Page 161 - HVAC Pump Handbook

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HVAC Pump Performance

158 HVAC Pumps and Their Performance

the computer calculations will determine the impeller diameter that
provides the maximum wire-to-water efficiency throughout the load
range for the pump. These computer calculations include taking into
consideration the flow and head ranges of the water system as well
as the number of pumps provided in parallel for that system.
Further, the pump efficiency and the wire-to-shaft efficiency of the
motor/variable speed drive combination must be computed as the
water system conditions vary from minimum to maximum water
flow. All of these factors will be discussed in the chapters on pump
application.

6.5 Pump Suction Limitations

Centrifugal pumps operate on water, a liquid stream that can change
its state under certain pressure-temperature relationships. Some par-
ticular characteristics of water were included in Chap. 1 to help under-
stand these conditions. One of the first rules of thermodynamics is
that there is a temperature for every absolute pressure at which
water will change its state from a liquid to a gas. A little understood
word is cavitation. Cavitation is the result of the changing of part of a
water stream from a liquid to a gas. It occurs when the temperature
of the water reaches the evaporation temperature for the absolute
pressure of that stream. The specific gravity of the gas (steam) is
much less than that of the liquid water, so the result is “hammering”
as dense water and then “light” steam hit the internal parts of a
water system. Such hammering can erode the internal parts of the
impeller or casing of a pump. Cavitation can damage many parts of a
water system; the damage usually occurs first in a pump due to the
fact that the suction of a pump may be the point of lowest pressure in
the water system.
For example, assume that a hot water system requires 240°F water.
From Table 2.4, the vapor pressure for 240°F water is 24.97 psia or
10.27 psig with an atmospheric pressure of 14.7 psi. At least a 10-psia
cushion, or 21 psig, should be maintained on this hot water system so
that no part of the system approaches the vapor pressure of the 240°F
water. If the pressure does drop, cavitation will occur whenever this
hot water pressure falls below 24.97 psia.
How can cavitation be avoided in a water system? Simply by ensur-
ing that the water pressure in every part of the water system is
greater than the evaporation pressure for any potential temperature
of water in that system. Tables 2.3 and 2.4 provide vapor pressures
for water at different temperatures. There is so much information
available and tools such as the pressure-gradient diagram that there

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