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Hello Crispin,<br>
<br>
I was using some reclaimed construction timber, pine/conifer, 2"x2"
and cutting it to short lengths.<br>
<br>
I was only opening the lid on top of the fuel hopper to push the
fuel through under the bridge, to look at what was happening inside
or to add more fuel. I appreciate that when I've got things set up
properly I only really want to open it to refuel. I was slightly
surprised to find the hopper full of smoke - I guess I was expecting
the air in the hopper to remain static - happy to hear that this is
normal.<br>
<br>
The wood on the hopper side of the grate was catching alight well.
The flames did appear to die back significantly when the wood/embers
that were on the combustion chamber side of the grate had burnt
away. I guess more testing will tell more. <br>
<br>
The original grate is not shakeable, the new one that is sitting on
top is. I did this so I can vibrate ;) the fuel down the grate.
I've now got 9cm from grate up to the bridge and this throat is
about 10cm wide.<br>
<br>
Hopefully I'll get a chance to fire it up with the new grate later
in the week. I was thinking about experimenting with covering some
of the back of the grate. I'll cut some metal to fit and try this at
some point also.<br>
<br>
I guess ideally it would run without needing to shake the grate.
Unfortunately, the way the thing is constructed, lifting the bridge
is not an easy option. I could more easily lower the grate which
would provide a bigger gap between grate and bridge and allow for
wood to fall through more easily, although I guess if this will also
effect the heat output? Also I guess a steeper grate angle will
help. <br>
<br>
Best<br>
<br>
Darren<br>
<br>
<br>
<br>
<br>
<br>
<br>
On 09/01/2012 14:52, Crispin Pemberton-Pigott wrote:
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<div class="WordSection1">
<p class="MsoPlainText">Dear Darren<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Thank you for your first burn report. My
compliments. People often do not say what happened the first
time they light something, but wait until most things are
favourable.<o:p></o:p></p>
<p class="MsoPlainText">>Things I noticed.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>I put some 5cm x 5cm lumps of wood,
then some smaller pieces then some paper in the combustion
chamber side of the stove and top lit. This worked well and
there was little smoke.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">So the
combustion is going well when the 'burning side' of the
stove is loaded and lit. That is a good start.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>I threw some more 5cm x 5cm lumps
into the fuel hopper.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">What is the
fuel?<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>Through the window I could see that
once the wood in the combustion <o:p></o:p></p>
<p class="MsoPlainText">chamber burnt away the flames died right
back down. I opened the <o:p></o:p></p>
<p class="MsoPlainText">hopper up - it was full of smoke. <o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">It should be full of smoke. You have a
hopper above a fire and the heat from below will be pyrolysing
the fuel above where there is no air. If you open the top of
the hopper a little, it provides combustion air at a section
where there should be no combustion. If you open it enough
you will have a hopper fire. The plan was to have the fire in
the combustion chamber. Do you get my drift? The hopper
opening is primary air supply. It should be coming under the
grate both at the bottom of the hopper and under the flaming
zone. My understanding is you are trying to establish a
self-sustaining (hopper-fed) fire in which case you want there
to be some burning at the bottom of the hopper but not in it.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>I closed it again, leaving it open a
crack and blocking the primary air inlet at the bottom of the
stove, which allowed the smoke to get sucked from the hopper,
it also reinvigorated the fire. <o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Yes, but with the air being supplied <i>through</i>
the fuel which will bring the fire into the hopper. Fire
follows air. If the main source of air is through the fuel,
the fire will run into the fuel.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>I tried pushing the wood through
under the bridge so that some was further down the grate. This
was difficult as these pieces of wood would readily jam
together blocking the throat (space under the ceramic bridge).<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">Is the grate
not shakeable? I understand from your later text you have
added a grate on top of this grate, and that the upper one
can be moved (shaken?) so the implication is the lower one
cannot be shaken. <o:p></o:p></span></p>
<p class="MsoPlainText">Once wood/embers were pushed through
under the bridge the fire would again burn vigorously.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>I chopped some smaller pieces of
wood but found that I still had to push them through under the
bridge to get the fire burning with more than a few licks of
flames. This was not easy to do.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">It seems that
the fuel dimensions (the ‘particle size’) will dictate the
gap under the ceramic bridge. The grate sliding back and
forth would be the easiest way to vibrate (is that the right
word?) forward.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>When wood and embers were across
most of the grate the whole combustion chamber would be full
of flames that would disappear into the heat exchanger.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">That is also
OK – you are looking to have that happen (not quite so
vigorously) on a continuous basis, right?<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>Other points to note. I tried
covering/uncovering the secondary air inlets at different
times which had a limited effect on the fire. I could see the
flames being blown away from the main secondary air inlet at
times.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">If you can see
it with your eyes it is definitely working.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>My attempt to have secondary air
wash the glass apparently did not work well. I guess this may
be due to the poor combustion at times - there was noticeable
amounts of smoke from the chimney at times. I may attempt to
fit some further piping/jetting to send the air more directly
at the glass. Wood ash and a little water does clean the
glass quite well.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">It is a bit
early to worry about that. You have good combustion when
there is fuel in place and the grate is not overloaded. So
you have combustion conditions at certain times. You also
have a fuel feeding problem which, when you overcome
manually, results in a good fire again. These are all steps
‘forward’.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>The ash [drawer] was not sealing
well. <o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">That is a problem. If you are not able
to direct the air where you want it, the result of leaks is
usually a general deterioration of performance because of high
excess air. At this stage of your development, just grab some
clay from a nearby garden. <o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Here is a stove which has lots of clay
on it:<o:p></o:p></p>
<p class="MsoPlainText"><img id="Picture_x0020_1"
src="cid:part1.04030301.04090005@vegburner.co.uk"
alt="Description:
imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.2&filename=image001.png"
height="265" width="299"><o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">The cast iron top in that case was so
leaky that it was difficult to test the combustion. All joints
have clay rubbed into them. The plan was to have a good
quality top, so was sealed with clay to simulate one. You can
deal with details like leaks later.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>The front of the stove is not flat
(much of the stove is not flat or square - the scrap metal I
used was not all flat and not enough care was taken during the
construction of the stove). I set the ash draw so that the
gap was under the hopper rather than the combustion chamber.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">That kinda
sounds like the right approach. It’s hard to tell where your
air is entering, though.<o:p></o:p></span></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>I managed to bend the front of the
ash draw today with a hydraulic press so that it now has a
reasonable fit against the fire rope/tape that is stuck to the
front of the stove.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">Good.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>I guessed that the reason the fire
was dying back when the combustion chamber part of the grate
was empty was due to excessive primary air passing up through
the uncovered (by embers and wood) grate and cooling the
combustion chamber burn. (I'm now not entirely sure that this
was what was happening)<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">That sounds
like the right analysis. To prove it you need to feed the
fuel forward as it is needed. If that solves all problems
except the fuel feeding (which should be by gravity) you
have made major progress. (To feed more fuel the bridge will
have to rise.)<o:p></o:p></span></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Yesterday and today I did some more work
to try and remedy this...<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>I made the hopper slope in from 3
sides onto the grate, rather than just one, to direct the wood
onto the grate and to give less places for the wood to jam.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">Tapering a
hopper in three directions just about guarantees that the
fuel will ‘bridge’ by which I mean it will create a hollow
space and not feed down. Back up one step. A hopper will
probably not feed fuel unless it is quite a lot larger than
the particle size. For example, if the fuel is 50mm it will
not feed into a 200mm funnel. 5mm fuel probably will because
it is 1/40<sup>th</sup> of the hopper dimensions. Looking a
lot of fuel feeding systems, I notice most taper larger, not
smaller to prevent bridging. Bridging is also more likely if
there is pressure (lots of hopper height) on the tapering
portion. Even large systems like boilers have bridging
problems.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>I made another grate out of 10mm bar
that sits on top of the original grate and can be moved
backwards and forwards with a handle.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">Now we are
talking….<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>I did this in the hope that moving
the grate will encourage wood under the bridge and across the
grate.<o:p></o:p></p>
<p class="MsoPlainText">>Still the moving grate gives more
variables to play with. I can also easily lift it higher.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">You fuel
feeding will be easier if the bridge is raised. I was
thinking this morning that one thing you can try to prevent
the system drawing lots of air through the uncovered (back)
portion of the grate is to drop a metal plate over that
portion of the grate. Cut a piece of sheet and plunk it on
the grate to force the primary air into the lower end of the
hopper, without letting in any through the hopper top. That
may migrate the fire into the area under the bridge and
provide a constant burn. Shaking the grate a little
occasionally would feed some fuel forward if the inserted
sheet is too narrow front to back.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">I moved the stove today from my workshop
(which is 50 miles away from where I now live)<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">When loading it into my truck I took the
doors off.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">>Despite twice noting that I had done
so and telling myself not to forget I unfortunately left the
combustion chamber door sat on the bed of the truck and it
slid off the truck as I was driving!!!! Luckily I heard the
glass break and realised what had happened and went back to
get the door.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span style="color:black">Oh well. Stuff
like that happens. Make a ½” observation hole and stick a
bolt into it when you are not looking through it. I usually
weld a ring on the head of the bolt. It is helpful to have
one on top directly over the flames so you can watch the
fire develop. It is even possible to take photos through a ½
inch hole.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">Here is a
photo of a stove similar in layout to yours, I think, (top
view)<o:p></o:p></span></p>
<p class="MsoPlainText"><img id="Picture_x0020_2"
src="cid:part2.03060204.07000405@vegburner.co.uk"
alt="Description:
imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.5&filename=image003.jpg"
height="184" width="352"><span style="color:black"><o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">And here is a
photo of the peephole, also top view<o:p></o:p></span></p>
<p class="MsoPlainText"><img id="Picture_x0020_3"
src="cid:part3.06000304.05070203@vegburner.co.uk"
alt="Description:
imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.3&filename=image004.png"
height="250" width="260"><span style="color:black"><o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">The square is
not steel, it is a piece of glass covering the hole. A photo
taken through the hole is below. Actually it was a video so
I am showing you two frames.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><img id="Picture_x0020_4"
src="cid:part4.03000004.07000401@vegburner.co.uk"
alt="Description:
imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.4&filename=image006.png"
height="122" width="147"><img id="Picture_x0020_5"
src="cid:part5.00080704.02070002@vegburner.co.uk"
alt="Description:
imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.6&filename=image007.jpg"
height="124" width="117"><span style="color:black"><o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText">>…I may temporarily fit some plate
metal where the glass should be while I await a new piece,
although the window has been very useful for assessing what is
happening.<o:p></o:p></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">It certainly
has. Without test equipment you are reduced to observing.
Good that you noticed the secondary air flow. Good that you
have what appears to be clean combustion at least some of
the time. You seem only to have a fuel feeding problem.
Cover part of the back of the grate – see what happens. Does
the fire move into the fuel piled at the hopper side of the
bridge? Does it burn longer without attention? Does the
developing fire have a gas path directly to the chimney or
are you making the heat exchanger work right from the
beginning? Extracting heat to soon and too efficiently kills
the draft. ‘Failure to thrive’.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">For the
moment, close the secondary air supply because the
development of the fire is not dependent on that portion of
the combustion. That will put all the draft power into the
primary air. At any time you can open the secondary air. If
it appears to immediately reduce the combustion rate, you
probably have too much. At that point you can start to give
it a little at a time. If you really do have excess primary
air (which is sounds like you do) you may not need any
‘secondary’ air at all. <o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">It is probably
worth chopping up wood into 1 inch chunks to see if it feeds
better. But keep the back of the grate partially blocked
until you know you have a self-sustaining fire at/under the
bridge.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">Regards<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">Crispin<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
</div>
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for more Biomass Cooking Stoves, News and Information see our web site:
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