Wanting to build a silent compressor from fridge compressor

[Franknstein]

Hi All

I am looking to build a small compressor to run an airbrush. I have a few old compressors lying around, some of which have seized motors (24L vessels), but everything else on them is sound, like the pressure switch, one-way valves, gauges, regulator etc... so was thinking if I can blag a fridge compressor I'd bolt it on where the old pump used to be and plumb it in....

Anyhow, does anyone know where I could source a really cheap (or even free!) fridge compressor?

Do they usually have start capacitors internally, or externally? I mean, would I need to buy a start/run capacitor?

 

[rtbcomp]

Go for a freezer compressor, Freecycle will be your best bet. You will have to have the refrigerant professionally removed and the carcass properly disposed off.

Small fridge compressors don't have a very high starting torque.

http://forums.lr4x4.com/index.php?showtopic=9710

 

[DrNik]

i have a scrap fridge, the motor works fine,if you wanted to get a courier collect you can have the motor for free

 

[Jim_gsxr1000]

A used air conditioning compressor could also be a shout generaly a bit more powerfull if u needed a bit more... puff

 

[mike 109444]

DrNik. For inspration take a look at the sushh (think that's how they spell it!) in MM. Have built one a long time ago for air brush. You might want to try and get 2 or more pumps for better reserve. Some use a thermal type device where you would normally expect a start capacitor (weard device about 12mm dia by 4mm thick) make sure you have a good oil trap as you can get oil work it's way up the pipe, not a problem on fridge as it can run back but not so good on a compressor. There was a chap on here about 12 mths ago that was building one from 4 or 5? very big air con pumps!! worth looking up if you can find it.

 

[DrNik]

i have a dentist compressor which uses a fridge type compressor for my waste oil heater, the motors are meant for short bursts really, definately not long term, mine overheats and the piston expands in the cylinder, wait till its cool and its fine again,

oh and the capacitor is external on mine

 

[madkayaker]

I work on oil free dentist type comp. they can run flat out all day. Trying to remember the make (some thing like guest?)

 

[rtbcomp]

Gäst ??

Some use a thermal type device where you would normally expect a start capacitor (weard device about 12mm dia by 4mm thick)

That will be a Klixon overload. If you plough through the thread I linked to earlier I think I put some circuit diagrams of the various types of setup there.

The Klixon cuts out if there's too much start current and cuts back in when it cools down. It is fastened to the body of the pump so it will cut out if the compressor overheats.

Another thing to watch is the dump valve in the pressure switch. You need the non-delayed type, otherwise it will never close because it dumps air faster than the pump can supply it, so pressure never builds up.

EDIT - A freezer compressor may well start OK under pressure, some of my fridge motors did once they had warmed up.

 

[madkayaker]

That's the ones. Lovely machine that has very little looking after but it just keeps going! The machine is used to provide dry air to keep a load of coax dry. Outlet pressure 0.4 bar but there is 10k of coax!

 

[Franknstein]

Thanks all for your responses.

Do you reckon either of these would work:

http://www.ebay.co.uk/itm/221172216615?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

http://www.ebay.co.uk/itm/221172216459?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

I could run two in parrallel if need be, but I'd prefer not to because of weight, size, and expense.
The TEE comp is 1/6hp and the Danfoss 1/5hp.

About the relief valve in the standard pressure switch not working - what do you think is the best way to overcome this? I could add a solenoid pneumatic valve to the circuit which purges the air in the charge pipe once the compressor turns off? - perhaps something like this:

http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2047675.m570.l2736&_nkw=pneumatic+solenoid

 

[rtbcomp]

The best way is to get the correct relief valve.

If you read that thread I pointed to earlier you will see how I wired in a washing machine valve in to do the job. At one time I had half a dozen compressors in parallel, all served by the same solenoid valve.

I wouldn't bother with new compressors, get an old freezer.

Another consideration is lubricating oil. They pass a lot of oil due to their design and the correct stuff is very expensive. I used 20/50 motor oil (because I have loads). The cheapest, lowest viscosity single grade, non detergent oil would be better, but the pump may not start to run until the oil has warmed up.

 

[Franknstein]

THanks RTBComp - I read only the first page of that, didn't see the other pages!
Nice thread that, and very helpful.
Is this suitable?:
http://www.ebay.co.uk/itm/Single-So...yers_Parts_Accessories_ET&hash=item19d284b8b9

What sort of size freezer would be good? Its just that I dont have a van - so collecting an old freezer and then disposing of it would cost me more than just getting a cheap fridge/freezer compressor.... Perhaps if I can find one small enough to fit in my X5?

About the oil, you say the pump wont run until the oil is heated, how do you overcome this? If it doesn't start then the oil will never heat surely?

My thought was to run an external pressure regulator/water trap to trap any oil on the output side - would that be sufficient? How often will I be emptying the trap vessel? Do you just pour oil in every now and then, or did you make a dipstick or attach a site glass?

One more thing, about cooling - do you just run a small 220v fan blowing onto the top of the hermetic housing?

 

[DrNik]

i have a jun air 50litre silent compressor for £40, works great after your messed about tinkering this would work out far cheaper and works

 

[Franknstein]

Right.
Thanks to DrNik for the silent comp. It works great - I stripped out the NRV and cleaned it up, adjusted the spring and now it holds pressure fine.

I've also collected a freezer from 'Spot the Welders Dog' - who so kindly gave let me have it for nothing. And I've since stripped it out of its compressor and started work on my silent compressor.

So I now have two. I love compressors, and cannot get enough of them - they are so useful.

Anyways, some photos below of the build.

The compressor is a Giobo - made in Italy, and its pretty old, but seems to work fine.
I've had no problems with it getting started up, and it even fires up under pressure of 6bar! So no need for me to plumb in a purge valve.

Basically, I unbolted the old mechanical pump/motor unit from a 24L I had lying around, and will use that pump on another vessel I have (50L).

Then drilled some new holes to mount the freezer comp onto the 24L tank (it damn nearly almost fitted the original holes!).
Cut down the inlet and outlet pipes with a tube cutter, and then used a piece of PVC hosing as a charge pipe. Had to make up a barbed fitting which connects to the NRV on the vessel, and its slightly leaky, but not too bad... basically found a rubber o-ring and inserted it inside the original nut from the original charge pipe, and then pushed a barbed fitting through it, I may have this fitting braised to eliminate the leak completely and then remove the o-ring.

Used two clamps on each side and when the pipe heats up they get a little softer, so its best to be on the safe side, not that anything bad would happen really if it did wander off.

bored out the plastic from an air filter and shoved it on the inlet.

Wired up the compressor to the original pressure switch, so it cycles as normal (6bar on, 8bar off).

Like I said, no need for a dump valve, as it starts just fine at 6bar even with pressure in the charge pipe. So for now, I just crimped the original tee'd pipe that would go to the pressure switch for the dump valve, but I am hoping to find a blanking fitting or to cut and braise the end of the pipe.

It works fine, but takes about 11 minutes to charge to 8bar - so its not especially efficient in that respect. But its whisper quiet. I will try my airbrush on it and see how it performs.

TO DO STILL:

1. Perhaps some cooling is in order, the compressor gets pretty warm after its 11 minute run, should I just bolt on a 230V 3" fan on top of the compressor blowing downwards?

2. Oil, I do see a bit of oil coming out of the outlet, as I have clear pipe as the charge pipe. However to me it does not look like much, and because the pipe work points upwards, it kinda pools up a bit on the crest. Not sure if this is an issue, but seems to be working fine for now. There is a third pipe extending from the compressor, which was crimped and braised closed, which I presume was the oil fill hole? I was thinking to cut this almost flush with the casing, then insert a rubber bung, then every now and then fill oil in there until it comes out the hole and then replace the bung - good idea?

Pics

 

[rtbcomp]

Oil coming out of the outlet pipe is a fact of life with fridge type compressors.

I put the fans underneath mine, with a tank thermostat to switch them on, this allowed a quick warm-up.

 

[shadow]

I'm in the process of building one of these from a second hand compressor I brought that the motor had been gummed up with paint! took the pump and motor off and have put on a pair of fridge/freezer pumps. so far it works fine as for the oil coming out the output I would think having it coating the bottom of the tank would be good for preventing rust?

 

[pidge]

You can expect around 15LPM (0.53cfm) from a danfoss FR7.5G. It doesnt sound like a lot but its 41% more than the Tee compressor. The danfoss should take 10-11 minutes to charge a 24L receiver from 0-100psi, if that helps.

Didnt realise there was a page 2 of the thread
A water trap with a spring loaded drain between the compressor and the tank does a good job of preventing oil and water reaching the tank. It`ll give you an idea of how much oil your using or losing too.