Service diagnosis and repairs:Compressor efficiency test

Compressor efficiency test

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.

Removing compressor valve plate assembly

On most compressors this will be a relatively simple task, but the procedure will differ with compressor design. The following is the procedure for a compressor with the valves in the head:

1 Start the plant, front seat the suction service valve and reduce the crankcase pressure to 3 psig or 0.2 bar if possible.

2 Stop the plant, isolate electrically and front seat the discharge service valve.

3 Slacken off the compressor head bolts slowly to release the high pressure from the discharge side of the head. Remove the bolts.

4 Gently raise the compressor head, with suction and discharge lines intact, sufficiently high to be able to withdraw the valve plate assembly. Care must be taken to avoid fracture of the pipework unions.

5 If the cylinder head and valve plate gaskets are damaged when the valve plate is removed, they must be replaced.

6 Distorted or broken reeds will obviously have to be replaced. Some manufacturers will recommend replacement of the entire valve plate assembly; replacement kits are available complete with gaskets.

Should the valve seats on the valve plate be eroded or pitted and a replacement is not readily available, they can be made serviceable by reseating or lapping. This is generally regarded as a workshop practice. The method is as follows:

1 Obtain some valve grinding paste or carborundum powder and some polishing compound. Spread a liberal amount of the paste on to a lapping block or a hard flat surface such as a polished steel plate or sheet of glass.

2 Remove the discharge reed retainers and reeds from the valve plate.

3 Place the valve plate on the pasted surface, exert a firm even pressure and move the valve plate in a figure-of-eight motion. Continue until the valve seats are returned to an original finish. When carborundum powder is used it should be mixed with refrigeration oil.

4 When reseating is complete, remove all traces of carborundum and paste from the valve plate with spirit or paraffin.

5 Repeat the operation with the polishing compound and clean as before.

6 Reassemble dry with new reeds if required. If the original reeds are used, they should be inverted so that the seat contact is made to the unused side of the reed.

The procedure for replacing the valve plate assembly is as follows. If gaskets are re-used they should be perfectly clean and dry.

1 Once the valve plate is located, replace the head bolts and screw them down finger tight.

2 Tighten the bolts diagonally across the head, care being taken not to overtighten and strip the threads.

3 Crack off the suction and discharge service valves from the front seat positions.

4 Leak test the compressor and pipework unions.

5 Purge the compressor through the gauge port unions.

6 Set the service valves to the operating positions and start the plant.

7 Operate the plant and observe pressures; these should now be normal.

8 Carry out the compressor pump test.

9 Reset the low pressure control.

10 Remove gauges, replace valve seal caps and wipe the compressor clean of oil.

11 Make a final leak test.

12 Clear the site.