[TheForge] Re: forge flues

[Peter Hirst]

Yep, a dogleg at the top of the hood and then 8' of stack above that.  The 
dogleg is as short as it could possibly be, and I figure the total stack 
height, or lack of it is the main issue.  After I get the present 8 feet 
supported a little better, I'm prepared to add another 8 feet to it.  Its 
already 3 feet above the nearest part of the roof, so support will 
definitley be an issue from here on up

pgh
----- Original Message ----- 
From: "Mike Spencer" <mspencer at tallships.ca>
To: <theforge at mailman.qth.net>
Sent: Thursday, January 17, 2008 12:43 AM
Subject: [TheForge] Re: forge flues


>
>> Any idea how to calculate sufficient stack size and height?
>
> You mentioned in another post that your pipe goes horizontally through
> the wall. Did I get that right?  That's going to significantly
> decrease the stack effect and require larger diameter flue and/or a
> taller one.  IMHO, 8" dia. is marginal at best and won't be enough if
> you have a dogleg in the flow.
>
> Back when I got my first big shop, I groveled through such engineering
> books as I could find to little or no avail and, after some clever but
> non-functional attempts, finally -- I think it was the Mark III :-) --
> came up combination of side-draught smoke box and ca. 13" flue.  That
> one was ca. 12' or 13' from smoke box to weather cap and worked fine
> except when the wind blew steadily from the direction of the big
> church across the street.  Then it periodically drew backwards and I
> just had to pack it in for the day.
>
> Presently I have the smoke box and about 16' of 13" diameter flue that
> only smokes if I forget and turn on the welding exhaust fan without an
> door open.  Oh, yes, or if I start the coal fire with a lot of green
> coal.  Then I have to stick a couple of sheets of burning newspaper
> into the smoke box to get it to draw the smoke that isn't yet hot
> enough to initialize the stack effect.  Once I have a hot fire there
> almost never a puff of smoke into the shop.
>
> But isn't the internet nice?  Here's the stuff I was looking for in
> the engineering books back when:
>
>    http://en.wikipedia.org/wiki/Flue_gas_stack
>    http://en.wikipedia.org/wiki/Stack_effect
>
> Not that it's altogether a cookbook procedure; they say:
>
>   The calculation of many of the above design factors requires
>   trial-and-error reiterative methods.
>
> The best forge flue I ever saw?  I knew a guy who rented a house in
> western Mass. that was on a property shared with an abandoned and
> partly demolished small factory.  The original brick boiler stack was
> still in good shape, maybe 10' or 12' inside diameter at the base and
> 4 or 5 storeys tall.  He put his workspace and forge inside and just
> propped the iron door open part way.  The draught was stupendous.  If
> he opened the door wide, he was working in a minor hurricane.
>
> There was a brick chimney like that near me that was demolished a
> couple of decades ago.  I wanted it but I couldn't figure out how to
> haul it home. :-)
>
>
> - Mike
>
> -- 
> Michael Spencer                  Nova Scotia, Canada       .~.
>                                                           /V\
> mspencer at tallships.ca                                     /( )\
> http://home.tallships.ca/mspencer/                        ^^-^^
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