Re: Fireplace Physics

From: Michael J. Strickland (qualityser_at_worldnet.att.net)
Date: 02/21/05 

Date: Mon, 21 Feb 2005 18:15:55 GMT

"Isaac Wingfield" <isw@witzend.com> wrote in message
news:isw-090366.21510120022005@comcast.dca.giganews.com...
> Our fireplace has a spark arrestor screen (I suppose almost all of them
> do).
>
> The screen is made of spiral wound wires. The "apertures" between the
> wires are almost big enough to stick a pencil through, and I'd guess
> that the cross-section area of the wires is well under five percent of
> the total area of the screen, when it's pulled all the way across the
> opening of the fireplace (IOW, it's almost all open space).

I question the above assumption. Most fireplace screens I've seen are pretty
finely meshed (i.e. they provide closer to 50% blockage).

For instance if the holes and the wires are equal sized, that is about 50%
right there.

>
> I've noticed that when the screen is closed, the amount of radiant
> energy coming into the room is significantly less than when it's open.
> In fact, you can pull one half of it closed, and tell the difference
> just by moving your hand between where it is, and where it isn't. It's
> difficult to estimate, but the reduction seems a lot closer to 50% than
> it does to 5% (which I probably would not notice).
>
> At the same time, there's not a noticable reduction in the amount of
> visible radiation from the coals.
>
> I know the IR wavelength is far too short to have trouble with those
> pencil-sized apertures. I also know that if it was convection (actual
> air movement), the reduction in flow would be out of proportion to the
> amount of area obscured by the wires, but this is radiation, not
> convection.
>
> What's going on?
>
> Isaac 

-- 
---------------------------------------------------------------
Michael J. Strickland
Quality Services                             qualityser@att.net
703-560-7380
---------------------------------------------------------------

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