Looking for a cheap compressor with a high pressure rating or for airbrush use? Answered
Today a friend of mine asked me if I know a way to reduce the noise level of his compressor in the work shed.
With the current heat he prefers to work in the evening and nights, which does not make his neighbours too happy.
His main use for several airbrush guns and sometimes for mormal airtools or the big spray gun for an undercoat or similar.
So his main concern is oil in the airline and the actual flow rate is of second concern as he has an old 25kg propane cyclinder as an additional air tank.
For relative low air volumes I would suggest an old fridge compressor.
With a thicker pipe at the outlet that is filled with stainless steel wool most of the oil stays in the compressor.
That is if this pipe is
a) long enough
c) of sufficient diameter so there is enough for the oil to avoid it being pushed up
A second, standard oil seperator will be enough for the oil level required for airbrush stuff - and most other things too.
If there is no pressure regulator on the airbrush system it is best to add a small air tank and shut off valve for it.
In our case however a fridge compressor would be just enough to keep the bigger airbrush gun running but not to fill the tank at the same time.
Not to mention the problem of fluctuating pressure levels.
Since we already had a tank and pressure shut off connected to the loud compressor it was only a matter of finding something that keeps the neighbours happy.
The first thing we did was to check how often the compressor comes on and how long it runs till the tank is back to pressure.
With that and the stated air volume on the compressor we guesstimated that something a bit bigger than the compressor of a window airconditioner should be sufficient.
The search begins....
If you don't know what to look for I give you a few hints:
Older airconditioners often run on R22 or R12 - both use quite high system pressures which is a bonus, but more on that later.
As a rule of thumb for these compressors you cans say: the bigger the higher the flow rate.
At the local wreckers and scrap yards we found a few units but noticed the bigger ones often used three phases and not just one :(
So we opted for the R22 compressor of a 4.5kW unit.
Keep in mind the 4.5kW is for the entire system, so the quite massive fans can be removed from the sum.
Usually the compressor alone is the 2.5 - 3kW range.
Ok, we found the big thing but how does this help us?
First things first ;)
The oil was removed as the housing stating the original oil amount.
This allowed us to use an oil rated for air use that has little to no water absorption qualities - you don't want water in your compressor.
With the usual heat the water should be no problem anyway.
Next was a pressure test to make sure the thing actually still works, so we added some plumping in the form of standard connectors to the inlet and outlet.
We got well above 200PSI and abondoned the test at this stage as it was more than enough already.
The air volume seemd to be well more than expected too so let'S move to the next stage.
A fridge or aircon compressor always needs to have a certain amount of oil in it as it will otherwise seize and overheat quickly.
But they are also designed so that the oil mixes with the refrigerant to cool all moving parts.
So the biggest hurdle is to make sure the oil stays where it should stay and won't enter or get lost in the tank.
Only real option for this to use something to catch the oil that is capable of releasing it into the compressor once it shuts off.
Now there are several options for this so I start with the most basic:
A "catch can" will get most of the oil, especially if filled with stainless steel wool or similar.
Downside is that you have to find a way to get it back into the compressor.
A step better is a thicker pipe filled with stainless steel wool to catch the oil.
If placed upright and the outgoing pipe can be bend a bit upwards you have a good chance that most of the oil will sweep through the valves and get back down into the compressor housing.
But only too often the cheap or even free compressor is better than expected and the oil won't get back into the housing as the vlaves are just too good.
The last and IMHO best option is a pressurised return system.
Most compressors for bigger aircons have a seperate filling port or sealed off piece of pipe.
In this case you can do a simple check to see if they are usable for our purposes.
Open the port of pipe and use a simple bike bump or similar to get some pressure in it.
With a dedicated oil filling port you are best off but they are hard to find.
The air you pump in should come out of the high pressure side - you might need a little pressure to overcome the valves.
If you hear any bubbling in the housing (use a pipe on your ear or a sensitive microphone) it means you are going through the oil inside the compressor - perfect!
You might not hear any bubbling but the port or pipe is still usable.
Get ready with your fingers and start the compressor.
The fill pipe should be sucking air in, same for the service port if there is one.
A dedicated oil port should not suck but instead force some oil up if you cover the high pressure outlet.
I assume all is good and no oil is splashing out of the open pipe or port.
Add a small amount of oil with a syringe or similar into the port/pipe.
If you see an oil mist coming out of the high side it is bad news.
Clean outlet air is good.
To get the oil back from the catch pipe or can we have to add a hose or pipe with a needle valve.
It needs to be adjusted so that there is only a very little airflow (or oil mist) coming out.
This regulated outlet is now being connect to the port/pipe with a bit of suction that we found earlier.
Now every time the compressor runs the collected oil is forced back into the compressor :)
Please double check the port/pipe used is not directly connected to the intake port!
The last thing you want is a puddle of oil going into the cylinder and damaging it!
They are designed to move gas but not liquid!
If in doubt use a hardened sttel nail or similar to create a small puncture in the top of the compressor housing if there is nothing else to use.
Check first if the material sound very thick, if so it might help to drill with a 5 or 6mm drill first - only about 1mm to make sure you won't enter the housing and conimate it with metal shavings!
Once you have a small puncture hole of about 2mm in diameter get some 2 component metal repair glue mix and add a suitable connection for the collecting pipe/can.
If you feel up to it you can of course use a blow torch and solder the connection on.
Now we have the compressor working with a oil return system that also gives up very little to no oil at all in our system.
You might now think you are good to go but you should at least add a decent and fine filter to the air inlet ;)
The compressor noise of a bigger system can still be an issue if thicker pipes are used that allow the noise to travel out.
Keep in mind they usually run in a fully closed system....
As we only need to match the noise level of the compressor itself a solid steel can like an old fire extinguisher in the 1kg rage is a good way out.
Fill it with filter wool and a fine filter pad after adding some hose connectors either end.
You can misuse the trigger nozzle and keep it to seal the top if you braze a connector on it.
If the intake here is about 5 times larger than the pipe connection to the compressor itself the air flow going into the thing is low enough for a cheap paper air filter can or box if you have a quite dusty enviroment to work with.
The real trick is to have a hose or pipe on the inside of the fire extinguisher connected to the compressor pipe connection.
A garden hose is great here as is reduces the noise quite good and is dirt cheap.
Make a lot of about 2mm sized holes in this pipe and close the other end of it off.
Now the compressor will suck it through the small holes and the soft garden hose reduces the noise, the surrounding padding brings it down to basically nothing.
The special case of clean air for airbrush....
If you read this for the sole purpose of airbrush use then this chapter is just for you, all other might want to skip it.
The two things you don't want to enter your gun is oil or water.
Both are a common thing in normal compressors due to lubrication and pressure difference resulting in condensation of the humidity in the intake air.
Oil free compressors of good quality can cost quite a few bucks and often require ongoing replacement of membranes or piston seals.
A refrigeration compressor with the above modifications already provides clean enough air for most airbrush users if a proper tank is used to store enough of the compressed air.
So you might just want to add a basic oil filter or very fine paper filter close to the regulator.
For very detailed work with very sensitive paints you might want to build a filter box containing of several layers of oil absorbent paper.
This stuff is often used in the industry to clean up minor oil spills and bind oil very well.
A PVC pipe (pressure rated please) with 5-8 layers of filter screens should last about a lifetime before the filters need changing if the diameter is in the 10-15cm range.
That leaves us with the dreaded problem of condensation and water contamination.
Depending on the type of paint and gun used a small amount of water vapour is usually no problem.
Solvent based paints usally show their disliking by unwanted drops or run offs caused by water droplets.
Of course you just go and buy a professional dehumidifier and accept the ongoing replacement costs for the cartridges...
But if you are in a climated that has above 30% humidity for most of the year than you will have to remove the water one way or the other.
A big enough storage tank for the air that is upright usually helps to release any condensated water prior to usage.
But if you use a homemade tank you might want to avoid this problem completely and forget about water in the system altogehter.
Silaca gel is the answer here, specifically the indicating variety that changes color once "full".
A spaghetti glas or similar should be big enough unless you are in a very humid climate - is so just use multiple in a row.
The air intake side for the compressor has to go through the silica gel to be effictive.
This mean we need two holes in the lid.
One with a pipe or hose going all the way to the botom - that is the air intake side.
The other right on the lid - this is the air outlet side which continues to the compressor intake.
With the color change in the silica gel we can estimate how much usage we have left until we have to heat it up to remove the water.
If this color change happens quite fast from the bottom to the top, let's say within three days or less than you really need to use more jars with silica gel in a row or a longer one - like using a long and clear acrylic pipe instead.
Of course you can always just cut holes and "viewing glasses" along the length to a PVC pipe....
No matter how wet your climate is you want to get at least 100 hours of compressor run time before you need to recharge the silica gel.
This brings us to the recharging....
Once the color changes and you only have about one quarter left to the top you want to get the water out of the gel and re-use it.
To do this you simply heat it up in your oven to around 120-150°C - the supplier should state the max temp for this.
If you use a gas oven or one with limited accuracy here it is best to stay within the 120° range.
You need to stir and mix the gel or use something big enough like an oven tray.
But be aware that these little balls are like glass!
The roll and bounce like no tomorrow!
IMHO it best to use an old cooking pot that has no plastic handles for this and not to overfill it.
This allows for easy mixing without making a mess that might cause a bad trpping hazard on your kitchen floor tiles!
Once the gel is back to original colr it is time to let it cool of to a safe temperature and to fill it back into our canister or pipe.
Tanks and shut off systems....
We have a refrigeration compressor working for us, and since it was for R22 we can use much higher pressures as a simple compressor from the hardware store.
The low pressure side is used to 70PSI or around 5Bar of pressure in normal working conditions.
The high side often works at pressure in the range of 200-300PSI or 14-20Bar!
The tank we used is a big propane tank that was restamped at some stage in his life for the use of LPG - so it was tested to quite high pressures.
The lower pressure limit is what keeps the stored gas liquid at the given temperature.
For Propane at an imaginary 30°C this would around 155PSI or 10Bar.
The stamped test pressure, although outdated, showed 600PSI or around 40Bar of pressure with no problems - and the thing was thick in the walls...
The old shut off switch from an old air compressor was adjustable after removing the safety cap with a bit of force and the help of few cold beer.
With a little tank attached we adjusted it to turn the compressor off at 250PSI or around 17Bar of pressure.
If your tank is old or has no test pressure stamped on do your own test in a safe location.
Make sure the area is secured so there is no chance of debris from a brusting tank can go anywhere - this includes to chain down the tank itself ;)
Use the aircon compressor to fill it up to 300PSI or 20Bar of pressure - this should be tolerated with ease by any propane or LPG tank.
Shut the valves and let it rest for a day or so.
It is best to do this in the early morning so the heat from the day will slightly increase the pressure.
At the end you still want to have a working tank and no major pressure losses.
All of our mods on this tank were done without actually harming the tank.
This was possible as the original valve had a release port for filling purposes - as it standard on most refillable ones.
Here we removed the valve and added a pressure guage instead - better to know what is happening than to assume things.
As this "port" had a seperate connection to the bottom of the brass valve we added as T-connection to allow for the connection to the compressor.
Just be be really sure a thin piece of copper tubing was brazed to the exit hole of this port so all incoming air will be going down and away from the outlet connection with the big shut off valve on top - which we use to actually isolate and close the tank when not it use.
Last thing required was something to connect the pressure shut off switch and regulator to.
That was the only major expense on this project as we had no old BBQ hose or similar to get a suitable connector to the tank.
We bought a simple adapter for the use of smaller hoses and cut the unwanted bits off we there was only the bottle conntector with the nut left.
After removing the rubber ring we brazed piece of copper pipe onto it.
Here we drilled holes and fitted severy connectors.
First for the pressure switch, then for the connection to the pressure regulator and two standard ones with a ball valve for air hose connections.
One air hose connection female, the other male so a standard compressor can be connected as well or "backfilled" for additional and mobile storage use.
As we wanted to avoid any reduction in the safety and burst pressure no release valve was added at the bottom on the tank.
The added silica gel filter stage was used instead so no water will get into the system to begin with.
Additionally, and painfully for me and me friend, the inside of the tank was coated with a layer of acrylic paint to prevent and rust as it was free from it when we checked it at the beginning.
This involved filling a suitable amount of paint into it, closing the top while keeping the thread clean and then to move the tank around to cover the inside evenly.
If you do this be prepared for some weird movements with your friends LOL
Once we were sure all ust be covered by paint at least three times we released the exxess paint and allowed the inside to dry with the assistance of some air forced to go in with a length of pipe.
This was repeated 3 times...
Then another two just for the bottom third of it where there might be some moisture after all...
Now you don't want to remove the brass valve with everything connected to it just to turn the tank over to releae the collected water.
Instead we made sure the added pipe on the former relese port would go all the way to the bottom of the tank.
If any water collection is suspected only the connection to the compressor needs an additional valve for the disconnection so the water will be force back out here.
To make this easy and fast we used standard quick connectors and a piece of flexible airhose rated to 20bar of pressure for the connection to the compressor.
We checked the performance of the moisture removal and oil removal only for a few hours of running time while priming some surface for later use.
The compressor oil used was very smelly to say it nice but nothing coul be smelled in the first paper filter after the pressure regulator.
To check for remaining moisture levels (65% humidity in the house) we used a 10m length of clear PVC tubing going through an ice bath.
After 30 minutes of moderate air release there was no condensation on the inside of the tubing visible.
Of course if you only need it for air supply and don't care about a bit of moisture and oil you can keep it simple ;)
Benefits of doing such a stupid thing:
For starters noise and the peace of mind that you can do a lot of airbrushing until the compressor needs to kick in again.
Then of course the benefit of an almost silent system compared to a standard compressor - something you can actually tolerate while doing art.
But the real deal is knowing YOU did it and you did it for cheap.
Warnings and some advise...
I know, it should be at the very beginning but I just hope you read till the end ;)
If the compressor fails from overheating you are up for a new one.
This means the tan size should be within the limits of what the compressor can handle - same for what you actually use on air.
You want an empty tank to be filled before the compressor feels hot to touch - quite warm is fine but if you can't leave your hand on it then it is too hot.
Same story for the usage.
There is no point in using a tiny 10 liter storage tank if you need that capacity every few minutes.
The compressor would only have little pauses and overheat quickly.
You want a good balance of usage time before the tank goes below supply pressure and running time of the compressor to get it to full pressure again.
This brings us to the safety of high pressures.
Where possible only copper tubing or sufficiently rate hoses should be used, the later as short as possible to avoid them acting like a whip if something goes wrong.
When it comes to the safety of the tank you want to make sure to stay withing it's rated limits.
All benefits of a compressor capable of producing over 500PSI otr close to 35Bar is wasted if your tank and pressure regulator can't handle it.
This must not mean that you try to use a gas cylinder of unknow age and pressure rating and assume it will work!
If in doubt use a lower shut off pressure and stay within the limits of normal air compressors - which is around 120PSI or 8Bar.
Never, ever use a tank that is compromised by inside rust or bad corrosion on the outside!
If you don't know how to braze copper tubing, pipes and connectors then check out some of the great Instructables about it!
Whenever you know you won't use any compressed air for more than a few hours close all valves especially the ones going back to the compressor on the high pressure side!
Some compressors really don't like a huge pressure difference constantly pushing on the reed valves.
If your tank is big enough to allow for more than one hour of operation before the compressor has to top it up you might want to consider a one way valve right on the compressor outlet.
This will prevent any massive pressures going onto the valves - especially helpful for modern compressors that only rely on the sealing capabilities of the clyinders or rotary system used.
One thing you should always consider is a pressure relief valve rated for about 50PSI more than your tank pressure - it can be added to the pipe ;)
If the shut off valve ever fails the relief valve gives you the ease of mind that it will blow before your tank does.
If modded correctly the compressor should stay in the compressor and the compressor itself should not overheat from use.
Having said that your compressor might force out a little more than your best catch system can handle.
If that becomes a problem it might help to use an oil with a lower viscosity.
If all fails it just means you need to top up oil once the last last paper filter is filthy or use slightly more to begin with so the intervals are longer.
The silica gel, if used should be recharged before all of it is wasted - no point in adding it if you use it once full of water.
If no gel is used there will be water in the storage tank.
Even with the added paint and a good air filter it is possible that nasty things grow in there.
Making sure the tank is emptied of any water after long uses and again before the next use is good practise.
If no pressure gauge is used on the tank you must make sure the shut off valve is always working fine and within set parameters.
I strongly recommend using a gauge and if not to perform a pressure check of the system every now and then to confirm all is within parameters of normal operation.
A compressor constantly running means you either use far too much air or you have a leak - same story if the compressos kicks in after some of forgetting to shut it off and close the valves.
If you keep the above in mind the salvaged compressor should work just fine for many years to come.
Troubleshooting and alternatives....
You put everything together the right way, double checked and something is till not right?
Maybe my crystal ball helps me to find something...
1. Always oil coming through the catch system.
It usually means you use too much of it.
A salvaged compressor, if the refrigent was removed legally from the system should still have a "correct" level of oil inside.
Too much oil would mean is being pumped through the system at an excessive rate.
Very thin compressor oils tend to do that in the compressor is misude like we do.
Changing to standard mineral oil can help here.
As a last resort you can use a pressure gauge or good judgement to allow more flow through the needle valve from the catch system back to the compressor.
Too much backflow here would mean we loose system pressure to the set level of this needle valve!
2. The R22 rated compressor seems to be unable to produce enough pressure.
First do a leak test using soapy water to rule out any leaks.
Do a back pressure test on the ports.
If you can push air through them in the reverse way with ease it means the valves are damaged making the compressor useless.
You need to replace it.
A regular cause with our type of usage is a constand back pressure from the storage tank to the compressor.
To prevent this it might help to mount an electric solenoid between the compressor and storage tank.
Such valve should be off when the pressure switch is engaged and on when the pressure switch is disengaged.
This prevents the coil from overheating but requires a "normally off" type of valve.
A good source at the wreckers are cars with LPG systems installed, they usually have suitable 12V valves somewhere on or near the tank and filler cap.
3. I am using several kg of silica gel but still get a lot of water in my storage tank.
Going overboard in a humid climate can be a good thing here but if moisture makes it into the tank even with great amounts of silica gel there are only two causes:
a) the tube or cylinder used is not long enough or not wide enough to allow the absorption of all the moisture going through.
b) the flow rate is too high and the temperatures are too.
For the first the solution is obvious enough.
The second is related to the first for the diameter and lenght but temperatures constantly above the 30°C while operating somehow limits what the gel can do.
Using a cooling coil on the intake side or simply putting the gel containers in icy water will help to a great deal here.
If that is not an option than I suggest to layer the gel and to seperate it with fine paper filter screens.
This will slow and even out the airflow allowing for more contact time with the gel.
4. The compressor gets very noisy after some time.
If "some time" means more than 30-45 minutes you simply have it running too much and it overheats.
If the noise increases too much when reaching the shut off pressure it can mean the pressure is too high for it.
5. Can I use multiple compressors from smaller units or refrigerators to get enough air volume?
Of course you can but it might mean you have to lower your pressure expectations.
Consider that each individual compressor would get the back pressure from all other compressors running while it's outlet valve is closed.
To avoid premature failure you want to make sure the compressors are shut off at a lowver pressure.
6. I don't want to use a big tank but require a good airflow for airbrush.
Two or three fridge compressors working one after the other with a small tank to keep the output pressure even can allow for about 30 minutes runtime per compressor.
With three it gives one hour for the the first to cool off and should be enough for ongoing work.
Downside is you need to make some sort of automatic switch to "rotate" to compressor working.
Is you find any spelling mistakes you can keep them.
However, if you use them in any way to make a profit with them I kindly ask for 10% of your earning from it ;)
Why did I not make an Instructable out of all this?
Well the day was very hot, the beer very cold and my mobile phone at home, so I did not take any pics.
To top it up the whole thing is now in a seperate box for additional noise reduction so it can be used in the same room where the guy is working.
Of course he just used a nailgun for the job without any regard of access or at least easy view of the two pressure gauges.
Typical if you have a great idea and the cold beer tells you to forget all about screws or hinges ROFL
Only comment was: You created it and it works fine, why would need more than the pipe connections for the gel and regulator?
Maybe he will reconsider when the service is due....