[Stoves] Heat / cook stove

ajheggie at gmail.com. ajheggie at gmail.com.
Sat Jan 1 16:57:41 CST 2011


I had another similar enquiry about a design for boats but I cannot find 
it now.

On Friday 31 December 2010 02:06:19 Darren Hill wrote:

> I think 2kw would be enough, its heating a small well insulated space,

Yes, I would have thought more than enough if insulated to modern 
standards (say about 0.2W/m2 deg C excepting windows).

In the my early days a lot of my trade lived in caravans and the 
traditional stove was a simple updraught device improvised from LPG 
bottles, they were by no means clean though. Efficiency wasn't a problem 
either as the resource was large.

> but more than this (4, 5, 6 kw) would not be a problem - as I am hoping
> to catch a fair bit of the excess heat in the heat storage.

What sort of heat storage? Any heating of this should be well separated by 
insulation from the combustion area which should be kept hot. So a 
massive steel stove with no internal insulation is to be avoided.


> Am I right 
> in thinking that kw output has quite a bit to do with the type of fuel?

Not really, we generally reckon all biomass to have about the same 
calorific value at a given moisture content, with exceptions for resinous 
or oily residues.

> I mainly plan to burn seasoned hardwood logs / sticks although possibly
> sometimes some softwoods and have the option of woodchip. 
> > How wide are you thinking of making the stove body?
>
> Approx 30 cm

From your previous post with a length of 60cms it's a good length to avoid 
too much cross cutting. The little Jotul 602 I have had for 30+ years has 
almost exactly these dimensions...

It's not a clean stove unless it is running very hot but them my heat 
store is a brick chimmney so I recoup some when I run it hot.

It has some internal baffles to reduce heat loss from the combustion area.

The principle is that it burns from front to back in the area under the 
arch formed by the cast iron plate you have drawn to separate the flue 
gases from the log area. The door provides both primary and secondary 
air, the primary is an opening directly to the bottom of the firebox, the 
secondary is high in the door and is baffled before it emerges through 6 
1.5cms holes. I think the aim is for the primary air to gradually eat 
into the loaded firebox and generate gases which are consumed by the 
secondary air. As the fire burns down a bed of coals forms. When 
reloading it is essential to rake these to the front and put the new log  
lengths to the rear so the hot coals are directly hit by the primary air 
and ignite the new charge.

>  It would
> be useful to be able to just throw in the large amounts of fuel.  The
> fact that the top is taken up by the stack and often the cooking/heat
> exchanger surface I think limits my options. 

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. 
As wet wood isn't a clean option this is not a viable control.

Many woodstoves get damped down to make the wood last longer, this is to 
be avoided, wood needs to be burned hot and fast. Damping down to 
smouldering with no flame just burns the char out, the pyrolysis offgases  
exit with the flue gases and are lost as wasted fuel or condense as tars 
in the flue. I've told this story before but I'll repeat it: a young 
lorry driver, from Oxford, who picked timber up from me in the late 70s 
left his front opening stove full of logs and damped down for the day 
whilst he was at work. One wet evening he arrived home with his anorak 
still on and opened the fire to see if it was still burning. The 
resultant draught from opening the door ignited the gases in the stove 
and there was a deflagration. he breathed the flame in and his anorak 
caught fire. He fetched himself to hospital but died two days later from 
the lung damage. I'm ashamed to say I cannot remember his name.

> Am I missing some 
> alternative methods?

Many, your idea has some concessions to better air control with that tube 
for preheating air but Crispin and I differ in that I don't want 
preheated primary air in a batch fed stove. Hot secondary air we agree on 
and the way it can be used to cool stove surfaces for longevity. Crispin 
doesn't like tubes in the fire because they burn away in the secondary 
flame, I think they have a use in lower temperature zones.
>
> As I understand it this is not ideal as the burn will progress through
> the material rather than the fuel falling/being fed to a set point for
> oxidisation.  Am I correct?

Yes, the heat from the fire gradually heats up the fuel until it reaches 
pyrolysis temperature, then the evolution of offgas fuel is unstoppable 
and you have a problem supplying it with enough air. That's why rocket 
stoves feed in the sticks gradually to control power and coal stoves have 
grates. The tlud has limited feedback to the fuel wood and the primary 
air supply also has a cooling effect, similarly with J type down draught.
>
> > Re the chimney, it is likely 100mm will be enough, even 90. It will
> > probably work much better if it is 3 metres long instead of 2.

Chimney serves two purposes: to create a depression in the stove and to 
get the flue gases out of the breathing space. UK building regs say solid 
fuel fires need to discharge above eaves and in fact 150mm is  a minimum. 
A 100mm is more than twice as likely to choke up with tar and risk flue 
gases getting into the living area.
>

>
Crispin> > Exiting at the top pretty much guarantees high smoke and CO 
(lost heat) the way the old stoves do.

Yes
>
> You mean that the air should draw through the oxidising material either
> sideways or downwards so that the gasses begin to burn as the pass down
> or sideways - I guess through some kind of "grate" into a "combustion
> area" where more air is added + ideally this area should be insulated
> to keep the temperature here high.  Is this right?

Yes but the grate isn't necessary.

Basically you need to avoid the fuel being between the fire and the flue 
exit, everything should pass through the hottest area before it can get 
out.

>
> Sorry I think you've lost me there.  Can you describe as the primary
> air flows?

Once full combustion is established the primary air reacts with the hot 
coals in the fire, this creates more heat and combustible gases ( CO and 
H2 ideally but we seldom get fully there in small stoves) which pyrolyse 
more wood and these primary products plus the pyrolysis offgas then burn 
as a flame with the secnodary air.

> I appreciate that and this has been useful.  I guess if I'm following
> you correctly the fact that I want a low height and front loading is
> really not in my favour...  Although I guess I may be able to rejiggle
> things - possibly allowing me to have a much taller stove....

Taller would allow a tlud approach or a downdraught one, our downdraught 
protagonist is Peter Verhaart, who I hope is on high ground in Eastern 
Australia.

I would have thought you could probably afford the luxury of a small fan 
in your situation.
>
> I really appreciate your help here.  Are there any good resources
> anywhere which give a good quick and easy oversight of the most crucial
> elements that require consideration?

I don't agree with them all for all stoves but for the rocket type Larry 
has some good rules:

http://www.bioenergylists.org/stovesdoc/apro/designp/Design%20Poster.pdf

Happy New Year

AJH




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