By Grant Laidlaw
Looking at the effect of this situation on the system, components, and capacity.
Gordon asks: Hi Grant, we have recently lost several compressors and it would seem that the compressor oil is all ending up in the evaporator. This then causes the compressor to fail. We can possibly fit oil cut out switches, but this does not solve the root problem. Can you please shed some light on this situation, thanks!
Hi Gordon. This situation sometimes occurs when an oil refrigerant mixture enters the evaporator but during the evaporation process the refrigerant separates from the oil leaving the oil behind in the evaporator. This reduces system capacity (evaporator capacity) and may drain the compressor sufficiently to cause lubrication issues as you have experienced, and possible oil pressure switch cut out.
There are several important factors to consider around keeping oil from accumulating in the evaporator. Typically, these problems can be avoided through proper design, including:
- Proper system capacity balance with the compressor
- Proper evaporator design
- Avoidance of improper traps
- Installation of properly designed traps+
- Correct refrigerant line sizing
High-side oil separators may help in some installations, especially low-temperature applications. Such separators are not 100% efficient, but they can keep the compressor from passing excessive amounts of oil to the evaporator. HFC systems are particularly prone to this problem.
As you are aware, in a refrigeration system, oil is carried through the compressor by the refrigerant. It circulates with the refrigerant throughout the system. A small amount of oil in circulation is not harmful, in fact it is desirable and is of some benefit in lubricating valves. Large amounts of oil, however, are a problem.
The following are the areas of concern:
Oil circulating with the refrigerant and is not in the compressor crankcase, where it should be, can be a problem. A shortage of oil may develop in the crankcase. As a result, bearings and other parts may not be properly lubricated. Adding more oil after start-up to compensate is certainly not a solution, because the amount of oil circulating with the refrigerant is not consistent throughout the system. Excess oil may accumulate in the evaporator and then rapidly return to the compressor possibly resulting in an overfilled crankcase and possibly damage the compressor.
Excessive amounts of oil in an evaporator decrease the capacity of a system. Heat transfer through the coil wall decreases. The coil area is less if part of the space is occupied by oil. Both factors produce a change in temperature difference (TD) between load and evaporating refrigerant and consequently a decrease in capacity. Capacity is further reduced to the extent that the vapour pressure of a refrigerant/oil solution is less than that of a refrigerant alone. If suction pressure controls the compressor, it stops running at a higher temperature when vapour pressure is reduced.
Oil in the refrigerant may cause restrictions in expansion valves or capillaries, and thus reduce refrigerant flow. Improvements in oil quality have greatly reduced this problem, but restrictions may still occur in certain circumstances.
In practice, it is difficult to find out how much oil is circulating with the refrigerant. On larger equipment, you can observe the oil level in the crankcase. However, if oil has been added, the crankcase level may not indicate the correct oil charge.
A loss of capacity might indicate excess oil in the evaporator. When there is oil in the evaporator, several factors determine whether or not it remains or is circulated by the refrigerant returning the oil to the compressor:
- The geometry of the piping (line sizing, bends, elbows, restrictions, vertical risers, slope).
- The velocity of the refrigerant gas.
- The viscosity or fluidity of the oil. Oil itself is quite fluid and has a relatively low viscosity at ordinary temperatures. At lower temperatures, the oil becomes more viscous (more difficult to pour). Whether refrigerant vapour can push oil out of the evaporator depends on how fluid or viscous the oil is. Remember that refrigerant itself has very low viscosity. Solutions of oil and refrigerant have a viscosity between that of the oil and that of the refrigerant. As more oil dissolves in the refrigerant, the solution becomes more viscous.
As a matter of interest, you might assume that oil is most difficult to move in the coldest part of the evaporator, but viscosity measurements show that this is not so. Refrigerant is more soluble in oil at low temperatures than at high temperatures at the same pressure.
So, Gordon, in summing up, remember that the compressor may not be properly lubricated if too much oil circulates with the refrigerant. In solving the issue: install oil separators, make sure your refrigerant line sizing is correct and correctly installed, make sure that the necessary P-traps are installed correctly, check that the system components have been correctly selected and finally check that you have used the correct oil for the system in question.
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