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air flow calculations
2

air flow calculations

air flow calculations

(OP)
I am trying to figure out how much air flow is going through a square duct.  I have airfoils in line and measuring differential pressure across them.  I have the range of the transmitter but don't know how many inches of differential is equivilant to lbs/hr.  I have tried to find venturi calculations but have only come up with calculations using pipe diameter.  I do have the dimensions of the duct.  But other than the specified information, I don't have a whole lot.  I would appreciate if anybody could point me in the right direction.

RE: air flow calculations

Check out the thread and posting in HVACR engineering thread403-157067 It has a link to Dwyer Instruments and some helpful formulas.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int.

RE: air flow calculations

Let me know if that helps the thread runs a little long but the Dwyer site has good data

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int.

RE: air flow calculations

2
mpullin,

Your description is a bit sketchy.  Differential pressure across some airfoils is not enough to calculate flowrate.  If you have a venturi tube, then it's easy and accurate and I can point you to the formulas.

If not, there are a lot of ways to measure airflow.  The Dwyer site is a good start.  The best method depends on the accuracy you need and the velocity range you expect.

ko  (www.ecooling.biz)

RE: air flow calculations

(OP)
Thanks for the replies.  They were a big help.  I have found calculations for the velocity and have figured that out.  I am working on a mass flow calculation now and the number I am coming up with is a little to high in my opinion.  I am still working on it and will eventually get it.  But I would appreciate if any direction could be given.

RE: air flow calculations

Are you using a pitot tube?  Just be aware they are very inaccurate below 600 ft/min or so.  Otherwise, finding massflow is easy:

1. Get average velocity by profiling at regular points over a cross-section of your tube (Dwyer and others have guidelines)

2. Volume flow = average velocity x section area (example: ft3/min = ft/min x ft2)

3. Mass flow = volume flow x air density (example: lb/min = ft3/min x lb/ft3).  
You can use this tool to find density: http://www.denysschen.com/catalogue/density.asp

ko  (www.ecooling.biz)

RE: air flow calculations

(OP)
Thanks for all the help that I have recieved.  With the help of your post I have calculated all my measurements.  Now I have to incorporate my calculations.

Thanks again.

RE: air flow calculations

When you average air velocity thru ducting be sure measurements are in a uniform air flow and be aware that the velocity distribution may be parabolic.  Read the instructions supplied by the manufacturer of the pitot tube to calculate average velocity.

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