Could somebody please explain the vapor compression cycle?

In summary: Summary: In summary, the refrigeration cycle involves heat being taken from food in the refrigerated space to the evaporator, where a saturated vapor refrigerant flows through and changes phase. The compressor then increases pressure and temperature, and the refrigerant enters the condenser to reduce temperature and become a compressed liquid. The pressure drops and temperature decreases in the expansion valve, before the refrigerant re-enters the evaporator as a liquid. The textbook used may have simplified the process, but further resources such as Modern Refrigeration and Air Conditioning and Fundamentals of Thermodynamics can provide more detail.
  • #1
Carbon273
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So I have a somewhat general understanding of this refrigeration cycle. But I have a few concerns.
242712


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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  • #2
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.

Carbon273 said:
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.

Carbon273 said:
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.

Carbon273 said:
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.
 
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Carbon273 said:
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.
Carbon273 said:
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
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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Related to Could somebody please explain the vapor compression cycle?

1. What is the vapor compression cycle?

The vapor compression cycle is a thermodynamic process used in refrigeration and air conditioning systems to transfer heat from a lower temperature source to a higher temperature sink. This cycle involves the evaporation and condensation of a refrigerant fluid, which absorbs and releases heat as it changes state.

2. How does the vapor compression cycle work?

The vapor compression cycle works by using a compressor to increase the pressure and temperature of the refrigerant gas, which then flows through a condenser where it releases heat and condenses into a liquid. The liquid refrigerant then flows through an expansion valve, which lowers its pressure and temperature, causing it to evaporate and absorb heat from the surrounding environment. The cycle then repeats as the refrigerant gas is compressed again.

3. What are the components of the vapor compression cycle?

The components of the vapor compression cycle include a compressor, condenser, expansion valve, and evaporator. These components work together to circulate the refrigerant and transfer heat from one location to another. Other components, such as a fan and refrigerant lines, may also be included in the system.

4. What is the purpose of the vapor compression cycle?

The purpose of the vapor compression cycle is to provide cooling or heating in refrigeration and air conditioning systems. By transferring heat from one location to another, this cycle can create a comfortable indoor environment, preserve food and other perishable items, and facilitate various industrial processes.

5. What are the advantages of using the vapor compression cycle?

The vapor compression cycle offers several advantages, including high efficiency, reliability, and flexibility. It can be used in a wide range of applications and can be easily controlled to meet specific cooling or heating needs. Additionally, the use of a refrigerant allows for the transfer of large amounts of heat with relatively small temperature changes.

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