Understanding Refrigeration & Reverse Carnot Cycle

[john t]

The refrigeration cycle is often likened to a reverse Carnot cycle. I pretty much understand the Carnot cycle, but in relating it to a household refrigerator, I am lost.

Where is the adiabatic compression done on the machine (inside vs. outside) , and where is the adiabatic expansion?

How is the change from insulated conditions (providing) q=0 to isothermal (providing delta E =0) accomplished?

 

[DrClaude]

Just like an internal combustion engine does not follow a Carnot cycle, a real refrigerator does not follow a reversed Carnot cycle. The Carnot cycle is just an ideal case, which happens to correspond to the highest efficiency/coefficient of performance possible.

For a real refrigerator, the actual cycle is (slightly) complicated due to the fact that the working substance undergoes a phase transition during the cycle.

 

[russ_watters]

https://en.wikipedia.org/wiki/Heat_pump_and_refrigeration_cycle#Vapor-compression_cycle

 

[Chestermiller]

When you refer to "inside" and "outside", I assume you are referring to central air conditioning, with the evaporator located above your furnace and the rest of the equipment outside the house. The refrigerant vapor (that has been returned by tubing from the Evaporator inside the house to the Compressor outside the house) is compressed "adiabatically" in the Compressor. Then, after the Compressor, there is warm outside air blowing over the tubes of a heat exchanger (also outside) to remove heat and condense the vapor. The condensed liquid refrigerant then flows through tubes into your house where it enters the Evaporator (at a lower pressure than the Compressor). The evaporator is just another heat exchanger where your house air from your ductwork is blown over the tubes. When the refrigerant evaporates in this heat exchanger, the house air is cooled.

 

[john t]

Thanks, Chestermiller. I guess the warm air brings about the "isothermal" stage outside the house, where the temperature of the gas remains relatively constant and heat is given up.