Could somebody please explain the vapor compression cycle?

  • #1
Carbon273
17
4
TL;DR Summary
So I have a somewhat general understanding of this refrigeration cycle. But I have a few concerns.
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Heat is taken from food in the refrigerated space to the evaporator which has evaporated (I am assuming saturated vapor) refrigerant flowing through it.

My first stumble would be, if this is true, why doesn't the temperature increase for the fluid coming out of the evaporator (unless it is saturated vapor or rather a saturated liquid-vapor mixture)? Next, the refrigerant flows through the compressor where the pressure is increased. If someone can correct me on this, the temperature increases too because the compressor's piston device does boundary work on the refrigerant vapor in the cylinder?

With quick closed system piston device analysis, we can confirm that the internal energy increases due to the boundary work? The next step would be a high pressure, high temperature superheated vapor enters the condenser to simply reduce the temperature, thus changing the phase to a compressed liquid. Once the compressed liquid enters the expansion valve (capillary tubes), the pressure drops and the temperature drops due to the throttling effect. Thus, entering the evaporator as a liquid (I am assuming saturated liquid or a sat liquid-vapor mixture)

Really, all I am looking for is for a smoother understanding of this process, the textbook I am using glossed over it in a couple sentences.
 
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Answers and Replies

  • #2
ChemAir
Gold Member
172
178
First, realize the diagram is idealized to an extent made to be an easy example to follow, and it is simplified. Variable load systems don't necessarily behave this way, exactly. I believe they just want you to get the basic understanding.

why doesn't the temperature increase for the fluid coming out of the evaporator (unless it is saturated vapor or rather a saturated liquid-vapor mixture)?
In the absence of additional information or written text to go with it, I believe they are implying a saturated vapor-liquid mixture, that is stably changing phase.

Next, the refrigerant flows through the compressor where the pressure is increased. If someone can correct me on this, the temperature increases too because the compressor's piston device does boundary work on the refrigerant vapor in the cylinder? With quick closed system piston device analysis, we can confirm that the internal energy increases due to the boundary work?

The compressor does work on the system, increasing pressure and temperature. I'm not sure what "quick closed system piston device analysis" is, and not sure it matters for this example.

Really, all I am looking for is for a smoother understanding of this process, the textbook I am using glossed over it in a couple sentences.

Wikipedias article is here. Modern Refrigeration and Air Conditioning by Althouse/Turnquist/Bracciano, does a very good job showing more detail on actual equipment. There are also the Trane Air Conditioning manual and the ASHRAE Handbook if you want more sources.
 
  • #3
nichmetsa
3
1
Summary: So I have a somewhat general understanding of this refrigeration cycle. But I have a few concerns.

My first stumble would be, if this is true, why doesn't the temperature increase for the fluid coming out of the evaporator (unless it is saturated vapor or rather a saturated liquid-vapor mixture)?
Its because if the fluid is a mixture of vapor and liquid, all the heat that it receives is considered latent heat, so it is only used to change the fluids' state (mixture ratio, in this case increase vapor and decrease liquid), therefore its temperature does not change.
Summary: So I have a somewhat general understanding of this refrigeration cycle. But I have a few concerns.

With quick closed system piston device analysis, we can confirm that the internal energy increases due to the boundary work?
No, the internal energy would change if there was a mass flow from the boudary, in this case you could say that this system is in a permanent cycle, it all depends on your initial hypothesis.
 
  • #4
nichmetsa
3
1
I recommend finding a copy of this book, its the best thermodynamics book you can find for studying and it has a LOT of exercises:
Borgnakke and Sonntag Fundamentals of Thermodynamics
 
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