air flow calculations 2

[mpullin]

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.

 

[Yorkman]

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.

 

[Yorkman]

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.

 

[ko99]

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 (

http://www.ecooling.biz)

 

[mpullin]

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.

 

[ko99]

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 (

http://www.ecooling.biz)

 

[mpullin]

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.

 

[chicopee]

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.