[TheForge] Burners

[Dave Brown]

At 09:48 AM 3/8/04, you wrote:
>The "true" flare is at the end of some burners, at the flame, and serve
>to reduce the velocity of the mixed gases, so as not to blow out the
>flame.  There are other devices that work as well - a flare is not the
>only possible "flame-holder."  Furthermore, a flame holder is not
>necessary on a burner directed into a forge, since the mixed gases will
>reignite on the hot surface of the forge.  Mineral wool is especially
>good at reigniting the burning gases.

Bruce,

This is a good point that many do not quite grasp, namely the purpose of 
the flame holder.  some, even one burner guru out there, have claimed that 
the flame holder was there to get hot and maintain ignition, even to the 
point of saying that the flare is not a flame holder.  I think you've made 
a pretty good description here.

>I understand the the bent tube is a mixing device.

In the Sandia Recuperative forge design they have a twisted strip of metal 
in the mixing tube to induce turbulence to mix the gas.  The bend in the 
tube does induce further turbulence but I think that this is by 
happenstance rather than by design.  I say this based on the description of 
the burners and their history as presented in the Sandia Recuperative Forge 
plans (available from ABANA) and comments that Robb Gunter made at a Guild 
of Metalsmiths conference during a very informative discussion of the 
origins of the design.  But regardless of the origin of the design, whether 
by design or happenstance, the bend does contribute to the mixing.

>principle, a blower design without a mixer could have a nice clean
>stream of fuel gas in the middle, and a nice clean stream of air on the
>outside, with minimal mixing between.  No good.

This might be the case in a straight line (from blower to combustion 
chamber), although I'm sure there is some, though minimal, turbulence  in 
the air stream as it exits the blower.  But the forced air burners, both 
the Hans Peot plans and commercial plans (e.g. Johnson Gas Furnaces) seem 
to have at least one 90d bend in the plumbing which induce a lot of mixing 
turbulence to the flow.  But I suppose that the amount of mixing also 
depends on the relative position of the gas jet  I've seen quite a few 
"improvements" to the blown design that are intended to generate 
turbulence.  But in my case I think that the jury is still out as to 
whether or not these mixers are necessary.  I can imagine designs where 
they would be an improvement, but I can (and use) a design (Hans Peot) that 
I don't think would be improved a bit with such an "improvement".

For those who don't know or who haven't guessed it ... I'm a big fan (no 
pun intended) of the forced air (e.g. blown) burners.  I also believe very 
strongly that the negatives that are bandied about against forced air 
burners are more myth and lousy setup than fact.

Dave Brown