Air compressor selection

So - I'm working backwards, I've picked out a 'mini' spray gun, the A.N.I. R150 with a 1.2mm tip. This gun needs only 3.6cfm (~108L/min) @ 29psi to operate. I will be spraying water based poly and shellac, mostly on furniture and boxes.

I've been driving myself slightly batty deciding what compressor to get. Using Trade Tools as an example, a belt driven 200L/min free air delivery compressor is close to $700. On the other hand is a less robust direct drive model with 220L/min for $250. I think I'll be using it for spraying one weekend a month so neither unit is going to get a serious workout. Would appreciate people's opinions on this and what compressors other hobbyists are using.

My limited experience is as follows
- manufacturers specifications of requirements for air driven tools and guns are usually understated.
- belt driven systems are usually quieter than the direct drive systems.
- free air delivery is usually more than air being delivered at a specific pressure.

I recently (6 months ago) purchased a direct drive 'silenced' 2 HP compressor. I was going to write up a review for it as I was wary at first but my tool supply guy was right - it's a beast. This is for use onsite as a carpenter so I'm not sure about painting but the recovery time is VERY quick, the noise level is rediculously quiet - for a compressor (can have a conversation standing next to it when it kicks over without much drama) and it keepS up with me in my high demand jobs (laying floor, using clout gun for sheet brace etc)
It's a touch more expensive than some belt driven models but if it stands the test of time then I think it's worth it.

I'm very impressed with how quiet it is

https://www.audel.com.au/rolair-2hp-...ssor-jc20plus/

Anyhoo.... Just thought it would be worth a look as a lot of us with small suburban sheds need to be worried about noise . But as I said - I don't know about the demands for spraying as I haven't delved into that yet.
Cheers
Gab

I have rarely made a mistake buying the bigger size but regularly end up regretting purchasing the cheaper one. Over the years Iv'e bought 2 air compressors that weren't up to running the tools I have because I went for the cheaper. Didn't realise that the air volume figures quoted often are at zero pressure and the compressor will put out mush less volume at operating pressures. Compressor needs deliver more air you want at its cut out pressure or it will run continuously.
I recently fixed my problem, picked up a 10hp model off gumtree which is twice the size I want but way cheaper than a new smaller one.

Tony A has the right idea.

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Originally Posted by Gabriel

I recently (6 months ago) purchased a direct drive 'silenced' 2 HP compressor. I was going to write up a review for it as I was wary at first but my tool supply guy was right - it's a beast. This is for use onsite as a carpenter so I'm not sure about painting but the recovery time is VERY quick,

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That looks like a really nice unit but I would expect the recharge time from occasional or pulsed use to be very quick as it only has a 20L tank.

However, even with this quick recharge time the 20L tank means it also has to run more often under heavy use.

Most DIY users have a poor understanding of compressor cycle times which is important as most lower end compressors are not designed to run continuously and absolutely rely on a decent cycle time to cool down and cannot cope long term with being on more often than being off.

A 20L tank @ 120PSI contains about 80L of air above the OPs guns pressure requirement of 30PSI. BUT only 30L of the air at is usable before the pressure drops below 90PSI which is when the compressor tigers a recharge. As the OP's gun requires 108 L/min this means the 20L tank at full pressure will last about 17s before it needs to recharge.

Now while the compressor recharges at the manufacturers claimed rate of 151 L/min, the gun continues to draw the manufacturers claimed rate of 108 L/min leaving only 43 L/min for recharge. To recharge 20L at 43 L/min should take ~30 seconds - but it will take longer because as the pressure approached the 120PSI cut off pressure the charging speed will drop. The compressor manufacturers claimed recharge time of 15s will be assuming no draw of air during this time - if 108 of the 151 L/min is being drawn then this translates to 151/43*15 or about 50s to recharge

This means it will be on for 50s and off for 17s which is a ratio of 3:1. if the compressor is designed to do this then this is fine but most lower end compressors are not designed to do that and they never get a chance to cool off enough between cycles. Long terms this leads to increased wear and tear, and reduced life times

Compare that to a 3HP compressor with 60L tank which under 108 L/min will be off for ~50s. The 3HP will have much more excess pressurised air available for recharge but the recharge time will be similar to the compressor with the 20L tank (50s) because a 60L tank is 3x bigger.

This translates to a ratio of 1:1 - even that is not too good but it is considerably better than 3:1.

This all assumes the manufacturers claims of the air draw for the gun really is 108 L/min and all the compressor manufacturers rates as correct, which from my experience has not always been the case.

This is why just looking at the L/min for the tool and the L/min deliverable by the compressors is often not sufficient and why, even for what sounds like a tool with a small air draw, a broader picture has to be taken.

The only way to be sure is a bigger compressor, a bigger tank and a compressor that can pump to higher PSI.
And the problem is not really solved by just adding a bigger tank to a small compressor as any gain in off time is wiped out by increased recharge time so you are back to where you started from.
Increased recharge times means the compressor will get much hotter than it needs to during the recharge time also leading to more wear and tear.

As I mentioned, I know nothing about spraying thus the load/expectation of a compressor during that operation. But running nail guns for a carpenter I am very impressed by it.
Thanks for that breakdown bob (me at of it still straight over my head/above my pay grade) - I'm glad you have analysed the specs as I wouldn't want the op getting bad advice - I was just expressing my opinion on a compressor which I am impressed with in the situations I utilise one.

Very true about running a compressor flat out for extended periods. Nailing always gives the motor a chance to catch up, but I guess in a spraying situation you can't stop the flow as it would leave to run marks.
Cheers
Gab

Thanks heaps for the input gents. BobL you aren't an engineer by any chance are you?

I think I'm pretty much decided that I'm going to go for a 2.5 or 3hp belt driven, 300 L/min FAD and a 60L+ tank should give me a bit of headroom as well.

Give me a few years(!) and I'll do a follow up on this once I've gotten all the bits and pieces and my garage sorted.

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Originally Posted by Alkahestic

Thanks heaps for the input gents. BobL you aren't an engineer by any chance are you?

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Close, in a former life I was an experimental physicist.

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I think I'm pretty much decided that I'm going to go for a 2.5 or 3hp belt driven, 300 L/min FAD and a 60L+ tank should give me a bit of headroom as well.

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Sounds like a fair decision

Your choice of compressor I think is sound but your choice of gun may not be. It sounds like a touch-up gun, which is a hard way to spray furniture.

Can you really get water based poly through a 1.2mm tip? In fact can you really spray it and get a quality job?

Also, spray guns need way more air then nailers.

Plus this post I wrote a few years ago might be interesting

https://www.woodworkforums.com/f9/com...t=Underpowered

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Originally Posted by BobL

Close, in a former life I was an experimental physicist.

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A bit like cousins that quarrel sometimes then ;)

Arron - thanks for your post and your link. I'll give the 1.2mm tip and poly a shot - if it doesn't work well (or at all) I'll use the 1.2 tip and gun for shellac. Got to start somewhere!

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Originally Posted by Alkahestic

A bit like cousins that quarrel sometimes then ;)

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Yeah, A bit like "Leonard" from Big Bang, but I reckon I would have more practical hands-on skill with every day practical problems than his character seems to have. I'm into electronics and computers but not computer games, comics. I like Sci fi but more Bladerunner than stuff like star trek.
In my immediate family there are 4 scientists (vet, ag, health and me) and 3 engineers (Mechanical, solar and electrical) and also a BIL who is an Al boat builder. We have some great arguments but the fella I most in common with is Al BIL.

I was using a 2.5hp direct drive compressor to spray using a LVLP and a HVLP gun and found the compressor would struggle to keep up. I recently went and got one of the below units which handles my spray gun needs pretty easy not to mention quietly.

Attachment 423028

Might be faster to figure this out by renting a compressor. I suppose there's a variety available.

As I did not know what carbon dioxide would do to my acrylic paints, I always only rented cylinders
of compressed nitrogen, about 3,000 psi. Have a good regulator to dial up any pressure I needed.
How many cubic feet of gas is in one of those? Never, ever ran out.

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Originally Posted by Robson Valley

Might be faster to figure this out by renting a compressor. I suppose there's a variety available.

As I did not know what carbon dioxide would do to my acrylic paints, I always only rented cylinders
of compressed nitrogen, about 3,000 psi. Have a good regulator to dial up any pressure I needed.
How many cubic feet of gas is in one of those? Never, ever ran out.

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Our "G" size cylinders are 50L
@ 3000 PSI or 200 Atm the total volume at atmospheric pressure 10000L or 10 cubic meters = 353 cubic ft.
At 3.5 CFM that will give you 100 minutes to vent the whole cylinder.

The Dynair compressor above is primarily a Medical air unit so it generates clean air and is very quiet (55dB).
I'm not 100% sure of this but my guess is that this unit has two independent pairs of super quiet twin compressors with each pair operating alternately so the other pair gets a chance to properly cool off and act as a backup if one pair fails - this unit should last for 40 years plus if treated well.
Each pair has 2 x 1.5HP? compressors and each compressor is 2 stage so 2 compressors acts like a 4 stage compressor.
Multiple stage compression is a very efficient way of compressing air
The price I saw on Alibaba was $2700 including shipping!

If noise and "instant back up" are not an issues then for about half that can get you similar specced unit i.e. 3HP, 3 stage, 120L tank

Hi

A touch up spray gun is actually very convenient for spraying furniture, I use mine regularly. It's size is very convenient for smaller areas like the inside of bookshelves without the spray gun container hitting the item being sprayed. For spraying boxes a touch up gun is ideal. I also use mine for spraying metal tube frames.