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Tool Talk Discussion Forum

Re: Determining CFMs on an old compressor

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Author  [Modern View]

03-15-2013 19:24:02

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rockyridgefarm said: (quoted from post at 09:19:58 03/15/13) Hey all,

Alls I got on the farm right now is a leaking old 10 gallon Montgomery ward 2 cylinder compressor. It works ok, but it sure take a long time to air up an 18.4-38 and forget about using an air impact or blowing off even a small piece of equipment.

Guys got a 60 gallon (he thinks) air compressor that's set to run on 240 for sale on CL. He says it takes four minutes from empty to 125 psi. I'm looking for a decent compressor to run an impact wrench, air up tires, and such. He wants $300. Should I bite?

Pictures are decieving but it looks smaller than 60. Take this formula with you though, you will need it anyway to get an accurate cfm rating.
1 gallon is 231 cubic inches.

The volume of a cylinder is given by volume = 3.1415 times radius squared times height. With that you can have a pretty accurate measurment of the tank size in gallons. Or, if you have a phone with you that has internet access, you can just punch in the numbers here:

Now that you have tank size, and given that you were told the time it takes to go from 0psi to 125psi, you can figure out the CFM of the compressor. Here is the link to that:

When I punched in your numbers, it came out to 9.57 CFM. Keep in mind that CFM changes depending on pressure. The only way to find out at the pressure you want is to do the calculations near the pressure you want. Check from 30-50 if you want the 40psi CFM rating or check from 80-100 if you want the 90psi CFM rating. If you are going to check it before you buy it, it wouldnt hurt to take a known accurate gauge with you that can be plugged in to the line.

I just went through this math stuff a little while ago when I bought a used compressor that was made in the lying years for ratings. Now with the new ones I think they still lie but with the HP ratings, not the CFM as much.

This post was edited by Rich_WI at 20:27:19 03/15/13.

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dr sportster

03-15-2013 20:02:27

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 Re: Determining CFMs on an old compressor in reply to Rich_WI, 03-15-2013 19:24:02  
Volume of cyl is 3.14 rsq x length [same thing as height]. pi r sq x ht
You forgot the PI

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03-15-2013 20:30:10

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 Re: Determining CFMs on an old compressor in reply to dr sportster, 03-15-2013 20:02:27  
Quoting Removed, click Modern View to see

Thanks for catching that, I copy pasted and the website I grabbed it from didnt allow the pi sign.

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