I recently acquired a used Vermont Casting Vigilant wood stove in almond. It needs repainting. My wife wants it an almond color, but I am concerned that the emissivity of the almond stove will be considerable lower than that of a black stove, yielding lower efficiency. Can anyone tell me how to figure the amount of loss I take in efficiency by using a light color for the stove and the chimney rather than flat black? (Or even just approximately what the difference would be?)
Black vs a light color
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Can't help you with the science invloved, but I can tell you that the heat's gonna get out of the stove into the room regardless of what color it is, and if I had to choose between some imagined optimum heat transfer property and a happy wife, it'd be no contest. I'd already have the almond paint on hand. :coolsmile: Rick
Just a side note...
The almond color is more than likely porcelain enamel and may not hold paint well without some sort of pre-treatment... but other than sanding with a very fine grit cloth or paper, I don't know what might be prudent.
Anyone else have experience painting over porcelain?
Peter B.
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The almond color is more than likely porcelain enamel and may not hold paint well without some sort of pre-treatment... but other than sanding with a very fine grit cloth or paper, I don't know what might be prudent.
Anyone else have experience painting over porcelain?
Peter B.
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Member Hotflame who hasnt' been around in a couple of years posted the science for a matte black surface:
"The total radiation emitted by the surface of a blackbody goes as the fourth power of its absolute temperature. Specifically, the relationship is,
R = (emissivity) * (Stefan-Boltzmann-constant) * (Absolute Temperature in Kelvin) ^ 4.
The emissivity is a property of the surface. Doesn’t matter in this case, 600 F is around 588 K, 700 F is 644 K.
So going from 600F to 700F, the total radiance will increase by (644/588)^4 = 1.44 times or 44 percent."
He didn't post the formula for almond.
"The total radiation emitted by the surface of a blackbody goes as the fourth power of its absolute temperature. Specifically, the relationship is,
R = (emissivity) * (Stefan-Boltzmann-constant) * (Absolute Temperature in Kelvin) ^ 4.
The emissivity is a property of the surface. Doesn’t matter in this case, 600 F is around 588 K, 700 F is 644 K.
So going from 600F to 700F, the total radiance will increase by (644/588)^4 = 1.44 times or 44 percent."
He didn't post the formula for almond.
fossil said:
Problem is, I think, that if it is not a black body, but has lower emissivity, it will take a higher temperature surface to radiate the same amount of heat, with the result that more of the heat goes up in the chimney and is lost.
Being finished in black doesn't make a stove a black body.BrotherBart said:
Drawing conclusions about the infra-red emissivity of a body based on its emissivity in the visual spectrum is bogus science. Anyway I digress: If the emissivity of the almond stove were indeed lower heat would be trapped in the stove. That trapped heat would cause a temperature increase which in turn would lead to greater radiance. In the end both stoves would radiate exactly the same, one through higher emissivity and lower temperature and the other through lower emissivity and higher temperature.
Ya know, I ain't no engineer (please hold your applause) but sitting here staring into the fount of all knowledge, a glass of beer, it would seem to me that the operative thing here is the color of the inside of the firebox, which is black. If that black surface absorbs the heat a coat of paint isn't going to keep it from coming out the other side no matter what color the outside of the stove happens to be.
Dix
Minister of Fire
It's quite funny really, the stove is lined with fire bricks that are 2" thick in an attempt to keep some heat in the firebox, and at the same time someone is worrying that a few microns of pigment is going to stop that heat escaping.BrotherBart said:
ChipTam
Burning Hunk
I'm no expert in this subject but, as a previous post mentioned, I would also suspect it would be difficult to apply another coat of paint over a factory-applied enamel coat. Also, you may be able to buy a heat resistant paint in some other color besides flat black but I've never seen it.
ChipTam
ChipTam
EddyKilowatt
Member
It is reasonable to give some thought to the emissivity of the paint, but the practical fact is that despite their visible-wavelength differences, light paint and dark paint behave nearly alike at infrared wavelengths.
Here's a handy chart of total infrared emissivities:
http://www.omega.com/temperature/Z/pdf/z088-089.pdf
This chart is published for infrared thermometry purposes, and the emissivity values for the various paints (p. 2) are around room temperature, meaning that they are for infrared wavelengths in the 10-micron range. (This is the peak emission wavelength of room-temperature objects.) A wood stove at 500F has an emission peak more like around 5 microns, but that is still different from visible wavelengths (0.4 - 0.7 microns) by a factor of ten... which often means that the infrared emissivity can be surprisingly different from the emissivity you'd guess based on visual appearance.
If you were looking at a stove made of polished metal, then you'd have reason to worry, since most polished metals have quite low infrared emissivities. My stove vents through a stainless stovepipe and my infrared thermometer reads this pipe way low, like by a factor of two, due to the low emissivity (and the fact that the thermometer is calibrated for everyday materials). As a side note, the phenomenon of the "hot tin roof" is based on the fact that the shiny roof absorbs some visible sunlight, but can't radiate it away due to the low infrared emissivity of the metal... thus the shiny roof which seems like it ought to stay cool can be nearly as hot as a black tarpaper roof in bright sun. And just to drag rocket science into it, space vehicles are pretty much limited to radiative cooling (and heating) by the vacuum in which they operate. The reason you see gold foil (think back to the Apollo vehicles) and other exotic materials on them is that the engineers are playing the hot-tin-roof game, trying to control the internal vehicle temperatures by balancing the visible and thermal emissivities of the various materials and pigments that they're covered with.
If you have an infrared thermometer and are a thorough kind of guy, you could do some testing of your proposed paints by painting a small patch of each somewhere inconspicuous on the stove (or even on a separate piece of steel), then heat the surface (e.g. fire the stove) and compare readings with your IR thermometer... my bet is that they'll be within a few degrees of each other, indicating very similar total emissivities.
Finally on a practical note, I have a Jotul with off-white porcelain (guess you could call it almond) and I must say that it throws heat like crazy. Much better than the above-mentioned stainless pipe, when both are at equal temperatures. I don't have a black paint patch on the stove (yet) to compare, but based on more decades of experience than I'd like to admit with black cast iron heaters, I'd say there's little to no difference in the heating ability of the white stove.
Eddy
Here's a handy chart of total infrared emissivities:
http://www.omega.com/temperature/Z/pdf/z088-089.pdf
This chart is published for infrared thermometry purposes, and the emissivity values for the various paints (p. 2) are around room temperature, meaning that they are for infrared wavelengths in the 10-micron range. (This is the peak emission wavelength of room-temperature objects.) A wood stove at 500F has an emission peak more like around 5 microns, but that is still different from visible wavelengths (0.4 - 0.7 microns) by a factor of ten... which often means that the infrared emissivity can be surprisingly different from the emissivity you'd guess based on visual appearance.
If you were looking at a stove made of polished metal, then you'd have reason to worry, since most polished metals have quite low infrared emissivities. My stove vents through a stainless stovepipe and my infrared thermometer reads this pipe way low, like by a factor of two, due to the low emissivity (and the fact that the thermometer is calibrated for everyday materials). As a side note, the phenomenon of the "hot tin roof" is based on the fact that the shiny roof absorbs some visible sunlight, but can't radiate it away due to the low infrared emissivity of the metal... thus the shiny roof which seems like it ought to stay cool can be nearly as hot as a black tarpaper roof in bright sun. And just to drag rocket science into it, space vehicles are pretty much limited to radiative cooling (and heating) by the vacuum in which they operate. The reason you see gold foil (think back to the Apollo vehicles) and other exotic materials on them is that the engineers are playing the hot-tin-roof game, trying to control the internal vehicle temperatures by balancing the visible and thermal emissivities of the various materials and pigments that they're covered with.
If you have an infrared thermometer and are a thorough kind of guy, you could do some testing of your proposed paints by painting a small patch of each somewhere inconspicuous on the stove (or even on a separate piece of steel), then heat the surface (e.g. fire the stove) and compare readings with your IR thermometer... my bet is that they'll be within a few degrees of each other, indicating very similar total emissivities.
Finally on a practical note, I have a Jotul with off-white porcelain (guess you could call it almond) and I must say that it throws heat like crazy. Much better than the above-mentioned stainless pipe, when both are at equal temperatures. I don't have a black paint patch on the stove (yet) to compare, but based on more decades of experience than I'd like to admit with black cast iron heaters, I'd say there's little to no difference in the heating ability of the white stove.
Eddy
He, he, he...I like this thread.
If I were a bet'in man, I would guess throwing a tooth pick into the fire will make more difference in the heat output of the stove, than the color of the paint. Just a bet.
If I were a bet'in man, I would guess throwing a tooth pick into the fire will make more difference in the heat output of the stove, than the color of the paint. Just a bet.
smokingolf
Member
I agree with Rick's first reply and Jags last reply. Who cares, what are we talking about .0001 degree difference in temps? I say clean it up first see how it looks and if the wife is happy, end of story, if not try painting it the color the wife likes or forever pay the price, if she dosnt like the color she won't like the stove.
Sorry, just my opinon.
Brian
Sorry, just my opinon.
Brian
johnsopi
Minister of Fire
Techstuf
New Member
I got a kick out of the following:
http://www.newton.dep.anl.gov/askasci/eng99/eng99320.htm
By the way....."This has nothing to do with black holes"
TS
http://www.newton.dep.anl.gov/askasci/eng99/eng99320.htm
By the way....."This has nothing to do with black holes"
TS
Dix
Minister of Fire
Dix
Minister of Fire
Dix
Minister of Fire
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