Page 142 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 139

GUARANTEED
EFFICIENCY POINT.

Figure 6.3 Total efficiency curve.

efficiency point occurs for each of the pumps involved on the water sys-
tem under consideration. It should be of interest to the HVAC system
designer as to how the efficiency curve of Fig. 6.3 is developed.
The efficiency of a pump at any flow is dependent upon a number of
conditions within the pump. The losses that exist in a centrifugal
pump and reduce the efficiency are: (1) mechanical, (2) leakage, (3) recir-
culation, and (4) hydraulic. Figure 6.4 describes these losses for a 12 in
double suction, volute-type pump with a specific speed of 1900 and
operating at a constant speed of 1750 rev/min. This diagram is called
a “power balance diagram” in the pump industry. The mechanical
losses are incurred primarily in the bearings and mechanical seals or
packing. The leakage loss, which is discussed in Chap. 5, is caused by
water flowing past the case rings from the impeller discharge back
the impeller suction.

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