[TheForge] Brick forges

[CGRAF]

On 11/22/2011 10:50 PM, Bruce Freeman wrote:
> A few comments on that brick forge design:
>
> The flue does not really need to be 12" square.  I have had excellent
> success with a 6" (round) metal flue pipe, perhaps 16' tall, if the
> lower end (in this case w/o hood per se) is quite near the fire.  6"
> round is less than 1/5 the area of 12" square.
>
> The original reason for large flues was so a boy could go down them to
> clean them.  The myth that large flues are necessary has become so
> ingrained in the culture that nobody will believe me on this point,
> despite Benjamin Johnson having debunked this in colonial times.  In
> any event, 12" square is excessive.  That's a 27% larger cross-section
> than a 12" round flue.
>
> The smoke shelf is only necessary if you insist in building an
> excessively large flue.  The function of the smoke shelf is to allow
> the hot air to pass in front of it, so as not to have to heat and/or
> lift the entire column of cold air before effective exhaust is
> achieved.  It also allows entry of outside air if you are so
> improvident as to not have any other way for air to enter your shop
> (which is more an issue in a home fireplace than in a shop coal
> forge).  A circulation is set up in the oversized flue in which cold
> air comes down the back of the forge and hot air goes up the front.
> You're better off with a small, hot flue than with a large, warm one.
>
> The opening of the "hood" should be as small as practical.  What
> constitutes practical is up to you, but the larger the opening, the
> more diluted will be the hot gases as they enter the hood, and the
> lower the draw.  Once the flue begins to draw, smoke will be drawn
> horizontally for a considerable distance into the hood.
>
> I prefer a hood that leans forward a bit in front, whether it be of
> metal or brick.  No doubt, this is a personal preference, but it's
> pretty easy to construct and brings a side-draft forge hood slightly
> over the fire, which can be an advantage -- or a disadvantage.  This
> effect can also be achieved or enhanced with a removable "visor"
> placed above the hood opening, to be used while lighting the coal and
> removed before beginning forging.
>
> On Tue, Nov 22, 2011 at 1:28 PM, Paul N<crosspein at sbcglobal.net>  wrote:
>> Found it. It needs a 'pdf' on the end:
>>
>> http://www.blacksmithing.org/CB-Archive/1995/1995-06-cb.pdf
>>
>
>
>
Flue capacity is a function of both functional cross section and height. 
( Functional in that a rough interior or radically offset 12" flue is 
not functionally a 12" flue)

Cross section effects velocity also. A smaller but taller flue will move 
the same volume of flue gases at  a higher velocity.

I used to know a rule of thumb that was pretty accurate for wood fires, 
but would be afraid to put what I remember out there as I am not 
confident that I remember correctly and the results would not be good if 
I was wrong.  Maybe a google search?

Mike Graf

Mike Graf