If a lead compressor is defined, the pump-down function can be enabled. In this explanation a VSD compressor is selected as lead compressor. This compressor will be the first compressor to start and the last to stop. The lead compressor will stop when the “Requested capacity” reaches “0 %”. The “Control temperature” will be lower than the “Reference” (“Setpoint”).
It is possible to set up a “Pump-down” function which will allow the lead compressor to keep on running until it has reached the “Pump-down limit Po” which should be set to a value somewhat lower than the “Setpoint”. Still the “Requested capacity” has to reach “0 %” before the VSD compressor stops.
Pros and cons using pump-down:
The lead compressor will keep running for a longer period which could lead to fewer stops/ starts. By keeping the lead compressor running at lower suction pressure, the COP of the application will suffer. This will increase the power consumption.
Capacity control can be done using an S4 media temperature sensor instead of the pressure transmitter. If S4 is used, the pressure transmitter is only used as safety e.g. as frost protection.