Air gun + Compressor - Guidance needed

[q9]

I'm in the market for a spray gun(s) + a compressor to drive it(them). The main restrictions are that the compressor needs to run on 240/15A. At a pinch I could probably add or change the 15A to a 20A, but I'd need a good reason to get the sparky back out.

I had a bit of a look at TotalTools yesterday morning to get a feel for what they had, there was a 3 gun HVLP set (Wagner I think) for about $290, or their own branded HVLP single gun for $99. Where it gets tricky is in knowing what size compressor I really need to run the gun. No point buying a $900 compressor when I really needed to spend $1200, and similarly no point spending $1200 if a $600 compressor would do it.

I'm not absolutely married to HVLP, though it seems like it might be the way to go.

Any advice or guidance on how to really make sense of how much air these things need?

[Dave50]

what you want to know is the volume of air that is required by that or those guns, cubic inches per minute is what you are after, ask the salesman what is required for the gun then add 15 % on his/her figure, this will give you an idea of the size compressor you need

hopefully someone will come out with a wee bit more knowledge than I

[q9]

Yes, at a basic level I have worked out that if the gun needs 9cfm then you need at least a 10cfm compressor and preferably more like 12cfm min. But I've also read people talking about volume @ psi eg 9cfm @35psi. I don't know if that matters or not...or perhaps that is just a new way to confuse the uninitiated?

[Dave50]

I'm not all that knowledgeable in this field I was hoping someone would come along who has far more knowledge to help you, but I will just tell you what I use and Hope it helps a wee bit, I use my gun (low pressure) at around 30 - 40 PSI, I have an old worn 10 cfm twin head compressor, I find this serves me well keeping up easily with constant spraying, I build furniture, all of which I spray lacquer over stain, as time allows, my main employment is tool sharpening, if I was to start over I dont think I would go below a 12 CFM compressor, just my thoughts anyway

[q9]

Ok, so I wonder if the McMillan at the top right of this page would do the trick. There are other twin pump units if I need more, but I'd need another 15A outlet. Initially I thought I might get away with something like the one on the bottom left of that page. Again, top right of this page and that is the gun I was thinking of, or the 3 gun set (which doesn't seem to be in the catalogue).

I probably should have asked about getting 3 phase into the shed...

[ian]

q9
Do you even need a compressor?

when it comes to spraying finishes, my understanding is that there is two systems

LVHP (low volume, high pressure) -- which needs a goodly supply of HP air

HVLP (high volume, low pressure) -- which usually uses an air impellor fitted to the end of teh air hose

[Dave50]

yes I often use a cig eager beaver spray unit simply because it is easier to fire up, it does a good finish and is easier to use, maybe something else to think about!

[q9]

q9
Do you even need a compressor?

Probably.

[ian]

q9

this table may help DIY Compressors
If I'm reading it correctly, you need a fairly large compressor and tank to run a convention spray gun

[munruben]

It also depends on what materials you are going to spray. Using a compressor with an air spray gun is not really suitable for spraying plastic. acrylic or water based paints satisfactorily. You need an ariless spray equipment for those to do a good job with little effort.. On the other hand if you are going to be using oil based materials, then the air spray is just fine. It also depends on the length of the delivery hoses you use, the longer the hoses the more pressure you need to maintain it at the head of the gun.
I have always found it better to overkill with any machinery when purchasing. You can always turn the pressure down if the compressor is giving you too much pressure whereas, you cannot turn the pressure up on a machine that is working flat out and not providing you with enough pressure.
If you need 9cfm compressor to do your job, buy a 12cfm compressor, it will definitely do the job much easier and last longer not having to work at maximum pressure all the time.

[ian]

Yes, at a basic level I have worked out that if the gun needs 9cfm then you need at least a 10cfm compressor and preferably more like 12cfm min. But I've also read people talking about volume @ psi eg 9cfm @35psi. I don't know if that matters or not...or perhaps that is just a new way to confuse the uninitiated?

when it comes to spraying, the psi doesn't matter all that much, the cfm is much more important, as that determins the size of he reservoir you need.

if your gun needs 9cfm (about 255 litres/min), a compresor with a 100 litre reservoir, pressurised to 95psi, will run the gun for something like 1 minute before the reservoir is empty, the compressor will cut in well before then to start refilling the tank.

to spray something like a table top might take upwards of 2 to 3 minutes, implying a reservoir in the order of 2-300 litres. (for a size comparison, a 44 gal drum is 200 litres)

this calculator might help Boyle's Law Calculator - Calculates Volume and Pressure

[q9]

this calculator might help Boyle's Law Calculator - Calculates Volume and Pressure

Thanks for that. I think I know how to read it, but I am not sure how to use the answer.

If I plug in P1 = 100, V1 = 90 and P2 = 35 I get V2 = 257

Now I am guessing the first P for convenience, but I take it that means 90 L at 100 has the same amount of molecules as 257 L at 35 psi. Which is great, but I don't know how that will translate to compressor performance. I take it that means it should basically be able to keep up...?

[ian]

Thanks for that. I think I know how to read it, but I am not sure how to use the answer.

If I plug in P1 = 100, V1 = 90 and P2 = 35 I get V2 = 257

Now I am guessing the first P for convenience, but I take it that means 90 L at 100 has the same amount of molecules as 257 L at 35 psi. Which is great, but I don't know how that will translate to compressor performance. I take it that means it should basically be able to keep up...?

The formula describes the relationship between pressure and volume.
For a given amount of air, if you double the pressure, you halve the volume.

so using your numbers, 90 litres of air compressed to 100psi will expand to 257 litres when the pressure is reduced to 35psi.
now if your gun uses 250 litres a minute at 35psi, then that 90 litres of air compressed to 100psi will run the gun for about 1 minute. However, long before then the pressure drop will cause the compressor to cut in to try and refill the tank.

I think a typical "home" compressor has a 50 litre tank, so basicly as soon as you start spraying, the compressor will cut in and run flat out till after you stop spraying. Never an ideal situation.

[q9]

I think a typical "home" compressor has a 50 litre tank, so basicly as soon as you start spraying, the compressor will cut in and run flat out till after you stop spraying. Never an ideal situation.

The biggest single phase compressor I can find locally is 307 L FAD with a 90 L tank. I can get one with a 100 L tank, but it is only rated at 274 L FAD. The next jump would be something like https://www.machineryhouse.com.au/Pr...stockCode=C345

It is only $200 more, but freight would add a bit I suspect.

The guns I am looking at seem to around 9 cfm = 254 L/m. 307 should perhaps provide enough headroom...?