[TheForge] Re: TheForge Digest, Vol 55, Issue 31

[Andrew Vida]

Mike Spencer wrote:
>> ...made a flue out of 14" dia. Pipe.  It is bolted to a footing on
>> the ground outside the shop.  A section of the pipe comes in through
>> the wall at a 45 degree angle in a "side-draft" approach to the
>> forge.
> 
> 12" to 14" diameter should be fine for the 25' to 30' of
> stack mentioned by the OP.
> 
> That'll work, of course, but it's a bit Mickey Mouse. 8-) I visited a
> guy in western Mass., circa 1968, who had the remains of a smallish,
> old Victorian, brick-built factory on his property.  He put his forge
> *in* the flue.  There was a 40- to 60-foot brick smoke stack with an
> iron door at ground level, maybe 8 or 10 feet in inside diameter.  Put
> the forge in there, open the door and the draft was stupendous even
> without lighting a fire.  IIRC, in practice he had to just prop the
> door open a crack so the breeze wouldn't suck anything less dense than
> iron up the and away.
> 
>        Powerful enough to suck prairie dogs from their holes, but
>        gentle all the same....  --  Tom Parmenter, "Desperado"
> 
> Baddest forge flue I ever saw.  Envy. :-)

	This is a different situation altogether - you have a large space 
acting as the "context" of the flue.  The volume of the shop space, 
whose air was presumably warmer than that outside the shop, is large and 
therefore the lfue will suck awhile.  The same flue in a small shop 
would most likely not function in the same way for long or even at all. 
  Flues work because of the temperature differential between the air in 
the shop and that outside.  Once that difference is gone (outside air 
fills up the shop) the sucking abates.  At that point you have to have a 
source of heat to induce a difference - that would be the fire.  The 
fire has to be properly sized to the flue volume in order for the stack 
to draw sufficiently.  Without it, little if anything will happen.