Purpose of Oil in a Refrigeration System

Oil is used in a refrigeration system to lubricate the compressor and keep the refrigeration unit running smoothly. Special formulations of oil are required to lubricate the internal components of a compressor, adding to the expense of manufacturing and operating cooling equipment while raising environmental issues such as disposal of waste oil and ozone depletion that contributes to global warming. While refrigeration systems will be required for the foreseeable future to preserve food and keep people comfortable, engineering advances into oil-free compression systems could change the way cold air is produced, but at a higher cost--at least for the short term.

The purpose of oil in a refrigeration system is to lubricate the compressor that produces cold air. Oil reduces friction on metal parts, reducing wear on the compressor and prolongs the life of the system.

Refrigerant oil is a special high-temperature formulation designed for use in cooling systems, so it is important to use the recommended lubricant for the equipment. Oil that is too heavy will not flow smoothly to all working parts. Oil that is too light will not adhere properly, causing inadequate lubrication.

The process of circulating refrigerant gas to cool a freezer, chiller or home refrigerator creates an increase in gas pressure and temperature. As the temperature rises, metal parts begin to heat up. This causes an increase in vapor pressure of the refrigerant oil, which advances through the system to lubricate the hot metal parts of the compressor. Cooling and lubricating the hot metal allows the compressor to operate efficiently while reducing wear and tear on the components.

The type of oil needed to lubricate a specific type of refrigeration equipment depends largely on compressor capacity, which is a measure of the system's ability to cool. This is measured in BTUs, or British Thermal Units.

Refrigeration systems depend on two main types of lubricant: synthetic and mineral oil.

While synthetic oil is longer lasting, mineral oil costs half as much and is typically used in industrial applications, such as air conditioning units for commercial buildings and grocery-store freezers.

The exact formulation of oil for a specific compressor depends on the size and power requirements of the compressor. Oil viscosity, which measures the resistance of the lubricant to various factors such as heat and pressure, is the main factor in determining the formulation of lubricant to be used in a compressor.

Compressors often are "seasoned" by injecting synthetic lubricant into the unit for an initial break-in period to test the equipment and ensure it is ready for use before installation. The compressor oil is then drained and a fresh charge of lubricant is installed before the unit is shipped from the manufacturer.

Some refrigerant compressors operate without oil, but are much more expensive to buy and maintain. As a result, most industrial refrigerants use oil, which poses environmental issues due to the periodic need to drain and replace the lubricant, and dispose of the old oil.

Compressor manufacturers are testing more environmentally-friendly lubricants, such as highly-refined mineral oil, as an alternative to fossil-fuel oils. Mineral oil is derived from crude oil, but contains fewer impurities. The goal of these compressors is to hold down costs while protecting the environment by reducing waste oil from heavy fossil fuels. However, refined mineral oil is also more expensive to produce.

These designs follow advances in refrigerant gas to reduce global warming by cutting back on the chlorofluorocarbons produced by older refrigerants.