# compressor cfm question for the mathmatically unchallenged as opposed to me! lol

anybody know how to figure the cfm for a homemade compressor? 30 gallon tank, emglo GU compressor head, 3 HP marathon motor running at 1725 ROM. motor has a 6 1/4 inch pully and compressor has a 10 1/2 inch pully. it goes from 0 to 145 PSI in 2 minuits and 27 seconds. im trying to figure out if it will run more than one sprayer. thanks all...
skeez
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
anybody know how to figure the cfm for a homemade compressor? 30 gallon tank, emglo GU compressor head, 3 HP marathon motor running at 1725 ROM. motor has a 6 1/4 inch pully and compressor has a 10 1/2 inch pully. it goes from 0 to 145 PSI in 2 minuits and 27 seconds. im trying to figure out if it will run more than one sprayer. thanks all...
BTW the compressor head is 4 cylinder.
skeez
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Might be hard to determine with out knowing the cubic inch displacement of the pump.
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
On Thu, 14 Oct 2004 23:38:12 GMT, "Leon"

yeh i know but since the big lemon yellow company bought emglo its near imposible to find info for the [real] emglo pumps. lol... gotta love B&D.the pump is a GU model with 4 cylinders but i cant seem to find online manuals for it anywhere.. hooked up 3 sprayers this morning and set em all to 40 psi filled with water and tied the trigers back with wire. the compressor cycled fine so i guess my question is answered. should fill my needs just fine. im still curios though if anybody out there has info for this model pump i would be glad to have it.
skeez
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

I'm curious skeez - what is it that you're doing that requires 2 or 3 guns spraying at once? I'm picturing three guns hung from the ceiling, triggers wired back and skeez pushing a cabinet through the mist...
--

-Mike-
snipped-for-privacy@sprintmail.com
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
On Sat, 16 Oct 2004 13:12:43 GMT, "Mike Marlow"

LOL............... i have three sets of cabinets going at once. its a pay job at their location. there is no compressor there and the contractor will be renting mine for a couple weeks. along with payin my help and myself. i wish i had not opened my mouth but too late for hindsite. moving the compressor aint gonna be easy. i am working on puting some wheels on it today.and it goes tomorrow. cant wait till it comes home to stay. i willl be using my portable here in the interm.
skeez
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

What kind of guns are you going to shoot with? You'll need to consult the manufacturer's specs for the SCFM requirements for the guns, but it does not take a lot of compressor to run two guns. I'm assuming you're going to be spraying woodworking projects? Most spraying does not present a steady load to a compressor - it's more on and off, so hooking up two guns is generally not a problem.
A question though - why two guns? Though the compressor may likely handle it, it's not good practice to shoot with two guns at the same time. Two guns means two people and seldom do two people shoot the same way. You're more likely to end up with a very noticeable difference where each person painted.
--

-Mike-
snipped-for-privacy@sprintmail.com
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
On Fri, 15 Oct 2004 01:06:20 GMT, "Mike Marlow"

the 2 gun theory is for a couple of large projects that need to be done in short time. possibly 3 guns going at once for a short [week or 2] time. then it will be just me for the most part on small projects.
skeez
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
A lot more than my 60g 4hp unit. If you can fill that tank that fast you pushing a lot of air. I'm sure you can calculate it but first one needs to know the displacement of the compressor.

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
The 30 gallon tank is about 4 cu ft but compressor air flow is measured as "Standard Temperature and Pressure" or STP. Not botering with adjustments for temperature and doing a rough calculation each cubic foot at 145# is about 10 cu-ft at STP so it looks like around 40cu-ft compressed in 2-1/2 min or about 16 cfm. All of this assumes I remember my college chem classes from 30 years ago. Us computer types don't run into compressed gas problems too often. Ken

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

I came at it from the other direction, and got similar results.
145psi (net) is 9.8666 atmospheres ( @14.6959psi/atm). The 30 gal. tank is 4.0104 cu. ft. Now the pump put out 9.88 atm * 4.0104 cu. ft., or 39.5690 cu. ft. of air in 2 min 27 sec; dividing gives 16.1506 cu. ft./min.
One has to de-rate that number somewhat, because the compressor was working into a variable pressure load. And, obviously, moves more air when the 'back pressure' is low, relative to when the tank pressure is high. In addition, there were probably secondary effects (heating) contributing to the pressure rise. With all that considered, a conservative rating would be 14 cu. ft/min @ 60PSI -- it will probably do somewhat more than that. :)
For a =real= engineering analysis, one has to know the volume in the cylinder with the piston at the top of the stoke, and at the bottom of the stroke. Just knowing the 'displacement' (the difference in those two figures) is _not_ sufficient. With both cylinder volumes, and shaft RPM, one can come up with firm numbers for any 'realistic' pressure level. *And* tell when the pressure level is -not- realistic -- i.e., the compressor cannot produce that pressure level, even at -zero- air flow.

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
I would think that figuring the area of the cylinders times the stroke times the speed ration times the ratio of the final pressure to atmosphere would get you in the neighborhood. Example 2 dia cylinders times 1 stroke times 4 = (1x1x3.14159x1) = 12.566 cu inches 12.566 times 1725 time 6 1/4 divided by 10 1/2 ,902 cu inches 12,902 x 14.7 (atmosphere pressure) divided by 145 (final pressure) = 1,308 cu inches 1,308 cu inches divided 1728 (cu inches per cu ft) =.75 cu ft per minute
Substitute your piston diameter and stroke and you should be able to figure out what you have. R. Wink

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
ok. now im mor confused! lol.... but looks like between 14 and 16 cfm. that should be more than sufficient for anything i throw at it. BTW i did a little test today that surprised the h!!l outta me. i held a blower nozzel wide open and the compressor cycled as normal. meaning it kicked on and off even with an open blower going as fast as the air would flow. i was ammazed that it caught up with itself like that. looks like a keeper to me. thanks for all the help guys.
skeez
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

## Site Timeline

• ### FA: Woodworker 5 Piece Kitchen Door and Drawer Set

• - next thread in Woodworking Forum

• ### Best infrared heating panels - Helvetic Heating Sunjoy Ireland

• Share To

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.