A purge unit is typically used to remove air and other non-condensables from the refrigeration system. Moisture is typically removed by a filter dryer. Referring to a pressure-temperature chart for R123, a common refrigerant used in lowpressure systems, for a common ambient temperature in a chiller room of 70 degrees a
corresponding pressure of minus 6.6 inches of vacuum is indicated.
When the chiller is off, the entire chiller is at this temperature and pressure; which means that, if there is a leak anywhere on the machine, air is entering the chiller. When the chiller is operating,
a common temperature in the evaporator would be 38 degrees with a corresponding pressure of -18.7 inches of vacuum.
The condenser of this system would have a temperature of approximately 96 degrees which would have a corresponding pressure of +4.6 psig. When the chiller is operating, and if leaks are in the evaporator or lowpressure sections, air would be introduced into the system; however, if the leaks are in the condenser or compressor sections refrigerant would leak out of the system due to the positive pressure in these sections.
