How to charge AC with propane?

[coolguy4]

The old R22 window AC that I filled with R134a is still running great, but it's time to try something new. So I bought R290 refrigerant (propane) to fill the system with. Yes, I know that it's flammable, so please don't go there. My question is what pressure do I fill it to? Or would I be better off just charging it in very small amounts with a thermometer in the vent and once the temp starts to go up, take a little out so it's the coldest? Thanks.

[Brian_UK]

If I remember your system has a capillary expansion device therefore charge it by sub-cooling.

[coolguy4]

If I remember your system has a capillary expansion device therefore charge it by sub-cooling.

Thank you. Yes, it has a cap tube. How do I measure the sub-cooling, and what should it be? Thanks again and sorry for the dumb questions, I am pretty new to HVAC.

[Brian_UK]

To measure sub-cooling you need to be able to measure the discharge pressure and the off-condenser pipe temperature. From a PT chart you can compare the two temperatures, perhaps try a start of 5K.

If you can't measure the above then a suction temperature above freezing is an aiming point, so probably about 60psi.

PT chart if you need one..... http://virtualmark.net/wx/pressure.htm

[coolguy4]

So I put in one 8 ounce can, and the low side pressure is around 55 PSI, and the vent temp is 48 degrees with an ambient temp of around 73, so the air temp is dropping about 25 degrees as it goes over the evaporator. The system originally took 13.6 ounces of R22, so how much propane would it need? I thought it took 42% of the original charge, so just under 6 ounces. However, the pressure still seems low after putting 8 ounces in. I don’t have a piercing valve on the discharge tube so I can’t measure head pressure, but I can install one if it would help.

[Rob White]

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Propane runs at a lower pressure.
@ 55 PSI it would be evaporating at about 34F - 35F

I would never use it in that system but that seems right to me

Rob

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[Rob White]

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Remember Propane work better at lower temps than higher ones,
freezer temps and such. It will run at a lower pressure.

Rob

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[Brian_UK]

Remember though that you are running an air conditioner not a freezer.

[coolguy4]

Thanks for your help. I bought a piercing valve to put on the tube coming out of the compressor so I can check head pressure too. I’ll report back when I have time to install the valve and get a head pressure reading.

[coolguy4]

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Propane runs at a lower pressure.
@ 55 PSI it would be evaporating at about 34F - 35F

I would never use it in that system but that seems right to me

Rob

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Just curious- Why wouldn’t you use propane in this system?

[Brian_UK]

Because hydrocarbon compressors have different sized pistons and cylinders as the gas needs to be pumped at a different rate from non-hydrocarbon fluids.
[Edit] The above is a rather generalised statement aimed towards the smaller compressors.

This document from Tecumseh might be of interest..
https://www.google.co.uk/url?sa=t&source=web&rct=j&url=http://www.tecumseh.com/~/media/South-America/Files/Catalogues/2017/English/Informativo-T%25C3%25A9cnico_Hidrocarbonetos.pdf&ved=2ahUKEwjdhf6S7sbaAhUqKMAKHWzEDHEQFjAAegQIBxAB&usg=AOvVaw2ZRBMveqcAhDvYO7fRiAxB

[Rob White]

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I agree with Brian.

As a toy and a test piece it is OK but I would
not choose to do it my self for the reasons Brian said
I know you only put 8 ounces of the stuff inside it but
the other reason I would not do it is the electrics on
the Compressor, motor, stat and other switches are
not rated for flammables and it might cause an issue
if the thing was ever to leak.

Rob

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[martinw58]

i thought you charged to super heat on capilery systems and sub cooling on txv systems without a receiver

[RANGER1]

i thought you charged to super heat on capilery systems and sub cooling on txv systems without a receiver

I would say both, so you know it’s not banking up in condenser.
Room would also be close to or in range of set point so stable conditions.
Propane to me same as R22 on P/T chart, just don’t need as much propane in system, as previously mentioned.