Air brush compressor.

[sureshot]

I'm looking at putting together an airbrush compressor. I'd  prefer the silent fridge type unit, but they cost a lot. I'm familiar with gases and gas laws, but wanted to run a fridge type compressor at better than 50% duty cycle which there designed for. Does anyone know if forced air cooling from below would allow longer running times ? I know it would cool the oil in the unit, but unsure about motor winding temperatures. As i understand it, start up is the highest current draw. So running shouldn't over heat the windings. Do you think if the oil is kept cool, constant run use can be achieved  ?
I've looked at on and off demand, and the motor is not suited to frequent stop start cycles where it would overheat.

Thanks for reading, any thoughts ideas appreciated.

[Miyuki]

They are gas cooled
You can open it and blow air to windings and also measure temperature for protection (some people do it this way)

As it is AC induction motor it draw almost no power without load and can draw like 10 times more when stall, so you can measure power at your pressure to see

Also I recommend to use big air tank, then it can run in long cycles and let it cool down with forced air cooling

[Ian.M]

A refrigeration compressor is designed to run with the gas return to its intake not much above 4 deg C and at fractional PSI.   Running it with 15 PSI air at 25 deg C to its intake is already overloading it and increasing its thermal stress so you'll have to drastically reduce the duty cycle to avoid overheating.

Getting a BLDC compressor (without a controller) and building a VFD controller so the pressure can be regulated by varying the speed would be the best option if you want to avoid the need for a large tank, but that's not going to be cheap.

Also, a refrigeration compressor will give you a lot of trouble with oil mist carryover, + the oil in a salvaged one will be unsuitable for reliable running open loop as its hygroscopic.

[bob225]

What is your budget ? the small air brush compressors are fairly quite as it is, The silent ones can run a tad hot but that's the trade off, I always run a filter with whatever air source I am using anway

what country are you from ? as its pointless posting any links.

In the uk there is everythingairbrush - https://www.everythingairbrush.com/compressors.html I have had great service from them over the years with parts for my DeVilbiss Dagr and some inks when I was playing with my Iwata brush

[DaJMasta]

I know it's not in the spirit of building your own with a project, but second hand good compressors can be had for cheap, sometimes.  When I was looking around and was in the market, I found someone selling a Badger brand compressor for like $80 because it had just been sitting around for a bit.  New start capacitor and an oil change and it's been going great for years since.

I also think tank size is going to be important if you are designing your own.  An airbrush doesn't actually have very high demands in terms of volume unless you're cranking up the pressure to use it as an air duster, so as long as you keep your tank small enough to not start overheating on the initial fill, the pressure maintenance in the tank should be a much lower duty cycle.  As mentioned above, I don't think external forced air cooling will have a big effect.  The housing does get warm in use, but at least on mine it doesn't seem to be relied on for cooling in the initial design, no fins or obvious heat transfer point mean it's probably not well thermally coupled to the hot components inside.

[amyk]

Older refridgeration compressors did have cooling fins.

That said, the assembly inside is usually mounted on a few springs, so there is certainly not much thermal contact.

[sureshot]

Thank you for your replys. Years ago i opened a fridge compressor carefully cutting above the hematic seal with an angle grinder. So yes I'm familiar with the anatomy of these units. I did a test on start up and measured just over 5 Amps on the unit i tested,  from memory that drop a lot lower once running. Only climbing a little under load, years ago i tried using one. Back then i found 15 minutes was about the same temperature as if it was run in a fridge, its intended location. I was hoping the filter trap would catch any would be fine oil mist, not a lot i don't think. But some oil in an open loop is possible. I've seen airbrush compressors that use these fridge motors, manufacture called Bambi i think. There rated at 50% duty cycle of a maximum of 30 minutes, they don't use forced air cooling. I did look in some detail years ago at frequent stop starts, but its just not possible with the split phase configuration. I was hoping to run a unit for continuous use with forced air cooling, i know its only the outer shell and oil that would get cooled. As you've said above the windings and compressor arrangment is mounted on sturdy springs. I'm  not sure what the vfd control for another motor type compressor is. My idea was to find a cheap normal work shop compressor, remove the original motor and drive. Then fabricate the fridge compressor in its place. Connect up the electrical control gear, but some how alter the stop start time, if it could be run with forced air cooling. Opening up the fridge compressor for cooling is not really viable as it would quickly rust badly due to its construction. Thanks again for the thoughts on this.

[coppercone2]

The main thing I think would be to ensure the output is sufficiently filtered from oil. Compressor oil getting into your paint is likely going to fuck it up, given how crazy people are about preparation and cleanliness with painting.

I don't know how to quantify air purity, you can go by commercial standards of 'clean dry air' maybe. Don't know what sensors you use to measure the vapors found in a compressor would be.

[Ian.M]

It will probably rust badly anyway unless you either have consistently low RH or have a large capacity air drier on its intake.   

If you want to go down the tankless variable speed route, 12V and 24V compressors from marine or RV fridges & freezers typically have low voltage BLDC motors that can be speed controlled by a low voltage three phase VFD drive.  As they can be used with eutectic holding plates, they support continuous operation to chill down the holding plate while power is available.  Use a RC ESC to drive it  that supports tuning to limit the max RPM to 3500, and implement the pressure control system with an Arduino and a 10 bar pressure transducer.

See https://www.mmair.com/wp-content/uploads/2016/09/BD35.BD50.Danfoss.pdf for the manual of a typical one (with OEM controller).


Its fairly common for the control board to get water or corrosion damaged in a marine installation, so you should be able to pick up just the compressor  without breaking the bank if you call around marinas with workshops, boat-breakers and second hand chandlers.

[DaJMasta]

Filtering the output air shouldn't be a big problem, you can get an inexpensive regulator/oil trap combo that just threads on the output of the tank and has done a good job with mine.  You could use a second stage trap or one with a filter inline with it for extra removal as well.

[coppercone2]

Filtering the output air shouldn't be a big problem, you can get an inexpensive regulator/oil trap combo that just threads on the output of the tank and has done a good job with mine.  You could use a second stage trap or one with a filter inline with it for extra removal as well.

Are you sure that will stop oil vapor, not just mist? Trace contamination might be enough to effect paint. Your not even supposed to let a work piece sit in air for more then like some short time period after sanding.

[sureshot]

Thanks again for the replys and the link there Ian. Yes I've come across Danfoss before, but not that type of motor. I will definitely have a good read up on this, VFD sounds an interesting alternative. I'm well versed in clean air as a diver. That has a few filtration stages. I did think for a split second on cylinder use as an air source. But realised quickly that airbrush artist's have some air on continuously, only adding paint where they want it to go. I would empty a 15 or 18 litre 232 or 300 bar in no time. And the initial outlay for valve regulator control gear is expensive. It was a nice thought while it lasted lol, complete silence...
If there was a "magic" way to alter that heat generated in that start winding, some how...
Stop starts would be possible, and problem solved. Any AC experts know if there is the slightest chance to take the heat of that start winding ?

[DaJMasta]

I haven't done any significant work on the subject, but my compressor made for airbrushing seems to be fine with it.  Cheap filter on the intake, compressor that dumps into the tank, then a drain vent in the bottom of the tank, then the output comes up from the tank and into a combined regulator/oil trap, which has done just fine in all of my painting.  Even if there's some vapor content in what comes out of the pump, it cools down in the tank and then any remainder is taken out in the moisture trap on the regulator (has a basic airstone like filter.)

[Ian.M]

If there was a "magic" way to alter that heat generated in that start winding, some how...
Stop starts would be possible, and problem solved. Any AC experts know if there is the slightest chance to take the heat of that start winding ?

The start winding phase shift is typically provided by its resistance being dominant, while the main winding's inductance is dominant so the current lags near 90 deg.   Therefore some heat buildup in the start winding from each start is unavoidable.  However, many compressors use a PTC thermistor to cut the current into the start winding to near zero after the compressor has started.  This is basically a timed start and cant be short-cycled because the thermistor and the start winding have to cool down before it can restart.

To improve the situation, if you add an unloader valve on the output, the compressor can run on, (on a timer) at virtually zero head, so minimal motor current and high flow, which will cool it down.   Also it wont have to start against any back pressure which will reduce the starting current and prevent stalling.   If you also add an electronic soft-starter it will operate the start winding for the minimum time required to get up to speed, reducing winding heat buildup and eliminating the PTC thermistor and its need to cool down.   You'll need something to sequence it all, so the motor runs up to speed before the unloader valve closes, and once the pressure setpoint is reached the unloader operates but the motor is allowed to run on for up to N minutes to cool down and avoid short cycling by closing the unloader if the pressure drops below the setpoint while the motor is still running.

[james_s]

My friend built a quiet air compressor using the hermetic compressor from his old refrigerator. He built it about 10 years ago and it is still going strong, so while there are various possible issues, his experience proves that it is indeed possible. If you let one sit too long they can rust and seize up but so long as you use it semi regularly it will be ok. The nice thing is that you can get fridge compressors for little or nothing so if it does fail you can get another one. The one special part he used is an unloader valve that releases the head pressure so the compressor isn't starting under load, they usually lack sufficient torque to do that.

His friend's dad is a retired dentist and had a small compressor which was commercially built using a refrigerator type hermetic design except the top of the dome was held on by a band clamp rather than welded and it had a threaded bung for a dip stick to check the oil.

[sureshot]

Some interesting replys, so thank you for that. I think if I can get a decent sized air receiver that will limit the start stop cycles and less run time between fills. 20 to 25 psi seems normal for spraying the majority of media, only one it two odd ball paints might need less or a bit more pressure. I think a really large air receiver would help. Maybe to compressors for efficiency, bold plans. The PTC  thing I didn't realise,  so thanks for that Ian. See if I can find a pair of matched compressors, and a large receiver. After seeing some YouTube stuff, I've been thinking air conditioning compressors. They must endure a hard work load.

[DaJMasta]

If it helps any, my compressor uses a 5L tank and pressurizes to about 110psi, then relies on the regulator to feed the brush.  I paint between like 18 and 25psi, and 50-80psi is where I use it for dusting stuff off.  The initial pressurization is 30-45s and even when using it to dust parts the duty cycle is no more than 50%... probably closer to 30%.   You could also consider looking at the design for dental air equipment (they were mentioned before), but the construction is essentially the same as an airbrush setup, just with larger airflow requirements.  The way they generally get around this is to use multiple compressors on the same tank, partly because compressors of this type above about 1/2hp don't seem to exist, so it's not uncommon to see a big tank with 3 or 4 1/2hp compressors on it with a little additional equipment to make sure the air is extremely dry (doesn't matter as much for airbrushing).

[Ian.M]

Hmm, my suggestion in reply #8 to use a RC ESC may be questionable, depending on the make of BLDC compressor chosen.  Apparently the Danfoss controller incorporates a boost converter, and runs the 3 phase H bridge driving the motor at a significantly higher DC bus voltage.  Most RC ESCs wont output enough voltage to drive a Danfoss BD35 BLDC compressor at anywhere near rated speed.  High end ones for 12S packs may be suitable.  This Youtuber is getting results using an eBike controller.

If you trawl through the Danfoss spec. sheets you can determine that for a BD35F, the absolute minimum speed is 1850 RPM, recommended minimum speed is 2000 RPM, max operating speed is 3500 RPM,  winding resistance is 2.2 ohms, displacement is 2 cc, and the max power consumption is 78.5W input to controller.   *IF* you can find one with a working or repairable OEM controller, speed can be controlled with a simple variable resistor.

[james_s]

You want to run the highest pressure that is safe for whatever tank you're using and then drop the pressure with a regulator on the output. This will allow a smaller tank to hold a lot more air and give you a consistent pressure.

Most small air conditioners use rotary compressors, I used one of those for several years as a vacuum pump for pumping down AC systems prior to charging. They have a compact can that fits tightly around the stator so it's easier to cool one externally but they are also noisier. Scroll compressors used in better quality central AC systems are nearly immune to seizing due to small amounts of rust but I've never seen one smaller than about 1.5HP (roughtly 1HP per ton of refrigeration capacity) and 240V.

Don't over-think it, an ordinary reciprocating compressor from a domestic fridge works fine. Add an unloader valve and an electromechanical or electronic starting relay in place of the PTC if it uses one and it'll be fine.

[sureshot]

Thank you for your replys and the video there Ian. Yes very interesting.  I've worked out a decent 24L receiver would set me back around £60 I've got to host compressors from our old fridge and freezer,  but not sure if to use those yet. I'd like to find a bigger retail type compressor out of a comercial chiller display unit. The control valve and regulator is £25 from a reputable seller. Then just hoses clips and hardware fittings. The cheapest Bambi purpose ready to go compressor is way over £300 I'd like that, but i also like £300 lol. Its far more cost effective to put one together from parts. I don't have the use of a decent work shop, so no welding or grinding indoors. I can get someone to do small welds if needed though. I will check out that video Ian, thanks for that. All your replys have been really helpful, putting real time use into perspective for me. Much appreciated 

[sureshot]

Had a quick look about on ebay and Amazon,  most Variable frequency drives look like single phase to 3 phase operation. Video below shows a guy using a VFD to power what looks like a general standard fridge compressor. I know little about VFD's other than they vary the frequency and control AC motors etc. I'm wandering how the compressors split phase start system would be with a lower Hz input on start up. It looks like, if it worked would be a great addition to an airbrush compressor build. But i need a single phase drive output, there not to expensive, and power ratings look doable as well. Do you think it would power a standard fridge compressor straight off the bat ? Worth a try right.

[sureshot]

Forgot to post these two, unsure on the red motor and fridge compressor motor differences.

https://youtu.be/GEWAbttNPmI

https://youtu.be/NuZsyDMP9os

Any thoughts on these appreciated.

[coppercone2]

Have you ever done any side by side tests of paint made by a oil filtered compressor and a oilless one? Like scratch test after curing for the same time period?

I am really curious. But I already have a quiet air compressor so there is no point in me doing this fun project. Maybe if I got into model making one day. Those are like 40dB. I am not sure what kind of delicate work I would need to be doing to benefit from the reduction from 60 to 40dB.

[Ian.M]

An abandoned 13 Kg propane bottle mounted upside down may be a cheap option for a tank of that size.  Don't use a butane bottle as their max working pressure is much lower.  A propane bottle should have been hydrotested to be safe with a max working pressure of 360 PSI @ 70 deg C, so unless its in really sh!tty condition has plenty of margin for use as a 150 PSI max air tank.   

Bore-scope it when you remove the valve to eliminate really sh!tty ones with deceptive exteriors.  Deodorise with neat bleach, rinse thoroughly then passivate with a dilute phosphoric acid soak, rinse again, drain and dry in a warm place opening down, preferably with an airline through the opening for circulation.

You'll have to braze up an adaptor, probably using the body of the existing valve to let you connect two T fittings  coupled TT on their side with one rotated enough so fittings on their legs don't mechanically interfere with each other, with a concentric pipe straight through both T's to take the air out from near the top of the tank.   You'll need a  solid plug reducer at the bottom of the bottom one.  Use the top T's leg for air in and the bottom leg for another T for safety valve  on the top and drain cock or auto-drain valve on the bottom.

Inspect regularly or replace every five years with a tank that was within its hydrotest interval when acquired.

[sureshot]

Yes I did look at the 4 cylinder oiless compressor, it has a 4 litre receiver,  on YouTube a video on it doesn't look to bad for noise. Price is £ 158 here in the UK, which is quite good. I've never done a side by side comparison of oiless verses oil compressor. I'm certain the tank and trap will remove any oil vapour. Most of the intended painting was going to be artist canvas or similar surfaces. I'm trying to get my Son interested also. He's got no interest in electronics sadly. Yes I can get hold of large propane cylinders that are empty, the problem there is finding someone to fill it with water before welding the feet and input output ports into it. Many welders are sceptical of welding old gas cylinders for obvious reasons, I know filled with water it's safe. As a retired diver, out of test cylinders look like an attractive option. Large twin set would do really well. They'd hold that pressure all day long, the walls are very thick indeed. But again it's the welding, welders see a cylinder and think oxygen, but it's not, it's compressed air or nitrox in it's previous use. EBay have a few sellers of air receivers but there expensive. There are also sellers of compressor / airconditoning motors I've found, but there not that cheap. A new bit of info above about the relay and PTC I was unaware of the PTC part of that, i knew it involved a relay, just asumed they used a centrifugal switch to disconnect the start winding and any capacitor. I'm wandering if there's a way to configure that PTC thermistor for frequent stop starts with out over heating any windings, but I doubt it...