[Stoves] briquette air feed

Rok Oblak rok.stoves at gmail.com
Wed Jan 5 02:36:20 CST 2011


Hi Crispin/AJH,

let me discuss some of my findings regarding to what you're saying on the
hollow briquettes

When i was testing different models for the side-fed briquettes in the lab,
i found out the briquette needs a little space to the wall for the air to
pass through. Now if the briquette is too small (hardly pushed in the feed),
it would burn unevenly, more on the top, while the hole area suffer from
less draft. The distance between the briquette and the side-fed wall was
around 3-5mm allaround, which makes a convenient buffer to push them in with
ease. When the briquette was too tight, there was smoke coming out at sides,
which was unpleasant and the briquette died many times as if this extra
wall- air would help it burn more evenly and keep the fire going

Now, later i drilled a half-inch hole through the bottom of the combustion
chamber right through the back tip of the elbow. Primarily i thought i would
clean the chamber of extra ashes with pushing the stick through the hole,
but i found out i never needed to do that as the briquettes would burn so
well - even after 6h of making a goulash, there was no need to clean the
stove even once. It would be interesting to see if this hole contributed to
the fire in any way, but i never specifically tested that.

When you mention *small 3mm telltale holes could be drilled in the feed tube
*  do you mean in the feed tube drilled vertically, so it would add up air
to the side of the briquette? but when you mention the visual contact - to
see the fire, i suppose you mean to drill holes horizontally in the
combustion chamber itself? I think the holes in the feed-tube could be good,
but they shouldn't be too big i suppose and it might contribute to an uneven
burn. I tried the holes in the combustion chamber, but it didn't seem to
help a lot - as you know, its difficult if not impossible to get to
gasification with burning the hollow briquette like this..

On the angled feed - i haven't tried that yet, but it seems tricky to make
it as well as not so practical and dangerous as the briquettes might fall
out while burning since they shrink a bit. So there's another mechanism
needed, which i was always trying to avoid even if there's better
efficiency..

I did try making bigger briquettes - like 15cm diameter and firing them in a
vertical position. It was pretty wild and very hard to maintain the constant
heat output. It needs more effort to fire it up, but when it starts it burns
like crazy with a very high flame out of the stove, while when it slows down
it tends not to burn out which creates enormous amounts of CO (i was testing
this at Aprovecho, specifically focusing on the latest phase of the burn -
if the briquette was not burning with a solid flame, the CO went sky-high).
So after this find, i really put the effort in a steady, efficient burn,
which was best achieved with shorter briquettes changed regularly in the
side-feed.

The size of the hole i ended up with was a generic hole that suited most
materials and stove types. Its probably true the dimension would vary a bit
with different briquette materials and stove types, but then again i don'
think so much. I made briquettes with different hole diameters and different
lengths, but it always showed up the hole being too big kills a fire in the
chamber since there's too much cold air i suppose, or if its too small it
chokes.. it would be interesting tho, to try smaller center hole and then
add 'secondary' air hole to the stove somewhere but i never got into this as
i was striving to at least changes to the existing stoves and briquettes and
really trying to make it simple to make..

Anyways, a big finding was that shorter briquettes are crucial to sustain a
constant fire. Besides the notion you are changing them more often, there is
always a 'fresh' one in the front, controling the amount of air - feed. Like
i said, when a briquette burns, it shrinks and changes the air-flow ratio.
And relating to CO and usability, shorter briquettes save the fuel (you
don't have to take it out and kill the flame) and limit the amount of smoke
in the last phase of the burn

there are some more details on the site, here is a new/old link, it appears
the rokstoves is broken

http://mdulastove.wordpress.com/

Happy to contribute if i can, unfortunately don't have time nor a shop to
continue working on this at the moment..

Greets from Vancouver, BC
Rok

On Tue, Dec 28, 2010 at 2:07 PM, Crispin Pemberton-Pigott <
crispinpigott at gmail.com> wrote:

> 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
>
>
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-- 
Rok Oblak, MAA Design

Gregorciceva ul.5
4224 Gorenja vas
Slovenia

temporary add.:
305-22 E Cordova St.
Vancouver, BC V6A 1K2
Canada

cell: +1 604 710 1615

www.holeyroket.com <http://www.rokstoves.com>
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