This is also referred to as a compressor pump test. It is carried out when the functions of the compressor suction and discharge valves are suspect; this will be indicated by high suction and low discharge pressures. The test should be carried out with the plant running at an operating head pressure of at least 100 psig or 6.5 bar if possible in order to prove the efficiency of the valves.
The procedure is as follows:
1. Front seat the suction service valve and note the cut-out pressure of the low pressure control.-The control must be reset to its original cut-out pressure after the test.
2. Alter the range of the low pressure control so that the compressor is drawing a vacuum.
3. Reduce the low side pressure to at least 20 in Hg vacuum or 0.7 bar.
4. Stop the compressor and observe the pressure rise on the compound gauge for 2 minutes.
If the suction reeds are seating properly and cylinder/piston wear is not excessive, the pull-down to a vacuum should be rapid. The front seating of the suction service valve isolates pressure coming from the evaporator, and only the compressor crankcase is being evacuated of the refrigerant vapour. A compressor is deemed reasonably efficient if the 20 in vacuum is achieved. If it is not possible to draw this vacuum, then the suction reeds are defective.
When a 20 in vacuum is achieved, the pressure rise should be minimal; the entire vacuum should not be lost over the 2 minute observation period. When the vacuum is drawn and the plant switched off, if a rapid rise in pressure is observed on the compound gauge then refrigerant vapour is leaking into the cylinder(s) via the discharge valve(s). The valves must be inspected for distortion of reeds or faulty seating.