Page 292 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 289

Noncritical installations such as some office and commercial build-
ings do not require as much redundancy or standby as the buildings
mentioned in the previous paragraph. For example, if a commercial
building had a system head curve as shown previously for a hospital
(Fig. 10.10a), only two 50 percent pumps could be installed, since one
pump would have a maximum capacity of 1750 gal/min, or 70 percent
of the total system capacity. It would be a decision for the water system
designer or the owner of the building as to whether two or three pumps
would be installed. With the reliability and reduced wear of variable-
speed pumps, the two-pump installation would offer a desirable
arrangement for this office building.

10.7 Mixing Constant- and Variable-Speed Pumps

It should be understood that pumps in parallel must always operate
at the same speed! There may be some unusual, sophisticated cases
where parallel pumps are operated at different speeds, but only expe-
rienced pump designers should make evaluations for such a proposed
operation. Also, it is better on most HVAC operations to use pumps of
the same size when operating them in parallel. This ensures that the
operating pumps are producing the same flow, and this simplifies
evaluation of the efficiency of operation of the pumps. Small jockey
pumps that operate the system on low loads are not justified on most
systems. Variable-speed pumps should be controlled so that pumps
operating in parallel never have over a 1 percent difference in actual
operating speeds.
Mixing of constant- and variable-speed pumps is encountered in the
HVAC field, often with disastrous results. Following is a discussion of
the problems that can occur when a constant-speed pump is operated
in parallel with a variable-speed pump (use Fig. 10.11a to follow this
discussion):
1. Assume that one variable-speed pump is operating at point A.
2. Next, assume that the flow in the system increases until the
variable-speed pump operates at full speed at point B.
3. Let the system load now increase to point C, where one pump can-
not handle the load.
4. The system controls will turn on the next pump in an attempt to
maintain the system flow.
5. Let the next pump be a constant-speed pump of the same size as
the operating variable-speed pump. It will operate at point D
2
because it cannot vary its speed.

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