FW: [TheForge] Burner angle

[Kathy]

Pete,
I didn't answer your question completely before; your specific concerns weren't
answered.


You wrote:
I. "A natural tendency to thermosiphon would work against the inflow of air to
mix with the fuel."

Air inflow to the burner is kept separate from the superheated forge gases by
proper sealing of the burner collar, and also by placement far enough from the
forge exhaust exits to permit the up-flowing exhaust gas to pass by the burner
without entering its air intakes (as little as two inches is a perfectly
sufficient distance from end exhausts). What does make a difference is buoyancy;
the superheated interior gases do cause buoyancy to come into play, which does
interfere with burner performance; in this case, the interference is used
deliberately to slow an unnecessarily high gas exchange rate (for small parts),
saving fuel.

Any concerns about exhaust gas can be laid to rest with an easy experiment.
Simply hold a running burner in your hand, and slowly turn it from vertical-up
position, through the horizontal, and finally to vertical-down; as you do so,
observe the change in flame velocity. The first surprise you will get from doing
this experiment with any good burner is that, while flame velocity is greater in
the vertical-up than in the vertical-down position, the difference is no where
near so much as you had expected. The second surprise comes from finding out how
close you can come to straight vertical-down position before the burner
destabilizes (about five degrees). The flame starts wavering and threatening to
go out at this point because exhaust gases begin entering the burner’s air
intakes. When the same burner is placed within a sealed burner collar on your
forge, exhaust gases are much further away and can do no harm even at straight
vertical down position. 

2. "The greater heat of the top of the forge would cook the burner faster."

The minuet the burner is shut down, interior gases can enter the burner through
buoyancy; this is called the "chimney effect," and is prevented by closing the
burner's choke completely during shutdown. While running, the burner prevents
the forge gases from entering. Refrigeration effect from the expanding fuel gas
is sufficient to keep the burner from overheating do to conduction and
convection gain from the forge body and nearby exhaust gas. These rules apply
equally to top mounted burners (the overwhelmingly dominant mounting choice),
whether they are placed at vertical down (90°) for spot heating, or 3” down from
top dead center (on a 14" diameter forge) to create a swirl.

Finally, why is vertical down the dominate choice even for swirl effect? Because
portable  forges use ceramic fiber for most of the interior insulation, and
placing a burner at the forge's top allows it to be aimed directly at the
firebrick or kiln shelf slab on the forge's bottom. Should you decide to use
Kastolite 3000 insulating refractory for the forge's interior, instead of
insulating fiber and a kiln shelf floor, than the burner placement can be
changed to whatever you find most convenient. Also, solid refractory is a much
better choice for multiple use forge/furnaces, or for a forge employed in billet
heating. A typical five gallon cylinder with a two inch thick Kastolite 3000
refractory lining will still be portable (approximately 65 lb.) On the other
hand the same forge built with ceramic fiber and a kiln shelf floor can be
lifted comfortably with two fingers; it's all a question of what features you
find most convenient.   

Mikey