DIY air conditioner (mostly)

[jazty]

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!

[cadillackid]

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

[ol trunt]

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

[jazty]

Please feel free to knock any of my misconceptions outta the park! I'm still brainstorming here..

Originally Posted by cadillackid

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..

Originally Posted by cadillackid

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.

Originally Posted by cadillackid

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.

Originally Posted by ol trunt

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.

Originally Posted by ol trunt

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

[jazty]

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....

[jazty]

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?

[family wagon]

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..

[Versatile]

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 .

[jazty]

Originally Posted by Versatile

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

[jazty]

Originally Posted by family wagon

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..

Originally Posted by family wagon

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

Originally Posted by family wagon

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!

[Arvik]

this way cool idea. i like it. use car ac for the bus, hm..
concerned about how much energy would have to go into turning the compressor though. after all said and done and time vested it may be cheaper to have gotten roof mount rv ac unit i think.
i am still toying with the idea of using my window mount ac unit from house in the driver window though.

[Tango]

Rotary compressors these days draw very little off the engine. I had a '67 Mustang ragtop with a six and automatic. When the huge old factory compressor kicked in, the motor would often die. I switched to a system with a Sanden rotary and it would freeze you out with the top down and the idle never seemed to change. Sometimes newer can actually be better.

[family wagon]

Originally Posted by jazty

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).

Yep. Actually the incoming power doesn't matter: the first thing a variable frequency drive does when fed with ac input is rectify into dc. Then, usually, there's a three-phase inverter converting the dc back to variable-frequency and possibly variable-voltage ac.

I'd love to find a source for the pieces out of one of those inverter air conditioner units.. specifically, the hermetically sealed three phase compressor and maybe the inverter board too. I guess replacement parts for an inverter mini-split shouldn't be hard to find.

[Versatile]

Reread my post. Do as you want. I will

[Jolly Roger]

Originally Posted by family wagon

Yep. Actually the incoming power doesn't matter: the first thing a variable frequency drive does when fed with ac input is rectify into dc. Then, usually, there's a three-phase inverter converting the dc back to variable-frequency and possibly variable-voltage ac.

I'd love to find a source for the pieces out of one of those inverter air conditioner units.. specifically, the hermetically sealed three phase compressor and maybe the inverter board too. I guess replacement parts for an inverter mini-split shouldn't be hard to find.

Mitsubishi.com
Pick a unit? I can give you model numbers for ac only or heat pumps/ac and can find a number for full tech. Support if needed?
They don't like when you start re-arranging there stuff but I can tell you it has been done and you might have to pissem off but they will support there equipment even if you have to move the circuit boards to get the hot gas bypass or to twin units together inside the units because they were shipped wrong.
TRANE,MITSUBISHI,LG is way above CARRIER,DAIKEN as far as equipment maintenance support.
Even on brand new install?

[cadillackid]

inverter units are in fact driven with a VFD... they are running 3 phase power to the compressor... so they have to rectify to DC then crate the A/C pohases with inverters and run a VFD...

there is LOTS of specs for compressors on the internet regarding KW capacity at various RPMs.. you have to find out who made the compressor you are looking at then look it up on the OEM site for it.. MOST automotive compressors are swash-plate type Sanden / Seltec style... sanden 508 (they have new numbers for the new ones) is onbe nunber that is real common.. most compressors have a minimum RPM of 600 - 800 (compressor RPM).. you can decent cooling in the 800-1000 RPM range..

to make that window unit work right you would need a TxV instead of the capillary tube the use now... with the capillary tube you have no ability to vary capacity.. even speeding up or slowing down the compressor wont help you.. too slow and you'll fail t omake liquid and wont properly return the oil to the compressor.. too fast and your head pressure is through the roof.. if you cycle out on high pressure you will have as TOUGH time restarting it...

ive custom built plenty of A/C units before but a window unit is not what id start with.. you likely have a single shaft fan motor so you would need a separate fan on your condenser(more power usage). and if you want to shrink the size of your unit to use the evaporator inside you have the condensate to deal with... most window units dont have a proper drain pan.. you'll have a double-shaft motor...

if I were dead set on usiong an inverter and that window unit.. id use a bigger inverter that could handle the cycle rather than attempting to rig something together that seemingly could work but definitely not be efficient and may be hard on said compressor...

like I say you have a SHOT if you run a TxV .. then you can run the compressor at various speeds and the TxV takes care of proper refrigerant flow and ensures oil return..

-Christopher

[cadillackid]

Originally Posted by Jolly Roger

Mitsubishi.com
Pick a unit? I can give you model numbers for ac only or heat pumps/ac and can find a number for full tech. Support if needed?
They don't like when you start re-arranging there stuff but I can tell you it has been done and you might have to pissem off but they will support there equipment even if you have to move the circuit boards to get the hot gas bypass or to twin units together inside the units because they were shipped wrong.
TRANE,MITSUBISHI,LG is way above CARRIER,DAIKEN as far as equipment maintenance support.
Even on brand new install?

have a couple of thoise mini splits in my basement.. one of them is torn apart as i used it for hardware abnd software DEV.. the VFD board in mine is not analog voltage controlled.. there is a special SPI bus protocol to control it.. you CAN use a full set of boards from one of these units to make the compressor run...

before the ducted ones were available.. I completely re-worked 3 of them and made ducted enclosures for the indoor units.. i also hacked the protocols to the indoor board so i could feed it whatever info I wanted to make the unit run in whatever mode and maintain whatever ideals I wanted.. I could set an arbitrary setpoint and then feed it the reading from the room sensor (my own value) so effectively i could control the compressor speed and output.. I use a zone blower damper system in the house and have one main blower.. I discarded the blowers fro mthe ductrless splits and by usiong dampers and my VFD main blower i can create whatever CFM i need across the coils..

the ONLY thing was I had to use a full set of boards from each uniut and i allowed their sensors to monitor the temps on my own coils .. each coil has 3 sensors... intake / middle / output so it knows the superheat and subcool this way of both coils.. the TxV is electronically controlled.. in essence by handing the unit manufactured sensor values for several inputs i can control them precisely.. been running this system 7 years in my home now without issue...

I would sooner start with a mini than I would a window shaker..

-Christopher

[jazty]

Originally Posted by cadillackid

to make that window unit work right you would need a TxV instead of the capillary tube the use now... with the capillary tube you have no ability to vary capacity.. even speeding up or slowing down the compressor wont help you.. too slow and you'll fail t omake liquid and wont properly return the oil to the compressor.. too fast and your head pressure is through the roof.. if you cycle out on high pressure you will have as TOUGH time restarting it...

Nice. A TxV will definitely be necessary to achieve my end result. Very nice. I've done some research and they appear to be relatively easy to incorporate into a system.

Originally Posted by cadillackid

ive custom built plenty of A/C units before but a window unit is not what id start with.. you likely have a single shaft fan motor so you would need a separate fan on your condenser(more power usage). and if you want to shrink the size of your unit to use the evaporator inside you have the condensate to deal with... most window units dont have a proper drain pan.. you'll have a double-shaft motor...

I suppose it's a bit silly to say that I was planning on reconfiguring the window unit. As I think about it further, the only things I'd end up using from the unit are the condenser and evaporator coils. Maybe the front end ductwork. I have a well suited DC PWM motor controller that could be used with some radiator fans I can pull off of an Outback. Based on the current design of the window unit, can I assume that the condenser fan should always be running at the same speed as the evaporator fan?

The other option I've been thinking about is ripping the A/C system out of a car, reassembling it into my preferred configuration and drive it with an electric motor. I've read that some modern car A/C systems use a TxV. I'm not sure I'd be able to pull the A/C from a modern car, though. But then again... Would the rental car company notice it missing?

[cadillackid]

a repurposed car A/C isnt a bad way to go... also you very well may be able to snag a condenser and / or an evaporator from a member here ..

some of the people buiy busses with A/C but most all remove it as they want electric / RV A/C and not engine driven... some busses have 2 small evaporators mounted and would work good...

a window A/Ccondenser and an electric radiator fan is a great way to go and more compact than a car condenser...

-Christopher