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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dear Steve’n’All<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>></span><span lang=EN-US> </span>No, the angle was about 40 deg to the horizontal, and there's no sign of back draught.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>That is basically the VITA stove from the late 70’s. I feel there is a generic problem with it (down-angled self-feeding wood): if the excess air flow is high enough there will be no backdraft up the fuel hopper, but this condition is only sustainable with high excess air.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>In every case I can think of, if the excess air is brought down to a reasonable level like 100-125%, there is far too much heat rising in the fuel at the burning end to be carried away by the measly air flow even is 100% of it directed through the fuel. The buoyancy of the gases in the burning zone overcome the downdraft.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Two solutions seem to have been worked out (excluding the Rocket stove which was developed in response to exactly this problem). They are a) run the stove with high excess air, sweeping the heat and gases and flames into the combustion zone and b) sealing the fuel supply gas tight to allow it to be treated as a self-pyrolysing bottom-burning fire with air admitted at the bottom somewhere. This is done with coal boilers and some pellet stoves and the GTZ/GIZ 7-series coal stoves. The fuel is replenished periodically.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>In fact the GIZ-7 stove is slightly different in that it has a crossdraft fire under the hopper (a pyrolysing zone) which leads to a combustion chamber that is not directly over that zone. The hopper is vertical. But these are details. The principle is the same: ‘inverted fuel’, fire at the bottom, and air entering somewhere near the fire (only) as a response the issues created if the excess air is brought down to a reasonable level.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>My prediction is this: if you limit the air moving past the pot to the point where there is low EA and good heat transfer efficiency, the flames will rise from the fire up the angled fuel hopper. It is a fundamental problem.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>David Hancock experimented with other at Aprovecho in 1982 using weights and string to make an ‘auto-feeding’ tray that moved wood horizontally into the fire as it burned. Not too successful apparently. The idea was to use David’s idea of a horizontal shelf with air coming from below to control the backdraft. Larry W developed this into the metal shelf with air passing under it in a compact and simple package (original Rocket stove). The result was a lower excess air ratio without backdraft, giving higher heat transfer efficiency than the VITA stove. However it did not solve completely two other problems: a) it was not self-feeding, requiring a significant amount of attention to be efficient and even in heat production and b) the excess air ratio was still too high. I might add a third which only emerged later: the flame runs to the back of the chamber if all the air comes from one side. This is what Andrew Parker has <i>not</i> observed with his new 16 brick stove when he adopted the Lion stove air supply arrangement at the back to counter-flow the primary air.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Variations on the theme have attempted to limit the airflow and provide a proper balance between primary and secondary supplies to reduce the PM and incomplete gas combustion.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>And that, ladies and gentlemen, is, over a 30 year period, a (very) Short History of Stoves.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Regards<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Crispin<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div></div></div></body></html>