Page 162 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 159

is no excuse for cavitation to occur in a normally operating HVAC
water system. Since the pump appears to be the HVAC equipment
most vulnerable to cavitation, it is a good place for the designer to
start the evaluation process needed to avoid cavitation.

6.5.1 Net positive suction head
Centrifugal pumps are tested very carefully to determine the net pos-
itive suction pressure that must be placed on the pump suction at
various flows through that pump to eliminate cavitation. This is called
the net positive suction head required (NPSHR) curve and appears in
Figs. 6.1a and 6.1b as the net positive suction head required by the
pump. Most of the centrifugal pumps used in the HVAC field cannot
pull water into the pump casing; it must be pushed. The NPSHR
curve merely defines the amount of push. If this amount of pressure
is maintained on the pump suction, cavitation will not occur in the
pump. Figure 6.1a provides the type of NPSHR curve found on most
pump curves; more precise NPSHR curves are shown in Fig. 6.1b.
Figure 6.15a describes the pressure gradient along the liquid path into
a pump.
How do we calculate the net positive suction head needed to avoid
cavitation? The actual NPSH on the pump suction must always be
equal to or greater than the NPSH required by the pump. The actual
suction pressure is called the net positive suction head available
(NPSHA), so
NPSHA ≥ NPSHR (6.7)

The problem for the uninitiated designer when calculating NPSHA
is the attempt to do so in absolute pressures (psia). If the calculations
are made in feet of head, they become much easier. This is reasonable,
because the NPSHR curve of the pump is always in feet of head.
Equation 6.8 is acceptable for HVAC operations at temperatures up to
85°F. The amount of “push” available, or net positive suction pressure
available, is simply the atmospheric pressure plus the system pres-
sure minus the vapor pressure of the water minus the friction of the
suction pipe. Figure 6.15b demonstrates this using the most common
point in HVAC systems for calculating NPSHA, namely, a cooling
tower. As shown,

NPSHA P P P P , in ft of head (6.8)
a s vp f
where P atmospheric pressure in feet at the installation altitude
a
(This can be done by reading the value directly from Table
2.1 for water from 32 to 85°F.)

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