[TheForge] coal forge exhaust system

[Jerry Frost]

You got me Paul, I wasn't going to chip in but pointing out how often
"venturi" is misused did it.

Naturally aspirated burners almost never incorporate a venture in any form
unless it's a commercial burner and some of those don't. The burner you cite
is an example but only incorporates a venturi to increase induction.
Naturally aspirated burners are induction devices using a stream of high
pressure gas to make a vacuum to draw combustion air, OR pump water,
transport grain, crush grain into flour or any number of cool jobs. Bournuli
observed and quantified how these things work, fluids flowing along or
across a curved surface accelerate and pressure drops against the surface.
Venturi,I believe, thought it only occurred across the outside curve but it
occurs on any curved surface in any direction. This means a fluid (gasses
obey the same principles) flowing down a tube, accelerate and offer less
resistance. The low resistance is low pressure and if applied properly will
vacuum up whatever's needed, be it sea water out of a bilge or combustion
air in your gas range.

Enlarging the burner tube at a ratio of 1:12 or less increases induction as
the same volume of fluid must fill a larger volume so pressure decreases and
induction increases. More change than 1:12 and bad turbulence occurs and
reduces draw through back pressure.

Anyway, it won't take a lot of blower to induce a considerable draft in your
stack, higher pressure is more effective than higher volume. A stack can
only carry so much volume unless you're pumping it while a smaller volume at
high velocity will make a much stronger induction for better draw.

Signing off now. 

Frosty the windy.

-----Original Message-----
From: theforge-bounces at mailman.qth.net
[mailto:theforge-bounces at mailman.qth.net] On Behalf Of Paul Sperbeck
Sent: Monday, November 18, 2013 9:49 AM
To: Blacksmithing List Sponsored by ABANA
Subject: Re: [TheForge] coal forge exhaust system

We in the blacksmith community tend to throw the term venturi around quite a
bit, when more often then not we are referring to things approximate
venturis.

Many gas burners for forges reference venturis, but to very few of them look
like this:

http://www.duncanshearer.co.nz/kilnplans/burners/venturi/venturi%20burner-01
.jpg

So when I referred to a venturi vent system I had in mind something more
like this:

http://www.quickdraft.com/products/venturi-exhauster

Which is more than a small pipe stuffed into a bigger one...

In our northern climate, we need to extract the noxious stuff with out
freezing ourselves to death. By using the quickdraft as a model, and drawing
outside air to supply the fan the heat loss in the shop can be held to a
minimum.

paul
--
"This is only temporary, unless it works."
--Red Green

No trees were killed in the generation of this message, but a tremendous
number of electrons were terribly inconvenienced

Try to notice the thing that happened just before the thing you didn't want
to have happen happened!

If you are using violence and it does not work, You are not using enough.

Life's tough, pilgrim, and it's even tougher if you're stupid. - John Wayne

Resistance Is Not Futile!  It's voltage divided by current.