babblefish said:
Now I see it, small line (high pressure?) and large line (low pressure?) connected together.
This is probably way too much information for what you want but it has taken me a while to figure out how an automotive AC system works on my Safari and I thought I'd share it in case it interests anyone.
Any corrections would be appreciated and you can call me a science troll if you like. :lol:
If you had a garden hose with water flowing through it and you had a fixture on the end of it like the orifice tube you'd get a spray, right? The pressure of the spray would be lower than the water coming down the hose as well.
When a compressor compresses the low pressure refrigerant gas into high pressure, the gas become a high pressure liquid. Butane gas can be liquefied at 75 PSI if temperature = 42C. R134a has a boiling point of -26C, meaning it converts to gas at that T at 1 atmosphere pressure (14.7 PSI).
If you raise the pressure of the gas enough, it remains as a liquid till a higher temperature. If it's already a gas, it reverts to a liquid.
The high side of the compressor can hit 200 PSI at least and 200 PSI is 13.6 atmospheres. At that pressure, the boiling point of the gas will rise to over 300 C, therefore it is in a liquid state coming out of the compressor high side. When the gas is compressed to liquid state it's temperature increases.
The liquid maintains it's high pressure through the condenser but as it loses heat in the condenser to the environment, the pressure drops a bit. The line coming out of the condenser is regarded a high pressure line so the refrigerant should still be in a liquid state as it reaches the orifice tube.
Maybe someone with experience could indicate what pressure to expect at the Shrader valve in that line.
For those interested in math:
Ideal gas equation: PV = nRT
Don't go away, it's easy.
P = pressure, V = volume, n = number of molecules of gas, R = gas constant, T = temperature
the volume is constant due to running through a pipe and so is the number of molecules (provided you keep your refrigerant at the correct level).
therefore, equation can be written P = (nR/V)T
since nR/V is constant, while P and T vary, it tells you P varies proportionately with T. In English, if the pressure goes up the temperature goes up, and vice-versa (provided the volume and weight of the gas remain constant).
Note that P varies with the number of molecules (essentially the weight on a planet like Earth) and if you let the weight of the refrigerant get too low, the P drops in proportion. That messes with the operation of the entire system.
When you compress the gas in the compressor, P goes up and T goes up in proportion. However, going through the condenser, the liquid refrigerant loses heat to the environment, therefore P goes down a bit as T drops. That would depend also on the outside environment, how hot or cold a day it might be.
At the orifice tube, it's still liquid, and if you opened that Shrader valve just before the brass nut, 'BRIEFLY', which you should not while the compressor is running, it would shoot out a liquid mixed with green PAG oil.
Don't ask me how I know. All I can say is that a good amount of pressure can remain in that line even with the gas bled out of the low pressure end, so be careful.
Besides being illegal to vent R134a into the atmosphere it's also very dangerous. This gas is seriously toxic in sufficient quantity and could kill you if enough is inhaled.
At the orifice tube, what's left of the liquid is atomized into a spray at lower pressure for entry into the evapourator. In the evapourator, the spray receives heat from the cab air and turns back into a low pressure gas as it expands in the evap. From there, the LP gas goes through the accumulator where dessicants dry it out. The accum also stores excess PAG oil, or whatever is used in the system.