Civil Engineering Reference
In-Depth Information
Chiller+condenser fan
Optimal point
Chiller
Condenser fan
Air flow rate
Figure 9.11
Power consumption vs. air flow rate in an air-cooled system.
optimal point of fan speeds to be controlled is that which allows the total
power consumption of chillers and condenser fans at the minimum at a cer-
tain load and ambient air temperature. Figure 9.11 illustrates the relations
between the air flow rate and power consumption in air-cooled systems with
variable-speed fans.
9.5.3 Cooling tower optimization
Control of cooling tower supply water temperature:
To minimize chiller
energy use, the condenser water supply temperature set-point (i.e. the cooling
tower supply water temperature) should be as low as possible. However, the
control set-point should be at or above the lowest temperature attainable by
cooling towers at a certain air (wet-bulb) temperature to avoid the waste of
fan energy trying to reach an unobtainable value. As the difference between
these temperatures increases, the cost of operating the cooling tower fans
drops and the costs of pumping and compressor operations increase.
The cooling tower fan speed control is the usual method of maintaining the
pre-set cooling tower supply water temperature (as shown in Figure 9.12).
In cool weather or partial load, a tower bypass valve can be used to further
reduce the cooling capacity when the fan speed is at its low limit. Besides the
application of variable speed fans which can increase cooling tower energy
efficiency at partial load, the cooling tower fans may be multiple (two or
three) speed fans and centrifugal units with variable blade pitch which can
provide continuous throttling.
Aims of cooling tower supply water temperature optimization:
Optimizing
the cooling tower supply water temperature is an important part of chiller
optimization. A decrease in the cooling tower water supply temperature
tends to reduce the cost of chiller operation but it increases the amount of
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