Refrigerator Cycle Compressor Function

[Chacabucogod]

I understand that you need certain work for the refrigerator to work, and that it comes from the compressor. Why isn't a pump used instead? Why do you have to compress and heat the air so much?

Thank you

 

[SteamKing]

In a refrigerator, the working fluid is generally not air.

http://en.wikipedia.org/wiki/Vapor-compression_refrigeration

The compressor acts essentially as a pump in this system, turning vapor at low pressure into vapor at high pressure.

The high pressure vapor leaving the compressor is cooled in a condenser and turned into a high pressure liquid. The high pressure liquid is then passed into an expansion valve and allowed to change into a mixture of liquid and vapor, which has a much lower temperature than ambient. This cooled mixture is passed through an evaporator which takes heat from inside the refrigerator to evaporate the rest of the liquid, so that only vapor comes out of the evaporator and enters the compressor.

 

[russ_watters]

I understand that you need certain work for the refrigerator to work, and that it comes from the compressor. Why isn't a pump used instead? Why do you have to compress and heat the air so much?

A pump and a compressor are the same thing, with the choice in wording being based on pressure vs flow. Higher pressure and lower flow? Call it a compressor. Not that's critical to the question...

Energy is transferred into and out of the working fluid via a temperature difference between the working fluid and the reservoir. Smaller DT means you need a bigger heat exchanger and more flow for the same energy transfer.

On the cold end, you don't want your coils to freeze, so your fluid should be safely above freezing. But you want the air to be cooled to 55F to dehumidify it enough, so the temperature should be somewhere between 40F and 50F. And the pressure determines the boiling temperature.

Your turn: how hot does the high temperature side need to be?

 

[jack action]

To cool the air, you heat up the fluid with the «inside» air. This removes the energy from the «inside» air, hence cooling it down, but then the fluid temperature will never be able to be higher than the temperature from the «inside» air.

So, to transfer the acquired heat to the «outside» air (which is at higher temperature than the «inside» air), you need to compress the fluid to raise its temperature such that there is a temperature difference and that the fluid can be cool down in the condenser, thus transferring the energy to the «outside» air.