Car AC theory question

[harry]

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I see you snipped the bit we are talking about.

Shitfer brains.

[harry]

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Exactly correct Duf.

[trader4]

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no it doesn't gain electrons. Where would these new electrons come from? The molecules just have more energy, so they move around more, increasing the seperation.

Uh oh. I hope you get that figured out...

[The Daring Dufas]

such.http://www.powdercoating.romerpp.pl/kategoria/3_50_84/Oven_with_force...

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That's what I was trying to remember. When energy is added to a molecule I believe it gains electrons which would account for the increase in spacing so the f/l itself gets less dense not the molecule. Hell, I wrote the original post after I had just awakened hoping my pain meds would kick in. I had a rough day yesterday since I figured out what was giving me chest pains, it was Naproxen Sodium. Now I must switch to another anti-inflammatory so I won't have so much trouble typing. o_O

TDD

[The Daring Dufas]

such.http://www.powdercoating.romerpp.pl/kategoria/3_50_84/Oven_with_force...

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Well, it's close, I forgot it's not the molecule itself that gets less dense but as Trader4 pointed out it's the spacing between the molecules that increases which makes the gas or liquid less dense. I was a physics major in college 40 years ago and I can't remember how to write a fancy calculation even if you put a gun to my head. ^_^

TDD

[The Daring Dufas]

such.http://www.powdercoating.romerpp.pl/kategoria/3_50_84/Oven_with_force...

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It could be more stuff I forgot after all these years. I can't remember the specifics but I suppose I was thinking the change in energy level was caused by added electrons but now I seem to recall the added energy is simply winding up the molecule increasing the speed of particles and changing their orbit around the nucleus. Since stable identical molecules repel each other perhaps this increase in energy causes the increase in spacing. Heck it's been so many years since I was immersed in physics and I been busy making a living so I've used those brain cells for other things. It's a good thing I have Internet access, now I can go back and refresh my memory. I remember living in libraries but I can't sit still that long anymore. ^_^

Thanks, my physician friend who helps me is a cardiologist and we discus what works best since I despise drugs and having to take the darn things. I'll probably wind up on another anti-inflammatory at a lower dose that won't melt my brain. o_O

TDD

[cjt]

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

I've recently been wondering the same thing. I can accept that under-the-hood is a hostile environment, but the disparity in price seems too large.

[trader4]

y a room AC to cool a room with the volume of a dozen cars for $100.

First, it doesn't always cost $1,200 to fix a car AC. If it's a bad switch or a corroded wire to the compressor it could be $100. If it's leaking shrader valves, it could be $250. Second, you're comparing the cost to repair something with the price of a new cheap, mass market appliance built on a high volume assembly line. If that $100 air conditioner had a compressor that failed, shred metal bits, contaminating the whole thing, what do you think it would cost to diagnose it, take it all apart, flush it, purge it, braze in a new compressor, recharge, test it, etc? It would be many times the $100 it costs for it to begin with, so it just isn't done. You can't throw away the auto AC as a unit and get a new one, so you have to deal with what's there. The factor in that half the system, ie the evaporator, TXV valve, blower, etc is buried under the dash where a lot of stuff potentially has to be removed if that part needs repair.

Also, how much it cost to fix a car AC is highly dependent on the parts used. Go to the dealer and pay for a new OEM compressor, receiver/dryer, etc and it's a lot of money. Kind of like buying a shelf or plastic part for a fridge. Use an independent shopt that will offer a rebuilt compressor, aftermarket parts, etc and the price can be hundreds less.

[Vic Smith]

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

It's mostly the labor costs. But parts can be costly too, compared to a window unit. Window unit compressors are hermetically sealed with the driving motor, cars compressors are belt driven. Pretty sure car condensers are bigger and more costly. Don't know why. Ever replace a car A/C compressor, condenser or evaporator? Lots of labor digging them out and putting them in. Pretty sure car A/C shops charge you for refrigerant, even if they pulled yours for recycling. But it mostly car mechanic manual labor vs automated factory labor.

[TimR]

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

A car AC is a three ton unit.

A three ton unit for your house is likely to cost you about $4000 installed.

[Ashton Crusher]

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

In fairness to the price disparity, the home unit also includes an EXTRA motor to drive teh compressor (in the car the motor comes with the car so it's "free") and the home unit also includes a condenser fan this is similarly "free" in the car. And the home unit also has a blower fan that, in the car, is again, "free". Likewise, the duct work in the car is "free". The extra cost to add AC at the factory to a modern car is really pretty low, I'd bet the parts don't amount to more then $500 and the extra labor perhaps $100.

[trader4]

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

I see folks throwing out all kinds of numbers for how many btus or tons a typical auto AC is, but I haven't seen a reference that I'd call credible or rely on. And I'd bet the typical auto AC isn't 3 tons. It's probably more like half that. A car is a tougher environment to cool than a house, but it's also a small volume. One thing that suggest it's not 3 tons is the size of the equipment it takes to get 3 tons of cooling. Anyone think for example the evaporator in a car is anywhere near the size of one in a home 3 ton HVAC?

[gregz]

It's not designed for efficiency. My 2.5 ton home compressor is small considering there is a motor in there. I recall in my air conditioning book talk about vehicles. Big difference in full size vans vs small car.

Greg

[Ashton Crusher]

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

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.

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...

[trader4]

e:

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

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.

 As I mentioned before, most add on system evaporators

[Ashton Crusher]

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

info.http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1269&context=icec

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.

[clare]

There comes a point where the better emissivity of the black and the poorer conduction of the paint cancel out. I expect that where air movement is adequate, the non painted unit would be more effective. Where air movement is limited, the darker colour MAY help.

I believe the black paint is for protection.

[The Daring Dufas]

I wouldn't discount that at all since I've seen a lot of corrosion damage done to condensers that were made from aluminum. Interestingly enough, the small condensing units that are under those soft drink vending machines are often painted black even if the fins are galvanized steel. ^_^

TDD

[jim beam]

galvanized steel cooling fins??? are you sure or are you guessing? steel's a poor conductor and it's much harder to make gas-tight joins as easily as brass/copper/aluminum. i can't imagine why anyone would use it.

[The Daring Dufas]

I own a small condensing unit from a vending machine that has steel fins that I've used for years as a recovery system for refrigerant. I purchased a new one a while back for an old Coke vending machine and that new unit has steel fins. Many soft drink vending machines have steel finned condensing units underneath them because they are more durable and less likely to be damaged by the myriad environments those types of machines wind up in. ^_^

TDD