Introduction:
Four common causes of compressor failures:
Liquid slugging
Flooding or liquid floodback
Flooded starts
Overheating
Focus on slugging and floodback.
Slugging:
Occurs when a large amount of refrigerant or oil enters the compressor.
Liquid is not compressible, causing damage:
Breaks pistons, rods, and valves.
Can snap the crankshaft and blow head gaskets.
Example: Slugging damaging suction read valves and discharge read valves.
Fully hermetic compressors require entire replacement; semi-hermetic or open drive compressors can be rebuilt.
Flooding/Floodback:
Smaller amounts of liquid entering the compressor, causing slower damage.
Air-cooled compressors:
Cooled by airflow across the body.
Suction vapor enters cylinders, liquid washes oil off cylinder walls causing wear.
Suction-cooled compressors:
Refrigerant suction vapors cool the motor.
Liquid washes oil off the crankshaft and connecting rods causing significant damage.
All hermetic compressors are suction gas cooled.
Example: Suction gas cool compressor experiencing liquid floodback.
Diagnosis and Repair:
Manufacturers encourage tearing down failed compressors to find clues of failure.
Progressive wear pattern from oil pump towards the motor end indicates flooding.
Ensuring proper airflow in the evaporator and checking superheat helps prevent floodback damage.
Causes of Slugging and Floodback:
Low evaporator heat load:
Too low space temperature or recirculation of conditioned air.
Cure: Operate at designed temperatures and ensure proper airflow.
Metering device issues:
Poorly selected device or improper superheat setting.
Frost on the suction line of an air conditioner indicates floodback.
For medium/low temperature systems, frost does not necessarily indicate floodback.
Example Problem:
Freezer compressor:
Suction line at zero degrees, suction pressure at 16 psig with R 404a.
Superheat of 20 degrees indicates no floodback issue.
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