Page 164 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 161

P static head of water level above the pump impeller (This
s
is negative if water level is below the impeller.)
P vapor pressure of water, in feet, at operating temperature
vp
(see Tables 2.3 and 2.4)
P friction of suction pipe, fittings, and valves, in feet of head
f
Equation 6.8 can be used for higher temperatures and elevations.
Assume the following:
1. The maximum cooling tower water temperature is 95°F, so P
vp
1.9 ft of vapor pressure. From Table 2.3.
2. The cooling tower sump is above the pump impeller, and P 5 ft.
s
3. The installation altitude is 1000 ft; From Table 2.1, the atmospheric
pressure, P = 32.8 ft.
a
4. The friction loss of the suction pipe, fittings, and valves P is 6 ft.
f
Thus

NPSHA 32.8 5 1.9 6 29.9 ft
Any pump handling this water must have an NPSHR of less than
29.9 ft at any possible flow through the pump.
Warning. Calculation of NPSHA for a cooling tower should be made
at the maximum possible operating water temperature in the tower, not
the design water temperature.

6.5.2 Air entrainment and vortexing
A companion to cavitation is air entrainment in the suction of cen-
trifugal pumps. Air can enter the water system at several points, as
shown in Fig. 6.16a. The effect of air entrainment on pump perfor-
mance is described in Fig. 6.16b; it indicates the drastic reduction in
both head and capacity when air is present. Every effort must be
made to ensure that the water in a pump is free of air.
Air entrainment is often confused with cavitation when drawing
water from a tank. It is thought that air entrainment cannot happen
when a tank of water is 10 to 15 ft deep. Air entrainment can occur eas-
ily when water is taken from a free surface of water regardless of the
depth of the water. Figure 6.17 describes the air entrainment that can
occur if water is not removed properly from an open tank. As shown
in Fig. 6.17a, a small whirlpool occurs on the surface, and this deep-
ens into a vortex that will extend down to the water outlet from the
tank, and finally, air will pass into the impeller of the pump, causing

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