Page 310 - HVAC Pump Handbook

Rishel_CH10.qxd 21/4/06 6:20 PM Page 307

Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 307

4. By emergencies caused by pump failure
5. Duty sharing such as alternation

Pumping systems can be interfaced easily so that they can be
started or stopped by the building management system. When a
chilled or hot water system is actuated, any pumping system involved
can be started automatically at that time.

10.11.1 Sequencing of pumps on system
demands of flow or pressure
Multiple-pump systems require care in the addition and subtraction
of pumps in order to achieve maximum pumping efficiency from mini-
mum to maximum system flow. Items 2 and 3 mentioned previously
list the procedures used for accomplishing this.
Which procedure is used to sequence multiple pumps can have a
great bearing on the efficiency of the pumping system. There are sev-
eral methods of sequencing pumps, and these are:
1. Maximum flow sequencing. This old method let one pump run
until it could no longer handle the system requirements; the controls
then started the next pump in the pumping sequence. This has proved
to be wasteful in energy because the pump operates far to the right of
the design point at a poor efficiency. Also, the pump then operates at a
point of high radial or axial thrust with greater wear on bearings,
sleeves, and mechanical seals. Maximum flow should be used as a
backup control system to maintain system flow on failure of more effi-
cient control systems, such as best efficiency or wire-to-water efficiency
control. Arbitrarily selecting a percentage of speed of the operating
pumps such as 80 percent to add and subtract pumps does not achieve
peak efficiency; this is an attempt to control pumps without evaluating
all the system characteristics that determine operating efficiency.
2. Best efficiency control. Recognizing the problems with maxi-
mum flow sequencing, this procedure was developed to eliminate the
preceding inefficient and high-thrust operation with maximum flow
sequencing. It has been called best efficiency control because the
pumps are sequenced so that they operate as closely as possible to
their best efficiency point. This is achieved by evaluating the system
head for the proposed system and selecting pump sequencing so that
the pumps do not “carry out” but operate near their best efficiency
point. This is demonstrated in Fig. 10.17, a system with two pumps
operating at design load. With maximum flow sequencing, one pump
could operate until 167 percent capacity of one pump was reached; at
this point, the pump efficiency would be very low compared with that

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.