Suction Accumulators

Whilst estimating the holding capacity of an Accumulator, is the entire internal volume of the accumulator considered or about 75 to 80% as is normally done for a Cylinder? The volume would be multiplied by the density of the liquid refrigerant at the evaporating temperature, I presume , to arrive at the weight of the refrigerant.
I would like to know if this assumption is correct.
Also, the weight calculated as mentioned above would be about 50% of the refrigerant charge inthe system, as recommended in most manufacturer tables. Some manufacturers talk of 65 or 70 % . What would be appropriate and what is the rationale behind this percentage factor?

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Originally Posted by rsiyer

Whilst estimating the holding capacity of an Accumulator, is the entire internal volume of the accumulator considered or about 75 to 80% as is normally done for a Cylinder? The volume would be multiplied by the density of the liquid refrigerant at the evaporating temperature, I presume , to arrive at the weight of the refrigerant.
I would like to know if this assumption is correct.
Also, the weight calculated as mentioned above would be about 50% of the refrigerant charge inthe system, as recommended in most manufacturer tables. Some manufacturers talk of 65 or 70 % . What would be appropriate and what is the rationale behind this percentage factor?

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Some designers would like to have the accumulator sized for almost 100% of system refrigerant charge on small systems and 80% of system refrigerant charge on large systems. The larger is the better for the accumulator this prevents liquid slugging back to the compressor provided it does not cause a severe pressure drop in suction line.

The usable volume of an accumulator is limited by how much liquid level it can carry and provide proper separation distance and separation velocity. It also depends whether the accumulator is vertical or horizontal and how many outlets are onthe accumulator.

There is a chapter in the ASHRAE handbooks on designing accumulators. The maximum separation velocity is determined by Stoke's Equtions which considers the relative density of the liquid and vapor phases and the surace tension of the liquid.

The whole idea of the accumulator is based on keeping liquid slugs and upsets out of the compressor suction. The accumulator should run dry and only have liquid in an up set.

To aide in emptying the accumulator, you can run a liquid coil (called a boil-out coil) fed by the high pressure liquid and then on to the evaporator. During normal operation, with a dry vessel, the vapor superheat and subcooling in minimal. If you have liquid there, the hot liquid will boil off the liquid in the vessel and subcool the liquid in the coil. The net effect on the system performance is close to zero.

Suction accumulator design is a whole different topic that can cover a lot of different designs and applications.
Ken