Page 107 - HVAC Pump Handbook

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Basics of Pump Design

104 HVAC Pumps and Their Performance

case wear ring. This reduces the flow of water from the discharge area
with its higher pressure back to the suction connection with its lower
pressure. This unwanted water flow is called bypassing or suction recir-
culation in the pump industry. Certified testing, which is discussed
later, proves the quality of the pump designer’s work and the pump
manufacturing to produce the lowest amount of water bypassing at the
case rings and therefore the highest pump efficiency.

4.11 General Pump Design Information
This chapter offers some very preliminary information on pump design.
The pump types used in the HVAC industry are very standard designs
that have been available for many years. Much has been done to secure
maximum efficiency from these pumps. Like any other industry, there
are practices that reduce the quality of pump design and manufactur-
ing. Poor casting and finishing of impellers and volutes as well as
eliminating casing wear rings on some designs reduces the possibili-
ties of sustained, high pump efficiencies.
There is considerable additional information available in the pump
industry for the HVAC design engineer who desires to study pump
design further. Most of the pump manufacturers provide extensive
information on their pump designs. Hydraulic Institute maintains
pump design and testing standards that are available to water sys-
tem designers.

4.12 The Quality of HVAC Pumps
Over the years, the industrial and municipal pump industries have
been derisive of HVAC pumps, calling them “throw-away” pumps.
Such may have been the case in the past, but most of the pumps sold
for HVAC installations now are of a quality equal to those of other
industries. Energy is of concern in this industry causing development
of pumps equal in efficiency to other pumps. As in any pump applica-
tion, concern should be made to ensure the quality of the pumps.
Figures 4.4 and 4.5 should be reviewed to discern the difference
between quality and inferior pumps. A simple inspection of the pump
suction can reveal the difference. Figure 4.8a provides how the quality
of an end-suction pump can be determined by inspecting its suction.
The throat of the suction should transform smoothly to the internal
diameter of the impeller as shown in Fig. 4.8a. There should be no
abrupt change from the throat to the internal diameter of the impeller
as described in Fig. 4.8b. On large installations involving a number of

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