April 2, 2007
A suction-line accumulator is a safety device used on a refrigeration system to prevent liquid refrigerant from returning to the compressor. Liquid refrigerant returning to a compressor can lead to several problems. One of these problems is bearing washout. The liquid refrigerant returning to the compressor mixes with the oil. This mixture is then used to lubricate the bearings within the compressor. It is, however, a poor lubricant and will lead to premature bearing failure.
The basic design of a suction-line accumulator, used on commercial refrigeration systems, is a Uâ€'shaped tube enclosed in a large volume vessel. One end of the U-shaped tube is connected to the suction line leading back to the compressor and the other end is open to the tank.
The suction line from the evaporator is then connected to the top of the shell of the tank. If any liquid refrigerant enters the accumulator, it will be exposed to a large volume of the tank, causing it to evaporate and return only refrigerant vapor to the compressor.
There are also two small holes drilled in this U-shaped tube. One hole is drilled at the bottom bend that allows a small amount of oil and possibly some liquid refrigerant that collects on the bottom of the tank to be safely metered back to the compressor. There is another small hole drilled at the top end of this tube, referred to as a pressure equalization orifice.
When the system shuts down, it is possible for liquid refrigerant to collect in the bottom of the accumulator, and because of the small hole drilled in the bottom bend of this tube, liquid refrigerant will also collect here. On startup, liquid refrigerant could then be sucked out of the tube and return to the compressor.
To prevent this from occurring, a small hole is drilled at the top end of the tube. This small hole equalizes the pressure on both sides of the liquid in the tube and prevents the liquid refrigerant from being sucked out on startup.
Some accumulators will have a section of the liquid line run through the bottom section of the accumulator. This warm liquid refrigerant helps to evaporate any liquid refrigerant that collects at the bottom of the accumulator.
The liquid refrigerant in the liquid line will gain additional subcooling, thus increasing the net refrigeration effect of the system, another benefit to this design.
Since accumulators are typically made of steel and may operate at temperatures below the dew point temperature of the surrounding air, the development of rust leading to a refrigerant leak is a common problem. Although manufacturers do coat the surface of their accumulators with a rust-preventive paint, if the steel surface is exposed, rust can develop. This can happen during the installation process of the accumulator.
If care is not taken, the paint near the line connections can be burned away, exposing the steel surface of the accumulator. If this does occur, the surface should be recoated with silicon or some other coating to protect the steel surface from rusting.
Accumulators are only a safety gap for liquid refrigerant returning to the compressor. They should not be used to solve a system problem that is causing liquid refrigerant to regularly return to a compressor. If liquid refrigerant is found continuously returning to a compressor, the root cause should be identified and repaired.
Publication date: 04/02/2007