[Stoves] Heat / cook stove - proposed design

[Darren]

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