Refrigerant Charge

Refrigerant Charge

Refrigerant R-12 is used in most transport systems at the present time, but R-502 is well suited fot 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 valves are required for each refrigerant, so the refrigerants are not interchangeable in a given system and should never be mixed. receivers for R-502 require higher maximum working pressure than those used with R-12, so normally it is not feasible to attempt to convert an existing R-12 unit fot 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.

System 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 chexking. 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 liquid subcooling. Manufacturer's published nominal working charge data should be used only as a genral guide, since each installation will vary in its charge requirements.

Refrigerant Migration

Refrigerant migration is a constant problem on transport units because of the varying temperatures to which the different parts of the system are exposed. On eutectic plate aplications, liquid refrigerant will be driven from the considensing unit to the plates during the day's operation, with the threat of floodback on start-up. On both plate and blower units not in operation, the body and evaporator inmmediately after operation will be colder than the condensing unit, causing migration to the evaporator . During daytime hours the body and evaporator will warm up, and because of body instasulation will remain much warmer than the compressor during the night hours when the ambient temperature falls, resilting in a pressure differential sufficient to drive the refrigerant to the compressor crankcase.

Excessive refrigerant in the compressor crankcase on star-up can cause slugging bearing washout, and loss of oil from the crankcase due to foaming. Dilution of oil with excessive refrigerant result in a drastic reduction of the lubricating ability of the oil. Adequate protective measures must be taken to keep migration difficulties at a minimum. Consideration shoul be given to keeping the refrigerant charge as low as possible, using a pump down cycle, use of a suction accumulator, and the use of a liquid line solenoid valve.

System Balance and Refrigerant

System Balance

If the compressor or condensing unit selected for a given application is to satisfactorily handle the refrigeration load, it must have sufficient capacity. However, over capacity can be equally as unsatisfactory as under capacity, and care must be taken to see that the compressor and evaporator balance at the desired operating conditions. Checking the proposed system operation by means of a compressor-evaporator-condenser balance chart as describet in selection 16 is recommended.

If fluctuations in the refrigeration load are to be expected, which could result in compressor operation at excessively low suction pressures, then some means of capacity control must be provided to maintain acceptable evaporating temperatures. If compressors with unloaders are not available or suitable, and if the load cannot be adequately handled by cycling the compressor. a hot gas bypass circuit may be required.



Refrigerant

Copeland compressors are primarily designed for operation with refrigerants 12, 22, and 502. Operation with other refrigerants in cascade systems may be satisfactory if the proper motor and displacement combination is selected, adequate lubrication can be maintained, and if adequate compressor protection is provided. All applications with refrigerants other than R-12, R-22 and       R-502 must be approved by the copeland aplication Engineering Departament.

R-22 is highly recommended for all single stage low temperature applications, and particularly where evaporating temperatures of -20 F. and below may be encountered. Because of the undesirable high discharge temperatures of R-22 when operated at high compression rations, R-22 should not be used in single stage low temperature compressors 5 HP and larger.

Different expansion valves are requiered for each refrigerant, so the refrigerants are not interchageable in a given system, and should never be mixed, If for some reason it is desirable to change from one refrigerant to another in an existing system by changing expansion valves and control settings providing the existing piping sizes and component working pressures are compatible. In some cases the existing motor-compressor may be satisfactory --for example, in converting from R-22 to R-502. If the conversion will result in higher power requirements as is the case in changing from R-12 to R-502. then it may also be necessary to change the motor-compressor.

The refrigerant charge should be held to the minimum required for satisfactory operation, since an abnormally high charge will create potential problems of liquid refrigerant control.