Principles of the Vapor Compression Refrigeration System

Adapted by: Harlan Bengtson • Edited by: Lamar Stonecypher
Updated Feb 8, 2011 • Related Guides: Refrigeration | EPA

The vapor compression cycle is widely used in refrigeration systems, including refrigerators, refrigeration air conditioning, freezers, and auto air conditioners. R22 and R12 refrigerant are the most commonly used, but are being phased out due to effects on the ozone layer.

Introduction to Refrigeration Systems

In very basic terms, refrigeration systems are used to remove heat from one area and transfer it to another location. This article gives some details about the vapor compression refrigeration cycle, which is very widely used for many types of refrigeration systems, including home refrigerators and freezers, refrigeration air conditioning, and automobile air conditioners. Read on for information about the vapor compression cycle, how it works, and how it is used in to provide refrigeration.

The Vapor Compression Refrigeration Cycle

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The diagram at the left shows the components of a vapor-compression refrigeration cycle: a compressor, condenser, expansion valve, and evaporator. A low pressure, low temperature liquid is converted to vapor in the evaporator, thus absorbing heat from the refrigerated space and keeping that space cool. The fluid is driven around the cycle by the compressor, which compresses the low temperature, low pressure vapor leaving the evaporator to high pressure, high temperature vapor. That vapor is condensed to liquid in the condenser, thus giving off heat at a high temperature to the surrounding environment. Finally, the high pressure, high temperature liquid leaving the condenser is cooled and reduced in pressure by passing it through an expansion valve. This provides the input to the evaporator which was the first step of the cycle described above.

The work and heat flows shown in the diagram are W_in, Q_H and Q_L. W_in is the work input to the compressor. The rate of work input to the compressor is most of the power requirement to run the refrigeration system. Power will probably be needed to drive one or more fans, but their power requirement will be small in comparison with that needed to drive the compressor. Q_H is the high temperature heat rejected to the surroundings by the condenser. Q_L is the low temperature heat absorbed from the cooled space by the evaporator.

Applications of Vapor Compression Refrigeration

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The vapor compression refrigeration cycle described in the previous section is widely used for a variety of cooling purposes, such as household refrigerators and freezers; home, industrial and commercial air conditioners; and automobile air conditioners. The locations of the components of a vapor compression cycle in a household refrigerator/freezer are shown in the diagram at the right. The evaporator coils are located in the freezer compartment, so low temperature heat is removed from that freezer compartment, keeping it as the coldest location in the refrigerator freezer. The compressor is shown at the bottom on the back of the refrigerator. It sends its compressed, high temperature vapor output through the condenser coils at the back of the refrigerator, where heat is given off to the air outside the refrigerator. Finally, the capillary coil shown at the upper part of the refrigerator back serves as the expansion valve, that cools the liquid leaving the condenser as its pressure is reduced, providing the input flow to the evaporator.

Image Credit: http://cooling device.net4.html

Improvement of Refrigeration Efficiency

In order to improve the efficiency of a refrigeration system, the following guidelines will be useful:

• The refrigerant characteristics should allow for a high condensation temperature for heat rejection to the surroundings and a low evaporation temperature for heat absorption from the cooled space.

• Air filtration through doors and other gaps should be minimized.

• The pressure drop of the refrigerant in suction and discharge lines should be minimumized.

• Good lubrication of moving parts needs to be maintained.

• Pipes of correct size should be used, avoiding unnecessary bends, in order to minimumize the pressure drop.

Phaseout of R22 and R12 Refrigerants

Freon 22, also known as R22, is the most common refrigerant in home air conditioning and refrigeration systems. Freon 12, also known as R12, is used in automobile air conditioners. Due to concerns about damage to the earth's ozone layer, the production and use of chloroflourocarbons like R22 and R12 refrigerants is being phased out around the world and substitute refrigerants are appearing to replace them.

References

1. Manning, L, The Ideal Vapor-Compression Refrigeration Cycle, Department of Mechanical Engineering, Univ of Nevada Reno

2. Shet, U.S.P, Sundararajan, T, and Mallikarjuna, J.M., Simple Vapor Compression Refrigeration System, Indian Institute of Technology Madras

3. U.S. EPA, What You Should Know abut Refrigerants When Purchasing or Repairing a Residential A/C System or Heat Pump.