Compressor Flow-Back Protection

In most air-conditioning and refrigeration systems, it is possible to control the refrigerant feed so that liquid refrigerant does not return to the compressor during operation. Some systems have poor operation of the expansion valve. Also, some have sudden changes in loading and will periodically return liquid refrigerant to the compressor. This is not usually in sufficient quantities to do real damage. However, there are certain systems that, by their very design, will periodically flood the compressor with excessive amounts of liquid refrigerant. Principally, these prove to be systems in which there is a periodic reversal of the refrigerant cycle to do one of the following:

  • Provide hot gas for defrosting an evaporator.
  • Furnish heat for release purposes in ice-making apparatus.
  • Change from cooling to heating cycle, or vice versa, in heat pump systems.

These systems are almost identical in operation and in the effect on the compressor. Consider an air-source heat pump for a typical analysis of what happens. The flow back occurs when reversing the cycle in either direction. During the cooling cycle, the outdoor coils act as condensers and have warm liquid continually draining from them. The lines to which the thermal expansion-valve bulbs are strapped are now hot-gas lines and are hot. When the cycle reverses to heating, the outdoor coils change from condensers to evaporators. The liquid that has been draining from them during the condensing cycle is now dumped into the suction line. Of even greater significance, the thermal expansion valves also open wide as the result of their bulbs being on warm lines (lines that were previously hot-gas lines). Therefore, these valves will flow through until control is re-established at the bulbs. The two effects constitute a substantial liquid flow back through the suction line.

When there is no way to control the hazard of periodic flow back of substantial proportions through the suction line, it is necessary to take measures to protect the compressor against it; otherwise, compressor life will be materially shortened. The most satisfactory method appears to be a trap arrangement that catches the liquid flow back and may do one of the following:

  • Meter it slowly into the suction line when it is cleaned up with a liquid-suction heat inter changer.
  • Evaporate 100 percent of the liquid in the trap itself and automatically return the oil to the suction line.

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