Page 132 - HVAC Pump Handbook

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Physical Description of HVAC Pumps

Physical Description of HVAC Pumps 129

distributed around the 360 degree area of the bowl. The pump
designer provides adequate bearings in the bowls and column to
ensure that any unevenness in flow will not disturb the rotating ele-
ment of the pump.
Such is not the case with volute pumps where the radial forces on
the impeller vary greatly at various points in the volute. This imbal-
ance varies with the flow of water through the pump. On most volute
pumps as shown in Fig. 5.15, the radial thrust is highest at the point
of zero flow in the pump. This is demonstrated in Fig. 5.19 that
describes typical radial thrust for a volute pump at 1,780 rev/min and
at 1,150 rev/min. This demonstrates that radial thrust is reduced
appreciably with reduction in pump speed. Therefore, double-volute
construction, Fig. 4.2c, may be needed for constant speed pumps that
run continuously at full speed and high radial thrusts. There may be
no need for it on variable speed pump applications where the pumps
run most of the time at lower speeds and, therefore, with very little
radial thrust.
Figure 5.19 is one of the most important figures in this book for the
water system designer and user of volute-type pumps. Is it not obvious
that it is imperative that these pumps be selected at capacities of not

Figure 5.19 Radial thrust curves for volute pumps.

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