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Old 01-17-12, 09:02 AM   #20
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Join Date: Jan 2012
Location: Michigan, US
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Back in my days as a foundry worker, I operated a natural gas fluidized-bed calciner furnace which ran at a constant 1500F. It was equipped with cross-flow exhaust heat recuperators that heated the incoming combustion air from the exhaust being drawn out of the furnace. They were basically stainless steel boxes with tubes for the exhaust to flow through. The combustion air was introduced to the bottom half of the side of the outer box, where it was forced across the bottom half of the tubes by a divider, then up and back across the upper half of the tubes, where it exited to the insulated hi-temp combustion air pipes. Once the recuperator temp reached about 500F it had sort of a turbocharging effect on the furnace temp, making it rise very quickly from 1000F to 1500F. The combustion air exiting the recuperators was usually 450F to 700F during normal operation. The combustion air was responsible for not only providing oxygen for the natural gas to burn, but also fluidizing (bubbling) the sand inside the furnace.

I'm sure some of you can see by now how this could be implemented on a wood stove. My idea is to use an air-to-air HX to preheat the combustion air from the flue gases. The most practical method is to put metal studs in a section of 6" stovepipe, surrounding that with a section of 8" duct. A divider should be run down the pipe on both sides so that fresh intake air from outside comes in one end, runs down the length of the HX, then moves across and up the other side of the pipe to the outlet on the same end as the inlet. The flue pipe up to the HX, the HX itself, and the heated combustion air should all be insulated. I would simply remove the ash pan on the bottom of the stove and make an insert in the end of the combustion air duct to fit inside the opening, able to be moved out of the way for removal of ash. Also, a damper would be located in the combustion air duct somewhere close to the stove to control firing.

The goal is to get the combustion air entering the stove to 400F, which should be sufficient to activate pyrolysis of the fuel on contact, creating the "turbocharging" effect that I'm looking for.

I like the aforementioned expansion chamber idea too. Maybe that could be a use for the extra 55 gal. drum that's been sitting in the garage . The HX could be mounted between the stove and expansion chamber. I'm thinking that after that the exhaust should be pretty well cooled down. Any creosote buildup due to low temps should happen in the expansion chamber and not in the flue beyond that point, hopefully.
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