[Stoves] Building my Heat / cook stove.

[Darren Hill]

Hello Crispin,

The bleed is at top bypassing the need for gases to go down past the 
heat exchanger.

Secondary air is flowing in through some 20mm x 20mm internal diameter 
square section tubing.  Is this enough or do I need more? Another 20mm x 
20mm square section tube? More?

I've attached another diagram which shows things as they are currently 
planned/progressing.

Looking at this diagram.  Are you saying to have some of the grate to 
the left of the red line I've drawn on the diagram?

Currently this is the thinnest point (or bottle neck, or throat) and has 
a vertical ceramic slab either side so that this 'throat' is not the 
full width of the hopper.  Extending the grate further to the left will, 
in effect, increase the area of the thinnest point.  I currently plan to 
have the lowest part of the grate level with the bottom of the ceramic 
block (that is the floor of the combustion chamber) so it is away from 
the thinnest/hottest area.

Is it best to shift the grate to the left?
If so what % of the grate left of the red line?
Should I also raise the grate so that the its level with the top of the 
ceramic block that is the floor of the combustion chamber?

I'm working on the stove today and tomorrow.  Going to build the air 
tight doors today.

Best

Darren



On 08/02/2011 21:13, Crispin Pemberton-Pigott wrote:
>
> Dear Darren
>
> First the cast iron grate is about right for the 4 kW (average, not 
> peak) power you want. I have been trying to make a 4 kW stove that 
> burned clean and the whole grate area is about 155 x 220 after 
> discounting the parts that are hiding under shields. What matters is 
> the volume of coal or wood burning at once. That sets the power level.
>
> I just want to confirm that the bleed hole takes hot fire gases from 
> the top of the combustion area into the flue before they go down into 
> the heat exchange, in other words like a GTZ 7.5 (now to be called a 
> GIZ 7.5 after their name change).
>
>
> ***** Do people think this should this provide sufficient secondary 
> air or should I put another similar secondary inlet in the front of 
> the burner??? *****
>
> Actually I can't see it clearly. How big is it?
>
>
> >With this 'door' wide open the bleed hole is 5cm x 10cm
>
> That is easily large enough.
>
>
> >My friend had a grate from an old coal stove that appeared to be a 
> suitable size and we intend to use - from memory this grate is approx 
> 15cm x 20cm.  Thinking of having the 15cm side in front of the 10cm 
> throat through into the combustion area.
>
> The whole grate does not need to be under the combusting zone. About ½ 
> is correct to give a crossdraft through the bottom of the fuel in the 
> hopper.
>
>
>
> >>Based on the hopper size, and presuming you have about 4 metres of 
> chimney,
>
>
> >If the chimney was shorter what would be the effect?
>
> Power is related to the draft, taller chimney means more draft means 
> more power. You can choke it of course with a damper.
>
>
> >The primary air hole should be 30x120mm?  With a valve so its adjustable?
>
> Yes. If you are quite short, it will be obvious in the thermal 
> efficiency number because too much excess air (secondary air 
> oversupplied) will give a cool burn and a high Stack EA (= low 
> efficiency).
>
>
> >Shes got plenty experience making air tight doors
>
> Good!
>
> The Hopper size:
>
> >Am I better reducing dimensions more than we are talking here?
> >Throat?
> >Hopper area?
>
> You can easily slim the hopper by dropping clay slabs inside it to 
> narrow it. Ditto with the grate actually.
>
> >- or do I not want the entire floor of the hopper to also be a grate?
>
> Yes, almost. As the air is not rising into the hopper other than 
> enough to get over to the combustion area, making it too small or too 
> much tucked under the combustion area will limit the power, maybe to 
> the point of it not working well. In your drawing you have the grate 
> nicely angled but stopping before it got under the combustion area. 
> With the angled grate extending from under the hopper (except perhaps 
> the bit at the very front) to the back of the combustion area (the 
> flame runs vertically from there) you can make a crossdraft fire that 
> is adjustable across a pretty good heat range.
>
>
> Perhaps you can make a good drawing in cross section of what you have now.
>
> Regards
>
> Crispin
>
>
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