REFRIGERANT CHARGE

  Refrigerant R-12 is used in most transport system at the present time, but R-502 is well suited for low temperature applications, and its use is increasing. Since R-502 creates a greater power requirement for a given compressor displacement than R-12, the motor-compressor must be properly selected for the refrigerant to be used. Different expansion valve are required for each refrigerant, so the refrigerant are not interchangeable in a given system and should never maximum working pressures than those used with R-12, so normally it is not feasible to attempt to convert an existing R-12 unit for the use of R-502.

 The refrigerant charge should be held to the minimum required for satisfactory operation. An abnormally high refrigerant charge will create potential problems of liquid refrigerant migration, oil slugging, and loss of compressor lubrication due to bearing washout or excessive refrigerant foaming in the crankcase.

  Systems should be charged with the minimum amount of refrigerant necessary to insure a liquid seal ahead of the expansion valve at normal operating temperatures. For an accurate indication of refrigerant charge, a sight glass is recommended at the expansion valve inlet, and a combination sight glass and moisture indicator is essential for easy field maintenance checking. It should be born in mind that bubbles in the refrigerant sight glass can be caused by pressure drop or restrictions in the liquid line, as well as inadequate nominal working charge data should be used only as a general guide, since each installation will vary in its charge requirements.