Sunday, December 6, 2009


by Roger Desrosiers

Many refrigeration compressors serviced today have positive-displacement oil pumps to help lubricate the internal compressor parts. Most compressors that have positive oil pumps also have a control that senses oil pressure and acts as a safety device whenever the oil pressure falls below a certain threshold level.

It is the action of the oil safety control we will discuss in this article.

There are several types of oil safety control devices on the market today. The two basic controls we are most familiar with are the mechanical differential control and the pressure-sensing electronic control.

The mechanical control uses tubing that senses the suction pressure of the compressor and the outlet oil pressure of the pump. The electronic control has a special pressure sensor that mounts in the outlet of the pump and connects only with an electrical cable.


In the mechanical control, such as above, the total pressure from the pump (less the suction pressure) is the actual net oil pressure. The control requires manual reset once it is tripped.

The oil sump is actually the suction pressure of the compressor. This means that the true oil pump pressure is the oil pump discharge pressure less the compressor suction pressure. So the oil pressure control is a pressure differential control which measures a difference in pressure to establish that positive oil pressure is present. A study of the compressor above shows how the net oil pressure is established. A study will show that when the system is using R-22 the crankcase pressure would be about 70 psig when the compressor is running and 125 psig when standing.

Most compressors need at least 30 psig of actual oil pressure for proper lubrication. This means that whatever the suction pressure is, the oil pressure discharge pressure has to be at least 30 lbs. above the oil pump inlet pressure, because oil; pressure inlet pressure is the same as the suction pressure. For example: if the suction pressure is 70 psig, the oil pressure must be 100 psig for the bearings to have a net oil pressure of 30 psig.

The oil pressure control uses a double bellows with one bellows opposing the other to detect the net or actual oil pressure. The pump inlet pressure under one bellows and the pump outlet pressure under the other bellows. The bellows with the most pressure is the oil pump outlet and it overrides the bellows with the least amount of pressure. This override is attached to a linkage that can stop the compressor if the net oil pressure is not established in a predetermined time.

When the compressor starts there is a time delay built into the control to allow the compressor to build up oil pressure in a short time and to prevent un-needed cutouts when oil pressure may vary for only a moment. This time delay is usually about 90 seconds. It is accomplished with a heater circuit and a bi-metal device or electronically.

Oil pressure safety controllers can also incorporate a pressure transducer to sense the combination of oil pump discharge and crankcase pressure.

Oil pressure safety controllers can also incorporate a pressure transducer to sense the combination of oil pump discharge and crankcase pressure. The pressure transducer has two separate ports for sensing suction pressure and oil discharge pressure and the difference between these two pressures is accomplished by the transducer. This pressure transducer is connected to an electronic controller by wires. The transducer is actually mounted directly into the oil pump.

The transducer transforms the pressure signal to an electrical signal for the electronic controller to process. One of the advantages of an electronic controller over a mechanical bellows type controller is that it eliminates capillary tubes, and eliminates the chance of a leak. Also the electronic clock and circuitry are much more accurate and reliable.  Net oil pressure varies from compressor to compressor. Net oil pressure usually ranges from 20 to 40 psi. Most oil pressure safety controllers will shut the compressor down if the net oil pressure falls below 10 psi.

 The following variables affect the net oil pressure.

·        Compressor size
·        Oil temperature
·        Oil viscosity
·        Bearing clearance
·        Percent of refrigerant in the oil

Below is a schematic diagram of an oil safety control circuit and three phase motor with internal overload. The oil pressure differential switch is NC and if it’s still closed after say 90 seconds, the “L” and “M” contacts open and takes the compressor off the line.
See FIG 3 also
Fig 4

Other reasons for oil failure:
  • Inadequate refrigerant in systems which will cause lower velocity and less oil return
  • If refrigerant charge is low, the expansion valve will not feed the evaporator properly, resulting in high superheat and low gas velocities.
  • Low head pressure resulting from the cold ambient can affect the thermal expansion valve.
  • Due to oversized suction line
  •  Blockage in the system usually produces no net oil pressure.
  • Smaller horsepower compressors can trip an oil failure if the compressor does not start when electrical power is applied to the terminals.
  • Oil contamination can cause a problem at the pick-up tube in the compressor crankcase.
  • In older compressors, oil failures can occur because of the pressurization of the crankcase due to blow-by from the pistons or piston rings.
When short cycling occurs, the compressor pumps more oil than normal and can cause the oil control to trip.

Roger J. Desrosiers

About the Author: Roger is a contributing faculty member He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified. More About Roger