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Open Cooling Tower Pumps

Open Cooling Tower Pumps 329

11.3 Cooling Tower Piping

The most basic and conventional cooling tower pumping system is
shown in Fig. 11.1a for one tower cell and one chiller condenser. This
configuration is for a constant-speed pump providing constant flow
through the chiller condenser. A three-way valve is included in the
return piping to the tower for cold weather operation; this is a two-
position nonthrottling valve. In very cold weather, the pipe to the cool-
ing tower may freeze when the water flow is in the bypass position for
a long period of time. An alternate way to pipe cooling tower bypasses
is shown in Figure 11.1b. The piping is arranged so that the velocity
head of the water is directed toward the bypass valve. Also, this valve
should be sized so that the pressure loss across the bypass valve and
the piping to the chiller sump is relatively low. This results in very lit-
tle water being backed up in the tower pipe. For example, if the
return pipe is 8-in steel carrying 1200 gal/min, the velocity head of
the water is 0.920 ft of head. If a 10-in bypass valve is installed, the
valve and exit loss to the tower sump should not exceed the velocity
head of the 8-in pipe. An advantage of this two-way valve is the fact
that the cooling tower piping drains when the condenser pump is
stopped.
Figure 11.1 describes relatively simple pumping installations, since
the flow through the tower is constant and equal to the needs of the
condenser on the chiller. The tower can be picked directly from a cool-
ing tower catalog or through computer software. The tower is sized to
the approach temperature and actual outdoor wet bulb temperature
at design conditions.
The cooling tower should be piped in such a way as to enable it to
achieve optimal performance. The piping also must aid the flow of
tower water to the using equipment. The water should leave a cooling
tower basin so that air entrainment or water vortexing is avoided.
Vortexing is discussed in Chap. 6.
Piping for cooling towers has been mostly steel. Recognizing the
great amount of oxygen carried by this pipe, many designers are now
using thermoplastic piping. This material is available in sizes up to
12 in in diameter. If steel pipe is selected for a cooling tower installa-
tion, the pipe should either be coated internally or the friction should
be calculated using a 1.2 multiplier on the Darcy-Weisbach data
included in Table 3.5.
It has been a tradition to install strainers on the suction side of
cooling tower pumps. As shown in Fig. 11.1, the strainer should be
installed on the discharge of the pump. The strainer is installed to
protect the tubes in a chiller condenser or process equipment such as
a heat exchanger; it is not there to protect the pump. The strainer in

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