| While a coal forge is a very useful piece of equipment in the smithy, it can generate copious amounts of coal smoke -- not only occluding vision but also impacting the health of the smith (directly via particulate contamination or carbon-monoxide poisoning or indirectly via engendering the displeasure of whoever ends up washing the shop clothes). The solution lies in the work of Count Rumford who perfected the design of a truly functional chimney. Roger Stuart, a fellow smith in Florida, generated the relationships cited here - I just followed Roger's lead. |
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The design calls for a curved smoke-shelf positioned at the bottom of the smoke chamber. A
reflector plane connects the leading edge of the smoke-shelf to the rear bottom edge of the main
chamber. The gap formed be the top edge of the reflector plane, the front and sides of the main
chamber is the opening into the smoke chamber. Air flowing down the flue is reflected back up
the flue and draws air from the main chamber through the gap. By controlling the geometry of
these parts, a venturi effect can result which literally will suck the flames, soot, and smoke at right
angles from the forge. In my shop, smoke rises no more than 6" to 9" above the fire before
making an abrupt 90 degree turn and then vanishes up the flue. |
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| W : the width and height of the main chamber (36" ). The depth of the main chamber is 0.5 W. |
| A: The distance from the opening to the top of the main chamber is typically 12". The actual dimension will be controlled by the size of the opening. Here, the opening is approximately 12" on a side, so A is closer to 24". |
| B:the length of the sloped side of the smoke chamber should be at least 12" long and should form a 30 degree or greater angle to the main chamber. |
| C: the length of the front sloping surface of the smoke chamber will be controlled by B and the width of the flue. |
| Smoke-shelf radius: the diameter of the curve should be W (note - this wil be a relatively shallow curve and is not critical but it must curve a bit). |
| Area of entrance into the smoke chamber should be equal to the flue area (no less than 67%). Assume the flue is 12" diameter = 113 sq.in. The area is then the area of the flue divided by the width of the box = 113 / 36 = ~ 3". So the curve section must fill the 15" of residual depth (0.5 W = 18 - 3" = 15"). So the smoke shelf is a section of a 36" diameter curve with a 15" chord. Or...bend a sheet of material to a gentle curve, run out 15" across the curve and fold the remainder into the reflector. Precision is not that critical - after all, we're smiths! |
| The flue area should be (1/8)(0.5) W^2 for flues 15' or taller. The area is (1.25 *0.5 * 36 * 36) = 81 sq.in. Since the area = pi * r^2 , then r = Sqrt(area/pi) = Sqrt(81/3.1416) = 5.07" or a diameter of 10.14", so use a 12" diameter flue. For a flue length < 15", you're suppose to use a factor of 0.25 instead of 0.125. That's a BIG flue (~20"), so take that with a big grain of salt. |
I built my flue from old 16-guage rusty sheet steel with the smoke chamber fabricated from used
roofing metal. It was assembled with pop-rivets. The lower section finally rusted out after 6
years in service while the smoke chamber seems untouched. Except for my disinclination to be
exposed to hot galvanized flumes, I could have built the whole thing from roofing metal. The actual
dimensions are: |
| Due to using a very fine powdered coal (I couldn't pass up a ton for $10), I've also added a hood over the fire proper. The forge is a 3' x 3' steel table with a Centaur Forge pot inset 6" from the left side and 3" from the from edge. The flue sits on the left edge of the forge with the opening in line with the forge pot. The cover is a light sheet steel shed, 18" tall right side, a sloping 'roof' running up to approximately 24" where it contacts the flue. The back surface is thus a 36" long x 18" on the right x the length of the roof x 24" on the left. The shed contains the fine fly ash and wasn't really needed when I was using good coal. |
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If you examine the picture of the flue in action (see below), you'll note that it does an
effective job pulling the flames out of the vertical. Also note that the smoke chamber
depth is actually a bit under 0.5W (more like 14" than 18"), so that the front simply
runs up to a cap rather than sloping back to the flue. The critical concept is have
a wide diameter flue and a curved smoke shelf that creates a venturi effect. Like with gas forges, there is a fairly wide zone of 'ok' around the theoretical dimensions. What is critical is a narrow gap into the smoke chamber, the reflector, and a gentle curve on the smoke shelf combined with a 10..12" flue of adequate length. | 
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If there are any areas than need clarification, e-mail me at the address given from the main page
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