SUCTION LINE ACCUMULATORS AND BURIED LINE SETS


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:

  1. 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.
  2. An accumulator should perform without adding excessive pressure drop to the system.  Excessive suction pressure drop reduces the capacity of the unit.
  3. 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!

Suction Line Accumulator

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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2 Responses to SUCTION LINE ACCUMULATORS AND BURIED LINE SETS

  1. Randy Chih says:

    Thank you for the informative post. I am planning to put central air into my home in Los Angeles. We were planning to run a lineset underground for approximately 15 ft (underneath a walkway).

    Based on my limited understanding it seems that I will need to ask the contractor to use armoflex insulation on both the suction and the lineset and then enclose both in a pvc conduit then use some kind of clamp at the end of the conduit in order to keep water out of the conduit.

    Would I still need a suction line accumulator? My thought was that Los Angeles weather is less extreme than East coast weather.

    Could you share your thoughts with me or point me to a resource that would help me ensure that the underground lineset is installed properly?

    • you absolutely need an accumulator. The purpose is not so much when the unit is running. During the OFF cycle, the gas in the lines will condense to a liquid and when the unit starts, without the accumulator, you would “slug” liquid into the compressor and eventually have a failure.

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