Vapour Pressure

[Stev]

I am a first year electrical apprentice and I am having trouble getting the refrigeration mechanics I work with to explain something to me. I thought I remembered my science lessons from 30 years ago fairly well so I asked the following.

In a standing system (where there pressure has been able to equalise throughout) for a given refrigerant you could say the following.

(a) if the pressure is below the vapour pressure for that gas and temperature, there is no liquid refrigerant remaining in the system. (This is assuming no contaminants)
(b) if the pressure is at vapour pressure, there is liquid refrigerant in the system but how much can not be known just from this information.
(c) the pressure can not be above the vapour pressure unless the system has been pumped full of liquid refrigerant or there are contanimants with a higher vapour pressure.

I am being told that refrigerants don't work this way but no one can explain why.

[SteinarN]

I am a first year electrical apprentice and I am having trouble getting the refrigeration mechanics I work with to explain something to me. I thought I remembered my science lessons from 30 years ago fairly well so I asked the following.

In a standing system (where there pressure has been able to equalise throughout) for a given refrigerant you could say the following.

(a) if the pressure is below the vapour pressure for that gas and temperature, there is no liquid refrigerant remaining in the system. (This is assuming no contaminants)
(b) if the pressure is at vapour pressure, there is liquid refrigerant in the system but how much can not be known just from this information.
(c) the pressure can not be above the vapour pressure unless the system has been pumped full of liquid refrigerant or there are contanimants with a higher vapour pressure.

I am being told that refrigerants don't work this way but no one can explain why.

As far as i understand it, your science from 30 years ago is absolutely correct assuming no contamination in the system.

[sparrow]

(a) if the pressure is below the vapour pressure for that gas and temperature, there is no liquid refrigerant remaining in the system. (This is assuming no contaminants)
(b) if the pressure is at vapour pressure, there is liquid refrigerant in the system but how much can not be known just from this information.
(c) the pressure can not be above the vapour pressure unless the system has been pumped full of liquid refrigerant or there are contanimants with a higher vapour pressure.

Stev, You need to try and understand the relationship between Pressure and temperature,in a refrigeration system .

It may help if you look up,the first and second law of thermodynamics, The pressure will stay constantly in proportion to the temperature as one or other is increased.

Regards

Sparrow

[Stev]

Stev, You need to try and understand the relationship between Pressure and temperature,in a refrigeration system .

It may help if you look up,the first and second law of thermodynamics, The pressure will stay constantly in proportion to the temperature as one or other is increased.

Regards

Sparrow


The first law is to do with work done on a system and does not relate to my question. The second law has to do with heat transfer and once again does not relate to my question.

The pressure/volume realtionship as expressed in the formula PV = nRT only applies to gases and is not appropriate for gas/liquid situations.

Assume that you have a vessel which holds a vacuum and you are able to maintain it at a constant temperature for the purposes of this thought experiment.

Add a gas gradually and the pressure will increase according to the PV=nRT relationship. For most gasses there is a dew point or vapour pressure at which the gas will begin to condensate. When this pressure is reached, the pressure will not increase when you add more gas because condensation will occur until the pressure drops back to the vapour pressure.

If there is liquid and gas present and you remove some gas, some of the liquid will evaporate until the pressure raises back to the vapour pressure.

Don't forget I am talking about a standing system and not an operating system.

[Gary]

In a standing system (where there pressure has been able to equalise throughout) for a given refrigerant you could say the following.

(a) if the pressure is below the vapour pressure for that gas and temperature, there is no liquid refrigerant remaining in the system. (This is assuming no contaminants)
(b) if the pressure is at vapour pressure, there is liquid refrigerant in the system but how much can not be known just from this information.
(c) the pressure can not be above the vapour pressure unless the system has been pumped full of liquid refrigerant or there are contanimants with a higher vapour pressure.


You are correct. The pressure/temperature relationship holds true only when there is both liquid and vapor.

For example, the low stage in a cascade system would burst at room temperature if it were subjected to the corresponding pressure. These systems contain only vapor when warmed above their normal operating range, thus the pressure is not excessive. These systems are vapor charged at room temperature for this very reason.

[chillymatt]

Hi,
You are correct in what you are saying in fact the same holds true for any liquid in a sealed container. The pressure temperature relationship is the fundamental concept to grasp where refrigeration is concerned.
Basically, a liquid under a higher pressure will 'boil' at a higher temperature.
One under a lower pressure will boil at a lower temperature. This is why you cant make a good cuppa on top of everest.. The air pressure is less and therefore the water boils at a lower temperature.
And also why pressure cookers work - the temperature of the water is higher than 100 degress when it starts to boil.

Consider that a refrigerant will always try and 'boil' away at normal ambient temperatures and atmospheric pressure.

Now consider the refrigerant in a sealed system such as a gas bottle. The liquid with no pressure exerted DOWN on it will 'boil' away. The vapour, not being able to escape will raise the pressure exterted on the liquid therefore raising its boiling point. This continues until the pressure is such that the liquid stops 'boiling' and the bottle stabilises with a mixture of liquid and vapour.

Now open the valve on the top... the pressure exerted down on the liquid is reduce therefore the boiling point of the liquid is lowered in relation to the lower pressure and the liquid starts to boil again.

So... which would have higher pressure? a cylinder with one inch of liquid in the bottom of it and the rest of the cylinder as vapour . OR a cylinder with 12 inches of liquid in it and the rest of the cylinder as vapour? ANSWER they would have the same pressure measured at the bottle provided the bottles were the same temperature.

Maniupulating the pressure temperature relationship through use of an expansion device and compressor is the principal of refrigeration.

Hope this helps!
Matt
ACRE Solutions LTD (Air conditioning and Refrigeration)

[paul_h]

I am a first year electrical apprentice and I am having trouble getting the refrigeration mechanics I work with to explain something to me. I thought I remembered my science lessons from 30 years ago fairly well so I asked the following.

In a standing system (where there pressure has been able to equalise throughout) for a given refrigerant you could say the following.

(a) if the pressure is below the vapour pressure for that gas and temperature, there is no liquid refrigerant remaining in the system. (This is assuming no contaminants)
(b) if the pressure is at vapour pressure, there is liquid refrigerant in the system but how much can not be known just from this information.
(c) the pressure can not be above the vapour pressure unless the system has been pumped full of liquid refrigerant or there are contanimants with a higher vapour pressure.

I am being told that refrigerants don't work this way but no one can explain why.
You are correct it's normal for that refrigerant to have some liquid at that ambient temp and pressure for a/cs, but that's not always the case in all refrigeration systems.
If you have some liquid in a system at a certain temperature, some of it will boil off until it reaches it's saturation point to reach equalibrium.
ie, If you look at an R410 system and the static pressure is only 500KPa, you know the sytem is short of refrigerant when the ambient is 20C
Likewise you know the system has contanimation/incondensibles if the static pressure was 2000kpa, as the pressure wouldn't be that high at saturation pressure for that ambient (edit: many use this info in order to tell if incondensibles are in the system after a pump down and letting the system cool to the ambient condtitions, then they read pressure and compare to PT chart because of this pressure/temp relationship)
Some refrigerations systems like gary pointed out don't have saturation pressures/temps that align with ambients due to the fact there's no equilibrium in a liquid/vapour state to compare to a PT chart and ambient, ie no liquid exists in the sytem for it to maintain a liquid/vapour relationship at high ambients due to it's design. But for a/cs and normal refrigeration units you are right, and it's something fridgies should know because its something that fridgies should use as a tool when diagnosing systems (daltons law, reading off the PT chart etc)

[Stev]

Thank you everyone for your replies. I think that there is mainly a communication problem happening which is causing a bit of confusion when I ask questions at work.