Page 131 - HVAC Pump Handbook

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Physical Description of HVAC Pumps

128 HVAC Pumps and Their Performance

Figure 5.18 Axial thrust vs. rate of flow curves for axial-flow pumps.

assembly and the water pressure developed by the pump, most axial-
flow impellers exert a downthrust on the rotating assembly at the
design condition. Upthrust, if great enough, can create problems in
motor bearings. Figure 5.18 describes typical upthrust and down-
thrust curves for various axial-flow pumps with different types of
impellers and, therefore, different specific speeds. Generally, open
impellers are free of upthrust while enclosed impellers have pronounced
upthrust when they run out to capacities in excess of 120 percent of
flow at the best efficiency point (BEP). It is obvious from the curves of
Fig. 5.18 that great care should be exerted to avoid operation of pumps
with enclosed impellers at flows greater than 110 percent of that at the
BEP. The actual thrust curve should be provided with the purchase of
an axial-flow pump.
Radial thrust is not a problem in most axial-flow pumps, since the
flow of water from the impeller into the bowl exit should be uniformly

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