Trane TRG-TRC016-EN manual Control of Condensing Pressure

period four

Chiller-Plant Control

notes

Control of Condensing Pressure

condenser

control

evaporator panel

Figure 107

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.

TRG-TRC016-EN 99