Thermal Radiation (physics)

[bokehman]

I read somewhere on the forum (I can't find the thread now) that a stove at 880F will radiate double the energy compared to a stove at 600F.

Can anyone verify this or point me towards the law that governs it, or provide an interesting link?

 

[savageactor7]

WOW! 880F...never been there before.

 

[Martin Strand III]

Is "hotter" better?

Consider this (I paraphrased from the Tulikivi website sometime ago).

To understand heat transfer by radiation, realize that any matter with a temperature above Absolute Zero (0* K, or -273* F) gives off “infrared radiation”, not seen in the visible spectrum of light, but explained by quantum mechanics as a stream of extremely small photons having properties of waves and particles, maybe both, maybe alternating between the two.  So small are these photons that it is debated (when I was in school, please, update me) whether they are pure energy or particles, either travelling at phenomenal speeds.  When these photons collide with other molecules or particles of matter, they cause increased particle movement and more heat.

As mentioned, gases, like air, have relatively enormous amounts of space between the molecules of the gas.  Liquids have less space between molecules and solids even less (solids being generally more dense than liquids or a gas).  The tighter the molecules are packed together in a piece of matter, the easier it is to absorb any radiant heat photon which may strike it, making it warmer.  Conversely, in air, since the molecules are far apart making the chances of a radiant energy photon hitting the gas particle much smaller.

Hotter objects radiate photons of greater amplitude (like a sine wave) than cooler objects.  A photon from a very hot source has a greater chance of colliding with a particle of matter, like an oxygen or nitrogen molecule in room air, in a given distance than a photon from a cooler source.
 
This explains why a hot metal stove at 550* F tends to heat the air around it and induce convection air currents versus a cooler stove (aka masonry heater) at 180* F which will radiate photons with less amplitude, have less chance of colliding with air molecules over a given distance and can heat objects further away from the source than the hot metal stove. 

That’s the story and I’m stickin’ to it:  why radiant heat has been heralded as “more healthy” (like the heat from the sun) and the air remains still at a more even temperature than convection heat which causes draftiness, temperature zones, etc.

Aye,
Marty
Albert Einstein once said, “Nothing happens, until something moves.”

 

[DriftWood]

Stefan-Boltzmann Law of Radiation

Energy radiated per second:

................. 4
H = esAT

e = emissivity (0-1)
s = Stefan-Boltzmann constant
= 5.67 x 10-8 J/(s-m2-K4)
A = surface area of object
T = Kelvin temperature
{a very simple form of this equation }

The Stefan–Boltzmann law, also known as Stefan's law, states that the total energy radiated per unit surface area of a black body in unit time (known variously as the black-body irradiance, energy flux density, radiant flux, or the emissive power), j*, is directly proportional to the fourth power of the black body's thermodynamic temperature T (also called absolute temperature)

http://en.wikipedia.org/wiki/Stefan-Boltzmann_law

Radiation Calculation

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html#c3

Fahrenheit to Kelvin calculator Temperature Calculator

(broken link removed to http://www.indiana.edu/~animal/fun/conversions/temperature.html)

 

[ModMan]

880 F vs 600 F:

((880 +460)**4)/((600 + 460)**4) = 2.55

880 F radiates about 2.5 time more than 600 F.

 

[bokehman]

Stefan-Boltzmann Law of Radiation

Thanks for that.

I see that's working from absolute zero. Is there anything that takes ambient temp into consideration? Obviously a body starts to radiate as soon as it is above zero but if it is at the same temp as it's environment it is going to be in equilibrium, absorbing as much as it radiates.

 

[webbie]

The exact formula aside, this is pretty interesting info, because it says what we already know (but can now prove) - that a 700 degree stove throws off vastly more heat than a 500 or even 500 one.

 

[tuolumne]

We excercized this principle while heating our house with just a wood stove. By burning hot and hard, we can dump more heat into the house with less ash as well. This results in much less firewood consumption over the season than smaller, steadier burns. The down side is the greater temperature swings between burns.

 

[michaelthomas]

So from a functional point of view with the radiant heat colliding with an object and storing heat energy, What can you do with your installation to harness as much heat energy as possible?

 

[RonB]

So from a functional point of view with the radiant heat colliding with an object and storing heat energy, What can you do with your installation to harness as much heat energy as possible?

I would think if you want to take full advantage of the radiant properties of heat that you would need a radiant heating stove such as a soapstone, cast iron, or unshielded stove. My old Vestal was a 6 sided steel box where all surfaces provided radiant heat. (No wonder it heated so well). My new Quad, best as I can tell, has about 1 1/2 to 2 sides only that produce radiant heat. The remaining are shielded and produce convective heat. And yes, I can really tell the difference between the two.

 

[Martin Strand III]

The fly in the ointment here, and what seems contradictory, is that while a hotter stove surface temperature (850* F) puts out about double the total radiant energy compared to a cooler stove surface temperature (600* F) - for reasons given above - it's the cooler surface stove that is able to heat objects further away from it - since the amplitude of the radiant energy coming from the hotter stove heats more air particles thereby converting much of it's energy to convection heat.

This ties into a topic we've hammered on before [my $.02]:

“Heating efficiency of any wood heater depends on 2 factors:

(1) Combustion Efficiency - how completely it burns the wood.
[**Many wood burning stoves do not reach high enough burn temps in the fire box to achieve a clean hot burn - especially in those slow overnight burns when air is choked way down. Therefore, typically, metal stoves have a comparatively low combustion efficiency - mostly to prevent self-destruction; i.e., over firing, not to mention overheating the room, creating drafts, dry air to breathe, etc].

(2) Transfer Efficiency - how much of the fire’s heat gets into the room rather than going up the flue.
[**Because metal conducts heat rapidly, the fire must be controlled by regulating air intake or else over firing occurs. In our example above where 'double' the heat output can occur from burning your stove at 850* F compared to 600* F, the fact is, you usually can't take it. A metal stove over 500* F - 600* F is usually not so comfortable to be close to for long. Therefore metal stoves demonstrate high heat transfer characteristics. Further, the irony is (because the stove can't take it either) more complete burning of wood requires a consistent high temperature of around 1000* F to get most of the BTU's out of your $$ spent for the fuel.]

[So, we see a metal wood burning stove has lower Combustion Efficiency yet high Transfer Efficiency which is, unfortunately, the opposite characteristics of an ideal wood burning heater. But, this isn't your fault, or is it (see below)?]

Moreover, how efficient your wood heater operates depends on 2 more factors:
(1) Installation - location on outside v inside wall. Heater too big for house? Flue draw, chimney offsets? What's your home floor plan?

(2) Operation - Is wood green, wet or dry? Firebox load? Adequate air?

Your operating technique accounts for the largest variations in your woodstove’s heating efficiency.”

www.baaqmd.gov “Woodburning Handbook”

I'm not intentionally bashing metal wood burning stoves. I' love'em. I've had'em - inserts and free standing - and still have one. But there is a better mouse trap out there and I feel we need to implore the industry to come up with a more affordable wood burning heater (vs the currently available rather expensive masonry heaters) that can take the required sustained heat in the firebox (has high Combustion Efficiency) and demonstrates moderate heat Transfer Efficiency.

I've enjoyed my rant. Thanks for being out there.

Aye,
Marty

 

[bokehman]

A metal stove over 500* F - 600* F is usually not so comfortable to be close to for long. Therefore metal stoves demonstrate high heat transfer characteristics. Further, the irony is (because the stove can't take it either) more complete burning of wood requires a consistent high temperature of around 1000* F to get most of the BTU's out of your $$ spent for the fuel

When a stove top is at 500-600 degrees it's well over double that inside.

 

[bcnu]

This has been very interesting. Thinking about the inside stove temp - is there a thermometer available that you could place inside to get the temps? Nothing permanent - just to experiment with. Since my soapstone should only get to 450-500*F, am I working with the same general principle (or law). In other words what is the difference between say, a stove top at 350 compared to 450?

 

[bokehman]

is there a thermometer available that you could place inside to get the temps?

I use an infra-red thermometer. You can use it for everything: stove, stovepipe, glass and internal (with the door open for a couple of secs). Reading only takes a couple of secs.

 

[kevin fitzsimmons]

So from a functional point of view with the radiant heat colliding with an object and storing heat energy, What can you do with your installation to harness as much heat energy as possible?

I would think if you want to take full advantage of the radiant properties of heat that you would need a radiant heating stove such as a soapstone, cast iron, or unshielded stove. My old Vestal was a 6 sided steel box where all surfaces provided radiant heat. (No wonder it heated so well). My new Quad, best as I can tell, has about 1 1/2 to 2 sides only that produce radiant heat. The remaining are shielded and produce convective heat. And yes, I can really tell the difference between the two.

My new pe is far more efficient than the dutchwets noncat i shuffled off the hearth. Event though it burns better, i am noticing that i am having a hard time when we get cold snaps. The old stove was cast, new one is plate with heat shields on left and right sides. Could the shields be reducing its ability to heat the room? can they be removed when side clearances are not an issue? I bet once removed the plate steel box is not not pretty.

 

[Martin Strand III]

A metal stove over 500* F - 600* F is usually not so comfortable to be close to for long. Therefore metal stoves demonstrate high heat transfer characteristics. Further, the irony is (because the stove can't take it either) more complete burning of wood requires a consistent high temperature of around 1000* F to get most of the BTU's out of your $$ spent for the fuel

When a stove top is at 500-600 degrees it's well over double that inside.

Yes. Well, maybe. Even if so, so what?

It still is uncomfortable to be close to for long because of the high heat transfer properties of steel/cast iron.

It just seems to me that Mfgs of these things could spend some R & D on increasing the mass by adding more ceramic materials to some kind of metal framework inside. Done right, this would allow wide open air for the burn cycle (more complete combustion of wood fuel) and dampen down the heat transfer property of metal to give a more moderate and cooler surface temperature to lessen burns, decrease clearances, soften heat-cool cycling and indoor weather. The thermal mass concept is valid and works in masonry heaters but they are just too expensive for mass appeal. With all the energy consciousness these days, it seems us common folk should have a better even more economical heater suited to burning wood (an easily obtained carbon neutral renewable fuel in most places) extremely hot without choking down the air to heat our homes more efficiently and conveniently than the current wood burning varieties of the metal stove.

We need a better Mouse Trap.

Aye,
Marty

 

[precaud]

I believe this is the thread you were remembering:
https://www.hearth.com/talk/threads/5835/