Author Topic: Air compressor size recommendation  (Read 21058 times)

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Online Mechatrommer

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Re: Air compressor size recommendation
« Reply #25 on: August 22, 2017, 12:27:13 am »
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....
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Offline BradC

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Re: Air compressor size recommendation
« Reply #26 on: August 22, 2017, 12:59:14 am »
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.
 

Offline Cerebus

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Re: Air compressor size recommendation
« Reply #27 on: August 22, 2017, 01:03:52 am »
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.
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Offline MacMeter

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Re: Air compressor size recommendation
« Reply #28 on: August 22, 2017, 01:17:28 am »
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.
 

Offline MacMeter

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Re: Air compressor size recommendation
« Reply #29 on: August 22, 2017, 01:20:14 am »
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.
 

Offline Cerebus

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Re: Air compressor size recommendation
« Reply #30 on: August 22, 2017, 01:40:08 am »
... 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".
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Offline MacMeter

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Re: Air compressor size recommendation
« Reply #31 on: August 22, 2017, 02:01:16 am »
... 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!  ;)
 

Offline tombi

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Re: Air compressor size recommendation
« Reply #32 on: August 22, 2017, 02:04:30 am »
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.

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Online Mechatrommer

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Re: Air compressor size recommendation
« Reply #33 on: August 22, 2017, 03:06:07 am »
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..
« Last Edit: August 22, 2017, 03:10:23 am by Mechatrommer »
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Offline BradC

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Re: Air compressor size recommendation
« Reply #34 on: August 22, 2017, 03:48:12 am »
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.
 

Offline tautech

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Re: Air compressor size recommendation
« Reply #35 on: August 22, 2017, 05:23:29 am »
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.
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Offline MacMeter

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Re: Air compressor size recommendation
« Reply #36 on: August 22, 2017, 05:48:33 am »
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.
 

Offline drummerdimitriTopic starter

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Re: Air compressor size recommendation
« Reply #37 on: August 22, 2017, 06:09:25 am »
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.




 

Offline Kjelt

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Re: Air compressor size recommendation
« Reply #38 on: August 22, 2017, 06:57:17 am »
There are also stainless steel tanks with a hefty price tag but at Ebay you can get second hand ones for decent prices.
 

Offline tautech

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Re: Air compressor size recommendation
« Reply #39 on: August 22, 2017, 07:06:00 am »
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.
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Offline BradC

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Re: Air compressor size recommendation
« Reply #40 on: August 22, 2017, 08:10:36 am »
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.
 

Offline MacMeter

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Re: Air compressor size recommendation
« Reply #41 on: August 22, 2017, 08:54:19 am »
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.
 

Online Mechatrommer

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Re: Air compressor size recommendation
« Reply #42 on: August 22, 2017, 10:28:58 am »
..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..
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline drummerdimitriTopic starter

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Re: Air compressor size recommendation
« Reply #43 on: August 23, 2017, 03:51:44 pm »
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.



 

Offline Kjelt

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Re: Air compressor size recommendation
« Reply #44 on: August 23, 2017, 04:21:39 pm »
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

« Last Edit: August 23, 2017, 04:27:43 pm by Kjelt »
 

Offline The Soulman

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Re: Air compressor size recommendation
« Reply #45 on: August 23, 2017, 04:34:01 pm »
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.
 

Offline nctnico

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Re: Air compressor size recommendation
« Reply #46 on: August 23, 2017, 09:22:05 pm »
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.
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Online Mechatrommer

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Re: Air compressor size recommendation
« Reply #47 on: August 24, 2017, 05:28:34 am »
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.
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Offline Cerebus

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Re: Air compressor size recommendation
« Reply #48 on: August 24, 2017, 08:45:46 am »
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.
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Offline nctnico

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Re: Air compressor size recommendation
« Reply #49 on: August 24, 2017, 11:58:52 am »
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.
« Last Edit: August 24, 2017, 12:21:47 pm by nctnico »
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