Why does the refrigerant in an air-conditioner's evaporator become cold?

  • #1

Main Question or Discussion Point

I believe that when refrigerant in an air-conditioner's evaporator boils into gas, the pressure on the refrigerant goes down tremendously. I believe that the drop in pressure on the refrigerant when it becomes a gas causes the temperature of the refrigerant to drop. I believe this because I remember that there is an ideal gas law that states that the Pressure of a gas is proportional to its temperature.

Does the refrigerant in an air-conditioner's evaporator become cold when it boils into a gas because the pressure on the refrigerant decreases?
 

Answers and Replies

  • #2
2,685
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The temperature is dependent on pressure and volume. If you increase the volume as you increase the temperature, the pressure won't change.

Now, for air conditioners they generally use evaporation to cool the air.

You are right, when the coolant evaporates the temperature drops. It is this cool gas that is used to cool the air.

Just a note: for evaporation to occur, the liquid does not have to boil - do puddles boil when they evaporate?

This article describes how air conditioners work with a nice image to accompany: http://home.howstuffworks.com/ac1.htm
 
  • #3
S_Happens
Gold Member
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The refrigerants are non-ideal gases, so even ignoring everything else there's more here to look at than simple decrease in pressure. Are you just guessing that there must be a pressure drop?

To me your post sounds unaware that there are a compressor and two heat exchangers in the system, or possibly unaware of the pressure difference across a compressor, specifically in a closed system.
 
  • #4
2,685
20
The refrigerants are non-ideal gases, so even ignoring everything else there's more here to look at than simple decrease in pressure. Are you just guessing that there must be a pressure drop?

To me your post sounds unaware that there are a compressor and two heat exchangers in the system, or possibly unaware of the pressure difference across a compressor, specifically in a closed system.
Who, me?
 
  • #5
russ_watters
Mentor
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You are right, when the coolant evaporates the temperature drops. It is this cool gas that is used to cool the air.
No, it doesn't.

The link you posted isn't correct in the description: When a substance exists simultaneously as a liquid and gas and you input heat at constant pressure, it evaporates without changing the temperature. This is similar to how ice-water is always at 0C.

An air conditioner passes the high pressure, warm liquid refrigerant through a throttling valve, where the pressure drops, some of it changes to gas and the temperature drops.

In the evaporator, the cold liquid/gas mixture is evaporated into all gas, at constant temperure and pressure (there may be some superheating at the end...) due to the input of heat from the air.

See the T-S diagram at the bottom of this link: http://engr.bd.psu.edu/davej/classes/thermo/chapter7.html

and the description:
for refrigeration air conditioning and heat pumps
Processes:

1-2 (compressor) Isentropic compression
2-3 (condenser) Constant pressure heat rejection
3-4 (expansion valve) throttling, isenthalpic
4-1 (evaporator) constant pressure and temperature heat addition
 
  • #6
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  • #7
russ_watters
Mentor
19,573
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Then I apologise for lacking the word "some".
Where would you put it? Evaporation is a consequence, not a cause.
 
  • #8
2,685
20
An air conditioner passes the high pressure, warm liquid refrigerant through a throttling valve, where the pressure drops, some of it changes to gas and the temperature drops.
Is this change not occurring by evaporation? (I honestly don't know.)

I assumed it was and so would have altered it as follows:

"when some of the coolant evaporates the temperature drops" - I wasn't really referencing stages, even in the original I was simply referencing a phase change as being related to the temperature drop. Didn't mean it as evaporation being the cause. I was a tad loose with the wording to say the least.
 
  • #9
4,662
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  • #10
2,685
20
read about the theory of the J-T (Joule-Thomson) throttling valve and temperature drop of compressed gases in

http://en.wikipedia.org/wiki/Joule–Thomson_effect

This J-T cooling applies only to real gases. ideal gases won't work.

Bob S
Marvellous, that's sorted it.

I do apologise, I was wrong.
 
  • #11
S_Happens
Gold Member
305
3
Who, me?
No, it was for the OP. It was obvious that they didn't understand the importance of the refrigerant being non-ideal, but also looked like they didn't even understand the components of a refrigeration cycle.

I wanted to work towards Joule-Thomson after getting an answer, but was beaten to it.
 
  • #12
PhilKravitz
I have not seen anybody include the equation PV=nRT so I will.

1) So more pressure more temperature, heat can then be lost to surrounding air through heat exchanger
2) Explained the volume lowering the pressure lowering the temperature, heat can be absorbed through heat exchanger from the food in the frig
3) repeat no condensing/evaporating is required but we could make use of the heat of vaporization to to make this move more heat per volume of working fluid
 
  • #13
The temperature is dependent on pressure and volume. If you increase the volume as you increase the temperature, the pressure won't change.
Yes; but I don't think that the volume changes when the pressure goes down.

Now, for air conditioners they generally use evaporation to cool the air.
I think that all air-conditioners use evaporation to cool the air.

Just a note: for evaporation to occur, the liquid does not have to boil - do puddles boil when they evaporate?
In a closed space such as an evaporator coil, I think that the liquid does have to boil to evaporate.
 
  • #14
2,685
20
Yes; but I don't think that the volume changes when the pressure goes down.

I think that all air-conditioners use evaporation to cool the air.
We established long ago that my post was incorrect, I see no purpose in discussing it further. The correct answer has been given to you (and me) by others. Just flogging a dead horse now.
In a closed space such as an evaporator coil, I think that the liquid does have to boil to evaporate.
I was simply commenting on it not being necessary for a liquid to boil in order for it to evaporate. Not that it isn't required / possible in certain cases, just that it doesn't apply to everything.
 
  • #15
The refrigerants are non-ideal gases, so even ignoring everything else there's more here to look at than simple decrease in pressure.
What are the other factors to look at than just the simple decrease in pressure? I think that the volume stays constant.

Are you just guessing that there must be a pressure drop?
Yes; but it's an educated guess. I believe that there must be a pressure drop because the refrigerant changes from liquid to gas.


To me your post sounds unaware that there are a compressor and two heat exchangers in the system, or possibly unaware of the pressure difference across a compressor, specifically in a closed system.
I am aware of the fact that air-conditioners have compressors. You say that there are two heat exchangers in the system. Are you calling the indoor coil (evaporator) and the outdoor coil (condenser) the heat exchangers? If not, what are the heat exchangers?

I'm not sure what you mean by "possibly unaware of the pressure difference across a compressor." I know that there is high pressure on the discharge line from the compressor, and I know that there is much less pressure after the refrigerant goes through the metering device on the suction line.
 
  • #16
In the evaporator, the cold liquid/gas mixture is evaporated into all gas, at constant temperure and pressure (there may be some superheating at the end...) due to the input of heat from the air.
This is my new understanding: The refrigerant does not get colder in the evaporator. The refrigerant loses most of its heat in the condenser, and then the refrigerant becomes even colder because of going through the metering device.
 
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