Air compressor size recommendation

[drummerdimitri]

So I currently have a 24L air compressor with a 2 HP electric engine for home use (air dusting, tyre pumping, spray painting etc...)

that I feel needs to be upgraded to a 50L or maybe even a 100 L compressor since the one I own is constantly working to replenish the air

in the tank and doeskin have enough air to get most jobs done.

I've owned a 50L air compressor in the past and it was more adequate for my needs but can anyone give me a reason to go for the 100L

model? I'd like to future proof it so maybe even consider running air tools with it but will the 50L one be enough is do I NEED the 100L

model to be able to run anything off of it?

[GreyWoolfe]

It's not just the size of the tank.  The amount of air it moves at a given PSI is important.  That said, I have basically the same compressor you have.  I not only wish I had gotten one with a bigger tank but also a bigger engine and the ability to move more air.  I have hand sander that runs the tank out amazingly fast so it is useless to me.  Go for the 100L tank but make sure it can move enough air for any tool you think you might want to buy in the future.

[drummerdimitri]

Are your referring to the CFM specification?

I don't what what tool I will be using eventually but just wanted to know if a 2 HP 50L air compressor would be enough for a spray gun and maybe even an air ratchet.

Not many compressors have that specification given and even if they did they wouldn't specify at what PSI it holds true...

Also, an Italian branded 50L would cost me 200$ whereas the 100L model runs for 400$ which is a bit on the expensive side for hobby use.

[eKretz]

CFM is definitely the most important thing to get right. Tank size really doesn't make a huge difference as long as you're within reason of where you need to be. If the pump can't keep up with the air tools you're using, the tank size will make no difference. I have an 18 CFM compressor and still sometimes run out of air. Of course it will depend on what type of tools you're using. If you have a tiny little CFM requirement then the larger tank size might be nice but then it will take a lot longer to pump it up as well - so you trade off running less often for running longer or vice versa.

[rx8pilot]

The way you use the system is the first thing to sort out.

If you plan to use tools that will run for a while consuming more CFM than the pump can deliver - you need a bigger compressor.
If you plan to run tools that need higher CFM than the compressor can deliver, but only briefly, you need a bigger tank.

The latter dominated my last air system design ($25k system). We needed a LOT of CFM but only in short bursts. The average CFM was rather modest. So I figured out how big the tank needed to be to deliver the burst at the specified PSI, CFM and time while the compressor was running. When the burst demand was done, the compressor was able to re-fill the tank before being needed again. I ran the tank at 175PSI with a 100PSI regulator for the system pressure. This allowed me to get a much smaller compressor and still run some big tools, but with a time limit as a compromise.

If you are running a paint system, sanders, or any other tool that pulls a constant CFM from the system that is above the capacity of the pump - you will lose pressure and have to stop work while the system recovers.

[Mechatrommer]

If you want to be a pro full body car painter, certainly go for 100l. I have 50l comp 3hp motor (upper and lower repumping psi limit adjustable currently at 60-90psi, 180psi max red line limit)  i've done personal painting for doors, house, big closet etc just fine,the key is better quality paint gun and err proper place to do the job. Above all, the most important part is you need a regulator, its 3 fold purpose, to regulate pressure hence constant pressure everytime, 2nd is you save motor pumping from excessive air usage (unregulated) and to trap moisture out of your gun...for electronic use, i think 50l is overkill to ok already..yours 25l should be ok imho, esp little work like airbrushing but ymmv...remember... the regulator something like this cheap one can make a better day, you can get more expensive and better than this.. https://www.ebay.com/p/Air-Filter-Pressure-Regulator-Airbrush-Compressor-W-Water-Trap-Gauge-Bracket/691365316?iid=261494054661

[drummerdimitri]

Lots of good information here thanks.

So here's the thing, I do not need to run tools for too long and I'm fine not using the air compressor while it is pumping air into the tank.

If I only used the the compressor while the motor is not pumping then the CFM specification is rendered useless correct?

CFM is how much air the engine can put out without a tank under constant use am I right?

HP will dictate how fast it recovers pressure and fills the tank and a higher maximum PSI would be better since I could get "more air"

with a pressure regulator valve if my tool does not need 145 PSI which is what the one I'm looking to get is rated at.

Should I get on of those silent versions (70dB) instead of the standard 95dB models? Are there any downsides to them?

[rx8pilot]

At my home lab, I use the California Air Compressors which are really quiet. It cycles about 10-15 times per day 7 days/week for the most part. As far as I can tell - it puts out about the same CFM per horsepower as the more typical loud compressors and doesn't really cost that much. The lower speed they run at is also cooler which is better for condensation. It still gets water in the tank, but not nearly as much as other low-cost home units.

[Mechatrommer]

So here's the thing, I do not need to run tools for too long

if you can live with that. with smaller tank, you need to run/operate slower in order for the compressor to catch up with your cfm need.

and I'm fine not using the air compressor while it is pumping air into the tank.

yes. you also fine to use during pump or not pump... you also fine to not use during pump or not pump.

If I only used the the compressor while the motor is not pumping then the CFM specification is rendered useless correct?

CFM is air volume cubic feet per minute... its related to tank pressure, low psi = low cfm, high psi = hi cfm, from spray gun point of view. what is your need? usually i dont care mine as long as its working. but unregulated pressure will make things harder for example you want a uniform layer of paint. its difficult to maintain paint hand speed if pressure (hence cfm) is keep changing, thats why you need a regulated pressure. the best is you set the regulated pressure to equal or lower than the compressor lower limit psi setting (ie when motor start pumping again to reach higher psi limit)

CFM is how much air the engine can put out without a tank under constant use am I right?

from motor/engine point of view, the larger/faster the motor, the higher CFM of air it can put into the tank, but it also a function/differential of atmospheric and tank pressure. simple rule = larger motor (and number of pistons) = higher cfm.

HP will dictate how fast it recovers pressure and fills the tank and a higher maximum PSI would be better since I could get "more air"

as said, higher HP = higher CFM hence you get your tank filled quicker to the set pressure. maximum pressure will depend on piston capability and its torque curve capability or efficiency, its mechanical thing, electrically, motor only provides the torque to the piston.

It cycles about 10-15 times per day 7 days/week for the most part.

this is a function of leakage, more leakage = more pump cycles everyday... with good seal, a tank can maintain pressure in weeks without re cycling. but this only if you remove hose and everything from the tank's outlet, given the tank itself and everything fixed to it is in good seal, its hard to maintain seal on older equipments except if you can spend time to do replacements on o rings etc. remember to drain liquid/water out of the tank at regular basis. there usually a drain valve below the tank for the purpose, accumulated liquid in the tank may rust and shorten the life of it.. fwiw...

[BradC]

I have a generic 2.5HP V-twin low pressure (115PSI) compressor with a 50L tank. I run the output of that through a water separator and into a pair of 20M 3'8" hoses which are buried in the garden to get air to my garage. From there it dumps into a 200L vertical receiver. Out of the top of that it goes into another water separator/regulator. So I have about 250L of reserve air capacity.

That is enough to run a small HVLP spray gun, plasma cutter (within its own duty cycle) or tiny sand-blasting cabinet. Any bigger users (like 1/4" die grinder or large air ratchet) relatively quickly run into the air delivery limitation of the compressor. The bigger reserve capacity allows the compressor to run in longer stints and the buried hoses work really well for dropping extra moisture out of the air.

Certainly enough for weekend mechanic rattle gun or cleaning up stuff in the blasting cabinet.

It recently developed a leaky seal on one of the water traps, so it's cycling about once every 2 days at the moment. When it's sealed up properly I can leave it on and it'll cycle once every other week or so, but you have to be pretty fastidious with the leak detection bubble fluid to get those sort of hold up times under pressure. Having ball valves to isolate each quick-connector helps too as they can leak a bit and are hard to dismantle to clean.

I had a little 2HP 24L direct drive screamer for a few years and it did an OK job, but they're not designed to work as hard as I was working it and if I forgot to turn it off I was alerted to it running in the middle of the night by one of the neighbours banging on my front door at 2am. The belt drive is quiet enough that it's no biggie if I forget to turn it off.

[rx8pilot]

It cycles about 10-15 times per day 7 days/week for the most part.

this is a function of leakage, more leakage = more pump cycles everyday... with good seal, a tank can maintain pressure in weeks without re cycling. but this only if you remove hose and everything from the tank's outlet, given the tank itself and everything fixed to it is in good seal, its hard to maintain seal on older equipments except if you can spend time to do replacements on o rings etc. remember to drain liquid/water out of the tank at regular basis. there usually a drain valve below the tank for the purpose, accumulated liquid in the tank may rust and shorten the life of it.. fwiw...

It gets used quite a bit and the distribution system has a few dozen outlets and quick connects, blow-off guns, etc that keep a constant small leak in the system. I have not tried to test the compressor itself for leaks. With all the stuff connected to it, normal use is a lot more than any other factor. When I am not using anything at all - it probably cycles about every 2 hrs or so with a tiny 6gal (22l) tank. Not horrible I guess.

[Mechatrommer]

you have few dozens outlet from 22l tank? man thats ironic.

[rx8pilot]

its all very low use stuff like solder despensers, low cfm blow guns that rarely get used, etc. The number of outlets is mainly about having what I need where I need it. The total system consumption is still quite low.

Sent from my SAMSUNG-SM-G890A using Tapatalk

[tautech]

Your air compressor choice and rating is ultimately determined by only 3 things:
Peak CFM usage
Budget
Single or three phase power availability.

The hungriest (CFM) appliances IME are:
Sand blasting rigs.
Die grinders
Big nozzle dusters
Air Drills
Air grinders
Large (1/2"+) ratchet guns.

I've built a few compressor setups for my own use over the years and the best was 2 cyl 2 stage ~25 CFM 3 phase monster that could run a duster with a 1.5mm nozzle continuously @ ~90 psi and that still wasn't enough to run a die grinder without giving it a break to catch up. Lost it in a fire. 

Currently I have a 3 cyl homemade job for general light workshop duties that supplies ~12-15 CFM that is just large enough to handle most duties if you're not in a hurry to get things done.
If you go down a DIY path, factor in ~6 CFM for each HP of motor required with a single phase supply, you can allow a little less HP if 3 phase motors are used.

[nctnico]

It's not just the size of the tank.  The amount of air it moves at a given PSI is important.  That said, I have basically the same compressor you have.  I not only wish I had gotten one with a bigger tank but also a bigger engine and the ability to move more air.  I have hand sander that runs the tank out amazingly fast so it is useless to me.  Go for the 100L tank but make sure it can move enough air for any tool you think you might want to buy in the future.

For things like drills, sanders, etc it is better to get an electric one. Compressed air is very inefficient.

[Kjelt]

You can always add a second bigger tank if you need one.
I have an airpress 2500W 230VAC compressor with a 100l tank at 12 bar , price was around $700 IIRC, but I can not use a sander or a wretchet for a longer period of time.
My supplier told me that the motor should not run for a long period of time or it will shorten its life (overtemperature).
It should run about 4-6x an hour max so it could cool down sufficiently.
So if you want to work continuously for an hour or so you should buy a professional compressor that is designed to run continuously or at least for a longer stretch of time, probably better aircooled and much more expensive or goto huge tanks.

[nctnico]

Bigger tanks just take longer to fill. Basically the size of the tank governs the interval at which the compressor will run (given a constant use of air). But if you need more air than the pump can deliver you'll always run out of air. I'd go for a small tank because it will be quicker to fill.

[Mechatrommer]

but small tank will usually come with small motor and only can fit single piston. unless you want to custom made your own compressor...

[MacMeter]

Funny I just began researching air compressors, but the small type, 2 gallon tank, 1 HP motor, for air blowing mostly. The rule of thumb from what I've read is keeping the HP requirement below 2 HP if plugging into US 120 V. mains, so I'm limited to outlet power.

I think this is the model I'm going to buy:
http://www.californiaairtools.com/ultra-quiet-series-of-air-compressors/1-0-hp-air-compressors/cat-2010a/

I like the idea of an aluminum air tank that won't get rusty, and using an inline water filter I hope to get fairly clean air for blowing out computer and other air blow gun tasks. For in home use, the sound output rating of 60 db is what makes this brand of oil less compressors so popular.

Here is a site PDF page, that lists CFM for various air tools:
http://webtest.appletoncompressor.com/wp-content/uploads/2011/04/Air-Tool-or-Equipment-CFM-Requirement.pdf

[SeanB]

Remember that steel tanks need to be pressure tested every few years ( depends on what your local regs are, here it is every 3 years) for safety, and that aluminium tanks, while not being as subject to corrosion, are however very subject to fatigue, and also need the same pressure test regime.  If you get a largish compressor, also get an auto drain valve, preferably an electrically timed one, and use it to drain the tank automatically periodically. That way the moisture is removed from the tank, and it does not build up at all.

Get an oil coalescing filter ( preferably 0.2micron or better) and place on the air outlet, it does a good job of getting the oil mist out and getting a bit more of the water out. However for really dry air you will need some form of air drier, either a refrigerant unit that condenses the water out, or a PSa drier that uses a zeolite and regenerates the substrate to recycle it. A larger tank as well also cools the stored air, and this also reduces the amount of water in the compressed air, as it gets to 100% RH at a cool temperature, instead of having hotter air which can hold more vapour.

[MacMeter]

Remember that steel tanks need to be pressure tested every few years ( depends on what your local regs are, here it is every 3 years) for safety, and that aluminium tanks, while not being as subject to corrosion, are however very subject to fatigue, and also need the same pressure test regime.  If you get a largish compressor, also get an auto drain valve, preferably an electrically timed one, and use it to drain the tank automatically periodically. That way the moisture is removed from the tank, and it does not build up at all.

Get an oil coalescing filter ( preferably 0.2micron or better) and place on the air outlet, it does a good job of getting the oil mist out and getting a bit more of the water out. However for really dry air you will need some form of air drier, either a refrigerant unit that condenses the water out, or a PSa drier that uses a zeolite and regenerates the substrate to recycle it. A larger tank as well also cools the stored air, and this also reduces the amount of water in the compressed air, as it gets to 100% RH at a cool temperature, instead of having hotter air which can hold more vapour.

Hopefully, a SMALL 2 gallon aluminum tank will last the the 3000 hour life of the dual pump motor, and I'm not too concerned about fatigue, if it's designed properly. Sure steel is strong material if the potential rusty air is not a factor.

Auto drain devices are too large for small compressors this size, but its super easy to manually drain the tank after use. I don't expect too get completely DRY air on a simple setup, but on an oil less unit, I'm not concerned with that element. The "air" in those expensive throw away cans is full of chemicals and depending on the angle of use can freeze up both can and object in front.

[rx8pilot]

I had auto drains on my 20hp and 10hp systems at my CNC shop. At home, I just have a line connected to a manual valve at the end to periodically drain it. I put the compressor in kind of a hard to reach area and it is elevated just because it was the easiest spot for it and right next to the power source. In southern California it is not nearly as bad as when I lived in Houston, Texas. Compressors fill up like a bath tub there.

For the really small and occasional use - manual drains certainly work as long as you actually do it. The auto-drains can save you from yourself if you tend to forget about those things. A ruptured tank can be a problem, but at 130-ish PSI they generally don't catastrophically explode. More than likely (if anything) you end up with a leak that is very easy to hear. It does happen. My 10HP system operated at 175PSI, which had me worried enough to play it totally by the book with safety valves, auto drains, and inspections.

[Kjelt]

In southern California it is not nearly as bad as when I lived in Houston, Texas. Compressors fill up like a bath tub there.

I only do manually every three months or so and get 3cc of water max.
Depends ofcourse on usage and climate.
I never ever saw something like this though 
https://youtu.be/ElQH4FrhDvY

[MacMeter]

In southern California it is not nearly as bad as when I lived in Houston, Texas. Compressors fill up like a bath tub there.

I only do manually every three months or so and get 3cc of water max.
Depends ofcourse on usage and climate.
I never ever saw something like this though 
https://youtu.be/ElQH4FrhDvY

I'll take my chances with an aluminum 2 gallon tank just to avoid rusty air!

[rx8pilot]

In southern California it is not nearly as bad as when I lived in Houston, Texas. Compressors fill up like a bath tub there.

I only do manually every three months or so and get 3cc of water max.
Depends ofcourse on usage and climate.
I never ever saw something like this though 
https://youtu.be/ElQH4FrhDvY

Wow - never seen anything quite like that before.

[Mechatrommer]

last time i did full scale maintenance on my tank. remove the drain valve, outlet fittings and air inlet everything open, let the inside tank dry, by air blowing and leave it for few hours or day, put the drain valve close again, and pour in a diluted oil based paint inside the tank, shake the tank like crazy until all the inside wall are covered by the paint, and drain the excess paint, let it dry again and reinstall everything... now i dont get rusted liquid out anymore like before....

[BradC]

I got my current compressor off my neighbour. He had never drained the tank. It had more than 10L of water in it.

I popped the inspection port and had a poke around with a bore scope. I was astonished at the lack of rust. A very light surface discolouration but that was about it. I put a hose in the drain and drain it after every use. Also, the synthetic oil I'm using now seems to very effectively coat the inside of the tank. The condensate is generally pretty clear.

This has reminded me though. It's time to pop the inspection ports on the tank and receiver for a look.

[Cerebus]

Hopefully, a SMALL 2 gallon aluminum tank will last the the 3000 hour life of the dual pump motor, and I'm not too concerned about fatigue, if it's designed properly. Sure steel is strong material if the potential rusty air is not a factor.

It's not a question of design, it's a question of the material science. Aluminium has no fatigue limit, meaning that there is no lower limit to the amount of force that will contribute to fatigue. With steel, if you keep below the fatigue limit loading (which you can design for) then you know there is no possibility of fatigue. You cannot say this for aluminium and you can't 'design fatigue out' the way you can with steel.

[MacMeter]

Hopefully, a SMALL 2 gallon aluminum tank will last the the 3000 hour life of the dual pump motor, and I'm not too concerned about fatigue, if it's designed properly. Sure steel is strong material if the potential rusty air is not a factor.

It's not a question of design, it's a question of the material science. Aluminium has no fatigue limit, meaning that there is no lower limit to the amount of force that will contribute to fatigue. With steel, if you keep below the fatigue limit loading (which you can design for) then you know there is no possibility of fatigue. You cannot say this for aluminium and you can't 'design fatigue out' the way you can with steel.

By design, I meant good welding. For a small home unit, the tank won't be getting hit or impacted. If in normal use, air in, air out, if the the 2 gallon aluminum tank gets "fatigued", then I would question why a respected manufacturer would chose aluminum in the first place. I could see a potential issue perhaps in larger sized tanks, and it seems most of those are steel. For my purposes a small aluminum tank seems ideal.

[MacMeter]

last time i did full scale maintenance on my tank. remove the drain valve, outlet fittings and air inlet everything open, let the inside tank dry, by air blowing and leave it for few hours or day, put the drain valve close again, and pour in a diluted oil based paint inside the tank, shake the tank like crazy until all the inside wall are covered by the paint, and drain the excess paint, let it dry again and reinstall everything... now i dont get rusted liquid out anymore like before....

Hmmmm. If that's all it takes to solve steel tanks getting rusty, WHY don't they sell them with the insides already painted? I'd bet there are downsides.

[Cerebus]

... if the the 2 gallon aluminum tank gets "fatigued", then I would question why a respected manufacturer would chose aluminum in the first place.

They use it because it's a great material, if you respect its limits; and in the case of pressure vessels that means regular test and inspection for, among other things, signs of fatigue. If you doubt what I am saying, and it sounds like you do, just Google "fatigue limit".

[MacMeter]

... if the the 2 gallon aluminum tank gets "fatigued", then I would question why a respected manufacturer would chose aluminum in the first place.

They use it because it's a great material, if you respect its limits; and in the case of pressure vessels that means regular test and inspection for, among other things, signs of fatigue. If you doubt what I am saying, and it sounds like you do, just Google "fatigue limit".

Not doubting you at all. If my main goal of getting as clean an air stream out of the tank was not a priority, I'd have no issue with a steel tank, which on many points is a better choice of material. I trust with California Air Tools years of experience in compressor design, and the years that small aluminum tanks have been on the market, that I hopefully won't get blown away by aluminum shrapnel, at least before the warranty runs out! 

[tombi]

One more thing worth mentioning (this confused me when I was buying a compressor to do small paint jobs).

Some places advertise the capacity as Free Air Delivery (FAD) and some in displacement. FAD is the litres of air pushed out of the compressor per minute at a given pressure (usually 120psi). Displacement is the air pumping capacity calculated by multiplying the cylinder area by the stroke length and number of strokes per minute. Usually this is about 1/3 higher than the FAD volume depending on the efficiency of the pump.

The units of both are cubic feet per minute or litres per minute so sometimes it often isn't clear what they are quoting.

I too ended up getting a cheap small compressor (135 l/m 36L tank) with the assumption I have to wait every so often when spraying. I only do small jobs with a gravity fed paint gun so this isn't too bad. The only thing is mine is a direct drive single cylinder machine and is noisy as all get-out.

Tom

[Mechatrommer]

Hmmmm. If that's all it takes to solve steel tanks getting rusty, WHY don't they sell them with the insides already painted? I'd bet there are downsides.

read china made, beauty on the outside,crap on the inside, we have to do a little bit of work, that is understood. iirc i bought it for around $100++.. most of the original fittings broke and replaced few years ago, luckily easily fixable, most critical parts still ok ... the brand is mr mark but i doubt it to be mr mark of the usa..general rule..rust is bad,regardless of the brand.. anyway even if it will take a long time before failure due to rust, i want to add value / quality to my stuffs, its only 1-2 days work..

[BradC]

read china made, beauty on the outside,crap on the inside, we have to do a little bit of work, that is understood. iirc i bought it for around $100++.. most of the original fittings broke and replaced few years ago, luckily easily fixable, most critical parts still ok ... the brand is mr mark but i doubt it to be mr mark of the usa..general rule..rust is bad,regardless of the brand.. anyway even if it will take a long time before failure due to rust, i want to add value / quality to my stuffs, its only 1-2 days work..

The problem you need to look out for with a painted inner is any pores or cracks that develop in the paint. They let the moisture through and trap it against the metal, so you get accelerated localised corrosion. Just keep an eye on it with occasional inspection.

[tautech]

As there's always some compressor oil condensate in the receiver tank corrosion is not of a significant issue. The tanks are pressure vessels and should have been tested to many times their working pressure.
My latest is an OLD tank that my father had set up, even had fittings welded on for the gauge, pressure switch and drain and has not given any issues over the decades of use. As these days compressed gas vessels are made of aluminum with working pressures to 170 bar, it's not the material used, it's the intended use.
The big boy I made some years back was from an automotive CNG tank and despite a fair few things welded onto it I was very confident the 8mm thick walls would have zero problems with the 10 bar pressures compressed air installations run at.

[MacMeter]

Agreed, there are even aluminum scuba tanks. The issue with a steel tank is the potential for rusty air on the new smaller tanks fitted to OIL FREE compressors. As you stated, on oil feed units you might get a coating of oil on the inside and of course drain daily. As I'm buying a small 1 HP, 2 gallon unit for mostly blowing "clean" air, the aluminum tank seems like a plus for my needs. I read an entire web site mostly devoted to why one should never buy an aluminum tanked compressor, and metal fatigue etc. Many of the points seemed logical, and since at first I could not find any, I assumed NO compressors were made with them. More searching proved that wrong. In the current catalog of California Air Tools, most of the smaller compressors have aluminum tanks.

Now I need to find a good quality air blow gun, looking at two models right now.

[drummerdimitri]

So I think I've made up my mind about what compressor to get.

I like the idea of a silent compressor as it is usually right next to me and gets annoying after a while of use.

I found one with a 50L Aluminum tank (unpainted) 1HP and a max pressure of 115 PSI. It's some Chinese one by the name of powerline. CFM is pathetic but since I wont use the compressor when the engine is working, that makes the CFM value irrelevant and will stop working when it switched back on to recover the pressure.

[Kjelt]

There are also stainless steel tanks with a hefty price tag but at Ebay you can get second hand ones for decent prices.

[tautech]

So I think I've made up my mind about what compressor to get.

I like the idea of a silent compressor as it is usually right next to me and gets annoying after a while of use.

I found one with a 50L Aluminum tank (unpainted) 1HP and a max pressure of 115 PSI. It's some Chinese one by the name of powerline. CFM is pathetic but since I wont use the compressor when the engine is working, that makes the CFM value irrelevant and will stop working when it switched back on to recover the pressure.

I suggest you post a pic for further member comment, just in case there's someone here that already has that one.

[BradC]

Agreed, there are even aluminum scuba tanks.

There are. I own two. I also need to have then hydraulically tested once a year. I've seen the aftermath of an Ally tank that let go and it wasn't pretty. They'd cleaned all the blood and human parts away by the time I saw it.

There are also stainless steel tanks with a hefty price tag but at Ebay you can get second hand ones for decent prices.

Have to watch stainless as it work hardens also. If you are buying a second hand tank, consider getting a hyrdro done on it before putting it into service.

The reality is unless you are in the right place at the right time, you really do pay for quality.

[MacMeter]

Agreed, there are even aluminum scuba tanks.

There are. I own two. I also need to have then hydraulically tested once a year. I've seen the aftermath of an Ally tank that let go and it wasn't pretty. They'd cleaned all the blood and human parts away by the time I saw it.

There are also stainless steel tanks with a hefty price tag but at Ebay you can get second hand ones for decent prices.

Have to watch stainless as it work hardens also. If you are buying a second hand tank, consider getting a hyrdro done on it before putting it into service.

The reality is unless you are in the right place at the right time, you really do pay for quality.

Getting an ALUMINUM scuba tank tested is smart, especially when filled to 3000 psi. I also plan on following the advise to empty my compressor tank when I'm done using it. The CAT with its dual pistons can fill the 2 gallon tank in 90 seconds.

[Mechatrommer]

..but since I wont use the compressor when the engine is working, ... and will stop working when it switched back on.

this statement is hard to meditate..

[drummerdimitri]

So after much shopping around locally, I narrowed my choice down to two or three models:

I have found two FIAC (Italian) air compressors with a 100 L tank and a 50 L tank that are belt driven and provide 8.8 CFM of air for 400 $(100L model),  a Newco 50L air compressor with a direct drive motor with 8.9 CFM for 250$.

I am not sure what the price of the 50L belt driven FIAC will cost but will find out tomorrow but if it's around the 300-350$ mark I think that would be the best choice since it is a bit quieter than the DD model, provides a higher maximum working pressure (145 PSI vs 130 PSI) and from what I've read will be more efficient and should last longer with proper maintenance.

I have dropped the idea of a silent compressor since it costs over 400$ and has a very low CFM value.

Which one is the right choice for someone who would like to futureproof their air compressor for air tools in the future. Currently will only use it for spray painting and high pressure dust cleaning.

[Kjelt]

That is aproximately what I have 350l/min (12cfm) and a 100l tank but at 10bar (145psi).
When the pressure drops from 10bar to about 8 bar it turns back on again and it takes minutes to go back to 10 bar then the engine is already quite hot, that is no problem but it needs to cool down at least 10 minutes if you continue the whole afternoon with that work that is.

Depending on the pressure ofcourse you can use all airtools but the ones that require a lot of air and high pressure can only be run for a short period at a time or the compressor will continue running and in the end overheat.
That is why you need to figure out which tools you absolutely want and how long you want to work with them without pauses.
So it is the other way around, which tool do you want to use and then look at which compressor you need for it to run for the amount of time you want.

For me I just forget about the sanders and drills I will use electrical power tools for those, I do have an impact wrench for changing the tires of my car, it needs 7 bar pressure and 4 tires and 5 bolts is enough to let the compressor kick in again. So no way I can use it all day.

http://www.engineeringtoolbox.com/air-consumption-tools-d_847.html

[The Soulman]

Which one is the right choice for someone who would like to futureproof their air compressor for air tools in the future. Currently will only use it for spray painting and high pressure dust cleaning.

Both (continues) spray painting and high pressure dust cleaning already require a lot of air compared to other hand tools in that regards you should be ok with a compressor that could drive those.
However a compressor with a higher cfm rating would have to run less for the same task, helping with durability.

Also get a particle filter and dehumidifier if not already on the compressor.

In a pinch for high air volume requirements you could always use multiple compressors together.

[nctnico]

As there's always some compressor oil condensate in the receiver tank corrosion is not of a significant issue. The tanks are pressure vessels and should have been tested to many times their working pressure.

That reminds me of my own compressor I got from a relative. It is a self-built compressor and he even welded the tank himself from 10mm steel pipe and end plates decades ago (maybe half a century). At first the compressor did not have an automatic shut-off so when he left it running for a while the end plates where slightly deformed due to the pressure. A few years ago the tank got a small hole which let all the air out. Fortunately this was just a small rust spot so drilling a larger hole and putting thread and a plug in was enough. This compressor goes to 8 bar BTW which is enough for pumping tires and blowing dust from equipment. In many cases I don't even let it get to it's maximum pressure.

[Mechatrommer]

you can even add over-pressure relief valve if you are too concerned about this matter. if the manufacturer is good engineering, they should intentionally make one welded joint to be the weakest point, usually joints where the fittings will meet, away from human operator. this of course required better engineering (math) and practice skills. in case of over pressure, rupture point will not be random, but its hard to recognize companies who practice this delicacy.

[Cerebus]

As there's always some compressor oil condensate in the receiver tank corrosion is not of a significant issue. The tanks are pressure vessels and should have been tested to many times their working pressure.

That reminds me of my own compressor I got from a relative. It is a self-built compressor and he even welded the tank himself from 10mm steel pipe and end plates decades ago (maybe half a century). At first the compressor did not have an automatic shut-off so when he left it running for a while the end plates where slightly deformed due to the pressure. A few years ago the tank got a small hole which let all the air out. Fortunately this was just a small rust spot so drilling a larger hole and putting thread and a plug in was enough. This compressor goes to 8 bar BTW which is enough for pumping tires and blowing dust from equipment. In many cases I don't even let it get to it's maximum pressure.

Two phrases you don't want to see in the same sentence are "home made" and "pressure vessel", unless they are accompanied by phrases like "professional welder" and "tested by an approved agency". The idea of a pressure vessel that has been seen to deform its end plates under operational pressures sends shivers down my spine. I've often heard qualified people equate "pressure vessel" with "bomb" and under the wrong circumstances the former becomes the latter.

In many jurisdictions it's a criminal matter to operate pressure vessels that haven't undergone official design review, individual testing and certification, and many examples of industrial accidents attest to why this is the case. For instance, in the UK it's the "Pressure Equipment Regulations 1999, which bring the Pressure Equipment Directive 97/23/EC into UK law", to quote from the relevant government website. Please folks, don't play fast and loose with pressure vessel safety.

[nctnico]

The end plates didn't deform under normal pressure. They deformed at tens of bar! How much pressure does it take to deform a 10mm round steel plate with an approx 30cm diameter? Right now there is a blow-off valve besides the automatic switch on/off.

[Cerebus]

The end plates didn't deform under normal pressure. They deformed at tens of bar! How much pressure does it take to deform a 10mm round steel plate with an approx 30cm diameter? Right now there is a blow-off valve besides the automatic switch on/off.

It deformed, that's enough isn't it? How much does it have to deform before you realise quite how potentially dangerous an object you've got there?

I knew you wouldn't take any notice, I've seen enough of your comments to know that, but please for once stop and think. There is a reason there are laws about pressure vessels. If you are strictly the only person at risk from operating this I'll happily wave you on and chalk it down as "evolution in action". But if there are any other family members, friends, co-workers or neighbours within the potential blast and shrapnel radius then I really do whole heartedly urge you to think about this again. If the simple "for safety's sake, your own and others" argument doesn't persuade you, consider your liability at law. I don't know what the penalties are in the Netherlands, but here in Blighty you can get a £5000 fine and/or 3 months in jail for operating a pressure vessel lacking the correct approvals - which clearly an old, home made, failed and repaired vessel doesn't have or you would have said so by now.

[Kjelt]

The Darwin award goes to......

But seriously I was the day before yesterday with my compressor dealer.
I wanted the autovalve but that was too expensive so I bought an extension pipe with manual bulletvalve so I have no excuse to drain the water each week.
I noticed that there was quite some rust when I removed the old watervalve, so I asked him if I should let the tank be inspected and what could happen when it ruptures.

He said that at 10bar when it rusts through there is a slight opening you will notice immediately, worst case it will rupture.
In the 25 years he worked there he never heard of an exploding tank with max 10 bar of pressure.

So are you serious with exploding schrapnell at 10bars or are you talking about gas tanks with 300 bar pressure, there I can imagine that it would give some blast.

I for instance would like to know how much bar was on this vessel before it exploded

[drummerdimitri]

So I think I will go for the 50L model as it is the same price as the 100L and much more suitable for home use IMO.

Found belt driven models from three brands: Newco (385$), Fiac (395$ )and Fima (425$). They are all Italian bur is one better than the
others? Spec wise they are all the same so I'm not sure which one to get?

[Mechatrommer]

The end plates didn't deform under normal pressure.

the failure point usually not on the metal surface area unless they are badly corroded, watch out at the welded lines, poor welding technique will make those spots as potential failure points.. for diy tank, preferably i will stress test it in controlled area at say 150% of normal operating point...

[SeanB]

Probably the FIAC, but they probably all are alike anyway.

As to pressure testing tanks the standard is to use water and a hydraulic pump, and this will be typically to 1.25 times the rated maximum pressure of the tank. The only exceptions to this are for oxygen systems, where the test medium is pure methanol or ethanol, so as not to contaminate the inner tank wall with corrosion, as this will act as a catalyst when pressurised with oxygen. Iron burns quite well in pure oxygen, you really do not want to start this though inside a tank with high pressure. one reason a lot of oxygen equipment is made in stainless steeld, pure nickel or aluminium, as all of these form a hard oxide layer on the surface, passivating it.

Of course if you want a nasty gas try fluorine, as literally everything burns in it, including things that you would normally consider non combustible, like asbestos, PTFE,  sapphire, concrete, brick, rock. The only reason you can store it and use it in process units is that slowly introducing it into a system, which is only made from pure nickel, allows the formation of a thin coat of nickel fluoride on all exposed surfaces, which is able to withstand further fluoridation and protect the underlying metal. If this wears away too fast you get a reaction that is best viewed from a distance, and it finishes typically with a large very hot hole in the ground when the fluorine finally all has reacted with what used to be your plant and property.

[nctnico]

The end plates didn't deform under normal pressure. They deformed at tens of bar! How much pressure does it take to deform a 10mm round steel plate with an approx 30cm diameter? Right now there is a blow-off valve besides the automatic switch on/off.

It deformed, that's enough isn't it? How much does it have to deform before you realise quite how potentially dangerous an object you've got there?

I knew you wouldn't take any notice, I've seen enough of your comments to know that, but please for once stop and think.

I'm thinking this: why replace a tank with 10mm thick walls with a new tank with walls which are a few mm (or less) thick? Only because someone (who can weld very well) welded it together a couple of decades ago and (unintentionally) tested it at many times the operational pressure? It got stress tested the sh*t out of it and it survived. It just doesn't make sense to replace it. As Kjelt noted it is not working at several hundred bar but at 8 bar and it is a small tank too so the amount of energy available to do damage is very limited.

The video linked by Kjelt above shows a commercial tank which exploded but without knowing why it exploded it is impossible to draw any conclusion. Was the tank bad or was the pressure cranked up way too high (due to a malfunction)?

BTW here is someone with a certified tank which blew open:

[CatalinaWOW]

Kjelts video shows the danger potential of these tanks.  But contrary to the commentary on the video it is not obvious that rust is the culprit.  While there is rust on the tank it doesn't appear to be enough to be the primary cause an of an explosion, though it may have been the last link in the chain.  If rust was the only failure in this tank I would say that there is clearly not enough initial safety margin.

Whatever the cause, the danger is real.  In most of the US periodic re-certification of these tanks is not required by law.  But world over it is required by good sense.  The bigger the tank the more the need.   A good check would not only verify the strength of the tank, but also confirm the auto shutoff and safety relief valve(s) are working properly.  Plural on safety relief because this is one place redundancy makes a fair amount of sense.  That same corrosion that is endemic on the inside of these tanks has an enormous tendency to freeze up relief valves.