Page 334 - HVAC Pump Handbook

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Open Cooling Tower Pumps

Open Cooling Tower Pumps 331

the cooling tower sump is normally adequate to keep rocks and other
large debris out of the pump. A suction strainer on a cooling tower
pump hazards a chance that the strainer could be obliterated with
algae; the pump could then overheat and be destroyed before it could
be stopped. Installing the strainer on the pump discharge protects the
pump. If the strainer becomes clogged, the pump will be protected
with only a small flow through the strainer. The equipment being sup-
plied by the cooling tower pump will indicate flow problems long
before the strainer is totally clogged.

11.4 Location of Cooling Tower Pumps

Cooling tower pumping is not severe duty for centrifugal pumps, but
applying pumps to cooling towers is much more detailed than most
HVAC pump installations. This is caused by the cooling tower loops
being open-type water systems with the possibility of oxygen, air-
borne dirt, and chemicals existing in the water.
Every cooling tower installation should be checked to ensure that
adequate net positive suction head (NPSH) available exists for the
pumps selected for that cooling tower. Chapter 6 provides information
on NPSH and how to calculate the available NPSH. Pumps should
not be selected for cooling tower duty without knowing the NPSH
required by them.
As the NPSH available equation (Eq. 6.8) indicates, the friction in
the suction pipe to end-suction and double-suction pumps should be
kept low. The pumps should be located as near as possible to the cool-
ing towers; the suction pipe should contain a minimum of pipe fit-
tings such as elbows (Fig. 11.2a). If there are doorways between the
cooling tower and a chiller, the pump should be located near the cool-
ing tower to avoid a loop in the suction piping (Fig. 11.2b). If the cool-
ing tower location is such that it is difficult to provide adequate suc-
tion conditions for end-suction or double-suction pumps, an alternate
is the use of turbine-type pumps located in the cooling tower sump.
The cooling tower should not be planned for installation at a specific
point with the hope that there is a way to pump the water from it.
Pumping the cooling tower should be settled before final location is
made for the tower.
Cooling tower pumps should not necessarily be located on the sup-
ply side of chiller condensers. On high-rise buildings where there is
adequate NPSH available under all load conditions, the condenser
pumps can be located on the discharge of the condenser, as shown in
Fig. 11.3a. This reduces the pressure on the condenser water boxes and
may eliminate the cost of higher-pressure construction. The maximum

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