The compressor is built to compress vapour; liquid cannot be compressed. If liquid arrives from the evaporator the valve plates break, the oil thins and the bearings seize. The suction accumulator is an upright vessel placed in the suction line: when the incoming mixture enters the vessel its velocity falls, the liquid settles to the bottom and the vapour is taken off at the top. The U-shaped outlet tube inside the vessel draws the vapour from the top; the liquid that collects at the bottom takes heat from the surroundings over time and evaporates. In this way the brief liquid floodback seen at start-up and after defrost is dealt with before it reaches the compressor.
At the lowest point of the U-tube there is a small hole 1–2 mm in diameter. Oil collects at the bottom together with the refrigerant; this hole meters the oil and a small amount of liquid into the vapour stream, at a rate that does not harm the compressor. Without the hole the oil would stay in the vessel and the compressor would run without oil. Blockage of the hole has the same result; that is why some models have a strainer in front of it, and why keeping the system clean matters. In dirty systems this is the first place to block.
A suction accumulator is necessary in the following systems: systems with hot gas defrost and reverse-cycle heat pumps, split systems with a large charge and long lines, capillary tube units, blast freezers whose load changes rapidly, and flooded evaporators. Its volume is chosen large enough to hold at least half the total refrigerant charge in the system. The connection diameter must suit the suction line and the gas velocity inside must be sufficient to carry the oil. In a vessel chosen too large, on the other hand, oil collects and does not return.
The vessel is installed close to the compressor, upright and with its inlet and outlet connections the right way round; connected in reverse it is useless. Because it runs cold its outside sweats and rusts; insulating it extends its life. The suction accumulator is an insurance, not a cure: if liquid is arriving continuously the real cause is sought. A wrongly adjusted expansion valve, low superheat, a frosted evaporator, a stopped fan or an excessive refrigerant charge are the most common causes. Permanent frosting on the shell of the vessel is the sign that there is always liquid inside it.