[Stoves] Heat / cook stove - proposed design

[Darren]

Its getting cold again, so I'm making time for the stove again.

First refinement on my list is to try to provide a fuel / hopper 
combination that will not 'bridge' jamming in the hopper causing the 
fire to die back for a while until fuel finally falls onto the grate 
giving poor combustion for a time.

My plan is to modify the hopper adding two walls to either side of the 
grate (shown in red on the plan) so the hopper is only as wide as the 
grate and the mouth through into the combustion chamber.

I've got a load of ash branches that I'm going to chop into short 
lengths (10cm, as wide as the grate) so that they can be stacked into 
the hopper in an orderly pile (see side view)

I am hoping this will help the burner to operate without requiring 
frequent grate rattling to get the fuel to self feed onto and down the 
grate.

Anyone got any thoughts on this?




Drawing was made using QCad on Linux Mint.



On 23/03/12 17:34, Darren wrote:
> Finally got a chance to write up my further experiences...
>
> I replaced the broken glass in the door into the combustion chamber 
> and I added the shakable grate on top of the original grate.  In order 
> to make this work without some very tricky welding I had to also 
> install the parts to make the hopper taper from three sides towards 
> the bridge (and onto the part of the grate inside the bottom of the 
> fuel hopper), rather than from just one side as it was previously.  
> With the current design the extra two sloping sides of the hopper act 
> to keep the shakable grate on top of the original grate and hold the 
> third sloping side in place.
>
> I chopped some very dry recycled pieces of pine timber into 
> approximately inch by inch and a half 'cubes' and ran the stove with 
> this.  This worked quite well. Approximately every 15 minutes I could 
> see the fire beginning to die back considerably.  I would then shake 
> the grate and it would flare up again.  From my observations it looked 
> as if this might in part be due to the fire having burnt back up the 
> grate so that there were only embers on the part of the grate under 
> the hopper or possibly because the fuel was bridging in the hopper - 
> shaking the grate also moves the tapered sides of the hopper and is 
> likely to encourage any fuel bridge in the hopper to collapse.  I 
> noticed after this die back, once shaken and the fire started to flare 
> up again that there would be a considerable amount of visible smoke 
> from the chimney for some time.  This led me to believe that I was 
> getting bridging in the hopper.
>
> Next I cut a piece of plate metal to sit on the back part of the grate 
> and cover about half of the part of the grate that is under the 
> combustion chamber.  I also extended the chimney from 2 meters to 3 
> meters.
>
> I ran  the stove again a couple more times, these times the wood was 
> probably 2 inch by inch and a half 'cubes'.  I had not intended to cut 
> the wood bigger but preparing the wood was time consuming and once I 
> had finished I noticed that the 'cubes' were consistently bigger than 
> on the previous occasion.
>
> On both these burns the stove appeared to run similar to the previous 
> occasion.
>
> Next I managed to get my hands on some recently cut ash branches. Ash 
> has a very low water content and burns reasonably when freshly cut.  I 
> put these branches through a tree surgeons wood chipper. I loaded the 
> hopper with this and started the fire on the combustion chamber side 
> using small 'cubes' of dry pine and paper.
>
> I ran the stove like this on three occasions.  The stove appeared to 
> 'work' reasonably well.  It gave a higher heat output and did not 
> require shaking, although I did notice at times the fire would die 
> back a bit, but it would self remedy.  I think that this was probably 
> fuel bridging that would collapse by itself.  I did notice that at 
> times there would be visible smoke from the chimney - I'm guessing 
> this was after fuel bridging collapse.
>
> Burning the chip was however providing a much higher heat output than 
> I require.  I'm not sure how best to try to reduce this output and 
> keep the stove burning efficiently.
>
> I think I need to try and address fuel bridging but I'm not sure how 
> best to do this - nor am I sure what type of fuel is going to work best.
>
> I can get lots of wood chip although generally the water content is 
> likely to be too high - and I cant think of an easy way to dry the 
> wood chip.
>
> I was thinking about laying up a lot of 1-2 inch diameter branches to 
> season that I could then saw to 1 inch lengths.  I could get some 
> seasoned wood and put it through a wood chipper - although I do not 
> always have access to a chipper and would prefer not to be dependant 
> on access to such a machine for my fuel.
>
> I was wondering how best to modify the hopper design.  As the sloping 
> sides guide the fuel onto the grate, if they were not there I'm 
> thinking that they would, in effect, be replaced by unburnt fuel as 
> air will not be flowing through these areas??? Unless I reduce the 
> hopper area so that it has upright sides going straight up from the 
> grate.  This would however seriously reduce the amount of fuel that 
> the hopper holds (and would be somewhat disappointing)
>
> The stove was getting through the fuel more quickly than I expected - 
> I guess that I can address this by reducing the burn rate (and the 
> output) which would be a double win for me, as long as the stove would 
> still burn efficiently.
>
> I currently have a butterfly valve in the primary air inlet - can 
> block the secondary air inlets (although this appears to have limited 
> effect on combustion) I intend to fit some kind of valves to these. I 
> also intend to add a butterfly valve in the bottom of the chimney.
>
> I wont be able to do any more testing/make more refinements for a 
> while but thought I would give a (late) update before the details had 
> escaped me.
>
> Best
>
> Darren

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