How to calculate the free area in a set of louvers? 2

[apMechE]

I have to calculate the free area through a set of 14ga 30-45 degree louvers and not sure what to take into consideration. It has to be more than just subtracting the area taken up by the material thickness isn't it? How do you take the bends into consideration?

 

[IRstuff]

see:

http://www.eng-tips.com/viewthread.cfm?qid=292054

or

http://www.archlouvers.com/Louver_Free_Area.htm

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[chicopee]

Also if it is possible get a catalog from the louver manufacturer and the free area will be stated.

 

[racookpe1978]

Why do you need this number? Flow through the louver?
Painting the louver surfaces?
Fabricating the louver?
Letting light through the louver (like a blind?) at various times of day?
Using the louver as a fence?

If flow matters, More important than "free area" is the entry and exit losses that reduce flow through the louver, and then (next important) is the flow losses across the length of the louver, and the form losses of the cross-section of the louver. 100 small narrow long slats close together will cause more pressure loss than 10 thicker ones further apart.

 

[apMechE]

@racookpe1978 ... Good questions. They are for the rear of a machine with a pretty hefty cooler fan sucking air across the engine and blowing it through the louvers. The current design was using 3/4 #13 expanded metal across the opening, though has been decided louvers would look more appealing. Louvers present a concern with increased flow restriction. The current louver design due to special constraints is 14GA roughly 1.5'' bent 45, 3/4'' in from the horizontal edge spaced X apart across the opening. Want to know how tight they can be spaced to be the same, if not more efficient than the 76% open expanded.

 

[racookpe1978]

14 gage metal is very, very thick if people won't be climbing on it, crashing into it.

Let me think about it a bit. Do you want the interior of the engine compartment to be completely "hidden" from external view, or can the louvers be short enough and spaced widely apart enough to "see through" but not "poke fingers through"? Commercial engine operated commercially, or will this be operated by the (very ignorant, very stupid, and very dangerous) public?

 

[Tmoose]

Like racookpe1978, My first hunch is the "free area" of the present expanded metal opening is downgraded quite a bit by the many inches of sharp edges presented by the fairly open 3/4 #13 expanded metal.

http://cdn.smokingmeatforums.com/5/5c/900x900px-LL-5c3a60f9_METDESP2.jpeg

Louvers are likely to be less restrictive.

How many of these will you be building?

How sure are you that the airflow you have is barely sufficient? I think some pressure measurements might be more important than spending a lot of time calculatin', contrasting and comparing areas.

 

[dbill74]

Are you trying to design the louver yourself to have someone construct it from scratch?!? If so, why are you trying to re-invent the wheel? Or at least, why are you not working with a louver manufacturer for a custom louver?

Being that you are blowing through the louver to the outside, limiting the free area velocity is about pointless as there is very little concern over water intrusion in such a case.

Contact a louver manufacturer such as Ruskin, Airolite, or Architectural Louvers and get some literature on their products. They will have all the information you need to size and select a louver including free area, and pressure drop vs. air flow curves.

To maintain a reasonable pressure drop and throw off your fan's system curve you will likely have to up the size of the louver face area. If so you will need to provide a duct transition ahead of the louver; not a big deal, is done all the time.