[Stoves] Heat / cook stove

[ajheggie at gmail.com]

On Friday 07 January 2011 12:43:06 Darren Hill wrote:

>
> I'm going to use a tank of water for heat storage and have a heat
> exchanger which I can place on/off the stove.  This needs to be
> relatively low so that I can use a thermosyphon effect to transfer the
> heat up into the tank.

Water is traditional because it has a high specific heat but it only 
(generally) stores at below 100C, Iron has a much lower specific heat but 
the density is better, higher temperature is available and it doesn't 
leak.

> I found this page which has a diagram which basically matches the
> design I'm now making in my head - sloping grate to help the
> fire/embers build up in front of the mouth to the combustion chamber.
>
> The Resolute Acclaim.
>
> http://www.hearth.com/econtent/index.php/wiki/Downdraft_Stove_Operation
>/
>
This is a sophisticated and complicated design but it is downdraught (once 
it gets going) so is better than the steel box approach, you will see the 
basic principle once it is in DD mode is all the gases are bound to pass 
through the small throat where the char burns out at high temperature,I'm 
not sure how air gets to the air holes under the sloping grate, much of 
the primary air seems to be washed over the glass and then descend 
through the logs, after being pre warmed, if so I don't like this aspect. 
In the Kunzel and Kob designs this throat area is protected by a water 
jacket, when we made a hot air version this area soon burned away in 
steel.

Even so the drawing looks strange as I would expect the log compartment to 
have little flame to be worth watching through the glass front if it was 
in DD mode.

> I was thinking of having the primary air coming in low down on the
> front of the stove and the combustion chamber insulated via a two
> skinned wall, with air gap.  I guess this air gap could be used to pre
> heat the secondary air which would be drawn down from a valve outside
> at the top of the stove.

Yes many of us do this to recoup heat and prolong the life of tincanium 
parts, I get over 200 deg C of preheat on my little swirl burners.
>
> "There is a big problem with loading large amounts of fuel, downdraught
> and tlud get around most of these but the type of layout you suggests
> will almost certainly tend to thermal runaway with large loads and dry
> wood. "
>
> *Is it not possible to keep the rate of burn in check by regulating the
> amount of primary air?*   *Or is this not desirable? *

It's both possible and desirable but it depends on the log mass not 
heating above pyrolysis temperatures before it gets to the combustion 
area, As I said the hopper needs to be cool and the stratified DD aims to 
do this with cool primary air. If the logs are dry then little heat is 
needed to get to pyrolysis temperature, at which point pyrolysis is just 
about self sustaining, so evolution of offgas happens suddenly. Of course 
if the logs are wet ( and the Kob happily burns 50%mc logs if a char bed 
is available, from a previous run, to start it). Then the endothermy of 
drying the logs prevents runaway but all the moisture has to be heated to 
throat temperature and discarded as steam.

>
> http://www.woodheat.org/technology/noncatalytic.gif
>
> I dont quite understand by what mechanism some of the gases are drawn
> back down from the front top, inside the front door, back down through
> the fire.  I guess this is however not an idea design for good
> combustion.
>

These clean burn stoves aim for two things, to keep the glass clean, 
because people like to see flames, and to clean up combustion. The latter 
is done with insulating the firebox to keep temperature up and providing 
jets of preheated secondary air ,  they do this with heat exchanging air 
paths.( again the three Ts Time by making the flame travel further before 
it reaches a heat exchange surface, Temperature by surrounding the 
combustion area with insulating fire brick and not allowing the flame to 
touch a cold surface and Turbulence with the secondary air jets to mix in 
the firebox). 

In the glass doored variety a portion of the secondary air is jetted down 
over the glass as an air wash, to keep combustion products away from the 
window. Power to drive the air is provided by the depression the chimney 
makes with the rising column of warm flue gases. As wood flue gases 
really need to reach the top of the chimney above 100C to avoid 
condensation there is no great heat loss incurred,m especially if the 
chimney is internal.

There is a strange reluctance  to use small fans in traditional stoves yet 
all gas boilers and pellet boilers use them to good effect ( and gas is a 
lot cleaner bunring than wood).

AJH