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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><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;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;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;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;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;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;color:black'>ɛ,</span><span style='color:black'> a temperature and an area. Then lower the </span><span style='font-size:14.0pt;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>