[TheForge] No More Smokin' Hoods

Thu May 1 17:48:24 EDT 2008

Bruce:

I have come to a similar conclusion about the conventional wisdom of the 
10-12" flue.  Draft clearly has more to do with placement of the opening and 
overall stack length than anything like the configuration of the smoke 
chamber, "expansion" chamber, etc.  I may modify the shape of the opening a 
bit, if only to seal off some slight leakage around the back of it, but I'm 
leaving the basic 8" opening and the 8" stack right where they are.
----- Original Message ----- 
From: "Bruce Freeman" <freemab222 at gmail.com>
To: "Blacksmithing List Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Thursday, May 01, 2008 4:15 PM
Subject: Re: [TheForge] No More Smokin' Hoods


> Hmmm,  Andy's insults are less vociferous than normal.  I think I'm
> being complimented!
>
> I think the apparatus Andy refers to goes back several years to when I
> had a coal forge in my garage shop.  I could only have a 6" flue, so
> brought it down to the fire much as Peter recounts, and my experience
> was similar - an incredible draw.  To control where the pipe was with
> respect to the fire, I developed a ball-joint for the middle of the
> flue.  This was two SS bowls, with holes in the bottom,  fastened
> together and to pipes on either side somehow.  Frankly I don't recall
> the details, but it enabled me to move the end of the flue anywhere I
> wanted.
>
> I gave this up because I live in suburbia, and now only use a gas forge.
>
> I think simpler devices than a ball joint would work as well, but may
> be more prone to leakage, reducing the draft somewhat.
>
> This experience convinced me that folks who think a 10" or 12" flue is
> essential are simply wrong.  No doubt there's an optimum diameter,
> depending on the sort of fire you're making, but 6" worked fine for
> me.
>
> Bruce
> NJ
>
> On Tue, Apr 29, 2008 at 12:51 PM, Ron Childers <munlaw2 at hcsmail.com> 
> wrote:
>> We have a slightly smaller pipe (12") suspended on cables and counter
>>  weights inside a larger pipe. We let it down close when starting the 
>> fire
>>  and lift it up when it starts drawing. the stack is 3' higher than the
>>  highest point on the roof. Works fine.... Ron C
>>
>>
>>  -----Original Message-----
>>  From: theforge-bounces at mailman.qth.net
>>  [mailto:theforge-bounces at mailman.qth.net] On Behalf Of Andrew Vida
>>  Sent: Tuesday, April 29, 2008 12:35 PM
>>  To: Blacksmithing List Sponsored by ABANA
>>
>> Subject: Re: [TheForge] No More Smokin' Hoods
>>
>>
>>
>> I will note that the reason for this is because the sudden transition
>>  from open air to the stack is what causes the right pressure drop.  If
>>  you have a large hood that tapers to the stack opening, guess what: all
>>  that air is being corralled into the same place and no drop in pressure
>>  and therefore no flow...  Bernoulli 101. :)
>>
>>  You could probably get a little better performance if you replaced that
>>  8" flue with 10 or even 12.  It should suck mo' bettah.
>>
>>  Bruce Freeman had a very interesting contraption set up at his forge.  I
>>  will leave it to him to disclose or not, but I think the basic idea was
>>  really good.  Almost fabulous, but if I go that far I'll never see the
>>  end of his big head, and it's ugly enough at its current size.  No same
>>  person would want MORE of that.  No insane person would either, for that
>>  matter.
>>
>>  Trust me.
>>
>>  Peter Hirst wrote:
>>  > Had an AHA! moment in the shop  today.  Since I got the forge up and
>>  running over the winter, my special custom-designed open hood has failed
>>  miserably.  It covers the entire area of the forge (2x2) and is only 
>> about
>>  18" above the fire.  It tapers beautifully to an 8" adapter and is 
>> topped by
>>  12 feet of 8" pipe with no cap.  Works beautifully on paper.  In the 
>> shop,
>>  hardly at all.  Some smoke stays under the hood, and some even goes up 
>> the
>>  stack, but mostly it just billows and swirls and spills out, even when 
>> the
>>  pipe itself seems to be  drawing well.     I have noticed several times 
>> that
>>  at the joint where the hood meets the pipe, it draws like crazy, and 
>> even
>>  with a  roar when anything flaming is placed near that spot.  Even then,
>>  smoke is going the wrong way at the edge of the hood. A lot of air goes 
>> up
>>  that pipe, but it doesn't take much smoke with it. SO today, when I 
>> couldnt
>>  stand it any more, out of sheer desperation I took a spare 4' section of 
>> 8"
>>  pipe and inserted
>>   it up under the hood at the joint into the existing 8".  That put the 
>> lower
>>  end, with about a 30 degree angle at the opening, just a couple inches 
>> over
>>  the fire..  As soon as I connected it, I had a winner.  The fire 
>> literally
>>  roared into the 8" opening, sucking smoke from at least a 12" radius 
>> around
>>  it. It may just have been because I was paying closer attention, but the
>>  fire seemed to burn hotter, cleaner, and more efficiently.  Things were
>>  suddenly so much better that just for grins, I got a nice clean, hot, 
>> deep
>>  open coke fire going and dumped a few fat shovels full of green coal  on 
>> it.
>>  No problemo.  All that thick, sickly yellow smoke -- and I mean all of 
>> it --
>>  disappeared like a wisp into the stack.
>>  >
>>  > So now my entire hood and vent system consists of just 16 feet of 8" 
>> pipe
>>  with  a 30 degree bevel at the bottom.  No side draft, no hood, no 
>> expansion
>>  chamber, smoke shelf, step down, step up, 12 " flue,  nothing.  Just 16 
>> feet
>>  of pipe.   It doesn't interfere visually (in fact I can see the work 
>> better
>>  than with smoke escaping from under the hood) or mechanically with the 
>> fire
>>  or the work,  and if it ever does need it for a particular piece, I can
>>  either swing it a few inches in any direction or remove it temporarily 
>> and
>>  rely on the hood.  I was on the verge of ripping out the whole thing and
>>  building a side draft with a 12 inch flue from scratch,  and investing 
>> in
>>  about 12 or 16 feet more of 12" duct for the portable rig for shows. 
>> Now
>>  I'm all set with the shop, and for the portable rig,  think I'll just go
>>  with the 8" pipe set about 4" over the fire, just like the shop, or 
>> maybe
>>  resting right on the hearth with the end cut at 45 degrees.  .
>>  >
>>  > Life just got a whole lot simpler.
>>  >
>>  > Keziah
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>>  --
>>
>>         -Andy V.
>>
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>
>
>
> -- 
> Bruce
> NJ
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