Preventing Refrigerant Floodback and Migration Before It’s Too Late
Liquid refrigerant flooding back to a compressor’s crankcase and mixing with the crankcase oil is a compressor’s worst nightmare. This situation, along with refrigerant migration, is the reason why a suction accumulator may be needed on a system.
Refrigerant floodback occurs when liquid refrigerant enters the compressor’s crankcase during the running cycle. Refrigerant floodback will dilute the compressor’s oil with liquid refrigerant, which will cause oil foaming in the crankcase, washing the bearings clean of their lubricating oil. This same phenomenon can also pressurize a compressor’s crankcase, causing leakage around rings, valves, and other compressor parts and passageways.
The compressor’s crankcase lubricant has many important functions. Even though the refrigerant is the working fluid required for cooling, the lubricant or oil is needed for lubrication of the compressor’s moving mechanical parts. Compressor floodback may be caused by a variety of issues, including low evaporator heat loads, defrost periods that have just ended, failed or misadjusted mechanical valves, or a lack of system maintenance.
Refrigerant migration is caused by a pressure difference and occurs when either liquid or vaporized refrigerant migrates back to the compressor’s crankcase during the off-cycle. The compressor’s crankcase is often the lowest pressure point in the system because it contains a low-vapor-pressure lubricant or oil in its shell. Both refrigerant liquid and vapor will be attracted to this low-pressure crankcase during an off-cycle, and once in the crankcase, the refrigerant vapor may condense to liquid. If there is not a way to prevent these unexpected, maybe even short periods of liquid floodback and/or migration to the compressor, a suction line accumulator is needed.
SUCTION LINE ACCUMULATORS
Suction line accumulators are designed to protect the compressor when refrigerant flooding and/or migration do occur. They should be installed between the evaporator and the compressor on the suction line, usually as close to the compressor as possible. In reverse-cycle systems like heat pumps, they should be installed in the suction line between the compressor and the reversing valve. Accumulators can also act as suction line mufflers to quiet compressor pulsation noises.
Accumulators act as a temporary reservoir for liquid refrigerant and/or oil. They are simply a vessel that collects liquid refrigerant from the suction line and stores it until the liquid either evaporates and returns to the compressor as a vapor or is metered back gradually. Their outlet tubes to the compressor are located at the highest point in the accumulator to let only refrigerant vapor enter the compressor. Hopefully, the refrigerant liquid level never reaches this highest point.
Most accumulators are designed to meter both the liquid refrigerant and oil back to the compressor at an acceptable rate that will not damage compressor parts or cause oil foaming in the crankcase. This is done while the compressor is running with a small metering orifice at the bottom of the outlet tube. This small orifice is mainly designed for oil return; however, liquid refrigerant can be slowly metered and vaporized through it also.
Another small orifice located near the top of the outlet tube, going to the compressor, is often referred to as the pressure equalization orifice. Refrigerant liquid and vapor may move from the evaporator and suction line into the accumulator during the off-cycle. The refrigerant vapor may condense and form liquid, and this liquid refrigerant will flow into the small metering orifice at the bottom of the U-tube, filling it up. The liquid refrigerant will seek its own level with the liquid already in the accumulator. Now, the U-tube inside the accumulator will have a column of liquid in it, and when the compressor starts up, this column of liquid refrigerant will be sucked out of the U-tube and into the compressor if it were not for the pressurization orifice at the outlet of the accumulator. This orifice will equalize pressure on both sides of the liquid column whether the compressor is on or off, preventing the entire column of liquid from entering the compressor.
In other words, the liquid column will have accumulator pressure on both sides of it because of the orifice. The column of liquid will slowly turn to vapor and enter the compressor without causing damage. It is important that the refrigerant system be kept clean and free of sludge, which is formed from a combination of acid, moisture, oil, and excessive heat. Sludge will clog these important orifices in the accumulator and render the accumulator useless. Compressor damage will surely follow.
Suction line accumulators add an extra safety precaution and can assist in flooding and migration conditions. When severe flooding or migration problems exist, suction line accumulators have been known to also flood. Always follow the manufacturer’s recommendations when sizing and installing a suction line accumulator.
Publication date: 9/3/2018