Air Conditioning and
Refrigeration Systems Oil Fouling (cont.)

As the oil and refrigerant mix, the chemical composition of both is drastically altered. Pressure and temperature characteristics of this refrigerant/oil mixture will be different from pure refrigerant. In addition, the viscosity of the oil is reduced by dilution with refrigerant, thereby increasing the probability of poor lubrication in the compressor. It matters little whether the refrigerant is miscible or immiscible with the oil. Contaminating oil films are deposited throughout the system, and it is these "oil fouling" deposits that ultimately cause multiple problems.

The result of oil contamination is frequently noticed at the expansion device. Materials dissolved in the refrigerant-lubricant mixture, under liquid line conditions, may precipitate at the lower temperature in the expansion device, resulting in restricted or plugged capillary tubes or sticky expansion valves. A few milligrams of these contaminants can render a system completely inoperative.* Any migrating oil in any refrigeration or air conditioning system is costly, both in kWh consumption and money and lost time spent on maintenance and repairs.

Equipment suppliers may state that in a particular system, migrating oil concentration has been reduced to only one percent. The one percent being referred to is one percent of the total oil volume. If a compressor holds four quarts, or 128 ounces of oil, then at one percent, 1.28 ounces is flowing through the system at any given time. Since a capillary tube, oil pressure switch, or expansion valve can be fouled with a few milligrams of oil, when one percent of any oil charge is flowing constantly through the system, the system will become oil fouled.

* 1998 ASHRAE Handbook, Refrigeration, Chap. 6.7