# Air Flow Calculations: Is this possible?
**URL:** https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144
**Category:** Factual Questions
**Created:** [September 22, 2011, 3:08am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144 "2011-09-22T03:08:35Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@RearEchelon](https://boards.straightdope.com/u/RearEchelon)
#### Post date: [September 22, 2011, 3:08am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/1 "2011-09-22T03:08:35Z")
Is it possible to calculate perceived pressure of a flow of air from a pipe (in say, psi) given volumetric flow rate, pipe diameter, and distance from the end of the pipe? E.g. if I were standing 18 inches from the end of a 6-inch pipe moving air at 1000 cubic feet/min, could I find out the pressure of the flow against my body (or, if it eases calculations, on a 1 sq. ft. area)?
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### Author:  [@GameHat](https://boards.straightdope.com/u/GameHat)
#### Post date: [September 22, 2011, 3:26am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/2 "2011-09-22T03:26:50Z")
Sure. I spent over 4 years getting a degree in Chemical Engineering doing just that, over and over.
Of course, it’s been five years since then, so I’m rusty 😃
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### Author:  [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)
#### Post date: [September 22, 2011, 4:15am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/3 "2011-09-22T04:15:48Z")
lets see 1000 CFM in a 6 inch pipe. That is about 28 sq inches. so that means the speed of the air is approx 35 ft per minute.
Ok I ve done my part. For me it was over 40 years ago.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [September 22, 2011, 11:57am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/4 "2011-09-22T11:57:24Z")
> [@RearEchelon](#):
>
> Is it possible to calculate perceived pressure of a flow of air from a pipe (in say, psi) given volumetric flow rate, pipe diameter, and distance from the end of the pipe? E.g. if I were standing 18 inches from the end of a 6-inch pipe moving air at 1000 cubic feet/min, could I find out the pressure of the flow against my body (or, if it eases calculations, on a 1 sq. ft. area)?
The distance from the end of the pipe complicates things quite a bit. If you assume the jet of air stays at the same diameter as when it first exits the pipe (a safe assumption if you are just a few diameters away from the end of the pipe), then things are pretty easy.
Is this a homework problem? :dubious:
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### Author:  [@RearEchelon](https://boards.straightdope.com/u/RearEchelon)
#### Post date: [September 23, 2011, 2:39am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/5 "2011-09-23T02:39:20Z")
> [@Machine\_Elf](#):
>
> Is this a homework problem? :dubious:
No, not at all. I haven’t had homework for 10 years. 😃 I was stepping out of the shower the other day, trying to visualize what it would be like if I could step through one of those super-powered multi-nozzle dryers like at the end of a car wash. My thoughts moved from there into the technical, as they are wont to do, and in guesstimating the airflow from the end of one of those nozzles, 1000 cfm popped into my head. I have no idea if this is close or way off but then I began to wonder if 1000 cfm out of a 6-inch pipe would efficiently blow excess water from my frame, or efficiently blow my frame onto the floor. I had no idea where to begin to try to calculate the pressure of such a flow, except to begin here.
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### Author:  [@The\_Niply\_Elder](https://boards.straightdope.com/u/The_Niply_Elder)
#### Post date: [September 23, 2011, 3:48am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/6 "2011-09-23T03:48:25Z")
You would realistically use CFD software to calculate this, as the pressure varies depending on the location of your body. Any other analytical equations are extremely simplistic and yield approximate answers.
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### Author:  [@Waterman](https://boards.straightdope.com/u/Waterman)
#### Post date: [September 23, 2011, 6:59am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/7 "2011-09-23T06:59:26Z")
> [@Snnipe\_70E](#):
>
> lets see 1000 CFM in a 6 inch pipe. That is about 28 sq inches. so that means the speed of the air is approx 35 ft per minute.
>
> Ok I ve done my part. For me it was over 40 years ago.
For me 42 years ago and the velocity of 1000 cfm in a six inch diameter pipe is 5,092 fpm. You’re dividing square inches instead of square feet (144 square iches per square foot)!
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### Author:  [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)
#### Post date: [September 23, 2011, 7:50am UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/8 "2011-09-23T07:50:26Z")
Perceived? By who?
Presumably you need to know something about the environment outside the pipe.
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### Author:  [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)
#### Post date: [September 23, 2011, 3:24pm UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/9 "2011-09-23T15:24:23Z")
> [@Waterman](#):
>
> For me 42 years ago and the velocity of 1000 cfm in a six inch diameter pipe is 5,092 fpm. You’re dividing square inches instead of square feet (144 square iches per square foot)!
oops.
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### Author:  [@treis](https://boards.straightdope.com/u/treis)
#### Post date: [September 24, 2011, 5:38pm UTC](https://boards.straightdope.com/t/air-flow-calculations-is-this-possible/597144/10 "2011-09-24T17:38:12Z")
> [@RearEchelon](#):
>
> Is it possible to calculate perceived pressure of a flow of air from a pipe (in say, psi) given volumetric flow rate, pipe diameter, and distance from the end of the pipe? E.g. if I were standing 18 inches from the end of a 6-inch pipe moving air at 1000 cubic feet/min, could I find out the pressure of the flow against my body (or, if it eases calculations, on a 1 sq. ft. area)?
Yes and no. You can’t calculate the force from basic physics like you would be able to do for tennis balls coming out of a pipe. The only way to figure out things related to fluid flow is by experimenting.
Luckily for us very smart engineers have done lots of experiments and generated a system that allows us to calculate numbers for generic cases without experimenting. So there is a way to plug in numbers and get an answer, but that is ultimately reliant on some guy with a wind tunnel taking measurements.