AC Gurus.

[BGKYK5]

And that means you have a leak ... if the car is a beater....well,,,you know what to do.....

Yes, the car is a major beater. In fact I think it will end it's life at a demolition derby someday - if it lasts long enough that I can replace it with a better beater.

I think I had a leak with the old condenser, but I don't think it leaks anymore.

Bottom line is if it's blowing cold and the pressures are close then leave it alone.

Music to my ears.

The system was vacuumed down with a pump after you replaced the condensor, right?

Uh.....


I know this thing is crap, but it would be better than nothing, right?

 

[Blue85]

Whoa dude !!!!!

that is the MAXIMUM it should be!!!! not a target range.....
the lower the better.. what your looking for is about 15 - 20 deg above the outdoor temp converted to pressure.

The correct way is to read the high side gauge, convert that pressure to temperature using a P/T chart.

http://www.glacierbay.com/ptchart134a.asp

Subtract the outdoor temp from this reading. your looking for about a 15-20 degrees difference.

At 300 psi w/ R-134 you would be at about 160 degrees condensing.
Subtract 20 degrees subcooling you end up with 140 degrees.

So unless it's 140 degrees where you live,,something would be wrong.


Now if your reading is about 155 psi that would = 114 degrees condensing.
subtract 20 degrees subcooling and you end up with 94 degrees.

so,, on a 94 degree day you would expect your head pressure to be about 155 PSI.

I agree that 300 is a max and not a target, but I don't agree with your calculations of what the temp/pressure should be. If your condensor is at the same temp as the outside air, little heat transfer will happen! The hotter it gets, the more heat is transferred out of the car.

High side pressures are usually over 200 when you are running, unless it's cold weather (but then why are you running it?). To have a high side pressure of 155 on a 94* day means you have an awesome condensor and huge fans.

 

[BGKYK5]

My beater; exhibit 1 (the white one).



That should provide some perspective on how much $$$ I am willing to dump into that POS.

 

[wasted wages]

but I don't agree with your calculations of what the temp/pressure should be. If your condensor is at the same temp as the outside air, little heat transfer will happen! The hotter it gets, the more heat is transferred out of the car.

Read it again......

Now if your reading is about 155 psi that would = 114 degrees condensing.
subtract 20 degrees subcooling and you end up with 94 degrees.

At 155 psi the gas temp is 114 degrees.
If it is 94 degrees outside you will be rejecting 20 degrees of heat.
The subcooled condensed refrigerant leaving the condenser will be 94 degrees or close to it ( no condenser is 100% efficient )

The BEST you could hope for is 20 degrees of heat rejection,
but as inefficient as car condensers are you would be good to get 8-12 degrees, so thats why car a/c head pressures tend to run higher.

Now when I size a refrigeration rack for a grocery store I work with the total BTU's that all of the compressors on the rack combined will pump.

Example:

8 compressors with a combined capacity of 1,200,000 Btu's @ 120 degrees
condensing and a -20 degrees suction.

I would then size my condenser with a heat of rejection factor based upon
10 degrees T/D or 15 degrees T/D depending on a few other factors (Type of refrigerant,,mechanical subcooling etc.)

The condenser would need to be able to reject the 1.2 million BTU's as well as the heat of compression ( from the compressors ) on a 120 degree day.

A condenser of this size is huge,,8ft wide X 24 ft long with 10-12 1 HP fans on it. So you can see how a small car a/c condenser will struggle to reject it's heat in such a small size.

Ever notice how much better the a/c works in a car when you drive into a rainstorm ?

Two things happen,,the condenser now being cooled with rainwater, is able to reject about twice as much heat,,,plus the load on the evaporator is reduced as the exterior of the car is now being cooled off by the rain also.

It's all about heat transfer.....

 

[Blue85]

OK, I see what you are saying, but we are talking about automotive A/C and I still think 155psi is low unless he has a strange vehicle with the high side service port after the condensor. Typically they are between the compressor and the condensor and on a 94 degree day that port will probably be over 200. I just don't want him to be alarmed by pressures like that.

And you made my case for me that car fans/condensors are not optimal.

Coincidentally I just charged the A/C in my car today with an ambient in the low 80's and the high side was about 220, which is fine.