Page 117 - HVAC Pump Handbook

Rishel_CH05.qxd 21/4/06 6:12 PM Page 114

Physical Description of HVAC Pumps

114 HVAC Pumps and Their Performance

Figure 5.6 Horizontal, self-priming, volute pump, flexible coupled for clear service.
(Courtesy Gorman Rupp Co.)

equal size. The impellers are usually of the open type without casing
rings, which reduces the overall, lasting efficiency of this type of
pump.
This pump is designed to operate on reasonable NPSH available
conditions and can handle gases; therefore, it can be self-priming. The
maximum dry-priming lift conditions for these pumps are usually
shown on the head-capacity curves for them. The maximum allowable
suction lift is the allowable lift or height of the pump above the mini-
mum tank water surface. Under this condition, the pump is assumed
to be dry. This means that within these lift conditions, the pump will
prime itself as long as the NPSHR conditions are not exceeded.
Manufacturers of self-priming pumps should provide information on
the priming, repriming, and suction lift capabilities of their pumps
before they are applied to an actual installation.
The suction pipe for most self-priming pumps should be the same
size as the pump suction. This adds friction loss to the pumping
installation and must be charged to the friction loss of the pump
fittings. Due to this and the overall efficiency of self-priming pumps,
the wire-to-water efficiency for such pumping systems is much lower
than that for other centrifugal pumping systems of equal capacity
and pump head.

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.