Trying to understand relationship between CFM and tank size

[Yasha Renner]

Hi everyone. This is my first post! So I'm considering a few small-shop compressors to do some hobby HVLP spraying. I was planning on using the inexpensive Central Pneumatic sprayer (6 CFM @ 40 PSI).

As for air compressors, there's a couple I've been considering. The first is an oil free Husky C041H, which has a 4 gallon tank and boasts 6.8 CFM @ 40 PSI. (I have no idea why I should even care that it does 225 PSI.) The second is an oil lubricated Rolair FC2002 (the BULL), which has a 4.3 gallon tank and is rated at 5.4 CFM @ 40 PSI, but has a 100% duty cycle.

My first question has to do with the length of time I can run the sprayer before the compressor turns back on. A 4 gallon tank has about 0.53 cubic feet of compressed air. The 6 CFM sprayer would require 0.1 CF per second. So, if my math is right, does that mean I could run the sprayer for only about 5 seconds before the tank is empty?! If that is true, then the compressor would be running constantly, right? And if that is so, then the Rolair, though slightly underpowered, might be a better choice. (I'm also a little skeptical of the Husky specifications.)

[Jim Becker]

Yasha, the pump plays a more important role in CFM/SCFM with a smaller compressor because as you indicate, there's a lot less storage of air under pressure exceeding your baseline level and you can conceivably use air at a rate that doesn't allow the tank to recover. A larger, beefier compressor is going to get a much higher volume of air under pressure in the tank which in turn, allows the unit to supply more air to whatever tool you're using before the compressor has to cycle back on. And the pumps on the larger compressors can generally supply air to the network while simultaneously compressing air into the tank during the on-cycle. Since your stated use will be for spraying finish which expects a continuous air flow for a longer period of time than tools like pneumatic nailers require, you are going to be better served with a larger compressor. I originally had a 10 gallon mass-market compressor and I did manage to spray finish with it, but it ran constantly during that process. It was loud and annoying. I also suspect it struggled to keep up with the HPLV conversion gun a little, too, but have no scientific evidence of the same. The bottom line is that if you go just by the published numbers, the compressors you ask about might do the job, but you ultimately are not going to be happy with them for spraying finish, IMHO. Something like the IR GarageMate at 25 gallons but still runs on 120v would be my choice for this purpose...or a turbine based finish spraying system.

[Yasha Renner]

Jim, I'd be pleased as punch to have the Ingersoll Rand. Unfortunately it's just a few hundred out of budget. But assuming I did have that compressor, I'm still trying to figure out how many seconds would elapse before the tank was empty. A 20 gallon tank equals about 2.67 cubit feet. An air tool that consumes 6 CFM @ 40 PSI uses 0.1 CF per second. So, if I'm not mistaken, even with the larger tank, the air tool would run for less than 30 seconds before the tank was empty and the pump kicked in. Does that sound right to you?

[Wayne Lomman]

The pump needs to be able to supply the air you need at the pressure you need it with a bit of capacity to spare. The function of the air receiver/tank is to act as a buffer that stabilises your air flow. This means you can plug/unplug hoses, tools etc without affecting system pressure to any great extent. Here are some examples.

If I am working on site doing abrasive blasting, I often use a 900cfm compressor. It has a 60 litre/13 gallon air receiver. This is all it needs because its function is to deliver a large volume of air at a constant rate through a blast hose with nothing else going on. If I add a second or third blast hose, I add in a portable air receiver as a stabiliser so that different operators can start and stop without the other guy being affected.

In the workshop, we have 24 CNC machining centres as well as the blasting and painting shop. There are pneumatic cylinders, die grinders, spray guns and blast hoses starting and stopping randomly all day. We have 400cfm in compressors and a 2000 litre/450 gallon air receiver. We need that volume to keep the pressure fairly stable otherwise the CNCs get grumpy and throw up alarms complaining of air pressure outside of parameters.

In your situation, you need to ensure that the pump can supply the volume and pressure that your gun requires plus about 25%. The extra is to allow for compressor and gun manufacturers lying a little bit on their data sheets. Check the data on your gun before you purchase. Different brands have different consumption so it is better to view your system as a package rather than buying the cheapest of each. If the 100% duty cycle one will provide enough air, that would be my recommendation. All small compressors are going to be working hard driving a spray gun so 100% is worth while. Oil lubricated will provide longer life as well. Cheers

[Stan Calow]

Yasha, the compressor will run more often than that as it will work to keep the pressure up to what you have it set for, not just when it is empty. I have a small compressor that I use for brad and pin nailing, and it will turn on every couple of minutes even if I'm not actively using the nailer, because of line leakage. When I was looking at buying spray equipment, I was resigned to the fact that you need a big capacity, more than a pancake compressor, to provide enough air. So I got an Earlex turbine for spraying, and the small compressor for nailing.

[Art Mann]

The simplest and most accurate answer to your question is that there is no relationship between the tank size and the cfm a compressor can deliver at a specific pressure. A larger tank will allow a longer time between cycles of the compressor running but it will not reduce the total amount of time the machine must be running for a given pressure and volume. For example, if you double tank size you will half the time between cycles but the duration of each cycle will double. In your case, if you need 6 cfm at 40 psi, the Rolair will not work at all because it can only deliver 5.4 cfm at 40 psi. It will run continuously and the pressure will quickly drop below the required 40 psi until you stop spraying for a few minutes to let it catch up. The Husky won't do much better because the amount of air you need at 40 psi is almost equal to the maximum required. The compressor will supply the 40 psi but will run nearly all the time.

I don't mean to make you feel bad, but your calculations only serve to illustrate that you don't have an understanding of how pressure and volume are related. It is much more complex than you are thinking. If you want to get an idea of just how complex the subject is, Google "Ideal Gas Law" or "Boyle's Law".

As far as compressor specifications go, you need to get a good understanding of the difference between tank pressure and working pressure. You are using those two parameters interchangeably and they are not the same thing.

[Yasha Renner]

Thanks for the education everyone. I've also been eyeing a couple older machines in my local classifieds that I think will suffice for my needs. One is a 25 gallon oil-free DeVilbiss (5 HP, 125 PSI; 6.9 CFM @ 90 PSI; 8.8 CFM @ 40 PSI). It's advertised as "fast and quiet." The other is a 27 gallon oil-free Coleman Powermate (5 HP, 125 PSI; 5.6 CFM @ 90 PSI; 8.2 CFM @ 40 PSI). Both appear to be in excellent working condition.

However, I'm reluctant to purchase one of these because if something breaks down on the unit I'll be on eBay looking up parts, and it may end up being money wasted. Am I being too cautious here?

The IR Garage Mate is definitely not within budget for me. Alternatively, the California Air Tools 10020C looks pretty enticing.

Thanks again,
YR

[Yasha Renner]

Thanks for the helpful information, Mr. Mann. I'm curious what you think of the Husky C041H and the manufacturer's claim that the increased PSI somehow creates three times more usable air. The product description states: "The 4 Gal. 225 PSI tank holds over 3x as much usable air as a 4 Gal. 125 PSI tank, which means you can fire 3x as many nails at any 1 time."

I don't understand this. The tank only holds so much air, irrespective of the PSI.

[John TenEyck]

You do care about what pressure a compressor stores the air at in the receiver. One form of the universal gas law is P1V1 = P2V2. The higher the stored pressure, the more volume you will have when released. So with your 4 gal, 225 psi Husky example that would be 4 x 225 = 900. Mixed units but it won't matter. When it steps down to 40 psi coming out of the regulator it will give you 22.5 gal of air. 22.5 gals = 3 cf because there are 7.5 gals in a cubic foot. If your gun needs 6 cfm @ 40 psi then you will have 30 seconds of continuous spray time before the tank hits 40 psi. Your pump will probably kick in at 90 psi, however, and since it makes 6.8 cfm at 40 psi it should be able to keep up with the gun and maybe even build a little pressure back in the tank, slowly. But you will have a lot of water to deal with as the air expands from 225 psi which isn't good. If the pump is not rated for continuous duty it's going to die a quick death if you do much spraying with it with a gun that needs that much air.

You really want a larger compressor if you plan to spray very much with a gun drawing that much air.

John

[Yasha Renner]

Thank you, John. Very helpful.

[Malcolm McLeod]

Thanks for the helpful information, Mr. Mann. I'm curious what you think of the Husky C041H and the manufacturer's claim that the increased PSI somehow creates three times more usable air. The product description states: "The 4 Gal. 225 PSI tank holds over 3x as much usable air as a 4 Gal. 125 PSI tank, which means you can fire 3x as many nails at any 1 time."

I don't understand this. The tank only holds so much air, irrespective of the PSI.

With all due respect, your last statement is not correct. If we measure the tank in SCF (standard cubic feet) as opposed to gallons it may make more sense? And lets assume 'standard' means the current atmospheric conditions at your location (temp/barometric press/%RH) ...and this pressure is typically around 15psia (absolute), which is the same as 0psig (gage).

Now, for the sake of round numbers, lets say your 4 gallon tank equals 0.5 SCF. This means with the plugs pulled there is 0.5 CF of air at zero pressure (0 psig, but 15 psia ) in the tank. If you pressurize the tank to 15psig, there are now 2 'atmospheres' in the tank - - still 0.5 CF of tank volume, but 1 SCF of air.

Add another 0.5 SCF of air - pressure is now 30psig, and 1.5 SCF. And so forth: 45psig::2.0 SCF; 60psig::2.5 SCF; 75psig::3.0 SCF; .... 225psig::8.0 SCF. All packed into a 0.5 CF tank.

Keep in mind that the 1st 0.5 CF of air in the tank is at zero (gage) pressure, so you would have to vacuum it out to actually use it... so while at 225psig there is 8.0 SCF in the tank, but only 7.5 is actually usable. And the sprayer needs 40psig minimum, so there is only roughly 6.0 SCF of usable air above this (8.0 - 2.0 = 6.0), so in this case, with 225psig in the tank and the pump is off, you have maybe 60 seconds of spray time? (I'd figure half that in real world applications.)

My numbers are rounded approximations and very rough (so hoping the perfectionists will go easy on me) - - I'm not much of an educator, but I hope this gives you a little clarity about pressure vs volume.

[Yasha Renner]

With all due respect, your last statement is not correct.

That's why I'm here. I have a lot to learn.

[John Ziebron]

Yasha, if your only reason for buying a big compressor is for using a HVLP sprayer I would suggest going the route I took a couple of years ago. I bought a LVLP gun which uses much less pressure - less than 4 CFM @ 30 PSI. You can buy a nice gun like this for around $45, so a little more than the HF one but the savings would be realized in a smaller, less costly compressor.

I've had good results using this type of gun spraying wood finishes. I've even sprayed slightly thinned latex using a bigger nozzle.

[Yasha Renner]

Hi John. Hmm, that is an intriguing solution. I had briefly considered an LVLP gun, but thought that perhaps the finish would not be as nice. My primary purpose is to spray paint some small book cases I'm building, but coincidentally, I'm also changing a timing belt on our minivan and so will be purchasing a pneumatic impact wrench to loosen a few tough bolts (torqued on at 162 ft. lbs.!). The impact wrench uses on average 4 CFM @ 90 PSI.

If I go this route, I could probably get by with that lovely Rolair double stack. Would you mind sharing a link to a quality LVLP gun you recommend?