[Stoves] Sidewinder 01

[Crispin Pemberton-Pigott]

Dear Dr M

>Crispin, what do you think of adding to Lannys Sidewinder 01 
>a bigger can without lid, the bottom with a circular cut-out of 
>the size of Lannys Sidewinder 01.

>This bigger can putted upside down onto Lannys Sidewinder 01, 
>so that the intake air is pre-warmed, as in your Vesto. 
>-Sufficient fresh-air intakes like one wants downside or in the wall.
>Would it make a better burn or would the pre-warmed air warm 
>up the fuel too much, that there was created too much woodgas 
>for burning, in comparison with necessary air (With effect of sooth)?

>From an overal efficiency point of view, preheating the air is a good thing.  Generating too much woodgas is the predictable result if the air flow is not controlled.  Thus there is a constant battle to get the fire as hot as possible either by keeping the heat in (insulation) or by recycling it through the mechanism of preheating the air.

The idea of downdrafting the air past the inner can serves two masters.  First it is one way to increase the efficiency of the fire and it allows the maintainance of a small, hot fire.  Second it increases the life of 'tincanium' stoves.  Especially when there is a vortex keeping the flame away from the walls and providing good mixing in the hot centre, the life of the stove can be increased a great deal by cooling the outside of the inner combustion chamber.

At some point the temperatures stabilise and there is a constant heat flux across the thickness of the metal, providing heat to the air coming in and minimizing the corrosion on the inside.

Downdrafting the air is easy (first of all) and second, it tends to counterbalance the natural 'runaway' fire that is seen in an uncontrolled Tsotso or perhaps coal stove with vents wide open, whereby the hotter the fire burns, the more draw there is and the hotter the fire burns, etc.

By adjusting the downdraft height you can get the air to flow at a reasonable rate, and it if starts to get really hot, that immediately increases the heat transfer to the incoming air (through the combustion chamber wall) and the preheat starts to increase.  This immediately increases the negative draft on the air reaching the fire, reducing the air flow, reducing the fire, balancing the system to some predetermined rate.

At least that is the idea behind that portion of a Vesto.  There are 'other holes' in a Vesto that assist this process by drafting in extra air at the secondary combustion stage if the draft increases too much.

Another approach is to put an air controller on the downdrafting air to mechanically restrict the flow to reduce runaway.

One of the easiest things to do with a tin can stove that is generating too much heat too soon is to place a second large can around it and force the stove to pull preheated air downwards.  It is easy to make and the effect can be shown by opening and closing a large hole in the bottom, akin to lifting a Vesto off its closed base.

Regards
Crispin