Aircon Pressure Question

[Ogben Schmutzel]

Hi All,

 

On the attched picture does anyone know what areas are high and low pressure?

 

And to what extent the pressure varies under load, i.e. in hot weather.

 

Cheers.

Oggy.

[sepulchrave]

Everything after the compressor is high pressure, every after the evaporator but before the compressor is low. The load should largely constant if the system is in use, but engine speed does make a difference, ambient temperature does not.

[Masked Marauder]

And with the engine off it is the same pressure through out

[Ogben Schmutzel]

Sorry guys - is the evaporator itself high or low?

 

Is the Thermostatic Expansion Valve the change point?

 

Showing my ignorance of how refrigeration works here!!

 

Regards.

Oggy.

[sepulchrave]

You are correct, the TEV is high pressure input, the evap. is low pressure out, transition occurs within these two components. As MM states, once the engine is off the system reaches equilibrium at static fill pressure.

[Alhambra]

When you look into the engine bay there are 2 black dust covers on 2 silver pipes.

 

The LEFT side is the LO pressure ... with a smaller diameter connector

 

The RIGHT side is the HIGH pressure side... ... with a larger diameter connector

 

These are different diameters so as you cannot make the wrong conncetion.

 

The Halfords re-gassing kit I used on mine has a Green zone, 0 to 25 psi & a Blue zone 25 to 45 psi.... above that is orange & red...... too high & very high pressure.

[Ogben Schmutzel]

Last question - I think!

 

Is the low pressure below atmospheric or is it just lower than the high??

 

Cheers.

Oggy.

[sepulchrave]

That is not an easy question to answer, however since the total pressure throughout the system will remain largely unchanged at about 3 bar (since CFC's are basically incompressible), then the high pressure side would need to be ramped up by the compressor to at least 9 bar in order for the low pressure to drop below 1 bar. I don't know how high it goes but this does seem possible given the nature of the pipework and connections.

 

The most important thing here is: Why on earth are you asking all these questions, and how will it help you fix your aircon?

 

It is a simple system containing needlessly expensive (but cheaply made) components and there is a LOT of mystification and nonsense talked about it. perhaps some form of tranquillizer might help reduce your anxiety levels. :16:

[Masked Marauder]

When running the low side is about 1.75 to 2 bar

[MrT]

That is not an easy question to answer, however since the total pressure throughout the system will remain largely unchanged at about 3 bar (since CFC's are basically incompressible),

 

Why do you believe that a CFC in its gaseous state is incompressible? Surely it is the compressing/condensing of the gas that heats it up and when it decompresses/evaporates it cools?

[johnb80]

In many air conditioning systems the gas is compressed and becomes a liquid, expansion roughly 1000:1 occurs when pressure is released (not quite so much when it goes through the evaporator).

[Masked Marauder]

Just for now, lets call a substance that is gasseous at room temperature at a pressure of 1 atmosphere a gas!

 

A gas that becomes liquid under low compression is not compressable.

 

IIRC R134a becomes a liquid at about 173psi, a pressure at which other gasses are still gas!

 

So refridgerant has been designed to be a poorly compressable gas.

[Ogben Schmutzel]

The most important thing here is: Why on earth are you asking all these questions, and how will it help you fix your aircon?

 

Fair question but is that really the most important thing here?

 

To answer your question; I am trying to understand how aircon works in a automotive situation. You may or may not be aware that I have a leak on my system somewhere, and yes there is a little anxiety attached to the fact that I may well have to spend significant sums replacing what doesn't need replacing.

 

The situation is that my climate control based aircon system held pressure for months over winter and then during the first few warm days it lost pressure. Had similar a while ago when it held pressure for a few days then on a hotter day the pressure went.

 

My measure that the pressure goes is three fold, 1) failure to cool, 2) whispering noise by the glove box, 3) followed a few days later by climate control unit flashing.

 

To date leak detection attempts have failed and before embarking on repair, or the process of elimination approach that dealers go for, I am trying to establish the most probable cause. So given the way that the pressure loss occurs I am wondering if that could be a clue. However to do try and do this I need to understand how it all works.

 

That, to me at least, is the most important thing!

 

Thanks for the responses to my questions, they really are appreciated, not sure that tranquilisers would be the answer though.

 

All the best.

Oggy.

[sepulchrave]

...Beta blockers? :16:

[NikpV]

I am trying to understand how aircon works in a automotive situation.

 

 

does this site help

 

http://www.familycar.com/Classroom/ac1.htm

 

 

[Ogben Schmutzel]

Just a bit :ph34r: :D :ph34r: :D :D

 

Thanks.

Oggy.

[NikpV]

Just a bit :D ;) :D :D :)

 

Thanks.

Oggy.

 

 

no problem ;)

[Guest rusper]

Right now we know how it works can enyone tell me where the low pressure fill port is on my '96 Galaxy?

I take it that its not the one just behind the bonnet slam panel, or Halfords sold me a duff refill kit. Tanks for any help.

Brian :) :)

[Guest rusper]

Right now we know how it works can enyone tell me where the low pressure fill port is on my '96 Galaxy?

I take it that its not the one just behind the bonnet slam panel, or Halfords sold me a duff refill kit. Tanks for any help.

Brian :) :)

Sorry to any one who was about to reply, found the answer in another thread, remove the right hand front indicator and there it is... obvious!!!!!

Thanks. Brian B) B) :) :)