[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. 
    Hi Dan  

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 
the tube. 
    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 
save water. 
    Never know how you will find the next  breakthrough.
    Dan Dimiduk 
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