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The refrigeration compressor is designed to compress vapor only. A suction line accumulator prevents compressor damage from a sudden surge of liquid refrigerant and oil that could enter the compressor from the suction line.
The suction line accumulator is a temporary reservoir for this mixture, designed to meter both the liquid refrigerant and oil back to the compressor at an acceptable rate. This prevents damage to the reed valves, pistons, rods, and crankshafts.
Accumulators have a metering ejector device that picks up liquid, vaporizes it, and returns it to the compressor. This prevents liquid slugging and controls oil return. It is particularly important on hot gas defrost systems, heat pumps, etc., where surges of liquid refrigerant frequently go back down the suction line.
Vertical accumulators use a U-- tube or tube-within-a-tube design to draw gaseous refrigerant off the top of the vessel.(See Figure 1.) At the bottom of the U-tube, an orifice picks up a small amount of oil and liquid refrigerant and meters it back with the gaseous refrigerant. The small amount of liquid refrigerant will boil off in the suction line. The oil will be carried with the gaseous refrigerant back to the compressors.
A vent hole at the top of the accumulator U-tube outlet acts as a vacuum break to prevent an accumulator that has flooded during an off cycle from slugging upon start up of the system. ASME accumulators over 6-in. diameter are manufactured to ASME Code, Section VIII. A female pipe connection is supplied for a pressure relief device. A pipe plug is installed in this connection for pressure testing and painting at the factory.
Vertical accumulators should have a heat exchanger coil or a heat element added on low-temperature applications (0 deg F and below) to help boil off liquid refrigerant and raise the oil temperature to help facilitate oil flow. (See Figure 2.)
The heat exchanger can be used on low-temperature systems to subcool the liquid line while helping to boil off liquid refrigerant in the accumulator by passing the liquid line through the heat exchanger coil. This can increase system efficiency while helping oil flow in the suction line. Do not use discharge gas through the heat exchange coil or there is a risk of overheating...