[Stoves] stoves 101

[Crispin Pembert-Pigott]

Dear Jock

 

>FYI: I reduced my secondary air slot by 50% and got better performance.  It is now about ¼ inch.  The larger slot, however, is required for photography ;-)

 

Agreed the photos. In order to know what the gap should be, it really is necessary to have an oxygen meter, unfortunately. The O2 level should be (with no air getting in from the sides to contaminate the sample) between 8 and 11% for most biomass stoves without fans.

 

1. If there is zero secondary air, clearly that is a fail.

 

You can pull air through the fuel bed – some stoves work like that. They should in theory always produce nearly no char. The reason is the high superficial velocity (Ref Tom Reed).

 

2. In my systems, if the draft can is removed, 100% secondary air, the pyrolysis fails -- insufficient draft.

 

Does it stop entirely? Even if there are lots of holes in the bottom? I would always try to have 100mm of draft tube above the fuel but I take your point.

 

3. So the challenge is to find the "just right" secondary air gap for a given TLUD of a given diameter with a given fuel type.  Adjusting the air gap, is, perhaps, another way to vary the output of the stove.  The more secondary air, the cooler the heat delivered to the target.  This suggests a draft can that can be set at a range of heights?

 

Aha – here is the hard part. It is quite easy to make a TLUD that will burn at one power level. Changing that power level is a big problem if the requirement is to maintain a clean burn and a steady % of secondary and excess air – constant proportions. This is best accomplished by (not a fan) (not movable slots over holes) downdrafting the secondary air (which also provides the excess air) using an outer sleeve. When the heights of everything are right, there is a balance that works across a range of primary air flows (and fire powers).

 

>When I see stoves in the field with soot covered pots I know that the combined stove+pot system is sub-optimal.

 

Yup. Right on. 

 

Thanks for your efforts

Crispin

 

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