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Fridge Compressors


rtbarton
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From time to time people have mentioned using fridge motors for a compressed air supply.

I've done quite a bit of work on this recently, with LandymanLuke, and thought a thread on the subject may be of interest.

For starters, this is a Zanussi 80 watt fridge motor with the top removed:

302918979_8250d1d773_o.jpg

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what are the benefits of using a fridge compressor over an a/c compressor?

I've never tried an aircon compressor, but if you mean an automotive unit then the AC unit would make a better compressor on a vehicle because you could drive it directly from the engine.

I'm using fridge compressor(s) in my workshop because they are quiet and I charge a receiver up overnight on Economy 7. Also because they can be used indoors the (substantial) heat they produce isn't wasted.

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how much do these things pump out in the way of CFM then?

Pretty low, but not insignificant.

There are two approaches, either run a few pumps in parallel as Roger intends, or have a large enough reservoir that you don't really care, like me ;)

I run the pump for a few hours before i need air, to fill the 300L tank. if i'm in a hurry, i cycle the Machine Mart (noisy) compressor to fill the big tank about half, and the 50L fully. the valve work i have allows me to do this easily.

I leave the freezer compressor going all the time, sporadic use of air tools is about equal to the constant drip feed from a single freezer compressor, i suspect two or more would easily match (my) demand.

One of Roger's intended uses is grit/bead blasting, so his air consumption is significantly higher.

Luke

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Evening,

My (slightly out of date) page on this project is here

this is my freezer compressor, with 12v PC fan mounted on the top for cooling

If you're going to run the unit for over an hour then a fan is essential, although a mains fan (£7 from Maplin) would be better as you could power it from the compressor supply.

I'd be inclined to mount the fan underneath to keep it cooler and maybe some sort of shroud to convey the air up the sides of the compressor.

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a mains fan (£7 from Maplin) would be better as you could power it from the compressor supply.

Agreed, i just didn't have a 230v fan, but did have a 12v, and i have a 12v supply in the garage.

I'd be inclined to mount the fan underneath to keep it cooler and maybe some sort of shroud to convey the air up the sides of the compressor.

again, in theory, agreed. However plonking one on the top has done pretty well so far. I've had a thermocouple on different areas of the pump body and it runs at about 30-35 degC with the fan on, >65 degC without.

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how much do these things pump out in the way of CFM then?

As Luke says, very little!

I have a nest* of five compressors and they will raise about 210 litres from 80 psi to 140 psi in about 1.5 hours, if I remember correctly. (I can't measure this at the moment because the system is closed down for modification)

If you're going to scrounge compressors from a Dumpit Site then don't bother with anyhting less than 100 watts and go for freezer rather than fridge compressors.

Any more than 5 the 13 Amp mains fuse blows, so you start to need some form of sequencer for more units.

* What is the collective noun for a group of fridge compressors? Somebody must know :D

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That's effectively like the Machine Mart Sssshhhh range of compressors.

He mentions in his article that you need a filter on the output. I reckon this is essential. The Zanussi (and ZEM) units generate an oil mist in the housing and suck air in from the same space, thus the air is very heavily contaminated. My receivers and pipes are full of white emulsion (makes very good chrome polish BTW).

Another implication of this is that you get very high oil consumption, Machine Mart sell synthetic compressor oil for £10 per litre which makes this an expensive system to run.

I tried running on veg oil, but it seems to thicken up over time so have reverted back to 20/50 engine oil, although 10/40 would be better. You need thin oil, I intend to look deeper into this.

Also the oil particles take up space in the cylinder and don't compress thus reducing capacity.

Further, the air in the housing gets very hot, so you compress less mass for a given volume, reducing efficiency.

My solution is to extend the inlet pipe to the exterior of the casing, this involves major surgery and I have a prototype running at the moment.

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That's effectively like the Machine Mart Sssshhhh range of compressors.

He mentions in his article that you need a filter on the output. I reckon this is essential. The Zanussi (and ZEM) units generate an oil mist in the housing and suck air in from the same space, thus the air is very heavily contaminated. My receivers and pipes are full of white emulsion (makes very good chrome polish BTW).

Another implication of this is that you get very high oil consumption, Machine Mart sell synthetic compressor oil for £10 per litre which makes this an expensive system to run.

I tried running on veg oil, but it seems to thicken up over time so have reverted back to 20/50 engine oil, although 10/40 would be better. You need thin oil, I intend to look deeper into this.

Also the oil particles take up space in the cylinder and don't compress thus reducing capacity.

Further, the air in the housing gets very hot, so you compress less mass for a given volume, reducing efficiency.

My solution is to extend the inlet pipe to the exterior of the casing, this involves major surgery and I have a prototype running at the moment.

a hermetic refrigeration compressor doesn't make a very effective air compressor.

Oil carryover is a feature of all refrig compressors. You will need an oil seperator in the discharge line at a minimum.

Refrigeration oil is extremely hygroscopic, with modern POE oils unbelievably so (as is the PAG oil used by Sanden in car A/C compressors)

Within 1/2hr your oil will become so moisture laden and acidic it will start to corrode things fairly quickly. A tip off here is when the colour starts to look a touch yellowy or worse, like beer.

Something around an ISO 32 or 46 grade air compressor oil would be a good substitute, or a PAO based refrigeration oil in the same viscosity. 20W-50 engine oil is just miles too thick and won't lube the bearings etc. properly. An ATF is about the right viscosity, albeit slightly on the heavier side.

Hermetic refrigeration compressors also rely on a ceratin % of cold suction gas to cool the windings. Sucking air out of the atmosphere isn't a good substitute, although a supplementary fan over the casing would help.

Displacements vary from about 2.25 cc's/rev to, well, how big do you want ?

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Magic Rick! That gives us some parameters to work with.

I realised 20/50 was too thick, but it was handy! Looks like ATF it is then.

Even with a filter on the outlet you're going to use oil like billy-oh, so I'll still go with extending the inlet, and pay extra attention to cooling, one method I was thinking of was to use the redundant inlet and service stubs to circulate the oil through an oil cooler, this wouldn't be cost effective for one or two units, but I'm thinking in terms of groups of 10 or so.

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That bit Rick mentioned about the oil being hygroscopic could explain the phenonomen whereby the motor switches on and off fine after first removing it from the fridge* but after a day or so it has difficulty starting - contaminated oil!

* NB Tree Huggers - the fridge I'm experimenting on got damaged whilst lifting it off the back of a lorry and no gasses were released into the atmosphere whilst it was in my posession.

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Hermetic refrigeration compressors also rely on a ceratin % of cold suction gas to cool the windings. Sucking air out of the atmosphere isn't a good substitute, although a supplementary fan over the casing would help.

In this application there is no cold gas to cool the motor so extending the inlet has got to be better than sucking in hot air and oil.

The downside of doing this is the noise - with the lid on the motor is virtually silent, lift the lid and there's a right old racket going on in there. The noise emanates from the inlet pipe so to quieten it down the end of the extended pipe needs to be connected to a silencer.

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I actually don't know if ATF will work, it's just that as I was writing it up this morning, I sudenly had an epiphany that my fridgy oil was roughly the same viscosity as ATF at 100*C and that you bodgy buggers would have some of that lying around ...... :P

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That bit Rick mentioned about the oil being hygroscopic could explain the phenonomen whereby the motor switches on and off fine after first removing it from the fridge* but after a day or so it has difficulty starting - contaminated oil!

* NB Tree Huggers - the fridge I'm experimenting on got damaged whilst lifting it off the back of a lorry and no gasses were released into the atmosphere whilst it was in my posession.

more than likely overheating and tripping out the thermal overload due to no cold suction vapour. This is usually a simple 'klixon' type bi-metal arrangement, arranged alongside the relay.

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Rog and Luke,

Far from me to criticise, Im game for anything which involves experimentation and general weirdness ( ;) ).

But in a practical world..

You have both played with the principle and proven the theory, but is it not a waste of time? I know Rog says he tops up at night time using economy 7, but it takes 1.5hrs to get from 80psi to 150psi? Surely it must be cheaper to use a 3hp motor that wold do that in a matter of mins?

Not digging or anything chaps, its projects like this which bring new ideas to the world, and Im so jealous of Lukes ability to wire that control system! The useage of "free" hardware deserves applause. Well done.

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