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Hi,
The shortest answer I can put forth is not "what pressure" but the "what temperature" the high side should be, for the fan come on!
Well, It's the Same for the "cabin temperature" and that fan!
When you are too hot and you want to remove heat!
For the condenser fan ... A "general rule of thumb" is ....
It's approximately 30 to 35 degrees above ambient temperature or to achieve about a one third to be a liquid filled condenser or less.
That's the bottom one third with liquid and the rest is left for cooling down the hot "latent heat" of super heated vapor gas!
The vapor gas will hotter because it has not lost its latent heat yet.
It's like condensing the "water medium" called steam!
The pressure in the condenser/vessel will be the temperature or vice versa, whether over liquid or the gases.
A physics law dealing with confined spaces. Book'em Dano is criminal law! (:-)
The pressures will be the same on the high side but the condenser temperature will vary from the top to the bottom or from the compressor discharge to the orifice.
Temperature is what the "white coat" lab boys use!
If you look at a temperature and pressure chart for R134 a or any other refrigerant in our future to be used, the answers are still there.
Here is an example chart I found while out just bumping out onto the Internet.
https://refrigeranthq.com/r-134a-refrigerant-pressure-temperature-chart/
You can pick your poison, like the "lab boys" and set it to where you will maintaining
the temperature of removing excess temperature and keeping both sides of the cycle with some liquid. Solid on the high side and big droplets on the low side.
In Kung Fu days, it was about balance! (:-)
Evaporator temperature is as low as 25 degrees to 35 degrees leaving an outputs of 55 to 60 degree from the vents. Remembering water vapor freezes.
Icing stop airflow!
The Condenser temperature will be held above 70 degrees as a lowest point and not needing cooling assistance.
Maybe using on a defogger/defrosting windshield day.
A condenser at 70 degrees day needs 30 degrees or 100 degrees. Just warm to the touch, ok?
A condenser, on a 100 degree day, will need that bump up by 30 more degrees to move heat out. Now that hot to touch or burn.
Add on engine heat under the hood heating of lines another 10 degrees, as a real possibility, and you are already at 140 degree condenser temperature.
You will notice that this chart stops at 150 degrees because that's about the limit of compression versus Energy costs, on electric motors to compress gases.
All Refrigerants have a working envelope or an efficiency zone.
On cars, its mostly for limits of the flexible hoses and horsepower available.
You will want the fan to be running a long before that, to expand the cooling range of the system.
You are moving heat from the evaporator in varying quantities so the outside fan can throw it away. Just like inside the car.
You'll notice no thermostat on late 240s. Cheaper to hire you!
You don't want to adjust it, pay more for the complications!
You can stop reading here as I'm throwing a wrench in a bucket to hear it rattle now!
On the late nineties cars they did away with high side service valves, saves money again!
I have said this before, that orifice's systems are cheaper, period!
Plain and simpler! (:)
Use the above in good health and maybe an infrared thermometer might help!
I could rattle on more but you get the idea.
Be a white coat rider?
(:-)
Phil
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