Ideally,
any liquid entering the compressor suction will be rich enough in oil and lean
enough in refrigerant that lubrication will be satisfactory. Yet, if any liquid ingested into a compressor
has too low a concentration of oil, lubrication may be compromised and wear
leading to compressor failure can ensue.
All compressors are vulnerable to lack-of-lubrication failure, either
from lack of oil or from too much refrigerant in the oil.
A
second type of failure is the result of injecting too much liquid refrigerant/oil
into a compressor that can damage or destroy the compressor by "liquid
slugging". Screw and scroll compressors
are rather more tolerant of liquid in the suction stream than are reciprocating
compressors. This is due to the
differing nature of the compression processes.
In
a reciprocating compressor designed for a three to one compression ratio, the
gas may reach the discharge pressure when the piston is only at half
stroke. At this point the discharge valve
opens and gas is discharged as the piston continues to rise even though gas
pressure in the cylinder no longer rises.
The final clearance volume may be only one tenth of the total swept
volume. This clearance volume is not
discharged, but is re-expanded on the suction stroke. One might say at this point that the true
compression ratio is ten to one considering a closed discharge valve (swept
volume divided by swept volume plus clearance volume). If a volume of liquid of 110% of clearance
volume is in the cylinder when compression begins, the piston will be
compressing only liquid at the end of its stroke and the liquid may not be able
to exit the discharge valve fast enough to avoid developing a very high
pressure in the cylinder. This high
pressure can cause failure of the connecting rod or failure of the head
bolts. For a reciprocating compressor to
be efficient, a small clearance volume is required. Yet, it is the small clearance volume that
makes reciprocating compressors susceptible to liquid slugging damage. Allowable levels of liquid in the suction are
determined by the ratio of clearance volume to swept volume.
In
contrast, screw and scroll compressors designed for a three to one compression
ratio capture a volume of suction gas (and some oil and maybe some liquid
refrigerant) and reduce its volume to one third its original value. But the compression process is completed
before the discharge port opens. Any liquid
in the suction stream will cause the compression ratio to rise above the design
value of three, but the rise is slower than in the reciprocating
compressor. For example, assume that the
suction stream for a screw compressor consists of 1 part liquid and 8 parts gas
by volume. The compressor will reduce
these 9 parts to 3 parts. At the
completion of compression, one part will still be liquid and two parts will be
gas. The pressure in the compressor when
the discharge port opens will be four times suction pressure (8 parts gas going
in divided by 2 parts gas going out). The one part of liquid remains one part
because the liquid is essentially incompressible. Thus,
the effect of liquid in the suction stream is to increase true compression
ratio. But a four to one true
compression ratio in a compressor designed for three to one is probably safe to
operate. Allowable levels of liquid in
the suction stream are determined by the design pressure ratio and the maximum
pressure that can be tolerated in the compression chamber.
- Ed Keuper
Editor's Note: CR4 would like to thank GEA Consulting for contributing this blog entry, which originally appeared on their website.
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