Air Conditioning and Refrigeration Systems Oil–Fouling

Abstract

Air conditioning and refrigeration systems have a mechanical compressor wherein, during the normal course of the refrigeration process, ½% to 8% of the compressor's lubricating oil is circulated throughout the system along with the refrigerant. Since the early days of refrigeration and air conditioning technology, the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) have addressed the results of "oil-fouling2"; where the compressor oil builds up on the walls of the refrigeration tubing reducing the heat transfer from the refrigerant to the walls of the refrigerant tubing.

Effects on Heat Transfer

Oil-fouling of the heat transfer surfaces of air conditioning and refrigeration systems, will cause a loss of about 7% efficiency the first year , 5% the second year, and 2% per year the following years. This loss will continue to accumulate until equilibrium is reached between flow force and (surface tension - ed) adhesion. At this point the oil boundary layer formed has achieved its maximum thickness, producing maximum loss of efficiency. Usually, the efficiency degradation will peak somewhere between 20% and 30%. Published ASHRAE information confirms these observations. According to ASHRAE, performance is degraded by as much as 30% due to the build-up of lubricants on internal surfaces3. Higher percentages up to 40% have been observed in systems 20 years old or older.

Refrigeration Handbook, published by ASHRAE states: "Oil-fouling of heat transfer surfaces of air conditioning and refrigeration systems, will cause a loss of about 7% efficiency the first year, 5% the second year, and 2% per year the following years."4

Effects on the System

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 and the entire length of heat exchanger tubing 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.

Managing The Problem

The oil that finds its way into the system must somehow be managed.5 The obvious question then becomes how to manage this troublesome oil. Some of the techniques used by manufacturers to control migrating oil include the 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 of 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 problems of restricted or plugged capillary tubes or sticky expansion valves, they do not resolve the boundary layer fouling over time.

The Solution

The thermal transfer efficiency loss is resolved by IceCOLD's® non-invasive synthetic catalyst compound that defeats / releases the surface tension forces (the powerful van der Waals’ force6) that cause the compressor oil globules to adhere to each other and the to refrigerant tubing walls, forming the oil-fouling. Releasing the surface tension attraction of a organic contaminant (compressor oil) holding to a metallic surface. The surface tension defeating catalyst tightly bonds to the metal surfaces in a one-molecule-thick layer and prevents the recurrence of oil contamination on heat transfer surfaces. The release of the oil contamination restores the lost 20% to 30% thermal transfer efficiency. An added benefit is that the capillary tubes and expansion valves are also cleaned and protected from future fouling.

6 Surface tension forces are 1X10e39 times stronger that gravity

Conclusion

Mechanical devices can ease oil-fouling yield efficiency improvements in the two-to-four percent range, with a Return On Investment (ROI) of three to ten years. The synthetic catalyst to remove the surface tension with a payback of under a year and an ROI of 600%+, presenting a radical shift from "how we've always done it." The new technology is gaining acceptance due to the ASHRAE published information and the potential for 20% to 30% energy savings and the recent SAE certified J standard lab testing results.

  1. 1986 ASHRAE Handbook, Refrigeration, Chap. 3.6
  2. 1986 ASHRAE Handbook, Refrigeration, Chap. 3.6
  3. "A Survey of Refrigerant Heat Transfer and Pressure Drop Emphasizing Oil Effects and In-Tube Augmentation;" ASHRAE Winter Symposium of 1987; Schlager, Pate, and Bergles.
  4. 1998 ASHRAE Handbook, Refrigeration, Chap.2.9
  5. 1998 ASHRAE Handbook, Refrigeration, Chap. 2.9

Added by USRT:

"The role of lubricants in refrigeration compressors is to reduce friction, prevent wear and also to act as a seal between the high and low pressure sides of the compressor. In the rest of the system, however, the presence of lubricants acts as a contaminant which reduces the system efficiency."7

Ulf Jonsson
Division of Machine Elements
Luleå University of Technology
S-971 87 Luleå, Sweden