# Maximum CFM through a pipe

1. Aug 18, 2008

### tc114

I am in he rock drilling industry and I am looking for a formula to be able to calculate what the maximum CFM is through a pipe. For example I have a 0.5” dia. pipe, 600CFM @ 150psi compressor. What is going to be the actual CFM that I will get at the end of a pipe?
Any help will be greatly appreciated.

2. Aug 19, 2008

### gmax137

It depends on how long the pipe is, and other details. Also, note that it depends on just what the "600 cfm" is...Is it at the intake of the compressor, or at the discharge? Is there an aftercooler? etc. The best thing for you to do is go to the compressor manufacturer's website (ingersol-rand, or atlas copco are big ones) and see if they don't have tech info pages. All this used to be in their catalogs, so I'm sure they have it on line by now. Another source is the Crane "Flow of Fluids..." book. Another excellent source of advice is http://www.eng-tips.com/

Good Luck

3. Aug 19, 2008

### OwenMc

Surely, if the compressor is delivering a particular CFM (which may or may not be the 600 quoted), this is what you'll get out of the pipe? You will have changes in density and pressure, but the overall mass flow rate has to stay constant, no?

Apologies if I've missed something!

4. Aug 19, 2008

### Topher925

I agree with Owen, depending on the type of compressor, it should deliver a constant CFM. Now whats coming out of the pipe will vary depending on the working fluid, length of the pipe, roughness of pipe, etc.

Not necessarily, it depends on what kind of pump and what the conditions are for its intake. If its intake is constant temp, and pressure, then yes mass flow rate will be constant. However if this is more of like an impeller kind of air pump, then things got a lot more complicated.

5. Aug 19, 2008

### gmax137

He didn't tell us what kind of compressor he has (but you're right, it is likely a recip or screw, ie, positive displacement machine). Which would lead you to believe that the mass flow is constant down the line. But, if it has an aftercooler, (they reduce the temp and take out alot of the water) that affects the "actual cfm" too. I still would go to the I-R or atlas site to see how they explain things (like, what exactly the "600 cfm" nameplate means). EDIT
And IIRC, actual CFM is the volumetric flow at the line pressure, so the ACFM may change with length of the pipe (since the pressure drops along the length).

6. Aug 19, 2008

### OwenMc

My mistake, too many years spent dealing with mass flow rates! Of course the volume flow rate will change with pipe length!