Compressor Tank Storage and Operating CFM

      How much is not enough? Thoughts on proper sizing of compressor airflow capacity and air tank storage capacity. June 30, 2009

Question
When a compressor lists X CFM at 40 psi and x CFM at 90 psi are these specifications what the motor delivers when the tank is filling? I guess my question is can a full tank at 135 psi deliver more than the top rating at 90 psi until the tank pressure drops to 90 or below?

The reason for the question is that I am looking to run an HLVP conversion gun that requires 11cfm at 23psi. With a 30 gallon tank and spraying small jobs like cabinet doors can a compressor rated at 5.5cfm at 90psi with a 30 gallon tank be rated at 130psi? Thanks for any responses - I tried searching for many answers but the tank size never seemed to be a factor in the responses but seems like it would be a factor for smaller jobs.

Forum Responses
(Finishing Forum)
From contributor J:
It will work, but won’t work well. The tank will drain very quickly when spraying and will start to fill. Once it starts to fill, you will need to stop spraying or else the compressor will run constantly and burn out the motor/pump. You can get away with it by spraying one small piece, wait for the tank to fill and stop running, then spray your next piece and so on.

You could get away with this a few times, but if you plan on spraying regularly, you really need a bigger tank (60 to 80 gallons) and something that will deliver a higher CFM (around 18 or so) than the gun requires (these are typically in the 5HP range).

I will try and explain the function. Your tank is full at 130psi, you start to spray, compressor kicks on at 90psi. Gun is taking 11 cubic feet per minute from the tank, but the compressor is only filling at 5.5 cubic feet per minute. Spray gun is taking 5.5 more cubic feet per minute than the compressor can deliver and will therefore run continuously and the tank will still be being depleted of pressurized air.

Your gun is asking for 23psi (dynamic pressure) so the restrictor/gauge on your gun probably needs to be set at around 40 psi without the trigger pulled. You need air flowing out of your gun when you set it at 23psi. With losses through the air hose etc, you probably can only spray until your tank gauge reads about 60psi or so.

For simple math, you can spray from 130psi down to 60psi, about 1/2 tank or 30 gallons. 30 gallons is about 4 cubic feet. A gun needing 11cfm is going to chew through those 4 cubic feet in about 20 seconds. You will then need to stop spraying and wait about a minute for your tank to fill up.

See where this is going, 20 seconds of spray time, to then have to wait a minute to fill. You are going to far exceed the duty cycle of your compressor and significantly shorten its life if you plan on doing this on a regular basis. But like I said, if this is a one time thing, you can get away with it.



From the original questioner:
Right now I am doing a few test panels to see work on laying down the finish. I wanted to make sure I was supplying the gun with the correct amount of air. It sounds like I can for a brief period which may work. I will be spraying some cabinet doors I am refinishing and will only do a few a day so I can spray one and by the time I hang it and get the next I should be able to keep going. With a few panels a day it should not be too much work on the compressor. That will buy me some time while I look into a larger one.


From contributor J:
You can get 80 gallons with horizontal tanks then hang them from the ceiling on a platform. It helps clear up some space.


From contributor O:
The problem is that the CFM ratings of both gun and compressor refer to volumes of uncompressed air. A 30-gallon tank holds about 4 cubic feet of uncompressed air, but if you take, say, 40 cubic feet of uncompressed air and compress it until it's at 130 PSI, its volume shrinks to only about 4 cubic feet (30 gallons). So, half of a 30-gallon tank of air that's compressed to 130 PSI is equivalent to about 20 cubic feet of uncompressed air, and will run an 11 CFM gun for almost 2 minutes.

Still, contributor J's right that your compressor is undersized for any significant amount of spraying. When I looked into appropriate equipment, I decided that a three-horse compressor was about as small as you could go for regular HVLP use, and that's real cfm, not the bogus "peak" output ratings they assign to cheap homeowner compressors. A real 3HP compressor is going to produce something more like 12 CFM at 90 PSI.



From the original questioner:
That make sense to me. I did do a test setting with the inlet pressure for the gun to 23psi with the trigger depressed and it worked for much more than 20 seconds which led me to believe I could paint a few panels with it.



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