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TRG-TRC016-EN 99
period four
Chiller-Plant Control
notes
Related to the issue of condenser-water-temperature control is the control of
condensing pressure. Every chiller requires a minimum refrigerant-pressure
difference between the evaporator and the condenser, in order to ensure that
refrigerant and oil circulate properly inside the chiller. This pressure difference
varies based on the chiller design and operating conditions. The chiller must
develop the required pressure difference within a certain amount of time, as
specified by the manufacturer, or the chiller controls will turn it off due to a
safety limit. During some start-up conditions, this pressure difference may be
difficult to achieve within the time required.
An example of such a condition is an office building that has been unoccupied
during a cool autumn weekend. The temperature of the water in the sump of
the cooling tower is 40°F [4.4°C]. Monday is sunny and warm, and the building
cooling load requires a chiller to be started. Because the chiller is operating
at part load and the tower sump is relatively large, the minimum pressure
difference may not be reached before the chiller is turned off on a safety.
If, however, the flow of water through the condenser is reduced, the minimum
pressure difference can be obtained. The lower flow rate increases the
temperature of the water leaving the condenser, which results in a higher
refrigerant pressure inside the condenser. After the minimum pressure
difference is reached, the flow may again be increased.
Either the refrigerant pressure in the condenser or the condenser-evaporator
refrigerant-pressure differential can be monitored and used to control the
temperature or flow rate of the condenser water, to prevent this pressure
differential from dropping below the limit.
Control of Condensing Pressure
condenser
condenser
control
control
panel
panel
evaporator
evaporator
Figure 107