Idea: How to Search for an Air Compressor

[WarthogARJ]

I would look at diving cylinders as, if you can buddy up with a testing shop they will tell you when they have cylinders that fail the inspection & if you guarantee that they will only ever be used at 150bar or under you may be able to buy them. You will be limited to 11 litre tanks but maybe able to get them at a reasonable price.

In fact there are a couple of 15 litre tanks on eBay that are out of test selling for £20 & £22. Ideally you would get them tested, that would give you a 5 year, in test, lifetime with the likelihood of them retesting again.

Hi,
Yeah, I was looking at a range of tanks: not all the following are GOOD options though, I'm just showing the possibilities:
(1) Scuba tanks: aluminium or steel
(2) Conventional gas cylinders; Nitrogen, oxygen, argon, air
(3) Fire extinguisher bodies:
(4) Propane gas cylinders:

I'm a Materials Engineer (started as Chemical Engineer), and have been involved in a wide range of industrial environments, many of which use pressure vessels. For instance, at age 22 in 1982 I was offshore as a Field Engineer, working with well head pressures in excess of 10k psi at times (H2S and CO2 as well).

For this, I would follow the general provisos of the relevant ASME Codes: for pressure vessels. And I think it would be a "Bad Idea" to use any pressure vessel that the scuba guys failed. Well, I suppose it would depend WHY they failed it, but their inspection/hydro test is pretty basic.

You do not want anything that is pressurised with ANY gas to suddenly decide to unzip itself. A liquid is a different story, but not compressed gas.

One issue is a compress air system tank is being continually pressurised, and then depressurised. Maybe not drained to zero very often, but it sees a lot of cyclical pressure. Whereas a scuba tank, or most of the other examples see very few pressure cycles. This will affect possible failure due to fatigue.

I would approach the Design of a Receiver Tank as follows:
(1) Determine my Design Pressure and Volume: maybe a range, going from high to low
(2) See what existing vessels are in that range, and what applications they are intended for.
As well as their materials, and dimensions.
(3) Unless the vessel was intended for an air compressor, you'd need to redo the design calcs for it.
And look at safety margins, material toughness, ability to inspect it for flaws etc etc, and to test it
It needs to be designed as "leak before break".
(4) Anything I used, I would only use iof after a full inspection.
Do something at least as demanding as a scuba tank test (hydro test, and visual inspection of internals). And I'd hydro test it to a LOT higher pressure than my Design Pressure: in effect, Proof Test it.
I might well get it X-rayed: that's not very expensive.
(5) Materials: I would try NOT to use aluminium. Nothing wrong with it, but for this purpose, unlike a scuba tank, you don't need the low weight.
And a good pressure vessel's steel COULD and SHOULD be a LOT tougher.
(6) Look at what safety valves, connections and pressure gauges are needed

This might sound $$$, but if you can do the design yourself, it's not.

Notes:
Scuba Tanks: if you rule out aluminium, then just steel tanks narrow your potential down a lot.
And they are pretty small.

Gas Cylinders: these are made to be banged around, are good quality/tough steel, and are well inspected/QC'd at manufacture, as well as regularily afterwards.
And they are BIG suckers: 50 litres water volume up to 200 litres.

They are hard to BUY though, most companies only want to RENT them.
One way would be to get it from a company that does a deposit, and no rent.
A full N2 cylinder costs you £135 then (deposit that you don't get back if you keep it).
The gas costs you £55, so you want to have someone else use that first.

See the attached graph, where I used prices from a big UK supplier of auxillary tanks. And i put on the point for a Nitrogen 50 litre cylinder (rated at 200 Bar). It's a LOT better. I'm expressing volume in cubic meters at room conditions (1 Bar).

The off-the -shelf tanks are BIG, but not very high pressure (10 - 11 Bar).

You would need to add a little bit for a relief valve, and a few connectors. And be clever about how you dealt with water draining. Maybe put the cylinder upside down, with a 4 way-T piece. Drain ater from the middle one, and have a gauge on one, and the outlet/intel on the other.

(3) Propane Tanks, and Fire Extinguishers:
I've not looked at how these are designed/made.
Perhaps they could be suiktable.
But I think too thin a wall thickness.
And not high enough pressures.
You want to get to 200 Bar I think.

I'm not saying this is the BEST solution, but it shows the idea is interesting to pursue.

 

[WarthogARJ]

There is a comparison method called multicriteria decision analysis which is really good technique for finding the most suitable choice based on your personal requirements. You first identify the parameters which are important to you and allocate a weighting factor and for each compressor, a score. Each machine gets a rating, which is the weighting factor multiplied by the score. Many levels can be nested if a parameter has sub-parameters. It's a lot simpler than my explanation, lol. You put in all the figures and the 'best' option gets the highest score.

Well, that's basically EXACLY what I did here.

(1) Identify important parameters: pressure, supply rate, price, receiver volume
(2) Why use an arbitrary "weighting factor" when Mr. Physics is there to help....I'm an engineer, we only use "Arbitrary Weighting Factors" when we need to guess.
No guessing needed when Mr. Physics is your friend.

OK, if you REALLY want Noise as a factor, you are welcome to try putting it in.
But then PRICE becomes irrelevant.
Or else you are willing to buy a REALLY low power machine.

But as i said, if Noise is important, just put it further away.
A 10mm section of pipe is CHEAP.

 

[WarthogARJ]

They really should quite it as part of the spec, but often you have to ask. Generally I would say places that specialise in compressors will give that information in their listings, places like Machine Matt maybe not.

Actually, the place I got my data from claims these values ARE indeed FAD.
For ALL these machines in my example.

But if you have a bit of data for ANY piston pump, you can do a pretty accurate estimate of FAD.
You need to estimate a few things, but can do it pretty accurately.

The big issue is not really FAD I think, but rather I suspect many (most?) people use too small a line from the compressor to their tool.
Something like 6.5mm ID costs you a LOT of Bar if you have anything longer than a few meters of it, and at anything bigger than say 250lpm.

Can work it out on the link I sent.

 

[WarthogARJ]

I bought one of these and it has been excellent, it's very quiet.
Wolf 100 Air Compressor 14cfm 3HP 150psi 10bar Twin-Cylinder Belt Driven 5060255482407 | eBay

Yeah, good choice.

I saw something like it.
In fact I think it's just a rebranded Abbac one.

It's a belt drive, and I didn't want to go over £250 if i could get what else I needed.
It was hard to find lower priced belts to put on.

I wonder how loud it's rated at?
I saw a YouTube video looking at noise vs Direct and Belt: need to watch that.

I'll add it onto the chart, and a few other ralated points.
I think in general belt drives have better duty-cyle than direct.
Maybe are just better cooled.

 

[WarthogARJ]

If you want ultimate ooooomph and quiet, get a hydrovane. I got one at work, it's quiet enough that you can hold a conversation next to it. Cost me £350 and all I had to do was service it.

BTW, what are its details?
I can add it to the chart.

But it needs its price "normalised", as in was that £350 a 2nd hand price?
Because the others are new prices.
Obviously at 2nd hand they cost less.

But then, maybe pretty worn.

 

[WarthogARJ]

looking for a compressor.....
for general use and want it to last
1, pump needs to be cast iron if poss, twin cylinder...better cooling if a V twin....
2, always a belt drive, much quieter and if the motor or the pump goes easy to replace the item.....small direct drives are just so noisey....gotta think of the neighbours....like it or not....
3, biggish tank but thats a personal choice depending on room and mobility....
4, costing, but I would be thinking of around £400 plus.....unless u go used....
5, there are some excellent used comp out there, mostly because people buy to SMALL.....remember the big ones wont like working on an extension lead...

I was lucky to buy this monster for not much more than a new 14 cu/ft comp....but it's 3 phase.....Atlas-CopCo....very quiet .....check out the size.... compare the chair....put it on caster as once a year I totally empty my workshop and sweep n wash the floors....not for everybody tho....hence cost...View attachment 117451

Awesome.....am jealous.
Err.....Tell me where you live in Crete, please....I have a friend with a big boat....:-}

 

[WarthogARJ]

OK, so thanks VERY much for all the EXCELLENT comments.
They heped me.

I'm not trying to sound as if I'm a compressor expert, I was really just trying to describe a way I think is useful to decide what could work.

I shall add in the comments that guys have made:
- Noise
- Reliability
- Multi-Criteria Analysis (for the fuzzy parts)
- Auxilliary tank
- Belt vs Direct Drive
- Materials: Cast iron

 

[WarthogARJ]

And NEXT, is for a WELDING MACHINE.
And THAT is a more complex subject.

 

[mikej460]

I've read good reviews about these
Hyundai 50 Litre Air Compressor, 11CFM/100psi, Oil Free, Low Noise, Electric 2hp | HY27550 (hyundaipowerequipment.co.uk)

 

[Sandyn]

Why use an arbitrary "weighting factor" when Mr. Physics is there to help....I'm an engineer, we only use "Arbitrary Weighting Factors" when we need to guess.

So why is this such a difficult choice for you??

 

[sometimewoodworker]

(5) Materials: I would try NOT to use aluminium. Nothing wrong with it, but for this purpose, unlike a scuba tank, you don't need the low weight.

That is in fact a false idea. A steel scuba tank is typically 10% lighter than its aluminium counter part For the same volume. Aluminium tanks don’t suffer from the rust problems inherent in steel tanks if there is any moisture in the fill. aluminium tanks are also cheaper for the same volume. So I completely disagree since the cost of a high pressure dry compressor and filters isn’t cheap.

the light weight tanks are the SCBA not the SCUBA tanks

Whereas a scuba tank, or most of the other examples see very few pressure cycles.

The ones I was using would see around 5 full cycles per day 7 days a week going from about 190 bar to about 20 bar whereas I question that you would be putting a compressed air tool tank through that kind of numbers of cycles.

The SCBA tanks would definitely not see anything remotely like that kind of usage.

 

[sometimewoodworker]

Gas Cylinders: these are made to be banged around, are good quality/tough steel, and are well inspected/QC'd at manufacture, as well as regularily afterwards.
And they are BIG suckers: 50 litres water volume up to 200 litres.

They are hard to BUY though, most companies only want to RENT them.

That would depend on your location and local conditions. Here you can’t easily rent, you have to buy but then exchange for full ones.

 

[BillK]

Couple of thoughts (Ok, I can't count) -

Doubling capacity via extra tanks might improve things. But say you double the capacity, the comp then runs roughly twice as long to refill. The more a comp runs the warmer & wetter the air. Might collide with duty cycle issues if you're caning the thing.
You can connect two comps and run them together if you rig up a Y-manifold type effort. This way you have the hp of two pumps going. But you have to juggle the pressure switch settings or one of them does far more of the work.

A typical 1/4" PCL connector with it's small port can do something like 40cfm @ 100psi, so personally I don't worry much about hose size - except make it at least 10mm, pref 12.
A lot of tools are supposed to have a lightweight 'whip' line at the end, to help with handling and wear & tear on hands etc. Those aren't big fat lines.

see spec here, 43cfm @ 100psi - ACA2593 Standard Adaptor Plug R 1/4 Male Thread | PCL (pclairtechnology.com)

I've got two 3hp/150 comps, one of them's is an Abac. It's the quieter of the two but builds full pressure slower than the other. I do like Abac, they carry parts, even though mine's 15 yrs old or more I can still get all sorts for it. They also used to have engineers you could book to come round for repairs, maybe still do.

Rough rule of thumb, 3cfm FAD per pump hp. It's enough to get an idea good enough to free you from possible dodgy specs/claims and save time poring over details. I don't get too hung up on things; for the budget, get the most hp as first priority, and look at tank capacity and is the seller reliable or accessible. If it goes bang, can you send it back (happened to me, brand new comp, exciting shower of fireworks emitted on turning on). Can you get parts.
Of course if it's just for occasional use then the seller & parts thing matters less, but I use mine virtually every day so it's high on my list.

 

[WarthogARJ]

So why is this such a difficult choice for you??

I wouldn't know.
Why does it seem like a difficult one for YOU?

For me, it was easy: I looked a bit online, including this Forum, and to be honest, nothing really that helpful in narrowing the choice down from the dozens, actually HUNDREDS of compressors available.

Sure, there were a few good, but somewhat isolated, comments on the Forums.

So i do what i always do if I'm buying a tool I want to use:
- Research the subject if I don't know enough first hand to decide already
- List the characteristics of a fairly wide range, and see how much the various charactersitics that are important, TO ME, vary
- Then narrow down on the range that looks good, and concentrate on price/availability for ones that fall in that general range
- I do it in Excel: makes it REALLY easy to add things, and organise it
- And as they say, "A picture is worth a 1,000 words", which is what a GRAPH or CHART is
- And right away, the best compressor for ME, that satisfied MY requirement, jumps right out at you: Bottom Right Corner

It all took me about an hour, and I'm pretty sure i'm going to get something that I'll like.
I'm an engineer, I don't really believe in impulse stuff based on vague ideas, or a few inconsistent remarks from someone on a Forum who might well not know what he's talking about.

Sure, if you are on a given Forum for a while, you start to see who knows their stuff, and who just fires off silly comments....
This Forum DOES seem to have some guys who DO know their stuff.

There are always the guys who sat in the back seats of the class and didn't listen, caused $hit in the class, and failed the tests/exams....best to ignore them.
But sometimes you do reply if it's worth your while and others.

 

[mikej460]

ooo fight fight!! now where did I put that popcorn

 

[WarthogARJ]

Couple of thoughts (Ok, I can't count) -

Doubling capacity via extra tanks might improve things. But say you double the capacity, the comp then runs roughly twice as long to refill. The more a comp runs the warmer & wetter the air. Might collide with duty cycle issues if you're caning the thing.
You can connect two comps and run them together if you rig up a Y-manifold type effort. This way you have the hp of two pumps going. But you have to juggle the pressure switch settings or one of them does far more of the work.

A typical 1/4" PCL connector with it's small port can do something like 40cfm @ 100psi, so personally I don't worry much about hose size - except make it at least 10mm, pref 12.
A lot of tools are supposed to have a lightweight 'whip' line at the end, to help with handling and wear & tear on hands etc. Those aren't big fat lines.

see spec here, 43cfm @ 100psi - ACA2593 Standard Adaptor Plug R 1/4 Male Thread | PCL (pclairtechnology.com)

I've got two 3hp/150 comps, one of them's is an Abac. It's the quieter of the two but builds full pressure slower than the other. I do like Abac, they carry parts, even though mine's 15 yrs old or more I can still get all sorts for it. They also used to have engineers you could book to come round for repairs, maybe still do.

Rough rule of thumb, 3cfm FAD per pump hp. It's enough to get an idea good enough to free you from possible dodgy specs/claims and save time poring over details. I don't get too hung up on things; for the budget, get the most hp as first priority, and look at tank capacity and is the seller reliable or accessible. If it goes bang, can you send it back (happened to me, brand new comp, exciting shower of fireworks emitted on turning on). Can you get parts.
Of course if it's just for occasional use then the seller & parts thing matters less, but I use mine virtually every day so it's high on my list.

Thanks: good points.

i MY case, I have NOT owned a compressor for 15 years.
I've NEVER owned my own, but borrowed them, or used one where I work, or study at.
So I wanted to get something good.
And yes, Abbac seemed OK to me (I'm happy you've found that too).
I think they are Italian, but have a UK presence to distribute them to suppliers.
You cannot buy direct: correct?

Yeah, I agree that you need to think out your design carefully if you are moving away from something "off-the-shelf".
As far as tank size, my view is that after a given size of tank, it ends up acting more as a "battery" for your pumping system.

You can work out for any system how much the compressor is going to need to work, based on:
- Your useage demand
- Pump specs: piston size, RPM, efficiency
- Receiver size
- Cooling of the unit (even if it's not ACTIVELY cooled by fans/fins etc, it is STILL PASSIVELY cooled, just by sitting in a colder room)
- Air supply (ambient temperature, pressure, relative humidity)

And your system itself can vary, because you can select:
- Primary pump size: piston size/number/type, efficiency, RPM (this gives you pressure output and volume)
- Secondary pump size: as above
- Primary Receiver: size, pressure rating
- Control system
- Thermal aspects: degree of active/passive cooling
- Humidity/water: you cannot really control the RH of your intake air for a smaller unit, and as you compress it, depending on the thermal aspects, the saturation level of the air changes

One aspect that's not been mentioned, is in some places the cost of electricity varies during the day.
And you might have some supply of your own: solar, wind etc.
I have not looked at it carefully, but in principle you MIGHT be better off having a BIG high pressure receiver that you pressurise when the sun is shining, the wind is blowing, ort rates are lower (say after 19h00).
The idea of using pressurised air as energy storeage IS used, and you'd need to see if you can get a big enough tank to make it worth your while.

If you have a bit of land, then you might well dig a hole, and pour yourself a big concrete tank, with a thin lining of steel, or fibreglass.
And then pressurise THAT to store in effect, cheaper electricity.
Not saying it's a good idea for all.

From what I've seen, the suppliers charge a LOT for a pretty basic pressure vessel.
I mean, a vessel that's made with low end structural steel, and is rated to 11 Bar is NOT a very difficult thing to make.
They get tube tubing/piping of the required OD/ID, and then weld on two dished ends (that are probably forged by the thousands in India).
Then weld on a few connections, and legs.
And slap some paint on.

The standard that most suppliers go by is:
DIRECTIVE 2014/29/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 26 February 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of simple pressure vessels

You can get a copy (free) here:
EUR-Lex - 32014L0029 - EN - EUR-Lex

It's NOT manadatory to follow it, which surprised me.

The design methods it specifies are pretty relaxed, basically:
- Design for 5 x Design WP
- No specific provision for corrosion
- Welds are not 100% NDE, but rely on welder qualification and batch tests
- Hydro/pneumatic test (with quite a relaxed requirement of 150% of Design WP)
- Material requirements pretty wide open, and some attention is given to toughness, but nothing very rigorous, and especially not to CONSISTENCY of mechanical properties throughout a BATCH of supplied steel
- Welds are NOT checked for mechanical properties, they rely on general certification, and in effect on the final hydro/pneumatic test

I'm not saying the STANDARD is poor, but that the QA/QC that goes with it is not very demanding.
And no attention to failure by fatigue.
So if a conscientous company follows it, what it makes could be good.
BUT, since those tend to be Europeran based, their LABOUR costs are high.

If it's been made in China or India, where labour costs are less, then I have a concern over the potential of failures from a bad weld, or out-of-spec steel.
And not all EU companies are so great either: I'm dealing with a Spanish one now that does HORRIFIC welds......

In my eyes, for a pressure vessel that sits in an area where people are, and sees fluctuating pressures (due to being pumped up, discharged somewhat, then re-charged), and is expected to have a lifespan of many years, even DECADES....well, I would like more attention paid to failure by fatigue.

For fun, you should work out the amount of ENERGY that's stored in a receiver tank of any size.
And compare it to a bomb.....
Your 90 litre tank has the equivalent of 30g of TNT: same as a WWII hand grenade does.
A 1,000 litre tank at 11 Bar has the equivalent of 0.3kg of TNT.

I think something that's sold for hundreds of $$/£££/Euros should have had a better NDE: like full X-ray or u/s of the welds.
And tighter specs on the steel used.

And done as PART of the EXISTING price.

Otherwise, sell it for LESS, a LOT less, and the end user can get it done.
I would.

 

[WarthogARJ]

ooo fight fight!! now where did I put that popcorn

Well, it's not a very constructive comment, and he DOES remind me of the lame brains in high school class.
Pretty funny actually, the head lame brain made a smart-ass comment, and the Geograhy teacher, very cool guy, said "Greg (said lmne brain), you know in 10 years time you will be working minimum wage, if you are lucky, and have a guy exactly like Alan as your boss".

And you know what.....well, I wasn't his boss, but Greg DOES work min wage pushing an idiot stick around....
Nothing WRONG with that, but he sure doesn't like it.

 

[mikej460]

Well if you come across as all high and mighty my friend expect people to poke fun at you. Keep it light, we are all here to help one another and casting dispersions like you've just made amount to personal attacks and you just end up letting yourself down. I'm sure you have a lot to contribute so just be kind.

 

[TFrench]

Well, it's not a very constructive comment, and he DOES remind me of the lame brains in high school class.
Pretty funny actually, the head lame brain made a smart-ass comment, and the Geograhy teacher, very cool guy, said "Greg (said lmne brain), you know in 10 years time you will be working minimum wage, if you are lucky, and have a guy exactly like Alan as your boss".

And you know what.....well, I wasn't his boss, but Greg DOES work min wage pushing an silly person stick around....
Nothing WRONG with that, but he sure doesn't like it.

You are being quite condescending mate. I understand that's probably a side effect of being an engineer but you might want to calm it down a tad. As someone who sat at the back of the class with the lame brains I'd like to think I'm an outlier for your theory.

 

[WarthogARJ]

That is in fact a false idea. A steel scuba tank is typically 10% lighter than its aluminium counter part For the same volume. Aluminium tanks don’t suffer from the rust problems inherent in steel tanks if there is any moisture in the fill. aluminium tanks are also cheaper for the same volume. So I completely disagree since the cost of a high pressure dry compressor and filters isn’t cheap.

the light weight tanks are the SCBA not the SCUBA tanks


The ones I was using would see around 5 full cycles per day 7 days a week going from about 190 bar to about 20 bar whereas I question that you would be putting a compressed air tool tank through that kind of numbers of cycles.

The SCBA tanks would definitely not see anything remotely like that kind of usage.

Aha.....OK, I was in correct to say that an advatage of aluminium SCUBA tanks was less weight.
Sorry for that.
Although when EMPTY they FLOAT, whereas STTEL ones do NOT tend to float.

But it's irrelevant for our application.

My main point is that the mechanical properties of aluminium are NOT an advantage for pressure vessels for ar compressors.
They are WORSE.
For many reasons:
- Toughness
- Plastic deformation: aluminium deforms under all loads, whereas steel only deforms above YIELD
- Aluminium alloys, especially those used for SCUBA tanks are very touchy about impurities in the alloy

As far as loading cycles, sure, the SCUBA and related tanks see a loading cycle that goes from high load, to low load.
But not that many of them, relative to the design of the cylinder (big thick walls, and that drops the stress down).
And it's fine: for THAT application.

And a compressor tank is going to see CONTINUAL series of loading cycles due to use.
I don't mean COMPLETE pressurise to depressurise, but pressure FLUCTUATIONS.
It's STILL a loading cycle.

These CAUSE fluctuations in the HOOP STRESS.
And THAT, if you have a big enough flaw/defect, and if material toughness is low enough, and with enough cycles CAN cause FATIGUE.

If your material is not tough enough.
And using a 2nd hand scuba tank, with a HISTORY of loading cycles due to use, and hydro testings doesn't help matters.

It's been identified as problem with SCUBA tanks, because a flaw that starts to grow, can remain sub-surtface, and thus not visible.
And can cause the dangerous case of "break BEFORE leak".
See attached papers (Page 54, Conclusions).

The aluminium alloys used for SCUBA tanks doesn't have an especially high fracture toughness.
Compared to what you can EASILY get in a low alloy steel.
Or even better, a stainless steel like 316.

In addition, the way you make an aluminium scuba tank causes its mechanical properties to be different in respect to the longitudinal and circumferential directions.

This is not an issue for SCUBA tanks, assuming you manufacture them properly, and hydrotest regularily, but it's NOT ideal for use in air compressors.

So to make it clear, if you design a compressor tank PROPERLY, with the correct material, and with goold QC in manufacture, ESPECIALLY of the welds, then fatigue should NOT be an issue.

However, I draw your attention to what USED to happen in industrial presure vessels before they started using the ASME codes for design.

Check the graph in Figure 3.
You can see when they started going by ASME design code ideas.

And I'm NOT saying the current compressor ar vessels are ALL unsafe, but I AM saying that the standard regulating them is NOT very rigorous. And if you start using the wrong tanks, you CAN go BOOM....
Or buy from a poor manufacturer.

But if you are competent, you COULD get a tank that is both well designed, as well as less expensive.