[Stoves] flames touching pot

[ajheggie at gmail.com]

On Thursday 15 September 2011 03:50:12 Crispin Pemberton-Pigott wrote:
> Dear Paul and AJH
>
>
>
> 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.
>
>
>
>
>
> That would be a bit high for stainless unless it is badly darkened.

Once they become discloured I doubt there is much difference between 
materials, the IR pyrometer I, which is only rated to 250C, doesn't 
discriminate much betwee common houshold surfaces and if I need them to 
be the same a thin layer of matt black paint suffices.
> 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.

I find the 316 used for flues goes matt brown if heated to dull red.
>
>
>
> Engineering toolbox says:
>
> Values of ɛ
>
> Steel Oxidized                                   0.79
>
> Steel Polished                                   0.07

Ok this one is essentially a mirror finish so would be low
>
> Stainless Steel, weathered          0.85

Which I would expect to be typical
>
> Stainless Steel, polished                               0.075

Agin a mirror finish
>
> Stainless Steel, type 301                               0.54 - 0.63

New?
>
> Steel Galvanized Old                      0.88
>
> Steel Galvanized New                    0.23

Agian silvery

In general a figure in the high 80s to 95 seem average.
>
>
>
> I have attached a photo of a fuel hopper made of 3CR12 which has a
> rough surface but is reasonably white. The ɛ is about 0.75 maybe less.
>
>
>
> 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.

Yes agreed ss is surprisingly different from mild steel and the low 
emissivity when still mirror finish can mean it is deceptive when hot. 
Also of course the poorere conductivity means the difference in 
temperature between the hot side and cool side is higher than mild steel 
of the same thickness.
>
>
>
> 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). 

You fail to indicatedthe outer temperature when glowing red like this? Yes 
the radiated heat goes up the the 4th power of the absolute temperature 
so radiative losses will be most significant in this scenario. What life 
do you expect in this ss?


> 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. 
I see all this except the low oxygen content, perceived wisdom is that ss 
has a corrosion advantage in non reducing atmospheres as these maintain 
the chromium oxide layer that resists oxidation of of the iron content. 
In a reducing atmosphere this oxide layer may be attacked and expose the 
iron molecules.

>
> For those who are not familiar with the term “ɛ” 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 ɛ * 5.6703*10-8* Temperature
> in Deg Kelvin * surface area in sq metres.

This formula confuses me, what is it measuring and what units are output?

The more genrrally used energy loss is the Stefan-Boltzman law which 
measures power loss in Joules from a surface and is the product of the a 
constant times the absolute temperature raised to the 4th power per m2. 
Differentiating this over time gives the power loss or transfer.


AJH