Air compressors 101

[Cesar Vega]

I’m about to buy my first spray equipment, air compressor + spray gun.
I’ve made some research already, but I still not sure about how certain things work.
I’d like to expose these doubts here, if anybody can help me to with one or two, I’d be gratefull.



How to re-calculate SCFM @ PSI? Say, the compressor I’m looking for is rated at 4.0 SCFM @ 100PSI, how do I know how much SCFM it will provide at say 40?




If I have a spray gun that’s rated to use 8.9 SCFM, and my compressor says it only gives 7.0. I understand that my compressor will not be able to catch up with the spray gun air expenditure, how ever, should I be able to use it at least for a few minutes? or am I unable to use it at all?




What’s the standard or most commonly used fluid tip size for lacquer? Is there such thing as a standard or default fluid tip? I’ve found many times the spray gun does not say anything about the fluid tip size.




What’s the main difference between gravity vs siphon feed? I’ve read gravity feed gives you more control, less overspray and more paint economy.




In the terms of the spray gun, what’s is continuous and non-continuous air flow?




For the siphon type, why there’s low, medium and high pressure spray guns and why are highger pressured more expensive? What’s the effect on finish? Say I get a low pressure spray gun box, and there’s a picture of a guy painting a car.... yet, the medium pressure still has another guy painting a car, and the high pressure also has a guy painting a car. If I can paint a car with any of the three, why I just don’t pick the cheaper one? What will the high pressure gun give me that the low pressure won’t?By the way, I won’t be painting cars, but spraying nitrocellulose lacquer over wood.




Does a very large hose (50 ft) cause to lower the pressure?




Does a humidity filter attached to the spray gun hose, cause to lower the pressure?


Thanks very much in advance for your help.

[Greg Portland]

I know a few answers:




How to re-calculate SCFM @ PSI? Say, the compressor I’m looking for is rated at 4.0 SCFM @ 100PSI, how do I know how much SCFM it will provide at say 40?

It is an actual curve that varies based on manufacturer. Additionally, the way they measure SCFM is suspect (i.e. there are no apples to apples comparisons). However, you will get higher SCFM at lower pressures. In other words, you can expect a lot more than 4.0 SCFM @ 40psi given your example above. Contact the equipment manufacturer for the actual data point (say you're running a spray rig @ requiring X CFM @ 45psi, will their compressor Y work or would Z be a better option).




If I have a spray gun that’s rated to use 8.9 SCFM, and my compressor says it only gives 7.0. I understand that my compressor will not be able to catch up with the spray gun air expenditure, how ever, should I be able to use it at least for a few minutes? or am I unable to use it at all?

Compressed air can empty out of your tank at that rate but the compressor will not be able to keep up with back filling the tank. So, if you have a huge tank in the instance above then you'd be OK for a few minutes. Of course, you'll need to wait longer for the pump to fill the tank back up.




What’s the main difference between gravity vs siphon feed? I’ve read gravity feed gives you more control, less overspray and more paint economy.In my limited experience a remote pressure pot setup gives the most flexibility and control. However, it is not ideal from a paint economy standpoint. Gravity guns are best for that (all the finish can drop straight into the gun).





Does a very large hose (50 ft) cause to lower the pressure?All plumbing and hoses lower the pressure. That is why better guns have pressure gauges @ the gun so you can set the pressure correctly (i.e. a 40psi gun may need 48psi @ the compressor to hit 40psi @ the gun). Note that you can buy/add pressure gauges in-line if your gun doesn't have one.





Does a humidity filter attached to the spray gun hose, cause to lower the pressure? yes/

[Van Huskey]

Although I am not really qualified to answer the questions well, I suggest you try the finishing forum, lots of good spray gun/compressor info there.

[Cesar Vega]

I also thought to post it there, but then I found out that most questions about compressors and spray guns have been made here, in the "General WoodWorking and Power Tools" section!

[Frederick Rowe]

If I have a spray gun that’s rated to use 8.9 SCFM, and my compressor says it only gives 7.0. I understand that my compressor will not be able to catch up with the spray gun air expenditure, how ever, should I be able to use it at least for a few minutes? or am I unable to use it at all?If your compressor pumps fewer SCFM than the gun consumes then the gun will not spray properly after the air in the tank is expended. How long will your tank last - this depends on the size of the tank. I estimate, based on spray guns I have used, that a 10 gallon tank at 150psi will last about 20 - 30 seconds of constant spraying with a gun that uses 8 SCFM. Don't let this lead you to conclude you can spray for 20 - 30 seconds. Your compressor will probably have a cut in pressure (the pressure at which the compressor kicks back in) of 100 - 120 psi. So if your compressor has a max capacity of 150 psi, and the cut in pressure is 120 psi; it will by cycling on after as few at 10 seconds of spraying.

Check the duty cycle of your compressor. Many consumer compressors have a duty cycle of 50% or less. That means if a compressor is rated at a 50% duty cycle, it needs 5 minutes rest for every 5 minutes it is compressing air. Exceed this duty cycle and you will decrease the life span of the compressor -especially oil-less compressors. Consider that even if your compressor is rated at the same number of SCFM as your gun, there will be drops in pressure due to connections and length of hose/pipe.

There are a number of low air or touch up HVLP guns which will work fine with a small compressor as long as the user does not require uninterrupted spraying - such as painting a car or spraying stain on a piece if furniture where pausing would cause a change in color tone. Loss of air pressure mid spray pass will cause the gun to spit the product being applied rather than atomizing. You don't want that happening mid-stroke. You won't be able to choose when you run out of compressed air. It may be more cost effective to buy an second tank, which will allow longer spraying between compressing cycles. This means that the time to recharge both tanks will be much longer.

Unless you are working with small spraying projects, or are certain you'd be satisfied with numerous pauses while spraying; get the biggest compressor and tank you can afford. There are lots of used compressors at discounted prices - check used first. Also budget for a good in-line oil filter and water separator - both will contaminate your finish.

Here's a good source of information on air compressors:
http://www.about-air-compressors.com/index.html

[Rich Engelhardt]

If I have a spray gun that’s rated to use 8.9 SCFM, and my compressor says it only gives 7.0. I understand that my compressor will not be able to catch up with the spray gun air expenditure, how ever, should I be able to use it at least for a few minutes? or am I unable to use it at all?

That's a "classic" question, which, if I had a dollar for, for every time a customer asked that question, I could run out and buy a Sawstop ;).

Based on your other post - where you mentioned how difficult it is for you to get things "down there" - I'm going to suggest an alternative.

Check out the Earlex HVLP systems.

If all you want the compressor for is spraying, then a turbine system like Earlex might be a better choice.

"Up here" in the US, we have easy access to other air tools that can justify the cost of a large compressor.

[John Coloccia]

I'm with Rich. HVLP is the way to go. HobbyAir is another supplier. You can get fresh air masks too, which is very convenient.

[Cesar Vega]

If all you want the compressor for is spraying, then a turbine system like Earlex might be a better choice.

"Up here" in the US, we have easy access to other air tools that can justify the cost of a large compressor.


I was in Texas last month and I was about to put an HVLP system on my shopping cart at a local Lowe's. I think it was a Graco... or Wagner, because my local Lowe's sells Wagner.
Well, the thing is that a full HVLP system makes would me feel like tied to the brand and spare parts, so I didn't bought it.


Now that I've researched some more, I realize that to spray nicely, I'd need a high SCFM gun, which would lead me to a big air compressor.
I'm just a hobbyist and more than the cost of a high-end air compressor, it's the scary size of them, and storage.

So, that's what's wondering how much could I paint with what's available here, 3.5HP, local brand, 8.9 SCFM, 10 Gals.

[Rich Engelhardt]

Fredrick covered it pretty well w/his explanation.

That jives 100% with my experience using a 3.5HP Coleman compressor w/about a 10 gal tank.
-Actually more than 10 since I plumbed a 5 gal aux tank inline with the tank on the compressor.

[Cesar Vega]

In the terms of the spray gun, what’s is continuous and non-continuous air flow?


I will answer my self.
This was a case of lost in translation.
In english this is what you guys call Bleeder and Non-Bleeder type.

Bleeder type gun is defined as continuous air flow regardless of trigger position. Non-bleeder type gun is defined as air flow shut off when trigger is released, i.e. non-continuous air flow.

Cool! :)

[Dave Lehnert]

I had the same questions as you and never did get a handle on it. I purchased a 220 volt 25 gal 5hp unit and it does anything I needed in my hobby shop.
Was told at the time not to get an aluminum pump, only a cast iron or cast iron sleeve unit. The aluminum would not last. I went ahead and got the aluminum pump because it fit my budget at the time. That was 20 years ago and my compressor runs like new. Again, In a hobby shop.