Ordinarily, suction line accumulators are not necessary on A/C units if the system is piped correctly and all of the precautionary guidelines are followed. Refrigerant suction line accumulators should only be installed on systems where liquid flood back to the compressor is highly likely.
One such scenario is when a line set is buried for 10 feet or more. During the off cycle the vapor in the suction line can condense back to a liquid. When the compressor starts, this liquid is pulled back into the compressor and can damage it. Compressors are designed to compress vapor only, they do not like liquid.
Compressor suction line size should never be used as a guideline for sizing an accumulator. Normally, the accumulator is sized for not less that 50% of the total system capacity of refrigerant. Careful consideration must be given when attempting to apply an accumulator on a split system. On any given unit, approximately 80% of the system charge can be found between the compressor and the expansion device during operation. When the system shuts down for long periods, the refrigerant will migrate to the low side of the system and accumulating in the evaporator coil and horizontal suction lines. In the case of buried line sets, this compounds the problem because the liquid is “trapped” in the suction line. The suction line accumulator prevents this condensed liquid and vapor from entering the compressor.
Think of the suction line accumulator as 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 valves, scrolls, crank shafts, etc. Accumulators have a metering device that picks up liquid, vaporizes it, and returns it to the compressor. this prevents liquid slugging and controls oil return.
Vertical accumulators use a U-tube or tube within a tube design to draw gaseous refrigerant off the top of the vessel (see attached drawing). At the bottom of the U-tube, an orifice picks up small amounts of oil and liquid refrigerant and meters it back with the gaseous refrigerant picked up at the top of the vessel. The small amount of liquid refrigerant that is drawn back will boil off in the suction line between the accumulator and compressor. The oil will be carried with the gaseous refrigerant back to the compressor.
Selection of a suction line accumulator should be made on the basis of 3 capabilities:
- The accumulator should have an adequate liquid-holding capacity that can vary with the system. Normally, this should not be less than 50% of the system charge.
- An accumulator should perform without adding excessive pressure drop to the system. Excessive suction pressure drop reduces the capacity of the unit.
- An accumulator should have the capability of returning liquid at the proper rate and under a range of load conditions.
Remember, as stated at the beginning of this post, most split system A/C units do not require suction line accumulators if proper piping is performed. Underground or buried piping IS an exception to that rule!