# compressor - evaporator matching

I'm build car and I plan to install an air conditioning system.
I'm thinking of using an electric compressor like those used in refrigerators/freezers instead of the belt driven type used in most
cars.
problem is, I'm not sure what size the evaporator should be relative to the compressor.
Is there any way to calculate the size of the evaporator based on the compressor's size or capacity?
what would happen if the evaporator is too big for the compressor? Will it still be able to make cooler air?
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It's gonna be big. Really, really big. As in more than a couple kilowatts.

Audel's Refrigeration Handbook will tell you want you want to know, but I believe your idea is misguided. --scott
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"C'est un Nagra. C'est suisse, et tres, tres precis."

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Scott Dorsey wrote:

No kidding! I remember reading back in the early 1990s that the cooling capacity of the Chrysler LH car AC system was equivalent to a 5-ton home AC unit!

Well, electric car A/C compressors ARE under development so that hybrid cars can shut down the ICE without shutting down the A/C, and so that plug-in electrics can HAVE AC in the first place (see http://www.sanden.com/products/electric.html ). But they're pretty specialized, and less efficient than direct-drive compressors. Just eyeballing the power and performance curves and assuming that they're being fully honest and the curves are under the same overall conditions, it looks to me like 2.5 KW input power gives you roughly 18,000 BTU/hr or around 1.5 tons of cooling. So that's over 3 horsepower electrical input for only 1.5 tons cooling.
So the obvious conclusion is that direct drive off the crankshaft is significantly more efficient. At this level of technological development the ONLY reason for an electric compressor in a car AC is if you're building an electric vehicle or hybrid.
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On Tue, 11 Sep 2007 16:06:03 -0000, snipped-for-privacy@gmail.com wrote:

I think you are barking up the wrong tree. The typical car compressor is a two ton unit as measured by BTUs. It's enough to cool a small house. It takes something like 4 hp to run it, that's a pretty good sized motor if you were running it by an electric motor. That's why two ton home AC units are 240 volts. So if you are truly thinking of taking a refrigerator compressor to put in a car I don't think you are going to get any usable cooling at all no matter what size evaporator you use. All that said, if you are building a car AC unit regardless of what you do for a compressor the evap and condenser sizes you would need will be pretty much the same as for any other car because all cars need more or less the same sized AC system.
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Are you guys sure? What about the wall-mounted air conditioners used in homes? There are even units rated at 1/2 hp.
I'm just trying to do something that Toyota did with their Hybrid Prius which has an electric (and not belt driver) compressor.

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That's miniscule. Try 5 hp or so for something more reasonable.

The Prius has huge amounts of current available, though. And also it is very well-sealed. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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On Wed, 12 Sep 2007 14:41:24 -0000, snipped-for-privacy@gmail.com wrote:

The ones that run on 120 volts are small units. Maybe 8000 btu to 12000 btu. A car unit is around 24000 btu.

There's nothing wrong with the concept, I just don't think you will find the parts to do it unless you are willing to pay some bucks for the right parts.

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Yes I can experiment on that since these compressors here are relatively cheap.
I still haven't figured out the piping of a refrigerator compressor since there are three. Unlike in an car air con compressor where there are only two: INLET and OUTLET.
What would happen if an evaporator is too big for a compressor?
Let's say I replace a car air con compressor with a much larger one. Will it still become cold?

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On Thu, 20 Sep 2007 01:18:52 -0000, snipped-for-privacy@gmail.com wrote:

I think one of those tubes is sort of a drain for oil that gets pumped upstairs.

Only part of it will get cold

Will the evap still become cold? Yes. But if the compressor is too large it will just cycle off a lot. Unless you have it hooked up so it can't turn off and then it will build up so much pressure something will blow.
I can almost guarantee a refrigerator compressor won't have enough capacity to do much of anything if hooked up to a car system. Notice the relative size of the piping in the two systems - little tubes in the refrigerator, large hoses in the car.

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Oh ya, I forgot. This air conditioning system will be used in a small car. 2 door, 2 seater economy car. It has a small motorcycle engine and batteries,electric motors.
And so I don't really need big capacity systems.

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Ashton Crusher wrote:

Hope your time is equally cheap....

Or a charge port that gets crimped and soldered shut when the system is charged at the factory.

And then, only if the expansion valve is sized for the tiny compressor. If you hook a refrigerator compressor to a car A/C system with its existing expansion valve (or orifice tube), then the refrigerator compressor won't even be able to develop any high-side pressure because the expansion valve's minimum flow is bigger than the compressor's maximum output!

Or it will suck in liquid refrigerant that hasn't had time to boil and "slug" itself and turn into scrap metal.

I can absolutely guarantee it.
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But wouldn't it be fun to see it all hooked up!
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