[Stoves] briquette air feed

[Crispin Pemberton-Pigott]

Dear Andrew

>Yes Crispin but it makes my case for using a tube to separate the primary
and secondary supply more intuitive. 

I am with you on that but exploring the more radical idea of tuning the hole
to the product to give correct air (maybe 100% EA) with no additional holes.
I am not yet convinced that an angled gravity-infeed won't work - the way
the VITA stove ran before David Hancock thought of the side feed that became
the Rocket Stove. David was at Aprovecho in the old days when that inclined
feed was the state of the art. The problem was the fire progressed up the
fuel to the air inlet. It seems that with proper air control (which was not
well understood in the early 80's) that would not have happened.

So back to the past it seems: get the hole size matched to the fuel
properties, then incline Rok's feed chamber and give the stove enough height
to pull like crazy on the air through the hole.

I don't suppose Rok has tried a nearly vertical bottom-lit briquette, has
he? Something like a downdraft at maybe 60 degrees up from horizontal?

If the briquette was hard enough, it may work really well.

The GTZ 7.4 stove in UB was running on wood briquettes (they call them)
which means a 50mm hollow log. It was working really we save for the fact
they really had the fire burning in the hopper which it was NOT designed
for.  I am suggesting that the air hole may be suitable for a high velocity
air supply.

It is probably worth my trying to run it in that mode. Hmmm...... I think I
will do that maybe in February.

>With a traditional burner all the primary air passes through the bed and if
the bed is thick and hot enough no primary oxygen survives into the
secondary combustion area.

Yes that is traditional but it is becoming a bit passé because there are
just too many alternatives showing that with hard fuel (which densified wood
logs are but one) it is not difficult to get secondary air past the fuel.
With a hollow briquette even light density fuels are going to see this
'exception' being reliable.

>I'm simply surmising that if the amount necessary for primary combustion
passes around the tube or along side the briquette and the remaining 8kg is
induced up the middle of the tube then we can have primary air control for
power. 

I understand. Having tried many times I am backing off recommending central
tubes in the combustion area until I see one that will survive the
temperatures. Generally, if a central tube is needed, something is wrong and
the tube is being inserted to overcome a problem that should not exist. When
the power starts to drop, the tube kills the combustion, big time. Cecil
Cook loves the idea and I think I was able to spike it with a number of
demonstrations.  It is an indication of failure, basically. 

>The secondary air will be proportional to the power and largely self
regulated by draught.

Yes, but only for a given power level. The domestic stove needs to vary both
primary and secondary automatically. If you try some things, try
down-drafting the secondary air with preheating like a Vesto to get that to
auto-balance across perhaps a 3:1 power range. That is an achievement in
itself.

>I see Tom has chimed in with similar thoughts.

Tom has some ideas worth trying.

>As I said before, if the geometry of the fire and hole can be made to do
theis then great. My experience of providing all the air via the same jets
is it leads to high excess air, yet it does look like modern pellet burners,
using a blast tube approach are tending that way.

Agreed - they are throwing the secondary past the fuel's primary burn.
Simplicity when you can manage it. Imagine how difficult that would be if
the secondary air was a fixed supply and the primary and fuel variable.

>Well just a couple more: some small 3mm telltale holes could be drilled in
the feed tube, they would supply some primary air but also indicate to the
cook when the fire was burning back and a new briquette needs screwing in.

I like the vision thing. Good point. We are leaving a single hole in the
ELCD stove fuel door just for looking to see how things are going. The air
is definitely not needed by people complain....

Holiday regards
Crispin in the snow