Page 341 - HVAC Pump Handbook

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Open Cooling Tower Pumps

338 Pumps for Open HVAC Cooling Systems

one, requiring wire-to-water efficiency analysis and first-cost calcula-
tions. The height of the larger cooling towers alone may dictate the
use of below-grade sumps with turbine pumps. The wire-to-water effi-
ciency analysis of Chap. 9 should be applied to the larger condenser
pumps to achieve the pumping system overall efficiency and the
energy input to the condenser pump motors throughout the load
range of the chillers.

11.6 Special Condenser Water Circuits
The advent of the variable-speed pump has introduced new and inno-
vative methods of pumping chiller condensers. The age-old problem of
operating the chiller and cooling tower at their desired flow rates has
been eliminated by the use of so-called primary-secondary pumping,
as shown in Fig. 11.7. The same procedure as primary-secondary
pumping for hot and chilled water systems is utilized with constant-
speed cooling tower pumps and variable-speed condenser pumps. The
constant-speed cooling tower pumps pressurize the supply to the
variable-speed condenser pumps so that there are no NPSH problems
with the condenser pumps. This pumping arrangement also puts all
the static head of the cooling tower on the tower pumps, letting the
variable-speed pumps operate on a total friction head. This may provide
a higher overall wire-to-water efficiency for the condenser pumping
installation. This procedure also allows the cooling towers to oper-
ate at their optimal flow rates as the water is varied through the
condensers.
Continued experience with variable-speed condenser pumps has
proved that variable flow can be accomplished in most open cooling
towers with proper control of variable-speed cooling tower fans. Most
cooling tower manufacturers can provide a flow range of water
through the tower with reduced airflow. Obviously, the water flow rate,
in percentage, should always be greater than the airflow percentage.
For example, one manufacturer allowed a reduced water flow rate of
30 percent of design with a minimum fan speed at an airflow rate of
25 percent of design. Such variations in water and airflow should be
approved by the cooling tower manufacturer on each application of
cooling towers.
The number and size of the variable-speed pumps for the condenser
flow can be selected for the most efficient installation. On individual
chiller applications where a variable-speed pump is supplied for each
chiller, the condenser pump can have its speed varied by the lift pres-
sure or the refrigerant pressure difference between the high and low
sides of the chiller (Fig. 11.7a). On an installation that consists of a

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