[Stoves] flames touching pot

Thu Sep 15 15:13:36 CDT 2011

Dear Frank

Good to hear from you.

During the early days of solar agitation (in the ‘70’s) it was discovered that one of the most effective colours for picking up (or radiating) solar insolation was a dull yellow. Big surprise. Some browns are also very efficient. They were not selective coatings such as are used in the evacuated glass pipes on Chinese coated solar water heaters.

Your bright red is probably a pretty good radiator, in spite of being slightly ‘white-ish’.

The main thing is to borrow an IR thermometer (which you in particular you should already have) and check the difference between the contact thermocouple reading and the IR reading with the emissivity set to 0.93 (most common default) or 0.95 (secondmost common).

You don’t really need to know the value, you want to know if there is a change, and in which direction.

I strongly suspect that your darkened paint will radiate more heat than the bright paint and the difference between the two readings will be reduced.

Remember that the heat convection from the same surface will be reduced if the radiation increases, and the surface temperature will drop. Don’t assume that if the dark area is cooler it is losing less heat.

This is either counter-intuitive or confuse-intuitive for most people. A higher surface temperature (with the same system insulation value) usually means a lousier heat loss. Low temperatures on the outside of a stove do not mean it is not losing heat, it may just be losing it very efficiently!

Always calculate…

If you buy an IR thermometer, make sure it has an adjustable emissivity setting, typically from 0.4 to 1.0 or 0.35 to 1.0. 

New galv metal has a very low ɛ value. Many small sheet metal stoves work much better when new than later because of this. They can’t get rid of heat easily when shiny. This leads to higher secondary air preheating and lower losses from radiation. Later the losses increase in both parameters.

For those who want to get rid of heat from hot surface (over 200 C) use a crosslinking silicone paint. It needs to be stirred a lot before use. Available in white and dark grey, it will survive 540 °C indefinitely. It is painted on and dries extremely quickly. Fresh black looks black. When heated over 200 is crosslinks, turns grey and is thereafter really hard to get off. It is not waterproof if painted on with a brush in only one coat. One brand name is HRA 540 from Dulux. We use it on the Basintuthu Stove and the guys making institutional baking ovens in Lesotho designed by Peter Scott switched to it once they were told it existed. Normal (enamel) black paint burned off quickly exposing things to rust.  In most cases you can use it on pot grates above the fire. It has the same heat resistant capabilities as silicone coated 600 degree electrical wire which is quite common so you know what to expect.

Regards

Crispin

From: stoves-bounces at lists.bioenergylists.org [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Frank Shields
Sent: 15 September 2011 13:34
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] flames touching pot

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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