Understanding Refrigeration & Reverse Carnot Cycle

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Discussion Overview

The discussion centers around the refrigeration cycle and its relationship to the reverse Carnot cycle. Participants explore the mechanics of refrigeration systems, particularly in household applications, and the thermodynamic processes involved, including adiabatic compression and expansion, as well as phase transitions in real refrigerators.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the adiabatic compression and expansion processes in a household refrigerator, questioning where these occur within the system.
  • Another participant argues that real refrigerators do not follow a reversed Carnot cycle, noting that the Carnot cycle is an idealized model representing maximum efficiency.
  • A link to a Wikipedia article on heat pumps and refrigeration cycles is provided for additional context.
  • A participant describes the process of adiabatic compression occurring in the compressor located outside the house, detailing the flow of refrigerant and the role of heat exchangers in the cycle.
  • One participant acknowledges the explanation and connects the concept of isothermal conditions to the heat exchange process occurring outside the house.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the relationship between the refrigeration cycle and the reverse Carnot cycle, with some acknowledging the idealized nature of the Carnot cycle while others focus on the practical aspects of real refrigeration systems.

Contextual Notes

Participants discuss the complexities of real refrigeration cycles, including phase transitions and the specifics of heat exchange, without resolving the implications of these factors on the idealized models.

Who May Find This Useful

This discussion may be of interest to those studying thermodynamics, engineering students, or individuals seeking to understand the practical applications of refrigeration technology.

john t
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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?
 
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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.
 
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.
 
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.
 

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