Oil Fouling is a contamination process which afflicts all refrigeration systems including both space conditioning and process refrigeration systems. All air conditioning and refrigeration systems that are based on the vapor-compression cycle contain a compressor in which, during normal operation, the compressor’s lubricating oil comes into contact with the refrigerant. Since the early days of refrigeration and air conditioning technology, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) has addressed the results of the interaction between the oil and the refrigerant. This mixture continuously migrates throughout the entire air conditioning or refrigeration system.
In the ASHRAE Handbook of Fundamentals Refrigeration Volume, it is stated that some lubricating oil is lost from all compressors during normal operation. Since oil inevitably leaves the compressor with the discharge gas, systems using halocarbon refrigerants must return this oil at the same rate at which it leaves. ASHRAE states that unless oil is removed periodically or continuously from the point where it collects, it can cover the heat transfer surface in the evaporator, reducing performance.
Effects on the System
As the oil and refrigerant mix, the chemical composition of each is altered. Pressure and temperature characteristics of the oil/refrigerant mixture will be different from pure refrigerant. In addition, the viscosity of the oil is reduced by dilution with the refrigerant, thereby increasing the probability of poor lubrication in the compressor. It doesn’t matter whether the refrigerant is miscible or immiscible with the oil. Contaminating oil films are deposited throughout the system, and it is these oil deposits that ultimately cause multiple problems.
The result of 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 .
Effects on Heat Transfer
Oil fouling of the heat transfer surfaces of air conditioning and refrigeration systems will cause a loss of about 7-8% efficiency in the first year, 5% in the second year, and 2-4% the following years. This loss will continue to accumulate until equilibrium is reached between flow force and adhesion. At this point the oil boundary layer has achieved its maximum thickness, producing maximum loss of efficiency. Published ASHRAE information confirms these observations. According to ASHRAE, performance can be degraded by as much as 30% due to the build up of lubricants on internal surfaces. Higher percentages, up to 40%, have been observed in systems 20 years old or older.
The figure shows the degradation of the performance of a 100-ton unit (chiller or DX unit) over time embodying the ASHRAE assumptions of annual degradation up to 30%. This degradation results in increased cycling time, and ultimately increases kWh and operating cost, in addition to additional wear and shorter equipment life.
Managing the Problem
The oil that finds its way into the system must somehow be managed. The obvious question then becomes how to manage this troublesome oil. Some of the techniques used by manufacturers to control migrating oil include use of mechanical devices such as separators, skimmers, drums, heat sources, suction risers, traps, and pumps. According to ASHRAE’s Handbook, these high-tech designs are not efficient enough to remove all the unwanted oil. Most of this oil can be removed from the stream by an oil separator and returned to the compressor. Coalescing separators are far better than separators using only mist pads or baffles; however even they are not 100% effective. Although the mechanical solutions may reduce the problem of restricted or plugged capillary tubs or sticky expansion valves, they do not resolve the boundary layer fouling over time.
ProaTEQ is an advanced, disruptive; nanotechnology developed by EnSaTEQ of Woodville, AL and is sold and installed by Georgetown Utilities and its affiliates. ProaTEQ is an additive to refrigeration systems lubrication oil that addresses the oil-fouling heat-transfer problem by the use of molecular level forces to remove and prohibit formation of the boundary layer. See our essays on PROAs and on PROATEQ elsewhere on this site.