Opening A/C system for repair while saving refrigerant?

[NeoPTLD]

In any system containing condensible vapor, the condensation occurs at the point of lowest temperature and the vapor pressure is that of the cold point.

So, theoretically, if I cool down receiver or the condenser with dry ice, the vapor pressure of refrigerant would what it has at the temperature or dry ice and it would accumulate where it is cooled by dry ice.


So, can I chill the accumulator with dry ice or liquid nitrogen and open the system to replace the expansion valve with minimal refrigerant loss?

It's an older car with R12 so I'd rather not lose the charge.

 

[imagoon]

R12 is liquid at around -30 degrees F so in theory that would work since dry ice is around -110 the problem will be air and water condensation contamination... beyond the issue of frost bite while trying to fix it. I am also not sure without a ton effort that you could keep the system cold enough. It would be easier to use an extraction pump and store the charge. That way you could pressure test the system afterwards and not lose the charge.

 

[CycloWizard]

It doesn't really make sense to say that "vapor pressure is that of the cold point." The vapor pressure will vary at different points in the system due to temperature variations. In other words, it's a local property of the system rather than a global variable. Thus, if you only cool part of the system, you will still see evaporation elsewhere. If you are able to cool all of the liquid, then you will be able to make this work.

That said, I would highly recommend against your approach even if you could find a way to make it work. Cooling your system that much could cause cracks to form due to thermal stresses, especially in an older engine.

 

[imagoon]

CycloWizard reminded me of another point.

A/C is a charge of R12 the in both liquid and gas form (plus some mineral oil) If you did manage to cool the entire system to -30F (at 1 atm, most R12 is around 2 atm so it liquifies at a warmer temperature in system) to make the R12 liquid (ignoring the vapor pressure that would force a much cooler temp than that to make vacuum) you will now have a negative pressure with relation to atmosphere so once you crack the system, ambient air would enter and further contaminate the system.

 

[leper84]

CycloWizard reminded me of another point.

A/C is a charge of R12 the in both liquid and gas form (plus some mineral oil) If you did manage to cool the entire system to -30F (at 1 atm, most R12 is around 2 atm so it liquifies at a warmer temperature in system) to make the R12 liquid (ignoring the vapor pressure that would force a much cooler temp than that to make vacuum) you will now have a negative pressure with relation to atmosphere so once you crack the system, ambient air would enter and further contaminate the system.

This.

Op, For it to work right you're gonna have to pull a vacuum to at least 29 in/hg which would cause a pressure drop, evaporate and remove any refrigerant left.

Rent a set of gauges and a vacuum pump at autozone. See if you can't find an empty re-sealable refrigerant canister, pull a good vacuum on it. Hook the evacuated can up to the gauges, leave the low side (compressor input) line closed and hook up the high side (compressor output) to your car. Open up the high side and line to the can which will let the r12 from your car into the can. Once the pressure equalizes between the car and the can you can very briefly turn the car on and turn on the a/c to get the compressor to engage.

The compressor should push most of the rest of r12 into the can, once your gauges show a vacumm on the car close em up and turn the compressor off. One problem with this is depending on the car there might/should be a low pressure shut off switch for the compressor, which you may have to temporarily bypass.

From that point do your work, always always always put on a new drier, and you might as well go for a new orifice tube/expansion valve. Pull a good vacuum, if you aren't going to go fancy with a vacuum gauge, let the a/c gauges max out in the negative then let the vacuum pump run for another 10-15 min to be safe. Then hook back up to the can of your old r12, open that and the low side valves and let the pressure equalize. After that turn on the car and the a/c and since you're going in through the low side, the compressor will suck the rest of the r12 into the system.

Its easier than I typed it out to be.

 

[philipma1957]

This.

Op, For it to work right you're gonna have to pull a vacuum to at least 29 in/hg which would cause a pressure drop, evaporate and remove any refrigerant left.

Rent a set of gauges and a vacuum pump at autozone. See if you can't find an empty re-sealable refrigerant canister, pull a good vacuum on it. Hook the evacuated can up to the gauges, leave the low side (compressor input) line closed and hook up the high side (compressor output) to your car. Open up the high side and line to the can which will let the r12 from your car into the can. Once the pressure equalizes between the car and the can you can very briefly turn the car on and turn on the a/c to get the compressor to engage.

The compressor should push most of the rest of r12 into the can, once your gauges show a vacumm on the car close em up and turn the compressor off. One problem with this is depending on the car there might/should be a low pressure shut off switch for the compressor, which you may have to temporarily bypass.

From that point do your work, always always always put on a new drier, and you might as well go for a new orifice tube/expansion valve. Pull a good vacuum, if you aren't going to go fancy with a vacuum gauge, let the a/c gauges max out in the negative then let the vacuum pump run for another 10-15 min to be safe. Then hook back up to the can of your old r12, open that and the low side valves and let the pressure equalize. After that turn on the car and the a/c and since you're going in through the low side, the compressor will suck the rest of the r12 into the system.

Its easier than I typed it out to be.

THIS you have to use a vacumn pump or you will contaminate the system.

 

[NeoPTLD]

R12 is liquid at around -30 degrees F so in theory that would work since dry ice is around -110 the problem will be air and water condensation contamination... beyond the issue of frost bite while trying to fix it. I am also not sure without a ton effort that you could keep the system cold enough. It would be easier to use an extraction pump and store the charge. That way you could pressure test the system afterwards and not lose the charge.

It wouldn't be necessary to chill the entire system.

Suppose you have an inverted U-tube filled with a gas that condenses at -10C. If you chill one end of the U to -30C, everything will condense to the cold spot.

The pressure within the entire tube will be the saturated vapor pressure of the contents at -30C. So, I'm thinking that if receiver is large enough to hold the entire system charge, if I just chill that part, all the refrigerant in the system will migrate to the cold spot.

 

[imagoon]

It wouldn't be necessary to chill the entire system.

Suppose you have an inverted U-tube filled with a gas that condenses at -10C. If you chill one end of the U to -30C, everything will condense to the cold spot.

The pressure within the entire tube will be the saturated vapor pressure of the contents at -30C. So, I'm thinking that if receiver is large enough to hold the entire system charge, if I just chill that part, all the refrigerant in the system will migrate to the cold spot.

Actually, no it won't. You will build a heat transfer loop which would accelerate loses. Add in vapor pressure loses. No different than how water will still evaporate [sublimate] from a cold cup.

This also still ignores the contamination issues.

 

[CycloWizard]

It wouldn't be necessary to chill the entire system.

Suppose you have an inverted U-tube filled with a gas that condenses at -10C. If you chill one end of the U to -30C, everything will condense to the cold spot.

The pressure within the entire tube will be the saturated vapor pressure of the contents at -30C. So, I'm thinking that if receiver is large enough to hold the entire system charge, if I just chill that part, all the refrigerant in the system will migrate to the cold spot.

The only place where temperature matters is at the gas-liquid interface. The partial pressure of refrigerant at the interface will be the vapor pressure at the interface temperature. The partial pressure elsewhere in the system depends on a lot of things (i.e. mass transfer driving forces and resistances). At steady state, if the system is sealed to the environment, then what you say is true and the partial pressure will be equal to the vapor pressure everywhere. However, the present system we're considering is the unsealed case where the steady state partial pressure is zero since the refrigerant will always be evaporating.

 

[C1]

Instead of going thru all that trouble, why dont you just have the system repaired by a licensed facility? They have the necessary recovery system, tools and equipment. Anyways, the system needs to be tested for leaks and proper operation after the valve replacement.

Also too bad as I have an unopened (still sealed) 15lb R12 cylinder that I bought years ago in anticipation of the R12 ban, but turns out never needed to use it. Apparently now it is even illegal to advertise selling it. Not sure what will happen to it.

 

[wirednuts]

ridiculous idea, op. just do it right. evacuate they system and recover the r12. you wont lose a drop and theres no chance you will hurt anything. dont try to complicate the simple

 

[Modelworks]

Why not just use R134A ? Most cars can be converted with just a vacuum pump and change out of orings.

 

[wirednuts]

Why not just use R134A ? Most cars can be converted with just a vacuum pump and change out of orings.

:/ not a terrible idea, but r12 is way 'cooler' pun intended

 

[bryanl]

Why not just use R134A ? Most cars can be converted with just a vacuum pump and change out of orings.

R134A requires a different and incompatible oil, it works at higher pressure that an R12 compressor may be able to tolerate, and cooling will not be good at all in a system regulated by an orfice tube and pressure switch rather tha TXV. R12 is expensive but still cheaper than the destruction caused by a bad R134A conversion.

 

[Shawn]

there are r12 substitutes that should work which are compatible with the r12 oil and compressor. never tried it myself though.

 

[imagoon]

there are r12 substitutes that should work which are compatible with the r12 oil and compressor. never tried it myself though.

I think R402A is the sub?