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Open Cooling Tower Pumps

Open Cooling Tower Pumps 327

falling water cascades over various types of fill or media that are
specifically designed to slow the water fall rate and thus increase the
time that the water is in contact with the air; this increases the evap-
orative process. There are special types of cooling towers that use nei-
ther media (fill) or air-moving systems (fans). The largest cooling tow-
ers without fans are the hyperbolic towers most often associated with
electrical generating plants. The most basic categorization of towers is
by the method used for air movement through the tower, whether nat-
ural or mechanical draft.
Cooling towers also can be characterized as to whether they are
factory assembled or field erected and can be divided into various
shapes such as rectilinear, round, or octagonal. Some cooling towers
are described by their basic material of construction, such as ceramic
towers.

11.1.1 Cooling tower fans
Cooling tower fans, until recently, were always constant speed or two-
speed. The availability and cost of variable-speed drives has made them
economical on many cooling tower installations. Some of the operating
problems with cooling towers have disappeared with the ability to
adjust the fan speed to the actual operating condition. This has
resulted in more precise control of the leaving water temperature.

11.2 Water Flow Conditions for Cooling Towers
A cooling tower has a very specific range of flow that provides accept-
able performance. This flow range can be only 80 to 100 percent of
design flow on most cooling towers. Some towers can have their flow
ranges extended by the manufacturer making modifications in the
cooling tower’s water distribution system. If the actual flow rate is less
or greater than the specified range, operating problems will develop;
the water flow through the tower can become nonuniform with uneven
airflow. It is the responsibility of the designer to ensure that the flow
rate of the water system meets the needs of the cooling tower.
Matching the pump to the cooling tower may compromise other
design requirements such as the flow rates required by chiller con-
densers. It must be recognized that this dichotomy exists, and the
water system design must satisfy both the flow rate of the using equip-
ment and that of the cooling towers. Revelation of this fact has led to
the development of a number of different chiller sumps and pumping
arrangements that satisfy both the tower and the equipment using the
tower water. Some of these solutions are provided later in this chapter.

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