HVAC Technologies-Understanding Oil Return in Refrigeration Systems - Part 3
Posted on Wed, Jan 05, 2011
Paert 3: Oil In The Compressor Section
by Ed Keuper Sr Consultant GEA
Ideally, in any chiller oil return system, 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).
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