Determining the actual pumping efficiency

What is the proper method of determining the actual pumping efficiency of a semi-hermetic or welded reciprocating compressor?

First, you need an accurate set of gauges, an accurate clamp amp/volt meter and the correct specification sheet for the compressor with the associated refrigerant in question. These data sheets are available from the compressor manufacturer (usually on their web site).

Next, obtain the suction pressure at the suction service valve or service port of the compressor, the discharge pressure at the discharge service valve or service port on the discharge line of the compressor, and the operating amperage and voltage of the compressor. Obtain the voltage reading at the load side of the contactor while the compressor is operating.

Now take the suction and discharge pressures and convert them to the equivalent temperatures. Use these temperatures to plot the operating amperage from the appropriate specification sheet.

For a given suction and related discharge pressure, a resulting amperage will be drawn by the compressor motor. Amperage from the clamp-on amp meter should match that obtained from the spec sheet within 7 percent. If amperage is outside of those tolerances the compressor might appear to be at fault. However, continue the process and see what might cause the differences.

If the amperage is above the specification sheet value by more than 7 percent there are some factors that effectively elevate the amperage. These might include low voltage, internal friction, turn-to-turn short, high oil level and bad run capacitor (single-phase motors). Of all five indicators, only two would require compressor replacement.

It is important to remember the voltage rating of the specification sheet and the voltage you have read at the leads of the compressor motor. If they vary more than 10 percent, the over amperage should be corrected by correcting the low- or high-voltage situation. If you are within the 10 percent range, the current must be corrected by the following formula: Ve x Ae = Vm x Am

Ve = rated voltage

Ae = current at rated voltage.

Vm = measured voltage

Am = measured current

If the amperage is below the specification sheet value by more than 7 percent, other factors could be the cause. These might include broken rods, broken valves, bent valves, worn rings or a blown head gasket.

These factors would cause the compressor to be inefficient. Still, only two would require the replacement of the semi-hermetic compressor. Valve plates and associated gaskets are available at wholesalers for semi-hermetic versions. Hermetic compressors would require replacement.u



Wesley Taylor, CMS, is training manager, Carlyle Compressor; chair, RSES Journal technical review committee; and a 30-year-plus RSES member.