successful:

1. Locating the differential pressure transmitter across the pumps
instead of at the end of the loops. There is little reason for using
variable-speed pumps with this control procedure. The design pump
head must be the set point for the differential pressure controller;
therefore, there is very little reduction in pump speed, whatever the
load on the building.
2. Temperature control by valve stem position. This method of con-
trolling chilled and hot water systems can be expensive and difficult to
calibrate. The position of a particular valve stem has no relationship to
the flow of water passing through the valve. Figure 10.23a describes
why this is so. Valve A close to the pumps will have a greater flow at a
specific valve position than valve B at the end of the loop. It is difficult
to correlate an analog representing valve position to pump speed with-
out excessive hunting. Pumps and their controls are designed to
respond to pressure changes; this is why the differential pressure con-
trol method has been so successful over the past 25 years.
A combination of differential pressure control and valve position
reset is possible, as shown in Fig. 10.23b. The differential pressure
transmitter is still the principal means of controlling the pump speed,
but the differential pressure set point is established by the energy
management system, which changes the set point as the valves recede
from full-open position. This is uneconomical unless the valve position
of all the control valves is being sent to the building management sys-
tem as part of the standard control system.
A typical differential pressure–valve position schedule would be
Any valve fully open: 25 ft
All valves 90 percent open: 22 ft
All valves 80 percent open: 19 ft

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