Find CFM Formula: Pipe Diameter & PSI

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    Cfm Formula
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Discussion Overview

The discussion revolves around finding a formula to calculate cubic feet per minute (CFM) of air flow through a pipe, given the pipe diameter and pressure in pounds per square inch (PSI). The scope includes theoretical considerations and practical applications related to fluid dynamics and airflow measurement.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant requests a formula for calculating CFM based on pipe diameter and PSI, indicating a need for clarity on the specific requirements.
  • Another participant seeks clarification on whether the inquiry pertains to determining airflow rate through a pipe using static pressure and pipe diameter.
  • A participant asserts that static pressure alone is insufficient for determining airflow, suggesting the need for additional measurements such as differential pressure across an orifice or velocity pressure from a pitot tube.
  • There is a mention of a compressor maintaining a constant pressure of +100 PSI and a pipe with a 3/8" inner diameter, indicating specific conditions for the airflow scenario.
  • Another participant emphasizes that static pressure does not directly correlate with velocity, questioning the application context of the pipe (e.g., whether it is supplying air or is open-ended).

Areas of Agreement / Disagreement

Participants generally agree that static pressure alone is not sufficient to determine airflow, but there is no consensus on the specific methods or additional data required to calculate CFM accurately.

Contextual Notes

Limitations include the need for more details about the setup to provide accurate measurements or estimations of airflow, as well as the dependence on specific definitions of pressure types and measurement methods.

shellback
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I just want the formula that shows how to find for CFM. I have the pipe diameter and the PSI that are available. I'm just using air, nothing special.
 
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shellback said:
I just want the formula that shows how to find for CFM. I have the pipe diameter and the PSI that are available. I'm just using air, nothing special.

Can you be more specific with what you want to know? I presume you mean how to determine the flow rate of air in cubic feet per minute through a pipe when knowing only the pipe diameter and back pressure?

CS
 
stewartcs said:
Can you be more specific with what you want to know? I presume you mean how to determine the flow rate of air in cubic feet per minute through a pipe when knowing only the pipe diameter and back pressure?

CS

Yes, that's exactly right. I know there's a way to do it. I just haven't been able to work that out.
 
The pressure is the static pressure in the pipe? That isn't enough to determine the airflow. You need a differential pressure across an orifice, pressure drop in a venturi or velocity pressure from a pitot tube.

Once you get some form of velocity pressure (and if you give us more details about your setup, we can tell you how to measure/estimate it), you can use a form of Bernoulli's equation to find the velocity:

V=4005*sqrt(Vp)
where
V= velocity in fpm
Vp= velocity pressure in inches of water gauge.
 
Last edited:
russ_watters said:
The pressure is the static pressure in the pipe? That isn't enough to determine the airflow. You need a differential pressure across an orifice, pressure drop in a venturi or velocity pressure from a pitot tube.

We're going to have a compressor maintaining a constant pressure of +100psi regulated to 100psi and a 3/8" ID for the pipe at its' most narrow.
 
That's static pressure. It doesn't have anything directly to do with velocity (consider an air tank pressurized to 100psi: no airflow at all). What is this pipe supplying? Or is it open ended?
 

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