[Stoves] Black radiating surfaces

[Thomas Reed]

Dear Crispin and all:

Most people don't realize the importance of radiation, but it increases 
as T^4, while convections and conduction only increase as T.  The black 
body radiation from any surface is given exactly by

W = 5.67 A E T^4

where W is in Watts/cm2, A is the area in cm2,  *T is (T in 
Kelvin)/1000, *E is the emissivity (1 for a black body or carbon, 0.1 
for silver and look up the emissivity for all others in the American 
Institute of Physics Handbook) and 5.67 is the Stefan Boltzman constant 
(5.67 Watts/cm^2-K^4 X 10^-12). 

If you use T in Kelvin you have to multiply by 10^(-12).  Most effective 
emissivities are closer to 1 than 0 (oxidized carburized etc.. surfaces). 

You can generally ignore any back radiation from nearby surfaces if they 
are cooler by more than 100 Cdue to the T^4 dependence. 

Hope this is as useful to you as it has been to me for the last 50years...
------------------------------------------------------------------------
Incidentally the new surface IR radiation thermomoters sold at  Harbor 
Freight ($10-$50)and other vendors  seem to be quite accurate unless you 
are looking at polished aluminum or stainless surfaces, in which case 
the mirror effect confuses the reading. 


TOM REED          BEF



Crispin Pemberton-Pigott wrote:
> Dear John
>
> Kevin raises an interesting point. What is the emissivity of a red hot black
> body?
>
> While agreeing with Philip, I feel it is also good to view this as a dynamic
> problem. By that I mean it is not just the red metal that we observe. We
> also see heat moving though the metal from one moment to the next.
>
> Suppose the emissivity of the steel drops when it is red.
>
> If you were able to increase the flow of air over the hot metal, the
> temperature would drop, increasing the coefficient of radiation. Also, if
> you were to use a metal with a lower thermal conduction coefficient, the
> amount of heat reaching the outside surface would decrease, similarly
> reducing the temperature and increasing the radiation coefficient. If the
> internal temperature was constant, heat would be retained.
>
> Conversely, given a fixed temperature inside the stove, the more heat you
> can conduct to the outside surface the higher the temperature and, after it
> goes red, the lower the radiation coefficient (emissivity). This means
> swapping mild steel for 3CR12 would decrease the outside temperature
> (presuming they were both the same colour).
>
> I have often suggested using 3CR12 for two reasons. The above is one. The
> other is that for any given amount of money, you can get a longer life from
> 3CR12 than from mild steel.  It is quite a bit stronger then mild steel so
> it can be thinner. The price relationship is about 1.5 to 1.8 times that of
> mild steel. For a 2.0mm piece of mild steel, you can substitute a 1.2mm
> piece of 3CR12.
>
> Its thermal conduction rate (metric numbers) is 0.23 against mild steel's
> 0.48. Slightly less heat will be conducted through the 1.2mm section, lower
> its temperature, perhaps keeping it 'black!
>
> Discussing this with Nigel for a minute, it seems that you might get more
> heat off the hot surface by putting an open ended vertical convection tube
> over the hot part to increase the air speed past the surface. This will pull
> off more heat than the radiation will, though it will deliver it in the form
> of hot air instead of radiation which may give the perception of 'not being
> as hot' to someone sitting nearby.
>
> I was given a stand-alone Peltier effect fan by my wife and it greatly
> increases the perception of heating because it blows hot air over the top of
> a stove and circulates it in the room.
>
> Regards
> Crispin
>
>
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