# Thread: static pressure to cfm?

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## static pressure to cfm?

Anyone have a calculation to convert static pressure to CFM?

Thanks.

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You can only convert velocity pressure (Total pressure - static pressure) to speed (fpm). The equation will consider engineering units, fluid (air) density and duct shape etc. That's why different VAV box has different flow pickup factor.
V=K*sqrt(dP)
V: speed
K: conversion factor, often from testing
dP: velocity pressure

3. Sounds like you need to look at the fan curves in the performance data for the unit you are checking the static pressure on. There you should be able to find what you are looking for.

Bobby

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Actually, I've got some VP sensors in a duct that are poly tubed to a Serta device that turns VP into a 4-20ma static pressure signal. Problem is, I don't need static pressure, I need CFM.

5. If i remember correctly you take the free square foot area of your duct multiply that by velocity pressure and then divide by 144.

6. Exactly,
AIR FLOW (cfm) = V x A
where V = average duct velocity (ft/min) A = duct cross sectional area *(ft2)

Read the FLOW section of the link I provided, also do yourself a favor and read everything on that page, tons of good info and a quick place to refresh the mind.

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I found some typo.

Velocity, not velocity pressure.
1 sq feet=144 sq inch. So the duct area is actually sq inch, not sq feet.
Originally Posted by lwarren
If i remember correctly you take the free square foot area of your duct multiply that by velocity pressure and then divide by 144.
Last edited by leaflying; 02-12-2007 at 10:48 PM.

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If you really wanna simple and ignore all calibration stuff.

Use K=4004 in my post at the 2nd floor. Pressure in "inch water", and velocity in "fpm". Then apply AIR FLOW (cfm) = V x A. And it's not static pressure, but velocity pressure. The relationship between vp and fluid speed has been decided in Bernoulli Equation. You cannot get speed from sp no matter how hard you try.

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I have done it the way Codewriter described. I am not sure what type of control system you have, but I have a screen capture of a calculation I have used many times to convert Velocity Pressure to CFM.

10. Originally Posted by leaflying
I found some typo.

Velocity, not velocity pressure.
1 sq feet=144 sq inch. So the duct area is actually sq inch, not sq feet.

What typo? Where?

EDIT: Nevermind, I need to adjust the eyes.

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## May need

The fan shape and size will create different condition and the design engineers provide the fan curve so you can look up how much air a fan will move for a given pressure. I think you will need that fan curve so you can cross reference your static.

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In this case, we need to measure total static pressure (TSP) on AHU unit, not a single point static pressure on common duct. And normally, the air flow is checked from a table/diagram rather than calculated in a controller.

Also, you cannot use this method to measure air flow on branch ducts.

Originally Posted by propmanage
The fan shape and size will create different condition and the design engineers provide the fan curve so you can look up how much air a fan will move for a given pressure. I think you will need that fan curve so you can cross reference your static.

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