Expanded Sheetmetal Resistance to Airflow

[TyLyon]

I am attempting to figure out how to determine the air flow resistance through a sheet of expanded sheet metal in regards to a pressure drop and velocity change. I figured I could determine the total area that is actually sheet metal and subtract it from the total size of the sheet to determine the area for the air to pass through. Would there be any type of loss coeffficients associated with this? I am trying to specify a fan and I am currently trying to calculate the required fan static pressure. thank you for any help you can provide!

 

[willard3]

Relative roughness can be quite high for some materials and higher than you expect.

Try here:

http://www.engineeringtoolbox.com/major-loss-ducts-tubes-d_459.html

 

[TyLyon]

so then you think the only loss through the screen will be due to friction?

 

[willard3]

It wasn't clear from you original post that this is a diffuser.

If the material is used as a diffuser, you should put this question to the maker. If you are doing it yourself and this is an important space, reserve some time for full-scale tests.

 

[TyLyon]

It is actually just an expanded sheet metal screen at the opening of a hood that will be taking in coal dust. The screen's main function will be to prevent large chunks of coal from coming up into the hood, I apologize for not stating that earlier. I attached a jpeg of my solid model of the actual hood with the expanded screen on it so you can see exactly what I mean

I'm trying to find if this screen will have a pressure drop across it or if it would decrease the velocity of the air coming through it.

Thanks

 

[berkshire]

Calculate the free area of the grid i.e. 60% open area, 70%open area, and compare that to your duct area as you have said. When you have the actual open area of the grid A, use that to calculate the rest of the problem.
B.E.

 

[IRstuff]

Seems like it should also depend on HOW it's expanded.

Can't tell from your graphic what the entrance angle is for your openings, if any, but I would think that would have some impact beyond just the ratio of areas, since the angles would deflect the air flow.

TTFN

FAQ731-376

 

[Compositepro]

You have to use the concept of hydraulic radius, which is the radius of a pipe with the same ratio of wetted perimeter to cross-sectional area.

 

[AbbyNormal]

I would carry the velocity head of the free area through the openings as the pressure drop and call it a day.

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[katmar]

This has come up here several times before. Click on "Forums - Advanced Search" at the top of this page and search for "ashrae screen".

If you do not have access to the ASHRAE manual you could try Perry's Chemical Engineers' Handbook. In my 7th edition it is on page 6-19. The Perry method is for incompressible fluids, but this will be fine for the very small pressure drop you will get - i.e. you can safely regard air as incompressible in this application.

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