High emissivity coatings to improve radiant heat from ultralight tent stoves

This post describes the effect of surface emissivity coatings for ultralight tent stoves that can be used for comfort and cooking while backpacking camping while winter trekking.

Introduction

I consider my tiny tent stoves to be ‘heat radiators’ rather than tent air heaters, although they do both. Consequently, this post focuses mainly on factors related to the optimization of radiant heat rather than conducted or convected heat. “In my experience, with tent stoves, the air in the top of a small tent can get unpleasantly hot. This is possibly good for drying gloves and socks and keeping the tent free of condensation but not much else.”

Most of my tent stoves burn 400g or less of damp wood sticks per hour. Consequently, I am not concerned about the efficiency of fuel use per se.

Stove surface temperature

A high stove surface temperature is crucial to provide strong radiant heat from a stove. Measurement of stove temperature is discussed in another related post.

Another post considers the strong effect of stove temperature on the radiant heat output of a stove. Lastly, increasing the emissivity of a stove surface will reduce the temperature of the surface as you will see in this post.

This all may seem ‘head bangingly’ complex and circuitous, but I think it will make sense.

Controlled testing of the effect of a refractory coating on a stove.

Stainless steel is an excellent material for making ultralight tent stoves. However, the surface of the stainless steel has low emissivity. This means that it is a poor heat radiator. In order to test the effect of increasing the emissivity of part of a stove surface, I applied a narrow strip of my DIY sodium silicate refractory render onto the relatively shiny stainless steel surface of an elegant new Simple Dome Stove shown in the photo below.

I ran the stove very hot and photographed the stove (photo below). I used the colour/temperature chart shown below to estimate the of the stove surface by radiant colour.

The above chart is from Reddit Inc

Simple dome stove night, showing the wide heat distribution that is characteristic of a Dome Stove with an inverted J-burner.

In the photo above, there is a dark band and running across the fire dome where the refractory was applied. In the darkest part of this, the temperature estimated by colour would be 600C or less. While adjacent to the band the temperature was appreciably higher est. 800C. This to me indicates that the refractory render coating has greatly increased emissivity of the stove surface. I think this caused it to radiate more heat and become cooler than the surrounding metal.

If the whole stove was coated with the render it should emit a lot more heat and have a less hot surface. “Unfortunately, it would not be so pretty in the day time or at night.”

The refractory render used for this experiment was not optimised for the highest emissivity, so ther is potential to make it ‘darker or blacker to come closer to the ideal emissivity of one.

The emissivity/temperature conundrum

A higher emissivity is supposed to increase the radiation rate of a surface. In doing so it will reduce temperature,as in the above experiment, if there is a steady source of heat energy with the stove.

On the other hand Stefan–Boltzmann law states that a body emits much more radiation as it gets hotter.

THE BIG QUESTION. Is it better to have a hotter stove for the best radiation? Or is it better to have a high emissivity stove with a lower temperature for the best radiation? “Oh….., and while you are at it what’s the meaning of life?”

Thought bubbles. If emissivity was near zero the stove presumably be very bright, but no useful radiation would come off the surface, it may just look good. On the other hand, if the emissivity was near 1, then the stove would emit a lot of heat and the rate of release would be limited only by the rate at which the heat from the fire can reach the surface.

I hope someone knows the answer to this? My hunch would be to go for high emissivity, as the lower metal temperature will aid its long term survival.

conclusion

I think this may just lead to a big improvement to the radiant heat power of my stoves. However, with certainty, it will make them ugly.

If my analysis is correct, it could mean that the lower surface temperature may add to stove longevity.

Tim