# Why do we need a compressor to heat vapor in vapor-compression refrigeration?

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:

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?

• 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 Commented Nov 18, 2022 at 18:41
• 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.
– Abel
Commented Nov 18, 2022 at 22:00
• 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. Commented Nov 19, 2022 at 21:01