In cooling systems that use cooling towers, hot water (typically 80 to 90°F [27 to 32°C]) is circulated from the condenser/heat exchanger to the tower, where it is cooled by direct means and by evaporation. The tower’s cooling capacity depends on the amount of both air and water flowing through the tower.
Airflow usually is controlled with a fan. When the cooling demand is low, the
fan often is stopped, and cooling is achieved only by evaporation as the water
circulates. When the cooling demand increases, the fan restarts to provide
additional cooling. In some cases, the fan might be speed-controlled.
The water flow through the cooling tower usually is controlled with a single-
or three-way valve and a pump (non-controlled). The valve controls the flow
through the cooling tower and thus the cooling power according to the
temperature of the return water.
In the case of three-way valves, the pump operates with an almost constant
flow, regardless of the load in the tower. However, if two-way valves are used,
the pump will be throttled when the cooling demand is reduced. A more
energy-efficient solution would be to use an intelligent pump, which would
reduce power consumption when the system is only partially loaded and would
make the control valve superfluous.
An intelligent pump that has a built-in PID controller can be connected
directly to a temperature transmitter, thereby allowing the pump to provide
closed loop control of the cooling system. It should be noted, though, that to
ensure a sufficient function of the nozzles in the cooling tower, a minimum
flow is required. If the flow is too low, the nozzles cannot atomize the water
satisfactorily. Therefore, the minimum pump speed must be set to ensure that
this requirement is always fulfilled.
Controlling Flow in Cooling Towers
September 8, 2008