Stove temperature and heat radiation from ultralight tent stoves
This post describes the powerful effect of stove temperature on heat radiation from ultralight tent stoves and considers optimization factors.
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 heat radiation 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 & socks, keeping the tent free of condensation but not much else.”
Most of my tent stoves efficiently burn 400g or less of damp wood sticks per hour. Consequently, I am not concerned about the efficiency of fuel use per se.
In a long series of Dome Stove developments, my most recent little stove is a 190g ultralight tent stove. I call it the Simple Dome Stove. I will use it for this discussion because it burns so brightly, but the principles apply to all tiny tent stoves.
A small stove surface area makes a higher stove temperature and more heat radiation
By any standard, my dome stove surfaces are very small. It is deliberately small so that the surface temperature is very high and it provides strong heat radiation to directly heat cooking and the campers’ bodies.
“I knew this rule-of-thumb from years of experience, long, long ago before I knew or maybe remembered good old Stefan’s law below.”
Me
Heat radiation increases as Temperature4(This means multiplying the temperature by itself four times). This comes from the Stefan–Boltzmann law. It describes the power radiated from a black body in terms of its temperature in degrees K.
To estimate this increase in heat radiation by increasing the stove surface temperature, we first must covert the temperature from degrees C to the absolute scale (K) by adding 273C to the degrees C. “F’US degrees are no use here”
The ratio of any two K4 values will give the relative heat radiation output of a stove at the two stove temperatures.
Using C1=300 and C2 400C as an example of such a difference, that I often see, these become K1=300+273=573 and K14=5734 =1.078*1010
similarly,
K2=400+273=673 and K24=6734 =2.051*1010
The ratio of the emission would be K24 /K14 =1.9
If I have the maths correct, this means that a 100C higher stove temperatures will cause a 1.9 fold increase in heat radiation heat that will be provided to campers from the stove.
Similarly, if the higher temperature was 500C, the heat radiation ratios would be 3.6.
The fullest delivery of the stoves heat radiation depends on the surface of my stoves having a perfect emissivity of 1.00 (Black body). In reality, my stoves are of stainless steel and the emissivity can be as low as 0.075 when new and shiny and up to o.85 when weathered and ‘well cooked’. (Source engineeringtoolbox). Nevertheless, for any given emissivity these large power ratios hold.
I have a separate post on refractory coating to improve emissivity of stove surfaces.
These are big and important differences that are largely invisible to us if we cannot measure stove temperature. “As the saying goes, if you can’t measure it, you can’t manage it.”
To manage and optimise this important heating factor, I need to have crudely accurate temperature measurements.
I have a separate post that discusses the robust determination of stove temperature. “And, would you believe that higher emissivity seems to reduce surface temperature.”
Conclusion
Ultralight tent stoves should be run at the highest possible temperature to provide maximum radiant heat for the comfort of the tent occupants and the speed of cooking.
An additional factor for consideration is increasing the stove surface emissivity to further improve the radiant heat output. However, I think it will (no, I know it will) also lower stove temperature. Now there is a conundrum and it may not be all that simple.
Addendum
After preparing this post I have obtained a thermocouple and meter that has allowed me to easily and accurately measure stove surface temperatures. It has confirmed my crude best estimates of stove temperatures differences use in the above considerations. I have prepared a post on monitoring stove temperatures with a cheap but effective thermocouple.
Tim
