Refrigeration Service

Low refrigerant charge on refrigeration equipment.

Scotty McGregor
HVAC Oracle
2005-07-09

If the compressor and the condenser fan are short cycling and the cooler temperature is too warm then this may indicate a low refrigerant charge.

Basic Refrigeration Circuit

Most coolers are equipped with a low pressure control for compressor protection and/or temperature control. This will shut off the compressor when the evaporator pressure falls below the cutout setpoint. When the compressor is off the pressure will rise if the case temperature is warm and restart the compressor. The compressor then runs and drops the pressure down below the setpoint and shuts it off again. This is often a indicator of a low refrigerant charge, but can also be caused by a clogged filter. If it is the filter that is clogged, there will be a temperature drop across the filter. If the temperature drops more then 2F degrees from the filter inlet and outlet this indicates a restricted liquid filter/drier.

On systems that use a TX Valve there should be a sight glass to indicate the system charge. Look for the sight glass on the piping. Normally installed immediately downstream of the liquid line filter. When the system is fully charged and the filter is OK, the sight glass will appears clear after the compressor has run for a few minutes. If undercharged, there will be a steady stream of bubbles in the glass. It may look cloudy or show a steady steam of bubbles going by. It could appear to be half full of liquid. Bubbles will also appear in the sight glass if the liquid line filter is clogged or if the system is overheating and running with high condenser pressures.

If the system is low on refrigerant, the leak must be located and repaired before gas can be added. Look for the leak with liquid leak indicator, electronic leak detector or an ultra sonic leak detector. If these method cannot locate the leak, the remaining refrigerant will have to be reclaimed and the system tested with CO2 or Nitrogen at a higher pressure to locate the leak. Pressure testing will allow you to isloate the different section of the system by closing any available valves. The can tell you if the leak is on the condenser side of evaporator side. After the leak is repaired, the system should be pressure tested 1 to 2 hours on smaller systems, less than 10 lbs of refrigerant and up to 24 hours on larger system. This will indicate if there are any additional leaks. On older systems there are often more then one leak. All leaks must be identified and repaired. After the leaks are repaired, install a new liquid line filter/drier and evacuate to 500 microns or less.
A system with a small leak cannot be properly evacuated. Even the smallest leak will draw in moisture as it is evacuated and will not remove the mositure already in the system. Moisture will cause the eventual failure or the compressor.