Previous in Forum: What is This?   Next in Forum: Microwave Intensity Meter
Close
Close
Close
11 comments
Power-User

Join Date: Mar 2011
Location: oz
Posts: 247
Good Answers: 10

Air Flow in a Vertical Pipe

04/08/2018 10:39 PM

Does air flow change at different positions in a vertical pipe. I understand it will change due to stack effect, but if there was a fan at the bottom of the pipe and the top is open to atmosphere ( fan sucking air from the top) will the flow rate change at different heights?

Login to Reply
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29985
Good Answers: 1673
#1

Re: Air flow in a Vertical Pipe

04/08/2018 11:27 PM

There would probably be some variation due to turbulence...but the air flow characteristics are different between negative and positive atmospheric pressure and it would be much more pronounced in a positive pressure situation, negative, not so much....

https://www.myodesie.com/wiki/index/returnEntry/id/3006

__________________
Break a sweat everyday doing something you enjoy
Login to Reply Score 1 for Good Answer
Guru

Join Date: Oct 2008
Posts: 42261
Good Answers: 1659
#2

Re: Air flow in a Vertical Pipe

04/09/2018 12:08 AM

How high is the top of the pipe?

Login to Reply Score 1 for Good Answer
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20877
Good Answers: 776
#3

Re: Air flow in a Vertical Pipe

04/09/2018 2:38 AM

Mass flow, no change; volume flow increases as height increases (and pressure decreases).

How does a fan at the bottom of the pipe equate to a fan sucking air from the top? Do you know what the real situation is?

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Login to Reply Score 1 for Good Answer
Guru

Join Date: Apr 2011
Posts: 7887
Good Answers: 279
#4

Re: Air flow in a Vertical Pipe

04/09/2018 6:03 AM

Can you characterize the heat flux along the pipe?

Surface roughness?

How about joints and other possible surface disruptions to the pipes interior?

What is the interior diameter as compared to length (height)? Does the fan impart something other than uniform straight flow across the diameter?

It the intake cut square?

If the air calm near the intake?

__________________
Eternal vigilance is the price of knowledge. - George Santayana
Login to Reply Score 1 for Good Answer
3
Guru

Join Date: Apr 2010
Location: About 4000 miles from the center of the earth (+/-100 mi)
Posts: 8762
Good Answers: 999
#5

Re: Air Flow in a Vertical Pipe

04/09/2018 11:00 AM

The mass flow rate would obviously have to be the same. Volume flow rate is related to mass flow rate by density (dV/dt = 1/ρ x dm/dt).

The two factors that determine density are pressure and temperature.

The two factors that determine pressure are the weight of the air and the pressure drop due to the flow. These two factors act in opposite directions.

The first causes a decrease of 0.00053125 psi per foot elevation. The second depends on the square of the flow velocity, the cross-sectional area of the pipe, the length of the pipe, and the characteristics of the inside of the pipe.

So, mass flow is constant, volume flow depends on density variation which depends on pipe dimensions, flow rate, and temperature variations.

Login to Reply Good Answer (Score 3)
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29966
Good Answers: 809
#6

Re: Air Flow in a Vertical Pipe

04/09/2018 3:38 PM

It would be useful to know the Reynolds Number that applies to this installation.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Guru
Engineering Fields - Nuclear Engineering - New Member

Join Date: Sep 2009
Location: Louisville, OH
Posts: 1461
Good Answers: 27
#7

Re: Air Flow in a Vertical Pipe

04/10/2018 9:05 AM

All those effects (surface roughness, turbulence, diameter changes, etc) will affect the actual flow rate. But if there are no entrances and exits along the pipe, the cfm at the bottom must be the cfm at the top. Pipe diameter changes will affect the air velocity, but not the cfm.

__________________
Lehman57
Login to Reply Score 2 for Off Topic
Power-User
United States - Member - New Member

Join Date: Mar 2007
Location: Wisconsin USA
Posts: 263
Good Answers: 19
#8
In reply to #7

Re: Air Flow in a Vertical Pipe

04/10/2018 12:19 PM

The SCFM will be the same at top and bottom.

The CFM will be different.

It is an important distinction (See here)

__________________
Troy
Login to Reply Score 1 for Good Answer
Guru

Join Date: Oct 2008
Posts: 42261
Good Answers: 1659
#9

Re: Air Flow in a Vertical Pipe

04/10/2018 1:17 PM

Does this question have any basis in reality? In other words, is it a real life situation* or just theoretical/hypothetical?

*"real life situation" for TINAC

Real-life | Definition of Real-life by Merriam-Webster

Login to Reply
Guru

Join Date: Apr 2011
Posts: 7887
Good Answers: 279
#10
In reply to #9

Re: Air Flow in a Vertical Pipe

04/11/2018 2:36 AM

Oh Lyn. don't feel threatened. Your position as the top real-life one-stop source of curmudgeonry remains unchallenged.

__________________
Eternal vigilance is the price of knowledge. - George Santayana
Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29966
Good Answers: 809
#11
In reply to #10

Re: Air Flow in a Vertical Pipe

04/11/2018 2:57 AM

Then the others aren't trying hard enough...

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Login to Reply 11 comments
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Lehman57 (1); lyn (2); PWSlack (2); Rixter (1); SolarEagle (1); Tornado (1); Troy36 (1); truth is not a compromise (2)

Previous in Forum: What is This?   Next in Forum: Microwave Intensity Meter

Advertisement