# Car AC theory question

On Sat, 16 Mar 2013 19:10:26 -0700 (PDT), " snipped-for-privacy@optonline.net"

I posted on this a couple days ago without any citations. But here's a paper that I was able to find and it's pretty much what I recalled from several years ago when I went looking for the same info. http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article 69&context=icec
basically, the typical Auto AC compressor will run perhaps 6000-8000 BTU at idle speed. When you are cruising at high speed on a freeway you can figure on between 20,000 to 30,000 BTU's depending on the type of compressor. As I mentioned before, most add on system evaporators are only 15,000 to 20,000 BTU capable and factory installed ones are around 22-24,000. But that's only when the compressor can supply them with enough freon. So at idle the system will only be as good as the compressor is at idle speed.
there's an interesting article from 1969 here... http://books.google.com/books?id=EyoDAAAAMBAJ&pg=PA117&dq=popular+science+1930&hl=en&sa=X&ei ¯b5Tq3AEc3eggfToOWeAg&ved DcQ6AEwAThu#v=onepage&q&f=true
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e:

can buy a room AC to cool a room with the volume of a dozen cars for \$100.

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talled.

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to

lib.purdue.edu/cgi/viewcontent.cgi?article69&context=icec

If that's true then there would be a 3 to 4X difference in cooling capacity between when the car is idling and when the car is at highway speed. There is some difference, but it sure isn't 3 to 4x difference, not IMO. And if you put a thermometer in front of the cold air outlets, there would have to be either a huge variation in the volume or the temp of the air exiting based on engine RPM. Again, there is some slight variation in temperature, but it's not 3 - 4x.
Â As I mentioned before, most add on system evaporators

ooks?id=EyoDAAAAMBAJ&pg=PA117&dq=popular+sci...- Hide quoted text -

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On Sun, 17 Mar 2013 04:47:00 -0700 (PDT), " snipped-for-privacy@optonline.net"

There are a couple issues on this. A LOT depends on the temperature and humidity of the air you are cooling down. A lot also depends on whether the vehicle body has been sitting out in the sun baking or not. My experience with my 92 explorer says there is definitely a difference of 3x. But that's when it's 105 in the shade and it NEEDS the full capacity. Then you can tell a huge difference between the cooling at idle and at speed, it's like night and day. This is pretty much true on almost every car out here in AZ. Now if you are talking about the difference when it's 85 degrees then sure, it seems to be working nearly as good at idle as at speed. But that's because you don't need 20,000 BTU's to cool things off, you only need 10,000. At speed the compressor is very possibly off more then it's on.
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m

It's to do with emissivity. Read this. http://en.wikipedia.org/wiki/Emissivity
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