CFM through Pipe

[bobegler]

I would like to be able to calculate the CFM of air that can flow through a 3" pipe at 70 PSI. I would also like to know how to calculate the CFM at other presures.
Thank you.
Bob

 

[TD2K]

If you just want a typical capacity, you can assume that the air will be moving at a maximum velocity of about 3,000 ft/min. Calculate the pipe cross sectional area in ft2 and multiply by the velocity, that will give you cfm at the flowing pressure and temperature.

If you want this flow rate in scfm, mutiply the cfm by (P + 14.7)/14.7 * 520/(T + 460) where P is the pressure in the line and T is the temperature of the air. There are some other factors but this will give you a reasonable value. 14.7 is atmospheric pressure, if you are somewhere where it's lower, use that value.

Caution, if it's a long line or you want to try very high velocities, it won't work in reality because of the pressure losses.

 

[KernOily]

TD2K you are always such a party-pooper . First you tell that dude he can't make his potato cannon. Now this! ;-) Thanks!
Pete

 

[TD2K]

I'm just trying to avoid everyone jumping on me going 'but, TD2K forgot to mention .....'. I know, you're just trying to help me out. ;-)

 

[TBP]

bobegler - You need to decide what air pressure you need at the other end of the system. If you're starting off with a 70 PSIG inlet, and you need 69 PSIG at the outlet 500 feet away, you won't get many CFMs. If you need 50 PSIG at the point of use, and it's only 25 feet away, you can stuff a lot more CFMs down that 3" line.

It's a balancing act. If you move the air too quickly, then any moisture and/or oil in the air won't be able to drop out. This is in addition to needing a LOT more power at the compressor to provide a higher initial pressure. If you decide on a very low pressure drop on a long line, you may have a relatively big pipe for a small load. The correct choice is usually somewhere in the middle. If it's a process plant, I usually find the appropriate line size, then go up one size. Production people are always adding things, and moving equipment around.

Determine what it is you want to have happen at the end of the line, then work backwards from that point.

 

[mucour]

The easiest solution is applying Darcy's formula if the following condition is met:

1) The difference in inlet and outlet pressure of the pipe is less than 10%.

Then by applying Darcy's formula.

DP=dens x f x L x (vel)^2 / 2D

Apply data to the variables and derive velocity.

Then apply Q = Area x vel.

This will give you the Q in CFM

I think this solves your problem

 

[ayfarm]

CFM=60VA; V=sqrt(25000DP/L)
L=pipe length in ft
D=dia in inches
P= pres. loss from table
V=fps
A=ft2