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Hi everyone

As a follow up to my questions about spraying equipment, I found a pretty good HVLP gun that requires 6.2 cfm. There is also available at the same store (Home Depot) a Dewalt compressor (http://www.dewalt.com/tools/compressors ... 55168.aspx) that is rated at 5.4 scfm at 90 psi. I can't find an accurate (and simple way) to convert the 2 to see if that compressor will fit the bill for that specific gun.

Does anyone know how to convert the 2 and determine if this compressor will work?

Thanks in advance

Pat

Pat, The "s" in scfm stands for standard temperature and pressure. Standard pressure is sea level and temperature is probably 68.
So if you live at or near sea level, you probably don't need to worry about the difference.
I live at 7000 ft, so I would need about 25% more cfm to be moving an equivalent volume of air. I arrived at that by just looking at the difference in air pressure between sea level and 7000 ft.
Hope that helps.

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Jim Watts
http://jameswattsguitars.com

Hi Pat .. 6.2 CFM for the gun, but at what pressure ??? I have never seen a gun that uses more than about 40-45 PSI .. so I am guessing that compressor will run that gun easily, as it probably delivers about 8-9 CFM at 40 PSI

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Hey Guys

Thanks Jim. I live at approx 200 ft above sea level, so I guess that is settled.

Tony: I forgot to include that it is 6.2 cfm at 40 psi for the gun. As the guy in the store suspected (he said he wasn't sure about it so he recommended I get a better opinion), 5.4 scfm at 90 psi should be well above 6.2 cfm at 40 psi.

Many thanks

Pat

Check with the compressor manufacturer. I bet they have the full specs on the unit. Most show both a high pressure CFM and a lower pressure CFM. The one thing you don't want it for it to run all the time to keep up, so some excess CFM is pretty important for the unit in use.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

A spray gun only has its trigger pulled about half of the time when spraying a small object like a guitar. And the whole coat only takes a couple of minutes. These breaks allow the compressor to catch up. Also the tank serves as a reservior. So in reality you can use a much smaller compressor than you'd think. I have been using a little 2 hp, 5.5 cfm unit for 30 years and it has never failed to keep up with tools often rated up to 10 cfm.

Barry said:



I was wondering about that. What kind of gun do you use, and how big is the tank on your compressor?

Pat

Thanks Barry. I've typed that so many times in the past my fingers are tired

I've got a big compressor (5 HP 2 stage) I rarely turn on. I spray with a 2HP with a 20 gallon tank. Walcom EGO or Sharp FX1000 guns, depending on material.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Pat-
If you just think of the situation without worrying about 'flow', it's pretty simple. If you halve the pressure , keeping temperature constant, the volume of a gas will double. So 5.4 cu ft at 90psi will be about 11 cu ft at 45psi.

In real life, the diameter and length of the hose will affect the results at the gun end, but you should be fine with the smaller compressor.

When you decide to get that air sander that needs 10cfm @90psi, then you will need a compressor upgrade....

Cheers
John

Hey guys

This is the gun I am looking at:

http://www.shopbot.ca/ps-campbell-hausf ... 59523.html

Pat

Pat, my compressor is a Speedaire and it has a 20 gallon tank.

My spray gun is a Walcom STM and I think the rating was about 6cfm.

I noticed that the STM is no longer sold and seems to have been replaced by the Walcom ES/RV which is rated at 1 to 2 cfm. That sounds like an efficient gun.

I was assuming that the compressor regulator would be set at 90psi and the gun regulator would control the gun pressure.

The relationship between pressure and volume at constant temperature is quite linear
http://en.wikipedia.org/wiki/Boyle%27s_law

As the under-40s say....'whatever'.....

Cheers
John

John,

The linear relationship you refer to is theoretically correct, but it does not apply to a compressor. All compressors and pumps have a performance curve that shows a non-linear relationship. This is due to the frictional resistance that increases at the higher rates.

As a EE taking flight test engineering courses I found that things sure do get complicated real quick. Not to mention, the math

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Steve Smith
"Music is what feelings sound like"