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Post by ringoism on Jan 23, 2016 5:54:52 GMT -8
Pinhead raised some interesting info on this point yesterday, but apart from one other brief comment of his, could find no earlier reference on this subject of emissivity (of materials) on the forum. A couple of informative / thought-provoking pages here: www.emi-coat.com/how_it_works.htmlwww.optotherm.com/emiss-increasing.htm (and related links on this page) What are the implications / applications for our stoves, whether they have masonry bells, cobb benches, or ferrous or other metal drums, when we're trying to control / minimize / maximize absorption and radiation in various parts of the system? In my case, I'm trying to build (maybe even mass-produce) a lightweight, portable stove with a small footprint for use in the small rooms typical here; so the question is one of how to maximize radiation while minimizing size/mass. Apart from the high-temp (1300C) industrial coating referenced, came across (for U.S.-based folks) RUST-OLEUM® 7778 BAR-B-Q-BLACK, being rated to 800°F (427°C). Not sure if this rating is sufficient for the upper portions of a primary drum, but lower parts and secondary drums should be okay. But anyway, all references I've seen to stove paints / oil-blackening or whatever only seemed to have physical appearance in mind, not thermal effects. Any thoughts / info / experience out there? At the least, I'm starting to see at least part of the reason that a hybrid traditional/rocket stove like Robert's (in Poland) could be so beautiful and so effective at the same time (ceramic coatings would provide very high emissivity value), maybe also why Matthew Walker noted (in the context of the D.C. competition a couple years back) how some well-designed box stoves observed there (iron/tile/soapstone/glass) were able to "throw off incredible amounts of heat". I can't even begin to grasp the physics of it all, but it gets me thinking. Thanks in advance, Eric
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Post by Deleted on Jan 23, 2016 7:10:16 GMT -8
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Post by hermetic on Jan 23, 2016 11:36:08 GMT -8
This is true Karl, but as the basic physics experiment shows, a matt black surface radiates heat faster than a polished silver one so coatings can have an effect if the heat is there to radiate in the first place.
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Post by ringoism on Jan 24, 2016 11:51:00 GMT -8
Seems that would depend on the surface quality of the steel. (Alleged) Emissivity Coefficient for "average" oil paint: 0.94; for fresh rolled steel (i.e., freshly paint-stripped 55-gal drums): 0.24!?; oxidized steel: 0.79; 301 Stainless: 0.60; Aluminum 0.09(!)etc. So unless your steel is optimized (a bit roughened or rusty or else coated somehow) it doesn't seem it could do as well as something with the special paint. Forgive me for being dense (I confess ignorance of some of the basics here), but are you saying that basic convective delivery (transfer) rate from gases is so low that high/low conductivity in the drum doesn't then matter much? The paper bag point was that a thin, insulative material still transfers (conducts) plenty and that an aluminum drum's higher conductivity would be of no real benefit? If so, point well taken, thanks for that and the links re: convection. I think I remember someone saying that above 100C convection had increasing effect vs. radiation (emissivity), I guess since the heat "waves" around the metal then begin to create air movement and do more of the mixing / transfer? The lower sides of my existing drum are surely below 100C (really am going to have to get a temp probe, for starters), meaning, perhaps, that radiation is a bigger player there, and mainly these lower temp areas could benefit from greater emissivity (coatings / surface prep inside and outside the drum)? Or would the fact of lower temp differential to ambient there again render it practically irrelevant? If this 100C (or whatever figure) is a transitional point, then are polished surfaces essentially heat reflectors (as Pinhead mentioned) only at temps lower than that, and more able to transfer heat once convection kicks in with higher temps? Meaning: Might I still get the (partially?) polished-metal glam stove I want, and have it actually heat a room? I'm feeling a little lost here. I don't want to limit the question to my current concern, either. I mean, if this coating is good to 1350, it could even work in risers / tunnels. Hard to imagine significant benefits, but then I'm also having trouble grasping exactly how it benefits a kiln/furnace as the example on the page showed, either. -Eric
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Post by Deleted on Jan 25, 2016 6:02:19 GMT -8
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Post by ringoism on Jan 25, 2016 8:35:03 GMT -8
All your points make sense to me except the last one. I thought that a higher surface temp would indicate a higher differential between it and ambient air, and thus if anything a potential for MORE transfer, not less. Clearly there are many things I'm not understanding here. Would read the technical paper, which looks like a good one, but it seems it costs $35 - I can only get the first page (abstract) to come up there. Optionally, I could enroll in a full semester Master's level course at what I guess is associated with the Free University of Brussels, which sounds compelling (if only I lived nearer Brussels): www.vub.ac.be/en/study/fiches/33215/heat-transfer-and-combustion Evidently it's not an essentially simple topic. I could give up the shine if need be, but still am tempted towards non-polished stainless (301 showed relatively high emissivity on the chart or can check with suppliers for something of a little rougher finish) for the sake of appearance, specifically rust-avoidance (I want these stoves to be good for around ten years, and fairly maintenance-free; and at least one forum member says his stainless drum radiates just fine). -Eric
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Post by Deleted on Jan 25, 2016 9:24:13 GMT -8
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Post by keithturtle on Jan 25, 2016 22:57:30 GMT -8
Higher surface temperatures on both sides on the other hand mean less heat can be harvested. OK then, this addresses my one of my current projects. I want to create a passive solar collector to heat air. My question takes the heat in the other direction. So, if I have available two materials (Copper sheet and aluminum sheet) both painted black on the sunny side, which one can I extract the most heat from by passing air underneath the sun-heated surface? Is the higher emissivity # the one that sheds the most heat into the adjacent environment? Thanks for posting those links, karl Turtle
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Post by eng on Jan 25, 2016 23:37:10 GMT -8
Gentlemen, Here is my penny worth. Radiators emit the same amount of heat whatever colour but emit less heat if painted with a metallic paint. Source. The Students Textbook of Heating Ventilating & Air Conditioning by L.M. Miller 1976. Highly polished surfaces emit or tansfer less heat to your hand. This is why coal ranges had polished handles. Solar water or air heaters should be painted with a special absorbing paint developed by the Israelis and others in the 1970's. I have never found a source for this and use black chalk board paint or mat black. I have been tempted to add some fine carbon powder but have not done so.
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Post by Deleted on Jan 26, 2016 5:18:29 GMT -8
keithturtle:
Emissivity does not play a significant role in heat transport from a solid to a transparent media. Maybe indirectly by heating up an opposite wall, which could heat the media by convection.
To take advantage of an opposite wall you would have to paint black both sides of the metal sheet and the opposite wall.
Under the conditions of your solar collector virtually only carbon dioxide and water vapour can absorb or emit small amounts of radiant heat.
Your blackened sheets have a serious disadvantage: They will throw back a large amount of the absorbed sun energy as radiant heat.
It would be much better to use a special glass, which lets the sunlight pass to heat a black surface, but reflects the radiant heat back.
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Post by keithturtle on Jan 26, 2016 10:33:11 GMT -8
It would be much better to use a special glass, which lets the sunlight pass to heat a black surface, but reflects the radiant heat back. Ah yes, those borosilicate evacuated tubes come to mind. You've saved me much effort karl, thank you. Back to splitting wood for now Turtle
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Post by daniel on Jan 29, 2016 8:04:48 GMT -8
keithturtle, Copper has a higher conductivity so it will transfer the heat better from one side to the other than aluminum but when we say heat we must distinguish between 3 different types of heat transfer forms. Radiant, convective and conductive. Radiant heat is very different from the other two and sun is giving out a lot of radiant heat in a certain spectrum which will pass more or less through a metal according to its emissivity :http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html These emissivity is different if the aluminum or copper is highly polished so it reflects back a lot of that heat or is heavily oxidized . In your case you want all three forms of heat transfer to occur to transfer heat to the molecules in the air (which by the way is not a very good heat transfer medium as it is with much lower density than water for instance) So basically oxidized copper or rough surface on the sunny side and shiny on the interior might be best in this situation. I suggest you look up thrombe walls for radiant heat accumulation. It would work like that; sun heats up some concrete or rocks on the other side of a double pane window which will allow the mass to be heated up and store radiant heat and will be insulated from the exterior. Beside the radiant heat that will be transmited to the interior after sundown you could also have convection heat by the colder air at the bottom entering this cavity and exiting warm at the top in slots created for that purpose.
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Post by russellcollins on Oct 28, 2019 0:29:56 GMT -8
Is this thread still active? I am seeking a surface coating for our range of Rocket Stoves made in India that meets the following criteria:
1. High Emissivity 2. High hardness at temps up to 600C 3. Non toxic for direct food contact 4. Non toxic with regards to off gassing 5. Able to be applied in a facility in India (where we produce).
Does anyone have any tips on what coatings are being used on commercial combustion heater products?
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