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I'm reading the Wikipedia article on vapor-compression refrigeration.

In the cycle, first the refrigerant absorbs heat from the source to be cooled, which causes it to be vaporized. Then this vapor goes through a compressor, which increases the pressure and temperature of the vapor. From Wikipedia:

Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor2 and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air flowing across the coil or tubes.

I'm slightly confused about this stage. Why do we compress the vapor to increase its temperature, only to cool it down again in the condenser? The refrigerant has first absorbed heat from the heat source. Why don't we just conduct this vapor into the condenser, causing it to reject heat to cool down? It seems strange that we are artificially adding heat to the vapor, when removing heat from the heat source to be cooled is our goal.

In this part it is said:

From point 1 to point 2, the vapor is isentropically compressed (compressed at constant entropy) and exits the compressor as a superheated vapor. Superheat is the amount of heat added above the boiling point.

From point 2 to point 3, the vapor travels through part of the condenser which removes the superheat by cooling the vapor.

So we are adding superheat, only to remove it in the next step? The diagram from the section also confuses me:

enter image description here

The text explains that the compression at 1-2 is isentropic, which can also be seen from the T-S diagram. But the text also says that superheat is added at this point. But doesn't isentropic mean that no heat is added or removed from the system?

So to summarize: are we adding heat to the refrigerant during the compression stage or not? And if we are, why are doing it only to remove it in the next step, rather than just conducting the saturated vapor through the condenser to give up heat?

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  • $\begingroup$ Why do we need a compressor to heat vapor in vapor-compression refrigeration? ... not quite correctly worded question ... it should be Why do we need to compress the working fluid in vapor-compression refrigeration? ... to achieve phase change in the working fluid ... that gives off the greatest amount if heat which is then dissipated $\endgroup$
    – jsotola
    Commented Nov 18, 2022 at 18:41
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    $\begingroup$ Heat flows from high temperature to low temperature. So how do you get heat to flow out of something cold? Give it something colder: refrigerant. Now how do you get it out of the refrigerant (you need to get it cold again to reuse it after all)? Get it hotter than the environment, but without heat flowing into it (more correctly minimal heat flowing into it), and let the heat flow out into the environment. Compression serves this process for specific materials used as refrigerants! We gain the ability to move heat by physically moving refrigerant. $\endgroup$
    – Abel
    Commented Nov 18, 2022 at 22:00
  • $\begingroup$ consider that the key thing a refrigeration cycle does is move heat from cold to hot, you need the phase changes to happen at temperatures that move heat from hot to cold the way God intended. $\endgroup$
    – Tiger Guy
    Commented Nov 19, 2022 at 21:01

2 Answers 2

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Because the compressor is to « push » the fluid around the circuit and as the compressors are designed to pump a gas part of the compression process is a temperature rise.

As the compressor output goes to the condenser then that is not a big problem, as long as the condenser can reject the heat.

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The compressor simply compresses the refrigerant gas. Compression itself increases the temperature. In order to "pump" heat, we need the refrigerant to be above the ambient temperature around the condensor (heat can only flow from hot to cold). So the hot gas leaving the compressor releases its heat to the hot region of the circuit and "condenses" into a liquid in the process.

The superheat portion has to do with the amount of heat the gas must give up at a given pressure before it begins to condense, reducing in volume and temperature as it goes. Once it reaches zero superheat, it remains at the same temperature and pressure and condenses to a liquid, after which it may continue to cool (subcooling) as a liquid. Superheat has nothing to do with whether the compression is isentropic - the paths taken by the refrigerant in a heat-pump cycle are constrained by thermodynamic limits - the gas can't be compressed into any desired state. The best (most efficient) compression possible is isentropic, everything worse will require even more work and push point 2 up and to the right, resulting in greater superheat, but it's not possible to push point 2 down and to the left beyond the isentropic limit. Note that refrigerant cycles do not require a phase change - it's possible to construct a heat pump where the refrigerant remains a gas at all times. For example, a Stirling engine can theoretically* achieve Carnot efficiency with no phase changes (*in practice not so much).

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