[Stoves] Use of preheated air for forging with wood.
Carefreeland at aol.com
Carefreeland at aol.com
Fri Aug 5 07:39:15 PDT 2011
The following is a reply I sent on the wastewatts list to someone,
describing how I can forge small pieces of iron while heating my greenhouse.
In the previous letter I was describing the very high temperatures I reached
with natural draft. Something can be taken from this to "warm up " some of
your colder operating stove designs and make them more efficient. I don't
see the need for forcing the air with a blower if the proper engineering is
done to enhance the natural draft.
The stove mentioned is an old cross flow brick lined coal/ wood stove
made from 1/4 inch plate steel. The incoming air ports are in the cast iron
front door. I have had trouble describing this stove to stove people in
the past. Paul Anderson has seen the stove but not in operation. Heat is
scavenged from the afterburner. On a high burn during warm up phase, the
afterburner produces more heat than the stove itself. My plans are to put preheat
tubes in the stove to enable warm air jets to make the low burn settings
more efficient. As the stove is configured now, unless I burn most of the
wood down to charcoal, the stove is not so clean burning on low settings when
I leave the greenhouse overnight.
Depending on the alloy, Iron starts burning around 1,800-1,900°F -- you
will ruin your work.
Reaching 2K°F or higher is neither necessary nor a good thing for
blacksmithing -- I use a propane forge (1,600°F) for 95% of my work and fire up the
coal forge only for metallurgical reasons, never for temperature reasons.
I usually hit the high notes while warming up the burner on very windy
zero degree F nights. Oak planks from skids or equivalent kindling burn
the hottest. Then as the stove burns down to coals I use the char bed to heat
my forging work. On a typical night the draft is not enough to reach those
very high temperatures. Adding length to the chimney would enable better
draft on less windy nights. Remember that this stove is heating a
greenhouse, so the primary purpose is to maximize the BTU output, not temperature.
Forging is just an added feature, recycling the heat.
Those temperatures are only on the surface of the charcoal bed with
some wood there and the door closed. The closed door enables the primary air
preheating to take place as the air coming through the ports in the door
passes over a hot brick. There is too much draft with the door open to
maintain that heat at the surface. Therefore unless my piece is small enough to
close the door, I bury it in the surface of the char bed which is a nice
stable bright red to orange heat. If I want an oxygenizing flame, I use the
surface of the charcoal and rotate the work but it is cooler.
One of the reasons I get such good natural draft I believe, is because
of the afterburner set up. As the flue gasses turn up into the vertical 6"
well casing secondary air is introduced, causing a swirl. As hot secondary
combustion takes place, the stove pipe expands from 6" to 8" diameter five
feet up. This expansion causes additional suction. By allowing additional
room at this key point, the expanding gasses do not have to create back
pressure on the draft. The combusting flue gasses accelerate vertically as in a
turbojet combustion chamber. That is what creates the signature howl in
There is a baffle inside the stove box that re circulates the
combustion gasses over the fuel bed until they are hot enough to be sucked out the
horizontal exhaust pipe. The thick steel exhaust pipe is in the hottest
part of the stove, acting like a gasifier throat. The partially combusted
gasses crack and reduce to lowest terms such as H. CH 4 and CO. These hot ions
are prime for rapid complete combustion when cold, dense secondary air is
The funniest thing was I put a firebrick in the front of the stove box
to hold more ashes and charcoal from falling out. The day I did that I
burned the wrought iron retainer out by accident. That was when I realized
that I had created a open hearth like checker, which was preheating the
incoming air very efficiently. It's like putting a brick in the toilet tank to
Never know how you will find the next breakthrough.
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