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Thermodynamics: Refrigerators

Refrigerators

Refrigerators are devices that that will transfer heat from a low temperature environment to a high temperature environment. Now, as stated in the previous article, heat will only flow from a hotter environment to a colder environment. Which initially makes it seem like the concept of a refrigerator breaks the second law of thermodynamics. However, work can be used to transfer heat from the colder environment to a warmer environment. Below a basic diagram of a refrigerator can be seen.

Refrigeration Diagram

From the diagram it can be seen that a refrigerator consist of four basic components. These are a compressor, a condenser, an expansion valve (also known as a throttling valve), and an evaporator. Work is input into the compressor to compress the refrigerant to a high pressure as it enters the condenser, which is the network of tubes seen on the back of the refrigerator. While the refrigerant is being compressed to a high pressure, the temperature also increases to a higher temperature then the environment outside of the refrigerator, which in turn will cause the outside environment to act like a sink. After the refrigerant has gone through the condenser it will reach the expansion valve. The expansion valve will cause the pressure and temperature of the refrigerant to drop drastically. The temperature of the refrigerant will now be lower than the environment inside refrigerator. The cooled refrigerant will than enter the evaporator, which is a series of tubes inside of the refrigerator. As the refrigerant flows through the evaporator, the refrigerators environment will act like a source and heat will be transferred into the refrigerator causing it to evaporate. The refrigerant, which is now a gas, will enter the compressor again, where it will be compressed back into a liquid at a high pressure and temperature, and the process will repeat.

So essentially, using work to run the compressor, the outside environment can be transformed into a sink where the refrigerant will dump waste heat, while the environment inside the refrigerator will become a source where heat will be transferred into the refrigerant. This all happens because of the process described in the previous paragraph.

To see how well a refrigerator is performing the coefficient of performance can be calculated by using equation 1.

Coefficient of Performance Regrigerator (1)

The coefficient of performance represents the amount of heat removed in comparison to work in, and the higher the number the better that refrigerator is performing. This number can be greater than 1, which means that this number doesn't represent the efficiency of the refrigerator and should not be confused with efficiency.

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