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Old 05-26-2016, 09:11 PM   #1
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DIY air conditioner (mostly)

I know we have a handful of A/C nerds around here , so I have a question for all of you:

To start with I have a working 5000BTU window air conditioner which I plucked for free at the local garbage dump.. I can get the thing working through the MSW inverter, but the start up surge from the compressor pushes the inverter near it's upper limit. If I ever decide to put this little unit in the bus I will split it apart. My soldering and fabrication skills are ok, so I'm not too worried about that part.

Then I was thinking, can I use an A/C compressor from a small car to drive this thing instead of the hermetically sealed unit that currently lives inside the magic box? My thinking is that I could potentially use a 90vdc/5a treadmill motor I have kicking around to drive the compressor via belts and pulleys. I already have a lovely little 600w step-up converter which I could use to set the motor speed. I could also mess with the A/C compressor speed by setting the pulley ratio appropriately. Additionally, automotive A/C compressors have a clutch, so I can get the motor spinning before engaging the compressor. In the end I would hope to have an A/C compressor that is always running slowly, instead of one that cycles on and off...

I'll admit to knowing VERY little about A/C. I haven't even lived in a house with it, but I'd like to learn!
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Old 05-26-2016, 10:19 PM   #2
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modifying window units is a tough endeavor.. the compressor from a car theoretically can pump the refrigerant.. however you might be surprised how much horsepower you need to spin a car compressor .. I dont know what the ratings of a treadmill motor are but it may have a tough time...

then you have the issue that the copper piping in a cheap window unit is thin and tough to braze on to get your fittings installed that youd need for a car compressor...

how do you plan to install the window unit? just take a bus window out and mount it in? if so, where would your compressor assembly go?

climax makes a portable unit that is soft startand so it doesnt surge an inverter.. by portable meaning that the unit sits inside on the floor and you run 2 hoses out of the bus.. you could modify their plate to fit into a bus window if you were inclined. or even through wall...that unit has a variable speed compressor so it rarely cycles

or the simplest way is to upsizee your inverter.. or add a second one.. one for the A/C and one for everything else...

-Christopher
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Old 05-26-2016, 11:02 PM   #3
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When I hear the word inverter I think of battery power. No matter the size of the inverter you are going to have to have a huge battery supply to carry an ac unit for more than a few minutes. That's not to say it cant be done----but I've yet to see it. maybe a combination of solar and battery could get it done, I just don't know.

As to the retrofit you propose, it probably will work to one degree or another and is just the sort of time burner I'd spend a bunch of time on just because. Jack
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Old 05-27-2016, 09:21 AM   #4
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Please feel free to knock any of my misconceptions outta the park! I'm still brainstorming here..

Quote:
Originally Posted by cadillackid View Post
however you might be surprised how much horsepower you need to spin a car compressor .. I dont know what the ratings of a treadmill motor are but it may have a tough time...
The treadmill motor in question is a 450w motor. The compressor motor inside the window unit is a 550w motor. Based off of those motor ratings I expect that with proper pulley ratio's I could run a small automotive A/C compressor at 80% of the window units current cooling capacity... potentially..

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Originally Posted by cadillackid View Post
then you have the issue that the copper piping in a cheap window unit is thin and tough to braze on to get your fittings installed that youd need for a car compressor...
I don't expect this to be much of a hurdle. I've become quite crafty in this field over the years.

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Originally Posted by cadillackid View Post
how do you plan to install the window unit? just take a bus window out and mount it in? if so, where would your compressor assembly go?
I would split the window A/C unit so that the evaporator coil is inside the bus and the condenser coil with compressor is under the bus.

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When I hear the word inverter I think of battery power. No matter the size of the inverter you are going to have to have a huge battery supply to carry an ac unit for more than a few minutes.
So the window A/C unit as it currently stands pulls ~50a @ ~12vdc when it's running. Of course, it's a basic model so it cycles on and off to maintain room temperature. If we can assume that it runs for 20 minutes every hour then we're looking at 16 amps per hour. My solar system can keep up with that as long as the sun is somewhat shining (which would be the times I'd end up using it anyway). On a clear, sunny day I can pull in between 30-40 amps @ 12vdc from the panels. I also have a 455Ah battery bank so 227.5Ah of energy to play with.

What I would like to achieve is a constant 16 amp load instead of an on-and-off 50 amp load. I can remove the inverter from the picture entirely by using the previously mentioned 600w step-up converter and the DC treadmill motor. A 16 amp load can easily be run all night if I care to.

Oh, it's also worth noting that I'm in a decently insulated short bus and I live in Canada so my cooling situation isn't extreme. Removing humidity would be a real nice plus, though.

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Originally Posted by ol trunt View Post
As to the retrofit you propose, it probably will work to one degree or another and is just the sort of time burner I'd spend a bunch of time on just because. Jack
Haha.. Exactly! It seems like a fun and educational Sunday afternoon project
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Old 05-27-2016, 10:34 AM   #5
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One of the big questions is how slow can I turn an automotive A/C compressor? My guess is that even a compressor from a smaller vehicle such as a Honda Civic is capable of more cooling than I need or want. I can reduce the compressor speed to several hundred rpm, but would it work at such low speed? I suppose the compressor works fine while the engine idles....
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Old 05-27-2016, 10:41 AM   #6
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I know how to test this! Anyone want to lend me their small car for an afternoon?

If I popped the accessory belt off of a car with A/C and made a quick and dirty mount for the treadmill motor over top of an engine I could see if it is capable of operating the A/C system at various speeds.. Maybe?
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Old 05-27-2016, 12:11 PM   #7
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I guess a compressor must have a minimum operating speed that would be related to the leakage of the rings on its piston (assuming a reciprocating style). There must be a similar leakage consideration with other pump styles. I'd think that as operating speed drops efficiency would get worse because the ratio of pumped output to leakage would fall.

Still, though, the idea clearly has merit. The inverter-based designs like the Climax unit caddilackid mentioned use an inverter because that's the road they picked to get to variable speed.

Maybe you could go to the local car rental place and borrow a test platform, oops I meant vehicle, for a day..
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Old 05-27-2016, 03:20 PM   #8
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Just go buy those little window AC units fr about $130.00 or so. Get the kind without a remote and they cool better than a 12,500 BTU .
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Old 05-27-2016, 03:54 PM   #9
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Quote:
Originally Posted by Versatile View Post
Just go buy those little window AC units fr about $130.00 or so. Get the kind without a remote and they cool better than a 12,500 BTU .
That doesn't solve any of the problems I outlined above.. In fact, it would likely make it worse considering that I already have a 5000BTU window A/C unit so a 12500BTU unit would be 2.5 times more of a problem
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Old 05-27-2016, 04:03 PM   #10
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Quote:
Originally Posted by family wagon View Post
I guess a compressor must have a minimum operating speed that would be related to the leakage of the rings on its piston (assuming a reciprocating style). There must be a similar leakage consideration with other pump styles. I'd think that as operating speed drops efficiency would get worse because the ratio of pumped output to leakage would fall.
Agreed. It sure would be nice if the internet was plastered with specs for compressors..

Quote:
Originally Posted by family wagon View Post
Still, though, the idea clearly has merit. The inverter-based designs like the Climax unit caddilackid mentioned use an inverter because that's the road they picked to get to variable speed.
I'm guessing that when the A/C companies say that their unit is inverter-based what they really mean is that they incorporate a VFD. That's an ideal solution when the accessible power is already AC, but since I have the option to start with DC I should be able to get better efficiency from using a DC step-up converter (the one I have has been tested to be 95% efficient). However, with all the mucking about I may end up with an entirely inefficient air conditioning system being run by a very efficient DC motor

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Maybe you could go to the local car rental place and borrow a test platform, oops I meant vehicle, for a day..
Haha.. yes... I like your thinking!
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