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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='color:#1F497D'>Dear Frank<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Good to hear from you.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>During the early days of solar agitation (in the ‘70’s) it was discovered that one of the most effective colours for picking up (or radiating) solar insolation was a dull yellow. Big surprise. Some browns are also very efficient. They were not selective coatings such as are used in the evacuated glass pipes on Chinese coated solar water heaters.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Your bright red is probably a pretty good radiator, in spite of being slightly ‘white-ish’.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>The main thing is to borrow an IR thermometer (which you in particular you should already have) and check the difference between the contact thermocouple reading and the IR reading with the emissivity set to 0.93 (most common default) or 0.95 (secondmost common).<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>You don’t really need to know the value, you want to know if there is a change, and in which direction.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>I strongly suspect that your darkened paint will radiate more heat than the bright paint and the difference between the two readings will be reduced.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Remember that the heat convection from the same surface will be reduced if the radiation increases, and the surface temperature will drop. Don’t assume that if the dark area is cooler it is losing less heat.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>This is either counter-intuitive or confuse-intuitive for most people. A higher surface temperature (with the same system insulation value) usually means a lousier heat loss. Low temperatures on the outside of a stove do <i>not</i> mean it is not losing heat, it may just be losing it very efficiently!<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Always calculate…<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>If you buy an IR thermometer, make sure it has an adjustable emissivity setting, typically from 0.4 to 1.0 or 0.35 to 1.0. <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>New galv metal has a very low ɛ value. Many small sheet metal stoves work much better when new than later because of this. They can’t get rid of heat easily when shiny. This leads to higher secondary air preheating and lower losses from radiation. Later the losses increase in both parameters.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>For those who want to get rid of heat from hot surface (over 200 C) use a crosslinking silicone paint. It needs to be stirred a lot before use. Available in white and dark grey, it will survive 540 </span><span style='color:#1F497D'>°</span><span style='color:#1F497D'>C indefinitely. It is painted on and dries extremely quickly. Fresh black looks black. When heated over 200 is crosslinks, turns grey and is thereafter really hard to get off. It is not waterproof if painted on with a brush in only one coat. One brand name is HRA 540 from Dulux. We use it on the Basintuthu Stove and the guys making institutional baking ovens in Lesotho designed by Peter Scott switched to it once they were told it existed. Normal (enamel) black paint burned off quickly exposing things to rust. In most cases you can use it on pot grates above the fire. It has the same heat resistant capabilities as silicone coated 600 degree electrical wire which is quite common so you know what to expect.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Regards<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Crispin<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0mm 0mm 0mm'><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> stoves-bounces@lists.bioenergylists.org [mailto:stoves-bounces@lists.bioenergylists.org] <b>On Behalf Of </b>Frank Shields<br><b>Sent:</b> 15 September 2011 13:34<br><b>To:</b> 'Discussion of biomass cooking stoves'<br><b>Subject:</b> Re: [Stoves] flames touching pot<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>Dear Crispin, <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>My GEK was painted bright fire engine red with some special paint. The paint changed color from red to gray-black around the lower area. What does that tell us? And can painted surfaces effect or be good indicators of how hot a surface is(?) or the e value? <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>Great info –thanks<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>Frank<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><o:p> </o:p></span></p><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>Frank Shields</span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>Control Laboratories, Inc.</span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>42 Hangar Way</span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>Watsonville, CA 95076</span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>(831) 724-5422 tel</span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'>(831) 724-3188 fax</span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><a href="mailto:frank@compostlab.com">frank@compostlab.com</a></span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial","sans-serif";color:navy'><a href="http://www.compostlab.com">www.compostlab.com</a></span><span lang=EN-US style='color:navy'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='color:navy'> <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='color:navy'> </span><span lang=EN-US><o:p></o:p></span></p></div><div><div class=MsoNormal align=center style='text-align:center'><span lang=EN-US style='font-size:12.0pt;font-family:"Times New Roman","serif"'><hr size=2 width="100%" align=center></span></div><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> stoves-bounces@lists.bioenergylists.org [mailto:stoves-bounces@lists.bioenergylists.org] <b>On Behalf Of </b>Crispin Pemberton-Pigott<br><b>Sent:</b> Wednesday, September 14, 2011 7:50 PM<br><b>To:</b> 'Discussion of biomass cooking stoves'<br><b>Subject:</b> Re: [Stoves] flames touching pot</span><span lang=EN-US style='font-size:12.0pt;font-family:"Times New Roman","serif"'><o:p></o:p></span></p></div><p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p><p class=MsoPlainText>Dear Paul and AJH<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>AJH >I don't for sure but dull red is around 600-700C for a black body, I'd guess stainless would be around 0.9 emissivity , so a bit hotter, of course the outside is losing heat by radiation, conduction and convection so the other side of the steel can be hotter.<o:p></o:p></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>That would be a bit high for stainless unless it is badly darkened. Before jumping in with a number I wondered if you had checked a few values with a surface contact thermocouple. The low chrome stainless steels darken quite a bit at high temp so the surface finish dominates (it can still be shiny). Discoloured 400 series is still pretty low.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Engineering toolbox says:<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Values of </span><span style='font-size:14.0pt;font-family:"Lucida Sans Unicode","sans-serif";color:black'>ɛ</span><span style='color:black'><o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Steel Oxidized 0.79<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Steel Polished 0.07<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Stainless Steel, weathered 0.85<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Stainless Steel, polished 0.075<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Stainless Steel, type 301 0.54 - 0.63<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Steel Galvanized Old 0.88<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Steel Galvanized New 0.23<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>I have attached a photo of a fuel hopper made of 3CR12 which has a rough surface but is reasonably white. The </span><span style='font-size:14.0pt;font-family:"Lucida Sans Unicode","sans-serif";color:black'>ɛ</span><span style='color:black'> is about 0.75 maybe less.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>I am speaking up on the subject because it can be quite dangerous for children if there are stainless steel components of a stove that can be touched. They have such difficulty getting rid of heat that the temperature is very high before it does so. On the other hand the inability to radiate away significant heat and the poor conduction coefficient (about ½ that of mild steel) means stainless is a pretty good insulating and therefore combustion chamber material if it is not further insulated on the back.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>In the attached photo of a BLDD5.2 from the SeTAR Centre the temperature inside is far above the outside surface in spite of the significant heat radiating away from the outside (making it visible to the camera). So it has the advantages of being strong, corrosion resistant, hot and insulating with a poor radiation coefficient. This combination works best when the whole system is small and when the oxygen level in the hot region is low.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>We make bread ovens from stainless steel with a bright finish and the </span><span style='font-size:14.0pt;font-family:"Lucida Sans Unicode","sans-serif";color:black'>ɛ</span><span style='color:black'> is 0.4 when it is new.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>For those who are not familiar with the term “</span><span style='font-size:14.0pt;font-family:"Lucida Sans Unicode","sans-serif";color:black'>ɛ</span><span style='color:black'>” it is used in the formula that calculates how much heat radiates away from a surface into the surrounding environment. It is called ‘grey body radiation’ because things are not truly black. The formula is </span><span style='font-size:14.0pt;font-family:"Lucida Sans Unicode","sans-serif";color:black'>ɛ</span><span style='color:black'> * 5.6703*10<sup>-8</sup>* Temperature in Deg Kelvin * surface area in sq metres.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>If you want to try something interesting, make up a case with an </span><span style='font-size:14.0pt;font-family:"Lucida Sans Unicode","sans-serif";color:black'>ɛ</span><span style='font-size:14.0pt;color:black'>,</span><span style='color:black'> a temperature and an area. Then lower the </span><span style='font-size:14.0pt;font-family:"Lucida Sans Unicode","sans-serif";color:black'>ɛ</span><span style='color:black'>. You will see the heat radiating drop proportionally. Then increase the temperature until the radiated heat equals the original case. You will be surprised. A low emissivity surface has to be a lot hotter to radiate the same energy as a high emissivity one. That is why cast iron stoves are painted and polished flat black. In real life the convective heat transfer increases with the temperature as well so it is not so straight forward to get accurate numbers.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Regards<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Crispin<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p></div></body></html>