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During the normal compression cycle, as the refrigerant is discharged from a compressor a small amount of refrigerant oil will also be discharged. It is important that this discharged oil returns back to the compressor. In order for the oil to return back to the compressor, good piping practices must be followed. If the refrigerant lines are not properly sized or configured, the oil may become trapped out in the system. If enough oil becomes trapped out in the system, the compressor could become starved for oil and bearing damage could occur.

An oil separator is an accessory used on some larger refrigeration systems to help limit the amount of oil in circulation. It is typically used on low temperature applications where the refrigerant’s mass flow rate is low. But it could be used on any system.

An oil separator will separate a major portion of the oil from the refrigerant as it is pumped from the compressor. It will also collect this separated oil and safely return it to the compressor’s crankcase. Oil separators are installed in the discharge line close to the compressor’s discharge. Sometimes they are heated and/or insulated to prevent the refrigerant inside from condensing at low ambient conditions.

As the refrigerant/oil mixture is discharged from the compressor, it enters the oil separator. The velocity of this mixture is slowed down from the use of internal baffles and impingement screens.

This slowing down causes a major portion of the oil to drop out of the mixture and fall to the bottom of the oil separator. At the bottom of the oil separator is a float assembly connected directly back to the crankcase of the compressor. As the oil level at the bottom of the oil separator increases, the float will cause a valve to open and, due to the pressure difference between the oil separator and the compressor’s crankcase, some of the oil will be returned to the crankcase until the float level in the oil separator drops and the valve closes.

While servicing systems with oil separators, one item to check is the temperature of the oil return line from the oil separator to the compressor. It should be just above room temperature most of the time. If it is hot all the time, this is an indication that the float assembly inside the oil separator may be stuck open and allowing hot discharge vapor to enter the compressor’s crankcase. This can lead to excessive crankcase pressures, overheating, and serious damage to the compressor.

Although oil separators can be quite efficient (some as high as 98 percent efficient), they do not separate all of the oil from the refrigerant. Some quantity of oil will always travel with the refrigerant throughout the system. They are not a cure for oil return problems. Good piping practices must always be followed while installing a system. Even with an oil separator, poor piping practices will eventually lead to oil being trapped out in the system causing a deficiency of oil in the compressor’s crankcase and possible compressor damage.

Publication date: 10/05/2009