Page 168 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 165

typical submergences for vertical turbine pumps operating at 1750
rev/min; these submergences should not be applied to any specific
manufacturer’s pumps.

6.6 Pumping Energy
Following is a review of the various energy equations that must be
used in determining the energy requirements of a water system and
the energy consumed by pumps and their drivers in satisfying those
requirements. It is important that the HVAC system designer under-
stand the difference between the energy absorbed by the water and
that consumed by the pump or its motor.

6.6.1 Water horsepower
The energy required by a pump, as indicated by the affinity laws,
depends on its speed and the diameter of its impeller. The energy
imparted to the water by a pump is called the water horsepower
(whp). Its equation is
Q h s
whp (6.11)
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where Q flow, in gal/min
h head, in ft
s specific gravity

Most HVAC water systems operate from 32 to 240°F water, where
the specific gravity can vary from 1.001 at 32°F to 0.948 at 240°F. For
these applications, the specific gravity is generally assumed to be
1.00; systems designed to operate at 240°F must be able to function at
startup with water temperatures near 50°F or at specific gravities of
around 1.00. Water horsepower for these systems usually ignores the
specific gravity and assumes it to be 1.00. This should not be the case
with medium- and high-temperature water systems, where the oper-
ating temperatures can vary from 250 to 450°F. The specific gravity of
450°F water is 0.825 and should not be ignored in calculating pump
operating energy for these systems. Again, pump motor or driver
brake horsepower should recognize that the pump may be required to
operate with colder water and specific gravities near 1.0.
Following is an example of water horsepower: If a pump is deliver-
ing 1000 gal/min at a total head of 100 ft, the water horsepower is
1000 100
whp 25.25 whp
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