Modified compressor. - Page 1
Forum section: electronics / general

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 24, 2021, 11:07:12 am
content: Not sure if this is the right category, I've wanted to convert two fridge compressors for air brush use for a while. The problem I have come up against has been start stop cycles and the start relay and PTC not liking it, and tripping out the compressor. So I've since found a couple of compressors that will play ball...So I wanted to attempt to add an oil glass level to show oil level, and an oil filler port. I have a few ideas how to do this, but want to avoid getting metal swarf inside the units. And as the hematically sealed casing isn't that thick, find a way to mount these two service ports, either adding some kind of boss etc.What I thought was silver solder with a torch, as it's got good tensile strength. But then remembered opening one of these compressors some years ago and realised the casing inside is not under positive pressure, but more a vacuum, or even nutural pressure state. And the original pipes as a refrigeration compressor are just normal solder joints. So debating if to soft solder, or even epoxy, although I'm not sure on longevity using epoxy. I know the slower curing epoxy Araldite is incredibly strong on a suitable surface, and any minor gap filling. And on a final, I'm not familiar with the clearances between the outer casing and the stator and windings, I could use another compressor as a sacrificial. Removing the top of it to establish clearance for two service ports (same model compressor) but at £ 40 a pop that's wasteful, and expensive. I was debating on drilling the casing as not toget any fine swarf inside the compressor, not easy to avoid when drilling regardless of how carefully it's done. And perhaps a Puch type tool maybe. The idea around all this is to create a silent airbrush compressor like the Bambi models available on the market, although the very expensive.Any refrigeration engineers out there have any thoughts on this task ? Sorry if this is the wrong section to post this question in, but it's not an electrical problem, so thought this section was the most appropriate to post this to. Thank for reading, any thoughts appreciated.

author: BradC
author_rank: Super Contributor
timestamp: March 24, 2021, 12:35:24 pm
content: Pressurize the shell with compressed air as you drill through and the swarf will just fly out. Best to remove the oil first thoughI'd use silver solder, but that's because I have the gear in the garage, but yeah the shell is at suction pressure rather than discharge.

author: nctnico
author_rank: Super Contributor
timestamp: March 24, 2021, 12:56:28 pm
content: Why would the oil get out? I'd just leave them as is. Any soldering on the shell likely turns any internal coating or oil residue into debris. If too much oil is coming out then the compressors likely aren't useful for air-brush anyway due to the oil getting in the paint. You'll want a filter on the intakes as well. In my experience the in and output pipes are solderable using leaded solder. Just make sure to include an overpressure vent valve though near the compressors in case the output hose gets clogged.Still, how much does a real airbruch compressor cost (from Ebay or Aliexpress)? By the time you are done with the modifications you might have spend more than a new or 2nd hand airbrush compressor costs.

author: amyk
author_rank: Super Contributor
timestamp: March 24, 2021, 01:40:12 pm
content: Quote from: davelectronic on March 24, 2021, 11:07:12 amIt depends on the model. Some are "high side dome" (compressor discharges into the housing) and some "low side dome" (compressor sucks from the housing).Likewise with where the stator and windings are. Reciprocating ones will usually have a separate spring-mounted internal unit, while rotary ones usually have the stator pressed into the housing.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 24, 2021, 03:34:44 pm
content: Thank you for your replies, I don't think oil will end up going through to the airbrush, as I'm using a trap to catch any water or oil residue. The compressor inside is mounted on springs, just a slight wobble and I can feel the enersure. For price and buying one, my Son and myself will be having a go at airbrushing, I have all the gear already. The compressor I purchased is border line on noise, ok daytime early evening, but to noisy after say 8:pm. We live in a block of flats, I have people above and below me, so I don't want to upset anyone, I know youg families putting out there kids to bed early evening. And a compressor do keep us both running simultaneously is upwards of £500 and a perfect retail compressor for both of us would be upwards of £900. It is costing a little to do the DIY option, but I'm using the air receiver and pressure control gear from the compressor I already have. That compressor is an oil less type compressor. The actual compressor can supply the air volume easily, but at just over 70db it's just to loud. Below is the compressor I have, but not yet modified. I wanted a bit bigger air receiver, that one is 24 litres it's ok for now.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 25, 2021, 09:37:34 pm
content: After some thinking, I've got a plan...So I'm going to open both hematically sealed compressors with a grinder. I have been looking above the weld line, with a thin disk I'm going to cut around each one, using a very large jubilee clip as a guide for cutting. This is really the only way to integrate an oil side glass fitting, and oil filler cap, tapping both. I know this will introduce undesirable swarf and disk cutting dust. Hopefully I can wash it all out with a very thin oil, I was thinking paraffin for this purpose. My only concern is the motor windings and the use of paraffin, do you think it would be safe coming in to contact with the insulation varnish ? If I get that far... I was going to use gasket sealant and copper strip, with enough overlap either side of the cut. Finally securing the copper strip with a few machine screws in tapped holes. I didn't want to start another thread, as it's still finding a way to add an oil site glass, and filler cap. Thoughts on washing out the grinding dust with paraffin. Thank for reading.

author: james_s
author_rank: Super Contributor
timestamp: March 25, 2021, 11:33:20 pm
content: Don't open up the domes, just leave them as-is.There is/was a series of dental air compressors utilizing refrigeration compressors. They installed a dipstick through a threaded bung in the dome, so if you wanted to have the ability to check the oil easily you could add something like that just by drilling a hole. I've used old fridge compressors before and never bothered with that though, I just tilt the thing until oil runs out the inlet to check the level.For starting, replace the PTC if it has one with a current relay or solid state starter.

author: amyk
author_rank: Super Contributor
timestamp: March 26, 2021, 01:46:53 pm
content: Quote from: davelectronic on March 25, 2021, 09:37:34 pmIt won't harm. Refrigeration oils that aren't mineral oil are harsher than paraffin.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 26, 2021, 01:51:25 pm
content: Yes I have thought about the troubles I could be getting into by opening these compressors up. I'm going to use a water trap to avoid any contaminants in the air supply. It was only for checking oil anyway, I guess I'd see if any oil accumulated in the water trap and access if it needed a top up. I have tried to find the oil quantity these compressors take, but it appears the lubricant is integrated with the refrigerant. The closest thing I could find was a Bambi compressor video showing the guy using 500 ml of oil in the new unit he'd just received. The compressors I have are about 2/3 the size of the Bambi compressor the guy is filling in the video. And I only need to add oil, as they came with about 50 ml oil in each compressor. I'm sure that can't be enough oil, I'm guessing a ball park figure of 250 ml oil for each compressor. I might have that wrong, I guess it's to much if it starts spitting oil out. Tried to think of a way to calculate the oil quantity needed, but it's not easy because of the shape of the housing. I would prefer not to open them up. For filling port, yes I could use the suction entry tube. But an oil site glass would be good. I have reservations on drilling where I can't se what's behind it. Although my drill control skills a excellent, so I could just break through with out the drill bit going to far. I had planned a friend to hold a vacuum cleaner nozzle close to the drilling site, I could at least minimise any swarf getting inside the compressor. I can see on the Bambi compressors, they have a U shaped oval clamp around what I expect is two flanges, one on each section, and removable for servicing purposes. Think I've orded the correct oil, probably have to guesstimate the quantity needed.

author: Refrigerator
author_rank: Super Contributor
timestamp: March 26, 2021, 06:13:58 pm
content: I've used fridge compressors for many things, mostly inflating car tires.They're ok as long as they don't suck any water in, because the reed valves will rust.The starters with thermistors are troublesome on quick restarts.

author: james_s
author_rank: Super Contributor
timestamp: March 27, 2021, 12:50:28 am
content: Quote from: davelectronic on March 26, 2021, 01:51:25 pmIf you can find a datasheet for the compressor it should tell you how much oil and what type it's meant to use. In this application the type doesn't really matter though, and the quantity is not too hard to guess, you want around 1.5" of oil in the sump though a bit more shouldn't hurt anything. The compressor head is at the top and there is a spiral groove in the shaft that pumps oil up from the sump and sprays it out from the top as the shaft spins. Excess oil will cause drag on the rotor but even that shouldn't really hurt anything.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 27, 2021, 11:06:11 am
content: It was many years ago now, but I did open one up out of curiosity. I found the oil pick up screw at the bottom of the shaft, one thing that puzzled me was how clean the oil was. As it was a fridge I had used for over 10 years, it hadn't broken down, I just upgraded it. So surplus fridge, I investigated the compressor. There is a component in addition to the relay in the two units I currently have. It's about an inch in diameter with two leads attached to it, it doesn't come in contact with the compressor shell at all, but sits above and to the left of the starting relay. No idea what that is, there is a start / run capacitor, but I can't see a PTC anywhere. I've tried frequent start stop cycles and it restarts fine.

author: amyk
author_rank: Super Contributor
timestamp: March 27, 2021, 09:30:37 pm
content: Quote from: davelectronic on March 27, 2021, 11:06:11 amThat's what you get with a hermetically sealed system. Unless the motor burnt out, there's essentially zero wear and the parts can look like new after several decades.User davida1hiwaaynet on YouTube has a series of videos on rebuilding a fridge compressor from 1937, whose only problem was leaking terminals:Code:https://www.youtube.com/watch?v=v0MLNuOF0Ds(I put the URL without embed so you can click through to the other parts.)Quote from: davelectronic on March 27, 2021, 11:06:11 amThat might be the overload protector. Post pictures if you want an identification.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 28, 2021, 12:31:46 pm
content: Yes interesting, when you think of a piston engine it's oil change always shows wear. Even other types of oil case crankshaft piston compressors show some oil wear going on during oil change. Yet these hematically sealed piston compressors show virtually no wear after decades. I'm not sure how or why, oil properties have excellent coating and surface film abilities... Or the materials, cylinder piston and all moving parts have some super wear protection properties, who knows. I will take a picture of this cylindrical two lead small devise that's mounted to the left of the starting relay.

author: james_s
author_rank: Super Contributor
timestamp: March 28, 2021, 05:46:51 pm
content: Quote from: davelectronic on March 28, 2021, 12:31:46 pmI've opened up several different refrigeration compressors, and aside from the two that had burned out they looked like new inside. I think it's largely because unlike an engine, there is no combustion going on to dirty up the oil, and unlike both engines and air compressors there is no ambient air with all of the various dirt and pollution it carries into the system. Also the oil circulates through the system as it is carried by the refrigerant and typically there is a filter, while most air compressors don't have any kind of oil filtration. The parts can also be more thoroughly oiled because the entire assembly is being constantly doused in oil and it is circulating through the compressor too, you can't do that with an engine or a standard air compressor or you'd be losing massive amounts of oil out the exhaust. It all adds up to a machine that can reliably run many tens of thousands of hours. Compressor failure in a refrigerator is quite rare, the vast majority of refrigerators get replaced because things like door seals, hinges or shelves break or they're just out of style. Still there are many 40-50 year old refrigerators out there that still work. We have a 1973 fridge at our cabin that has been in continuous service since then, it has never had a problem with the hermetic system.

author: nctnico
author_rank: Super Contributor
timestamp: March 28, 2021, 06:12:59 pm
content: Quote from: james_s on March 28, 2021, 05:46:51 pmI disagree here. At some point the bearings in a compressor will wear out which causes a refridgerator to become very noisy.

author: james_s
author_rank: Super Contributor
timestamp: March 28, 2021, 07:19:26 pm
content: Quote from: nctnico on March 28, 2021, 06:12:59 pmAt some point yes, nothing lasts forever but how many compressor failures have you seen? It's a fact that most refrigerators (at least in the USA) are replaced for reasons that do not involve the hermetic system. It's also a fact that there are thousands if not millions of decades-old refrigerators still working. The refrigerator, despite being an electromechanical system with moving parts that runs for hours every day is one of the most reliable and long lived appliances in the average home.Even in the failed compressors I've torn down, only one of them had a bearing failure and it was a super cheap water cooler. It had only a single bearing in the middle between the compressor and the motor with the other end of the shaft just floating. The rest have been other things, refrigerant charge lost resulting in a burnout, saw a blown gasket on the discharge line in one, broken reed valve in another. None of those failed ones were in domestic refrigerators though.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 28, 2021, 08:52:40 pm
content: Quote from: james_s on March 28, 2021, 07:19:26 pmThe shelves are really cheaply made nowadays. I have to replace shelves every few years as the plastic simply cracks eventually.Next time, I'm going to pour some epoxy into them to add some strength...

author: nctnico
author_rank: Super Contributor
timestamp: March 28, 2021, 09:35:45 pm
content: Quote from: james_s on March 28, 2021, 07:19:26 pmWell our own a couple of months ago (after 20 years of service) and I heard the one from the neighbours loud & clear a couple of years ago.

author: amyk
author_rank: Super Contributor
timestamp: March 29, 2021, 02:15:16 am
content: Maybe there's something about these relatively newer (more "environmentally friendly"?) systems that causes them to fail more often --- they could be giving them barely enough oil to begin with, a carefully calculated amount that ensures the bearings will wear out.

author: BradC
author_rank: Super Contributor
timestamp: March 29, 2021, 05:10:27 am
content: Quote from: amyk on March 29, 2021, 02:15:16 amThe older systems all used mineral oil and an incredibly stable refrigerant. It was pretty bulletproof *and* they were built to last. They were really tolerant of sloppy manufacturing / assembly practices.The move to HFC refrigerants and synthetic oils results in a system that is far less tolerant of contamination. Pretty much every failure I've seen over the last 10 years has been leak related. Cheaper materials in the evaporator or pipework results in a small leak which eventually results in a bit of moisture being pulled into the system. On the old systems, a good vac and replace the drier and it was all good. With the newer refrigerants and oils a bit of moisture starts to hydrolyze the oil and refrigerant leading to acid, and the rot starts from there.I've seen a swathe recently of aluminium pipework buried in foam where something in there has caused mass corrosion of the aluminum leading to catastrophic leaks. For all intents and purposes it looks like the blowing agent used for the foam has formed acid over time. They're just built down to a price these days. I don't recall ever seeing a compressor failure that was wear related. My neighbours split seized a compressor, but on teardown it was a bit of manufacturing leftover (looked like either swarf or solder) that rattled around for 15 years until it lodged in the compressor and jammed it.The move to a hydrocarbon refrigerant should help with acid long term, but it still depends on the oil used and how cheap they can engineer the remainder of the system.

author: Berni
author_rank: Super Contributor
timestamp: March 29, 2021, 05:30:57 am
content: Leave the oil as is, these things don't need oil changes.We got a DIY altitude simulation chamber that uses a refrigeration compressor to pump it up and down. Has been running for many years just fine. Just stick filters on the ports to keep dust out and have a oil separator close to the output so that the captured oil can flow right back down into the compressor.Corrosion is however a problem due to moisture. You cant grantee dried air when you are using it as an air compressor while the parts inside have no form of corrosion mitigation and made from cheep materials so it does rust. But it appears that if you keep running it on a regular basis it keeps the thing covered in oil and mitigates it. But if you leave one of these things sitting for a long time they might seize up. I tried to free one of those up by giving it plenty of mechanical whacks, feeding it more voltage, pulsing the power, turning it over using compressed air... etc but to no avail.

author: amyk
author_rank: Super Contributor
timestamp: March 30, 2021, 01:41:04 am
content: Quote from: BradC on March 29, 2021, 05:10:27 amPOEoil, yes. Definitely do not leave that in there and change it over to a mineral type if you're converting a compressor to pump air.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 30, 2021, 01:27:56 pm
content: I've bought some polyoester oil ISO 64 I found a data sheet for these compressors, but oil quantity isn't on there. I guesstimate around 250ml, going by a Bambi compressor video first use oil fill. That compressor took 500ml but it's much bigger than the 1/6 hp units I have. If it starts spitting oil, then I will empty it and lower the quantity. It's tricky to work out exactly how much is needed. I know the oil pick up is at the bottom end of the rotor, I'm not adding an oil site glass, any oil loss will show up in a water / oil filter trap. I can just measure any oil loss, if I had access to welding equipment I would probably open it up and add an oil site glass and filling port. I've removed the piston compressor from a brand new unit, just be happy to get it up and running with these pair of hematically sealed compressors. I should get a reasonable duty cycle with a 24 litre receiver pressurised to 8 bar, I had detected a small leak before dismantling the original noisy compressor. Acquired some loctite thread sealer, so will dismantle and reassemble the pipe work and fittings.

author: BradC
author_rank: Super Contributor
timestamp: March 30, 2021, 01:43:57 pm
content: Quote from: davelectronic on March 30, 2021, 01:27:56 pmTake it back. It’s terribly hygroscopic and will just turn to acid. Flush it out and replace it with plain old mineral or synthetic air compressor oil. I’d use 4GS but that’s only because I have some. Pretty much any light mineral oil will do.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 31, 2021, 09:54:59 am
content: The oil a purchased is in the image below, I was going on the ISO 64 standardisation. So is that oil not suitable ? I did find an oil product Bambi compressors use, but the same oil, but generic so not a named brand. It was twice as expensive as the oil in the picture, that was purchased off Amazon.

author: BradC
author_rank: Super Contributor
timestamp: March 31, 2021, 11:31:29 am
content: Quote from: davelectronic on March 31, 2021, 09:54:59 amNo, that oil is fine. It's a refined paraffin not a POE. You said you'd bought Polyolester, but that isn't.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 31, 2021, 03:30:12 pm
content: Oh ok, I must have got it wrong. I've looked at so many different oil types, I know it can't be as viscose as 10W 40 engine oil. The polyoester oil idea was just what I'd been reading on general functioning of hematically sealed compressors. If it's the right oil that's great. I've run into some mounting to the air receiver, with the position both compressors sit at. Got to get a larger plate to mount them on. Current configuration leaves it to unbalanced with weight to much to one side. Going with longer threaded rod M10 and get them centred above the receiver. And a hose thread to receiver is being difficult, I though it was 1/4 bspp female. That doesn't fit, 1/4 npt doesn't fit, I can only imagine it's 1/4 unf maybe.

author: Refrigerator
author_rank: Super Contributor
timestamp: March 31, 2021, 05:14:23 pm
content: Quote from: BradC on March 30, 2021, 01:43:57 pmPlain motor oil should work also, afterall there's neither heat nor combustion in a compressor so it shouldn't degrade.

author: davelectronic
author_rank: Frequent Contributor
timestamp: March 31, 2021, 06:51:40 pm
content: The main reason I didn't want to put the wrong oil in was possible motor windings damage. I was finding it tricky to find the exact same oil as when it was a refrigeration compressor. Just filled both compressors to 250ml, no oil spitting. So nudged it up to 300ml in each compressor. If it's to much it will end up in the water trap I've yet to buy. I have waited over 20 years to try silent airbrushing. Over 20 years ago I got started on building a similar compressor, but for one reason or other it came to nothing. Shouldn't take much longer now.The compressor I purchased as "silent" is not, in terms of garage / workshop compressors it is very quiet. But the oil less unit was over 70db, still to noisy for an apartment with people above and below me. When I purchased it, it was the silent part of the description had me convinced it would be fine. This is not a cheap option, the original compressor was £ 175 and a pair of refrigeration compressors where £ 88 posted. Still the cheapest Bambi compressor is just shy of £400 So if it works out it's not to unreasonable. Just have to mount them, plumb in and wire up.

author: james_s
author_rank: Super Contributor
timestamp: March 31, 2021, 08:59:20 pm
content: You won't damage the windings, just use regular compressor oil. Enamel insulation is really durable stuff, and they don't use different wire for different oil. Also you can get compressors for scrap value or even free if you talk to a HVAC guy. Lots of systems get scrapped when there is nothing wrong with the compressor.

author: MadScientist
author_rank: Frequent Contributor
timestamp: March 31, 2021, 09:04:59 pm
content: I have a Chinese dual head airbrush compressor , nice and quite , will support upto 5 airbrushs, was about £120.00 various models from £55, will drive two brushes etcgoogle bartsharpbuilding one from fridges units will never be good

author: james_s
author_rank: Super Contributor
timestamp: March 31, 2021, 09:07:57 pm
content: Quote from: MadScientist on March 31, 2021, 09:04:59 pmWhy do you think that? Lots of people have built compressors with salvaged fridge compressors, they work fine. Why buy something new when you can reuse something that would otherwise go to scrap or a landfill?

author: davelectronic
author_rank: Frequent Contributor
timestamp: April 01, 2021, 01:49:08 pm
content: I can't see a problem using refrigeration compressors to be honest. I've seen the smaller airbrush compressors on Amazon, with the 3 litre air receivers. I expect they are fine for one, maybe two airbrushes at the same time. But they work exactly the same as the oilless compressor I removed from the unit I purchased. There less powerful in terms of watts and piston displacement. They maybe marginally quieter than the beast I removed from the unit I have. I'd have liked a Bambi 24 litre compressor, I just don't have the extra cash laying around at the moment. This idea is a compromise, but as best to the same as a Bambi compressor, just a homebrew version. I would have liked an oil site glass and oil filler port, but it's not a deal breaker. Any oil loss should end up in the airline trap. I don't expect there will be to much. Nothing mechanical last for ever, but hopefully I can get good service life from this diy variant airbrush compressor. If I can find this thread, once it's functional I will post a picture of it. Thank you for all the help and replys, much appreciated.

author: Berni
author_rank: Super Contributor
timestamp: April 01, 2021, 04:11:03 pm
content: Yep fridge compressors are fine, just do put a oil trap right after the output as over time they put out quite a bit of oil, the stuff is messy and gets absolutley everywhere. I simply 3D printed a 3 or 4 stage vertical oil trap from thigyverse and it works really well, not a bit of oil makes it trough, all of it flowing back into the compressor via gravity.

author: james_s
author_rank: Super Contributor
timestamp: April 01, 2021, 08:42:35 pm
content: You 3D printed something that you are running compressed air through? That's slightly frightening. Hopefully it's over-engineered.

author: Refrigerator
author_rank: Super Contributor
timestamp: April 01, 2021, 08:44:26 pm
content: Quote from: james_s on April 01, 2021, 08:42:35 pmCheck out Tom Stanton and his 3D printed compressed air engines.

author: Berni
author_rank: Super Contributor
timestamp: April 02, 2021, 12:04:28 pm
content: Quote from: james_s on April 01, 2021, 08:42:35 pmThe walls are reasonably thick.I was only using it at pressures up to about 1/4 of a bar so no worries. But i tried testing at much higher pressures just in case by blocking the output and letting the compressor have at it with all it can give. At that point the hose started pooping itself off the output from the pressure, so i held the hose down to prevent it from doing that, that made the pressure go even higher but eventually the hose expanded enough to start leaking around connection point. So it most definitely can hold quite a few bars with no issue. And even so my hoses pop off before it gets there as safety.If you wanted to approach something like 10 bar tho, then yeah you have to be careful, but at a few bar its no biggie.

author: james_s
author_rank: Super Contributor
timestamp: April 02, 2021, 07:51:32 pm
content: Ah ok, my air compressor is typically set to around 125 psi, and a refrigeration compressor, if not monitored can produce several hundred PSI in some cases so it's worth being careful. Don't underestimate the amount of carnage you can have with the failure of any sort of pressure vessel. It may not occur to someone that their system is running much higher pressure than yours.