Air Conditioning specifications: What do they mean exactly?

[Fred Byrne]

I looked up the a/c data on page 56 of the Workshop Manual the figures given are extraordinarily high. Namely, Pressure high side = 35.3 Bar and pressure low side 16.7 Bar. Are these the pressures that my manifold should record with the engine running and the a/c on full blast? If my memory is correct 1 Bar= 15psi approximately. These pressures terrify me. The trinary switch pressure is slightly lower at 31.4 bar.

[COLVERT]

Well you do have to squash that gas back down into being a liquid in order to use it again.---

[Fred Byrne]

It still seems like one helluva lot to me

[T-Cut]

Originally Posted by Fred Byrne

I looked up the a/c data on page 56 of the Workshop Manual the figures given are extraordinarily high. Namely, Pressure high side = 35.3 Bar and pressure low side 16.7 Bar. Are these the pressures that my manifold should record with the engine running and the a/c on full blast? If my memory is correct 1 Bar= 15psi approximately. These pressures terrify me.

Yes the pressures are approximately what a typical aircon system will show when operating. The refrigerant is a gas at normal ambient temperatures and pressure, but will compress into a liquid (normal boiling point about -26C). So, when it's in the aircon system it's an 'equilibrium mixture' of liquid and gas with a pressure somewhere around the 'low' figure. When the aircon's running, this mixture is compressed further by the compressor so it's all essentially liquid (and hot). That's the upper pressure quoted. The liquid is injected into the evaporator, where the pressure is much lower, so it evaporates and in doing so absorbes heat from the airflow passing through the evaporator. The airflow is chilled. The trinary valve is set at the upper and lower operating pressures for the system and will prevent the compressor working if the refrigerant pressure is not in the preset operating range. The pressures aren't to be feared unless you intend opening valves that shouldn't be opened.


TC

[SD1too]

Originally Posted by Fred Byrne

... Pressure high side = 35.3 Bar and pressure low side 16.7 Bar. Are these the pressures that my manifold should record with the engine running and the a/c on full blast?

No Fred, they're not. They appear to be the maximum pressures achievable by the compressor, i.e. its specification.

To get an idea of normal service pressures look lower down at the trinary switch data and you'll see that the condenser/radiator fan speed is increased at a high side refrigerant pressure of about 15 bar (noting that the figures are printed the wrong way around in RAVE). The fan speed returns to low at a high side pressure of about 12 bar. This is approximately the figure which you will normally see at the high side Schrader valve.

Note that the trinary switch disengages the compressor if the pressure rises to about 31 bar, therefore this is regarded as abnormal and potentially harmful to the system.

I hope that I have put your mind at rest!

Simon

[Fred Byrne]

Thank you very much for that Simon. As a physicist I had checked the pressure against enthalpy curves and obtain the same result. It seems late in the summer but I will check those pressure readings tomorrow.