[TheForge] FAQ

Tue Apr 18 17:21:28 EDT 2006

Well, I'm not sure there's anything here to disagree with. When discussing
dragon's breath, I'm referring to the end result exiting the forge. That is
not the same as the high velocity flame entering the equipment from a
naturally aspirated burner. I like to design forges and furnaces so that
that furious little flame is pretty well spent and puny by the time it exits
:-) Tell us more about diffusers please; greedy minds want to know.
Mikey
P.S. "Pandora's Tums...catchy. Now all I need is a text to go with it :)

-----Original Message-----
From: theforge-bounces at mailman.qth.net
[mailto:theforge-bounces at mailman.qth.net] On Behalf Of Jerry Frost
Sent: Tuesday, April 18, 2006 12:56 PM
To: Sponsored by ABANA
Subject: Re: [TheForge] FAQ

Once the Tums are out of the bottle you can't put them back you know.

Hey! There's the title of your next book, "Pandora's Tums." <grin>

Oooh cool! A little disagreement we can chew on. Burning fuel is a matter of

proper fuel air ratios; X amount of fuel to X amount of air produces X 
amount of BTUs. Period.

Whether you do this with a gun or naturally aspirated burner a stoichometric

burn will require approx 16.5 parts air to every part of propane. (Air:Fuel 
or A:F ratio) There are a couple other factors involved the biggest being 
propane's resistance to properly mixing with air that make a rich 
(carburizing or reducing) fire necessary. It is actually hard to get 
propane/air burners to completely consume the oxy, even running quite rich, 
say in the 12-13:1 range.

Okay, the properties of combustion rely heavily on the A:F ratio, then 
there's delivery and this is where gun vs. N/A (Naturally Aspirated) burners

differ.

A gun burner has a positive air flow with the blower forcing air through it 
regardless (within reason) of obstructions, corners, outside conditions like

wind, etc. etc. This means a gun burner is a much more turbulent system 
which mixes the A:F more thoroughly. It also means you don't need the high 
velocity a N/A burner does.

Even with a gun's higher turbulence you need to introduce the propane 
upstream of a corner for a good mix and clean burn. This is also a safety 
measure as a backfire finds it very hard to go around a corner against a 
wind.

A naturally aspirated burner on the other hand relies on the primary 
pressure (propane from the jet) to induce air into the system. This makes 
N/A burners very susceptible to obstructions, corners, outside conditions, 
etc. There are ways around some of their sensitivity though there are 
tradeoffs.

You can use a smaller jet and longer tube for instance. The upside is a 
burner that's less sensitive to conditions, mixes better and is a bit more 
efficient. The downside is burner that's louder and produces more dragon's 
breath.

In the last example the increased dragon's breath results from the higher 
velocity output from the burner, not more air and fuel in the firebox. It's 
just moving through faster so the trend to make N/A burners with as low a 
primary pressure as possible is a good thing. . . Generally.

Basically there shouldn't be a lot of difference in dragon's breath between 
a gun and N/A burner with equal BTU/hr outputs. If there is a lot of 
difference, look to the burner's output velocity rather than a fundamental 
difference in type.

This brings up a real advantage to gun burners, they don't care (much) about

obstructions so you can put all kinds of cool stuff on the output end like 
diffusers. A good diffuser will drastically reduce the output velocity and 
keep the fire in the box a little longer. You can also run them through a 
manifold to any number of nozzles. You can do this with N/A burners but they

don't perform as well as I'd like. Still.

Regardless, if you're burning something you have exhaust and it has to go 
somewhere. The higher the burner velocity, the greater the dragon's range 
and temp if not volume.

Begin speculative ramble. <grin>

A thing I want to experiment with when I can is a heat exchanging forge 
opening. It'll be pretty simple, just a double walled little tunnel in front

of the forge openings with an exhaust vent on top and a hookup to the 
burner's air intake.

My shop forge will be a variable volume jobby like Ralph makes and I'm 
considering building an exhaust hood for it so I don't have to breath more 
CO than necessary. I'll vent it through my subfloor exhaust system to 
scavenge some of the heat into the slab. However, if I make it a double 
walled hood it just might make a spanking good source of hot air for the 
burner. (Greedy ain't I?)

Frosty
-------------------------------
If it ain't forged
it ain't real.
Wrought iron is.
The FrostWorks

Meadow Lakes, AK.

http://www.artmetalradio.com/

From: "Mike Porter" <michael.a.porter at comcast.net>

> Frosty,
> Oh boy was that a mouth full or what? I will not write anything in regard 
> to
> your DEMANDING people comment, not because I disagree with it at all, but
> simply in order to keep the Tums in their bottle. 

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