Electric or mechanical expansion devices lose capacity as the pressure drop across them decreases.

The percent loading of a valve with low condensing pressure is greater than it would be with high condensing pressure. For instance, an expansion device — electrical or mechanical — has roughly twice the capacity with 200 psig drop (50% loaded) as with a 50 psig drop given the same entering liquid temperature.

A rule of thumb on selecting valves for minimum loading is as follows:

Conventional - 35%

Balanced port - 25%

Electronic - 5%

With air conditioning systems, the compressor is usually the limiting factor of how far head pressure can be lowered, not the expansion device. There is a minimum compression ratio (absolute head pressure divided by the absolute suction pressure) that is determined by the compressor manufacturer and is application specific. Attempting to drop head pressure below that minimum ratio may result in damage to some compressors. The manufacturer of the compressor should be consulted to determine the minimum head pressure for the application.

Capacitors

QUESTION:
By John West
Chesterfield, VA

My question involves capacitors. I understand how to check a capacitor for open-shorted and grounded.

Would it be possible to use the voltage rating on the capacitor to determine if it is going to fail? Would you be able to go to common on the compressor and the hermetic side of the capacitor to see if you are achieving the rated voltage? Would this have to be checked with the unit running or could you check it while the unit attempts to start?

My understanding is to match capacitor ratings exactly. Years ago there was a 10% rule.

ANSWER
From Dan Kramer, P.E.
Specialist Grade Member of RSES,
Professional Engineer

You have two questions:

1. You propose observing the voltage from common to the hermetic side of the capacitor (I believe that would be the voltage across the start winding) and comparing that observed voltage with the capacitor voltage rating both during starting and during running.

Of course, the voltage across either the start winding or the start capacitor could be measured only during the brief starting period. This might be only a fraction of a second. Unless that transient voltage was measured with an oscilloscope, it would not be reliable.

Assuming that the capacitors have been correctly selected initially, I do not believe it would be possible to predict the failure of either start or run capacitors by comparing the voltage across the capacitor with the capacitor voltage rating. It might be possible to predict capacitor failure if the capacitor voltage ratings were lower than the normal starting voltages, but that would suggest that the wrong capacitors had been applied initially.

2. When replacing failed capacitors, it is important to use capacitors having voltage ratings equal to or higher than those recommended by the motor manufacturer. Electrolytic capacitors, the type used in the start capacitor applications, generally have a wide capacity tolerance, possibly in the ±50% range from the capacitor nameplate. While the capacity of the start capacitor affects the motor starting torque, this wide capacitor capacity tolerance appears to be tolerable. The metal can capacitors used for the run applications, I recall, are made with a ±10% tolerance.

Capacitors are cheap compared with the replacement cost of a motor. If your capacitor tester shows a low internal resistance or a microfarad capacity that is much different from the rating, I would simply change the capacitor.

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Publication date: 12/02/2002