Understanding Oil Return in Refrigeration Systems
by Ed Keuper & Dick Cawley
With few exceptions, all compressors that are lubricated with oil will discharge oil into the gas stream. The rate of discharge can be as small as parts of oil per million parts of refrigerant for direct drive hermetic centrifugal compressors and as much as several percent for screw compressors. Oil discharge rates are usually expressed in terms of lbm of oil discharged per lbm of refrigerant compressed or in mass percent of oil in the discharge gas.
On screw chillers using eductors for oil return, when operating at low load conditions, it appears that the eductor does not operate efficiently enough to return a sufficient amount of oil to the oil separator or sump to maintain its oil level, which then causes the chiller to shut down on low oil.
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.
This sequence deals with direct expansion unitary systems that are designed to move
lubricant around the system and back to the compressor sump by momentum, in the case of refrigerant gas transport (suction and discharge), and solubility where refrigerant is in liquid form. Three areas discussed include:
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Equilibrium and Minimum Run Time
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Maintaining an Adequate Lubricant Level
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Minimum Velocities in Suction and Discharge Gas Risers
Hermetic motors, seals, elastomers,compressors, thermal expansion devices,distributors, capillary tubes, short tube orifices, line valves, service valves, switch over valves, check valves, filters / driers, accumulators, heat
transfer surfaces and connectingtubing could cause a refrigerant related failure.
(PPT file - 1.2 MB) Slide presentation at Dairyland Power Annual Conference 3/24/10 by Larry Butz . Includes discussion of reasons for increased use in colder northern climates.
The Smart Grid Consumer Mantra
"Ask not what the grid can do for you. Ask what you can do for the grid – and prepare to get paid for it.
Imagine it’s a hot, humid day and the temperature is rising. The local electric company is reaching it maximum generating capacity, so it sends a signal to your home automation system to reduce the electrical consumption in your house. Your home automation system responds by raising the set point of your thermostat and only energizing the water heater when absolutely necessary. For your ability to shed some load, you get a discount on your energy bill. Welcome to the "Smart Grid".