SINGLE STAGE LOW TEMPERATURE SYSTEMS

  Low temperature single stage system become increasingly critical from a design and application standpoint as the desired evaporating temperature is decreased. The combination of high compression ratios, low operating temperatures, and rarified  return gas can cause lubrication and overheating problems, and make the compressor more vulnerable to damage from moisture and contaminants in the system.

  The compressor selection, suction temperature, and application must be such that the temperature of the discharge line measured within 1¨ to 6¨of the discharge line service valve does not exceed 230° F. for Refrigerants 12,22 and 502. Under these conditions, the estimated average temperature at the discharge port (measured at the valve retainer on the valve plate) will be approximately 310° F. for R-12 and R-502, and 320° F. for R-22.

  The compressor displacement, pressure limiting devices, and quantity of cooling air or water must be selected to prevent the motor temperature from exceeding the limits stated below:

 A. 210° F. when protected by inherent protectors affected by line current and motor temperature.

 B. 190° F. when protected by motor starters.

  The temperature of the motor should be determined by the resistance method and should be determined when the compressor is tested in the highest ambient in which it is expected to operate, at 90 per cent of rated voltage, with 90° F. return suction gas temperature of 170° F. to 190° F. are highly recommended.

  In order to prevent the discharge and motor temperatures from exceeding recommended limits, it is very desirable, and in some instances absolutely necessary, to insulated the suction lines and return the suction gas to the compressor at a lower than normal temperature. This is particularly important with suction cooled compressor when R-22 is used. (Approximately 30° F. superheat suggested.)

  Suction cooled compressors required auxiliary cooling by means of an air below 0° F. evaporator temperature.

  Either the evaporator must be properly designed, or a pressure limiting device such as a pressure limiting expansion valve or crankcase pressure regulating valve must be provided to prevent motor overloading during pulldown period, or after defrost.

  Copeland now recommends R-502 for all single stage low temperature applications where evaporating temperature of -20° F. and below may be encountered. Now that R-502 is readily available, R-22 should not be used in single stage low temperature compressor, 5 H.P. and larger. The lower discharge temperature of R-502 have resulted in much more trouble-free operation.

  An adequate supply of oil must be maintained in the crankcase at all times to insure continuous lubrication. If the refrigerant velocity in the system is so low that rapid return of the oil is not assured, on adequate oil separator must be used. The normal oil level should be maintained at or slightly above the center of the sight glass. An excessive amount of refrigerant or oil must not be allowed in the system as it may result in excessive liquid slugging and damage to the compressor valve, pistons, or cylinders.