Page 337 - HVAC Pump Handbook

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Open Cooling Tower Pumps

334 Pumps for Open HVAC Cooling Systems

static pressure must be calculated to determine the working pressure
for these water boxes.

11.4.1 Remote cooling tower sumps
Many of the problems involved in operating cooling towers in cold
weather and with chiller condensers are solved through the use of
special cooling tower sumps. These sumps can be located indoors to
prevent freezing, as shown in Fig. 11.3b. When the sump is located
remotely from the tower, as in this figure, the added height Z must be
included in the pump total dynamic head. This indoor sump must
have a drain-down capacity to handle the water of the system when
the pump is stopped. Most tower manufacturers provide the drain-
down volume in gallons for their towers; to this must be added any
pipe capacity that is drained at the same time.
A sump that solves many application problems is the hot-well, cold-
well type of Fig. 11.4a. This sump offers freezing protection and also
separates the cooling tower loop from the process or chiller loop. This
provides maximum flexibility for modulating the heat load that is sent
to the cooling tower. The cooling tower receives the correct water flow
regardless of the load on the equipment being cooled. A hot-well pump
circulates the cooling tower, while the process or condenser pump cir-
culates the process equipment or the condenser on the chiller. In warm
climates where there is no concern about freezing, the primary-
secondary circuiting that is described later in this chapter eliminates
the need for this sump.
The concrete sump and turbine pump of Fig. 11.4b is the typical
installation for large cooling towers. A battery of turbine pumps oper-
ating in parallel can develop an efficient flow program with a sub-
stantial load range.
Large cooling towers require careful evaluation of the cooling tower
sump. The intake of water from the sump to the pumps requires cor-
rect design to avoid suction problems for the pumps.
The Hydraulic Institute has summarized the entire subject of
intake design in their ANSI/HI 9.8-1998 American National Standard
on Pump Intake Design. This document should be considered before
any serious design is initiated for pump intakes. They have intro-
duced a new structure known as a trench-type intake that may sim-
plify sump design for large cooling towers, Fig. 11.5. It is urged that a
designer of large cooling tower installations should have a copy of the
above standard to assist in the design of any type sump, whether rec-
tangular, circular, or trench configuration.
The exit of water from cooling tower sumps should be designed
carefully to avoid vortexing. Figure 6.16 should be reviewed to ensure

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