[TheForge] Brick forges

[Bruce Freeman]

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
>



-- 
Bruce
NJ