Chimney design / draft question

[edennis]

I'm wondering how a temporary increase volume affects the draw on a coal forge chimney:

A 10" diameter round chimney that goes into a box measuring 14"x"14"x12.5" and then leaving as a 10" round again. 

Will this a affect the overall draft?  

[Latticino]

Short answer: yes.  A rapid change in crossection like that one will induce additional turbulence and add to the duct friction for the escaping flue gasses.  It will have an effect, however it may not be a large enough effect to appreciably change the draft for your forge.

[edennis]

Ok, that's a helpful way to think about it. I guess it would depend a lot on the overall draw before the box was put in to the system. 

Thanks,