[Stoves] flames touching pot

[Frank Shields]

Dear Crispin, 

 

 

My GEK was painted bright fire engine red with some special paint. The paint changed color from red to gray-black around the lower area. What does that tell us? And can painted surfaces effect or be good indicators of how hot a surface is(?) or the e value? 

 

Great info –thanks

 

Frank

 

 

Frank Shields

Control Laboratories, Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 tel

(831) 724-3188 fax

frank at compostlab.com

www.compostlab.com

 

 

  _____  

From: stoves-bounces at lists.bioenergylists.org [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin Pemberton-Pigott
Sent: Wednesday, September 14, 2011 7:50 PM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] flames touching pot

 

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. 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.

 

Engineering toolbox says:

Values of ɛ

Steel Oxidized                                   0.79

Steel Polished                                   0.07

Stainless Steel, weathered          0.85

Stainless Steel, polished                               0.075

Stainless Steel, type 301                               0.54 - 0.63

Steel Galvanized Old                      0.88

Steel Galvanized New                    0.23

 

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.

 

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). 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.

 

We make bread ovens from stainless steel with a bright finish and the ɛ is 0.4 when it is new.

 

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.

 

If you want to try something interesting, make up a case with an ɛ, a temperature and an area. Then lower the ɛ. You will see the heat radiating drop proportionally. Then increase the temperature until the radiated heat equals the original case. You will be surprised. A low emissivity surface has to be a lot hotter to radiate the same energy as a high emissivity one. That is why cast iron stoves are painted and polished flat black. In real life the convective heat transfer increases with the temperature as well so it is not so straight forward to get accurate numbers.

 

Regards

Crispin

 

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