Will my compressor Keep Up? CFM vs PSI

[Mark Crenshaw]

I have a Dewalt D55168 air compressor that is 200 psi max, 15 gallon tank and rated at 5.4 cfm @ 100 psi.

I'm looking into purchasing an HVLP spray kit that runs at 30 psi @ 13 cfm for the production gun and 30 psi @ 9 cfm for the detail gun. I need to figure out if my compressor will provide this in a usable duty cycle. Dewalt doesn't list any other flow/pressure ratings for this model. Is there a simple formula to calculate this with the given specs?

Peace,
Mark

[William M Johnson]

Ideal gas law PV=znRT. We want to know V (cfm) at a different P (30 psi)

(P1)*(V1)=(P2)*(V2) now filling in the numbers

(100psi)*(5.3 cfm)= (P2)*(V2)

V2 = (5.3*100)/30

V2= 18 cfm is the maximum your compressor will output at 30 psi.

For all you other engineers out there: Yes I know I skipped a bunch of steps and made a bunch of assumptions. I also know that V is not a rate in the Ideal Gas law but it works. Feel free to elaborate f you feel the need.

Bill

[Mark Crenshaw]

Ideal gas law PV=znRT. We want to know V (cfm) at a different P (30 psi)

(P1)*(V1)=(P2)*(V2) now filling in the numbers

(100psi)*(5.3 cfm)= (P2)*(V2)

V2 = (5.3*100)/30

V2= 18 cfm is the maximum your compressor will output at 30 psi.

For all you other engineers out there: Yes I know I skipped a bunch of steps and made a bunch of assumptions. I also know that V is not a rate in the Ideal Gas law but it works. Feel free to elaborate f you feel the need.

Bill

Thanks Bill!

That's great! I'm gonna stash this away for future use. Looks like I'm good to go on the spray guns.

Peace,
Mark

[keith ouellette]

Ideal gas law PV=znRT. We want to know V (cfm) at a different P (30 psi)

(P1)*(V1)=(P2)*(V2) now filling in the numbers

(100psi)*(5.3 cfm)= (P2)*(V2)

V2 = (5.3*100)/30

V2= 18 cfm is the maximum your compressor will output at 30 psi.

For all you other engineers out there: Yes I know I skipped a bunch of steps and made a bunch of assumptions. I also know that V is not a rate in the Ideal Gas law but it works. Feel free to elaborate f you feel the need.

Bill

I'm not gonna pretend I can do anything but simple syferin' like jethro can do but I was wondering....

Even if the compressor can put out 18cfm at 30 psi how much is the compressor going to have to run in order to keep up with the output? If the gun needs 13 and the pressure is constantly going down as air leaves the tank won't it have to run constantly?
I really have no idea but I am curious as to the answer.

[Faust M. Ruggiero]

I would not buy a gun that consumes more air than the compressor supplying it. You can't wait for the compressor to catch up when you are spraying finish. Your thirty gallon tank is insufficient to store any meaningful amount. Have you thought about buying a turbine unit. They are not cheap but may be a better choice. for you.
fmr

[Matt Armstrong]

It probably will not, no. Time to get a bigger compressor.

[Jim Becker]

Mark, honestly, even if the CFM is close, the constant air demands for shooting finish make that smaller compressor less than ideal. 30 gallon, give or take as a minimum, is my suggestion to keep the thing from constantly running. I had a 20 gallon unit when I first got my Wagner HVLP gun (and it's less demanding on air than some guns) and it was working constantly. I don't have that situation with the 60 gallon IR I have in the shop now. It doesn't run often and recovers quickly when it does need to recharge the tank. The more the compressor runs, the more moisture you have to deal with in some cases, too. That can be a factor depending on what type of finish you are shooting. I only use waterborne finish, so it's not as critical, but still...I try to minimize it anyway.

[Terry Welty]

All that is very, very true... but it also depends on what you're paining. If you are going to try to paint cars and large pieces, you are going to be at or beyond limits... however, if you are going to paint small pieces and you won't have a problem as the compressor will have time to catch up as you go from piece to piece. Spray painting is all about rhythm... at least that is my experience...

[Mark Crenshaw]

My thanks to all who responded. There are some very helpful replies here. Buying a new compressor at this point isn't an option, so I have to deal with what I have. Of the 2 guns I'm buying, I will likely use the detail gun the most. To Terry's point, all I'm spraying right now are guitar necks and bodies. Coats are sprayed in a matter of minutes. So I think I'll be okay with the compressor I have. I feel pretty confident it will provide the 9 cfm @ 30 psi for the few minutes I need it to run. If I move on the furniture and more complex shapes, I'll have to look at another compressor.

Thanks again and Merry Christmas.

Peace,
Mark

[Rick Fisher]

It sounds like it will work fine.. Your compressor will be working if you spray for a long time.. but it is what your compressor is supposed to do.. Work.. lol.

I am never afraid to make a tool work.. As a hobby guy, my tools get lots of time off .. lol.

If your gonna spray 3 hours a day.. sure ... If its an hour a month.. make it earn its square footage in your shop..

[Glen Butler]

I think you will be fine. I have sprayed things much larger than guitar bodies, in fact even nearing 100 sq ft, using a 4.3 gal. Rol-air bull 4.0 CFM @ 100PSI.

[David Epperson]

Ideal gas law PV=znRT. We want to know V (cfm) at a different P (30 psi)

V2= 18 cfm is the maximum your compressor will output at 30 psi.

For all you other engineers out there: Yes I know I skipped a bunch of steps and made a bunch of assumptions. I also know that V is not a rate in the Ideal Gas law but it works. Feel free to elaborate f you feel the need.

Bill
The main thing you skipped was the fact that 5.4scfm was all his compressor can suck in at the speed it runs while outputting 100 psi. The out put at 30 psi will not be much different, some, but nothing like triple.
What you have is 60 gallons at some reserve pressure - that is what determines the number of standard cubic feet of air you have to start with (and a "standard cubic foot of air" is always the same regardless of pressure - it only takes up a cubic foot at sea level - it's defined that way)
60 gal = 8 cubic feet (give or take a bit). one atmosphere is about 15psi so if we assume a 150psi charge on the tank - that is 80 scf of air to start with - the 16scf vol @ 30 psi or 64scf to work with. Being drained at a rate of 13scfm says that it will work for about 5 minutes before having to stop and recharge - it will take longer to drain if the compressor runs while this is happening since you're putting 5.4 back in for every minute you're taking 13 out - the net drain is only 7.6scfm so about 8 minutes before the compressor cannot keep up.

The smaller sprayer will of course work for longer. 9-5.4scfm=3.6scfm - 64/3.6= almost 18 minutes

I have no idea where I got the 60 gal figure...At 15 gal of course your times will be a lot less.
15 gal = 2 cubic foot as standard pressure
@150 psi = 20 cubic feet
@ 30 psi = 4 cubic feet
16 cubic foot of usable air
At 7.6 drain rate - a little over 2 minutes
at 3.6 drain rate - almost 5 minutes.

I found this little tidbit hidden in the reviews of this unit on amazon.
It takes 3 1/2min. to fully recover to 200lbs after I stop sanding at 70 lbs of tank pressure.