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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dear Frank<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I will use Deans rather helpful list as a starting point to expand the topic and perhaps provide additional insight.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div><div><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span lang=EN-US><span style='mso-list:Ignore'>1.)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span lang=EN-US>Air is very light and by volume does not hold much heat. <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Air has a low thermal capacity and once its temperature has dropped to near what the pot temperature is, there is nothing to gain by holding it in contact with the pot. If the heat needs more time to transfer into the pot, the air velocity (gas velocity actually) should be decreased. You can check with a thermometer to see what the exit temperature is. You will recall I have often mentioned the chimney temperature for a chimney stove. This is <i>not</i> the same thing. That has to do with draft and pulling the air effectively. For a small cooking stove the exit temperature factored by the excess air quantity determines the thermal efficiency. In other words, if you have a lot of excess air cooling the gas stream, you will get a lower exit temperature but that does not mean automatically it is getting the heat into the pot more efficiently. Just keep that in mind.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US style='color:#1F497D'>…</span><span lang=EN-US>So a lot of hot air needs to contact a surface to get it hot. <span style='color:#1F497D'><o:p></o:p></span></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I believe that taken with the first line, this is a misconception. There is as I mentioned previously, the <i>heat transfer rate</i>, which means heat getting into the pot to make the water temperature rise rapidly. Then there is the <i>heat transfer efficiency</i> which is the effectiveness which any parcel of hot gas cools against the pot. It get a high transfer efficiency, you need to give the gas time to cool against the pot. To heat the pot rapidly, you need to move lots of gas through that space. The combination of volume x temperature x time to cool against the pot determines two things: the overall heat transfer efficiency, and the overall quantity of heat that is transferred. It is important to keep in mind the difference between the rate of heating and efficiency of transferring that heat from a parcel of gas.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US style='color:#1F497D'>…</span><span lang=EN-US>Slowing down the air generally decreases the heat transfer for this reason. <span style='color:#1F497D'><o:p></o:p></span></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dean is saying that the total amount of heat transferred per second (the quantum of heat) decreases, not that the % efficiency of the transfer drops. The efficiency of the transfer of any quantity of heat increases if you give it more time to make the transfer. Transferring ‘more heat’ to a pot in 1 second does not mean it was ‘more efficient’. That is the difference between rate and effectiveness of the heat transfer.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='color:#1F497D'><o:p> </o:p></span></p><div><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span lang=EN-US><span style='mso-list:Ignore'>2.)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span lang=EN-US>The boundary layer of still air is punctured more effectively by high velocity hot air that heats the molecules near the pot surface and replaces them as they cool with new hot molecules.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>There isn’t really a boundary layer of ‘still air’ except for mathematical convenience. The term ‘layer’ is used but it applies to things like aircraft and missiles and things travelling at high speed. For convenience when making heat transfer calculations, it is imagined that there is indeed a ‘stationary layer of air’ next to the surface which is only conductive – in other words it has a heat conduction capacity and no convective movement. <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The standard thickness use to make such a calculation is 0.1 mm. If you are calculating the heat transfer from a turbulent gas flow under a pot, that theoretical 0.1 mm of still air can be considered to be ‘not moving’ but this is for convenience only. It is in fact moving and even at the molecular level, there is no stationary air. It all moves but at a varying speed. There are plenty of charts on the internet showing the distance-velocity relationship. The same principle applies to liquids as well, and the same formulas are used for both (air is a ‘fluid’).<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The transfer of heat from a flying missile is very much unlike the transfer of heat to the bottom of a pot so I will explain why the ‘scrubbing’ idea is misleading, even though Dean likes it. Dean, I hope you and Damon are reading this and then you talk about it with some math on paper because it is important to include in your new book.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Hot gases travelling under a pot have two effect at play: the forces driving the gases sideways tending to make a reasonably well-mixed gas flow with the temperature about the same everywhere, with the stream of gas being cooled on the top side by the (relatively) cold pot and warmer underneath where it is no cold, then re-mixing. That is one effect and the ‘boundary layer’ idea comes from the similarity between this and a missile or jet plane flying through the air with (often) clearly defined temperature and velocity profiles.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The second effect is buoyancy, or the effect of hot gases rising if they are in a cooler gas. This effect dominates cooking but is completely absent in a flying missile. With aircraft, the velocities are high and there is no real transfer of heat – the skin of the missile is in equilibrium with the air around it and no heat transfer is taking place once that equilibrium is established. There is no water being boiled inside the missile. The effect of ‘hot air rising’ so to speak, is that hot molecules rise quite forcefully in the gas stream impinging on the bottom of the pot and transferring heat to it. This takes place mostly in the 1-2mm space immediately below the pot. The gases in this space are moving vertically, rapidly. This is not a ‘boundary layer’ it is just the vertical space in which the heat loss is happening. This cooling zone was shown experimentally by Dale Andreatta, if you recall his paper on it subject.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>So the question is, which is the more effective force: the horizontal movement tending to keep things well mixed, or the hot air rising that brings hot molecules into contact with the pot? The best effect of hot air rising would be to have all of the hottest molecules rapidly rise to the top where the pot is, and the cooler ones to fall down, even though there is a sideways flow which is probably pretty turbulent. Which is the stronger effect?<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I put this question to Prof Chris Snow from the University of London (Mechanical Engineering Dept). He calculated for me on a single sheet of paper a comparison between the tendency to maintain laminar flow horizontally and the buoyancy effect tending to break that laminar flow and send the hottest molecules to the top. We used gas speeds of 100 to 200 mm per second as the region of interest.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The long and short of the calculation is that the buoyancy effect is 30 times the laminar flow effect. This means that there is nearly no effect at all of ‘insulating with a boundary layer’. Any ‘insulating layer’ forming is immediately broken up by the rising hot molecules clawing their way through the gases trying to get above the cooler ones. The movement is very rapid and involves a lot of collisional energy transfer. The overall effect however, is to break up any tendency to form a non-heat transferring layer of gases, with an overpowering energy ratio of 30:1. Under a pot there is no such layer.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>This effect is not present on a missile or on the side of a pot. It only applies to the underside of pots or other heated surfaces. Because the effect is so strong, gases under a pot can be used as an insulator to stop heat losses downwards. By deepening the space under the pot the amount of heat transferred (by ‘convective heat transfer’) downwards can be limited and the overall efficiency raised. In this case the space below the pot is enlarged and the designer is counting on the depth to get the bottom cooler and leave the top just as hot (because heat still rises). This design was CFD modeled by a company in Cape Town and the design used in the BP/Arivi paraffin stove – a novel product with a very high heat transfer efficiency. This is one of the proofs that the buoyancy effect is stronger than the laminar flow/insulating effect.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span lang=EN-US><span style='mso-list:Ignore'>3.)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span lang=EN-US>Extending the time that hot gases flow next to the pot is good. Make the skirt length longer. Don't slow the flow.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>This cover two points: extending the heat contact time increases heat transfer efficiency, and possible interference with the gas flow. Dale’s experiments were done using a Rocket Stove type product that had a simulated fire (using gas). Dale showed and reported that the sensitivity of the pot-skirt’s radial distance was low. In other words, the gap between the pot and the skirt was not really very important. The main problem (or benefit) arose when the gap was so small that it began to constrict the flow of gases and affect the way the fire burned. Rocket stoves, as we all know by now, tend to have a high excess air ratio (typically between 800 and 2000% if you follow their design manual – Dean, I hope you are going to fix that). The ideal for a wood fire is about 100% so there is typically a great deal more air going through the stove than is needed. If you put a tight skirt around a pot and seal it to the upper surface, you can use the skirt as an air controller. This was shown repeatedly by Aprovecho when they reported that there were significant changes to the efficiency when making small changes to the pot-skirt distance at the range of about 8mm. <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>What was happening was the skirt was choking the fire effectively enough to reduce the excess air without interfering with the combustion efficiency – might even have improved it. Peter Scott and others tried a lot of different gaps and found that at certain critical gaps the stove worked most effectively. This is not the effect of ‘scraping stationary air’ off the pot, it is the effect of controlling the largely uncontrolled air flow through a Rocket stove. You can accomplish the same thing by reducing the size of the fuel hole, adding preheating & descending counter-flow primary air (as per the Lion Stove) or choking the combustion chamber by reducing its diameter near the top, or reducing the pot-stove gap (lowering the pot). Any of these has the same effect – to limit the air flow to what is needed. Lowering the excess air flow also increases the gas temperature leading to a higher heat transfer efficiency and thus a higher heat transfer rate for any given fuel burn rate.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dale’s work clearly showed that the pot-skirt gap could be 10 or 20mm and have nearly no effect in the heat transfer. If the gap was wider, the flow rate was slower. If it was narrower, the flow was faster. The overall difference was hardly detectable, unless the gap constricted the whole gas path. If someone tells you that the skirt has to be a very particular gap, you can rest assured that the stove already has too much excess air passing through it and the skirt is being used to compensate for that design fault.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span lang=EN-US><span style='mso-list:Ignore'>4.)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span lang=EN-US>The most effective heat transfer technique is to decrease the channel gap until velocity of very hot gases starts to diminish.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Correct. I reiterated this in detail above. If the stove <i>already has the correct air flow</i> then the skirt can be run to that limit. If you have a stove with high excess air (an O<sub>2</sub> level in the exhaust above say, 13%) you have to take a good look at what to do about getting it reduced. A tight skirt is not a good choice.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span lang=EN-US><span style='mso-list:Ignore'>5.)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span lang=EN-US>A smaller fire creates less hot gasses that can flow successfully through a quite narrow channel (5mm-6mm) resulting in generally higher theoretical thermal efficiency.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dean is confirming the point that a high heat transfer efficiency can be obtained by having a longer ‘residence time’ in contact with the pot. The more time, the better the transfer. When designing an air-to-air heat exchanger or a water boiler, the residence time is a major consideration. The physical orientation also plays a role. If one were to compare a series of horizontal channels like a stack of dinner plates with a vertical array like a set of dishes in a dishwasher, there is a different ratio of forces at play. If the surface area is limited (a pot) and the surface is vertical, a small gap might be most efficient at transferring heat, but not if the velocity is so high as to create a ‘gain’ for laminar flow ad a ‘loss’ for buoyancy. Speed it up enough and the hot gases will zip through the laminar flow in the centre of the gap. We induce this effect deliberately on the SeTAR BLDD Coal stove to use combustion gases insulate the combustion channel walls without resorting to an insulating liner as Charlie has just done. We do it by spinning the gases to create a hot central punch-through combustion zone. In that state, the buoyancy effect is almost nil.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span lang=EN-US style='color:#1F497D'><span style='mso-list:Ignore'>6.)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span lang=EN-US>Larger diameter pots have an advantage because narrow channel gaps add up to bigger constant cross sectional areas. <span style='color:#1F497D'><o:p></o:p></span></span></p><p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Here Dean is referring to a gap under the pot – the ‘narrow channel’ being the gap between the (horizontal) bottom of the pot and the top of the stove body. As described above, CFD modeling shows this is not the case. It is not necessarily more efficient to have a small gap based on the theory that the velocity is higher and therefore there is a ‘scrubbing’ action taking place. (The capacity to disrupt the gases near the pot is already completely overwhelmed by the buoyancy effect.) <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>There are in any case two additional problems with that idea. The first is that the velocity is not constant in a constant area profile because the gas is cooling and shrinking in volume. If one were to hold that a ‘constant velocity gives maximum heat transfer’ one would make a profile that took into consideration the temperature of the gases as they cooled under the pot. The gap would be much smaller at the periphery of the pot (about ½ the ‘constant area’ value) and it would not be a straight tapering cone.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>But suppose we used a straight tapering cone with a gap based on the combustion chamber area and the pot diameter. The calculation is done in degrees Kelvin in order to use the universal gas law which holds that there is a linear relationship between temperature an volume. <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Let’s use typical numbers: Input 1073 degrees, output 548 degrees, 10 cm combustion round chamber, 30 cm diameter pot.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Initial cross sectional area = 100 cm<sup>2</sup> at 1073 degrees<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Final cross sectional area = 548/1073 x 100 = 51 cm<sup>2</sup> (This compensates for the shrinkage in gas volume) <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Initial pot clearance at the combustion chamber lip = 100 cm<sup>2</sup>/(10 cm x </span><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>ð)</span><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> = 3.2 cm vertically<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Pot clearance at the lip = 51 cm<sup>2</sup>/(30cm x </span><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>ð) = 0.54 cm vertically</span><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Compare this with an ‘equal area’ calculation of 100 cm<sup>2</sup>/(30 cm x </span><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>ð)</span><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> = 1.1 cm for the vertical gap at the pot edge. Quite a difference.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>If the theory of scraping gases off holds, then the drop in the height at the outer edge of the pot should be calculated considering the drop in the gas temperature. The modeling of the Arivi and in fact the proof with the physical design shows that none of this applies – not Larry’s constant cross section nor my constant velocity. In fact the best heat transfer occurred when most of the gap for the Arivi was deeper than the nominal diameter of the combustion chamber! Why? Because buoyancy overcomes all need to disrupt laminar flow. The cooler gases acting as an insulator gave more net gain than loss for ‘architectural’ reasons. I hasten to add that the gap at the outer edge of the pot was quite small – about 8mm. <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Some of you may have seen the chart of thermal efficiency I circulated a few weeks ago. The highest heat transfer efficiency (93%) was achieved with quite a large gap, low velocity and small flame. Why so high? Lots of contact time under a horizontal pot at a very low speed with insulating cool gases under the pot, trapped by a metal shield. Interesting. No disrupting from speedy gases taking place there.<o:p></o:p></span></p><p class=MsoNormal><sup><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></sup></p><p class=MsoListParagraph><span lang=EN-US>Larry's rule of thumb, when beginning a design, is to maintain equal cross sectional area throughout the entire stove/pot. <span style='color:#1F497D'><o:p></o:p></span></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>There is no scientific basis for this rule of thumb, even though I like Larry a lot. I like Sam Baldwin too – both are nice guys. The idea is wrong. There is nothing to support the idea that the cross sectional area through a stove should be constant. Stoves built to this plan and which have an air entry at the bottom and an exit at the top <i>almost invariably</i> have far too much excess air passing through the stove and have an overall cooking efficiency topping out at about 27%. Peter Scott’s rather nice bread baking stove built in Lesotho on that basis were immediately improved by dropping the constant area idea and going with a constant velocity. It worked not because of any pot scraping effect, but because the heat transfer is mostly on vertical surfaces and not so dominantly from the bottom. I believe part of the improvement resulted from accidentally decreasing the excess air at the same time by choking.<o:p></o:p></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The added control for secondary air purposes of having the restriction on the air flow being at the entry point cannot be over-emphasized. We were discussing here a few days ago the need for secondary air injection with the correct amount of draft (I think it was here). This is best accomplished by having plentiful draft available at the injection point (suction) rather than having a choke point at the exit with the secondary air port under slightly positive pressure. In the latter case one is very dependent on getting the hole sizes exactly right, and even then, it only holds for a certain fire power level. Choke on inlet, pull secondary air in. COI-PSA.<o:p></o:p></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph><span lang=EN-US>Sam Baldwin has a chart on page 48 of his book (Biomass Stoves:) showing firepower/channel gap/length of channel gap/thermal efficiency.<o:p></o:p></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>That if course only holds for those stoves operating on that fuel under that power level. There is no general case one can make on the subject, just as there is no such thing as ‘typical emissions’ of PM from fuels. (PM is a result of the stove's ability to deal with the fuel, or not.)<o:p></o:p></span></p><p class=MsoListParagraph style='margin-left:0mm'><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US>7.) Use radiation, too!<o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>If heat is being lost to the stove body and not recovered (recycled) or otherwise desired, it is lost. Calculations shown here on this list regarding the radiant wire gauze that Paul Olivier tried indicate quite clearly that the maximum one gets from radiant heat (in that example) is low – about 14%. If you are only getting 5% then you have something to gain. If you are losing 10% because you have high excess air, a little radiation is not going to save your design. If you are losing 50% from high excess air, getting 100% of the available radiation is not gonna save you. I have a heat exchanger at the house which is 92-93% efficient and it has nearly no radiant heat involved at all. Grab it if you can; don’t sweat the small stuff.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Best regards to all<o:p></o:p></span></p><p class=MsoNormal><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><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>PS Buy a combustion analyser, a thermometer and a scale<o:p></o:p></span></p></div></div></div></div></body></html>