[Stoves] Heat / cook stove - proposed design

Mon Jan 9 08:52:49 CST 2012

Dear Darren

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.

>Things I noticed.

>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.

So the combustion is going well when the 'burning side' of the stove is
loaded and lit. That is a good start.

>I threw some more 5cm x 5cm lumps into the fuel hopper.

What is the fuel?

>Through the window I could see that once the wood in the combustion 

chamber burnt away the flames died right back down.   I opened the 

hopper up - it was full of smoke.  

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.

>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.  

Yes, but with the air being supplied through 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.

>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).

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. 

Once wood/embers were pushed through under the bridge the fire would again
burn vigorously.

>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.

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.

>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.

That is also OK – you are looking to have that happen (not quite so
vigorously) on a continuous basis, right?

>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.

If you can see it with your eyes it is definitely working.

>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.

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’.

>The ash [drawer] was not sealing well.  

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. 

Here is a stove which has lots of clay on it:

Description: cid:image001.png at 01CCCEB0.92503E00

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.

>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.

That kinda sounds like the right approach. It’s hard to tell where your air
is entering, though.

>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.

Good.

>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)

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.)

Yesterday and today I did some more work to try and remedy this...

>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.

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
1/40th 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.

>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.

Now we are talking
.

>I did this in the hope that moving the grate will encourage wood under the
bridge and across the grate.

>Still the moving grate gives more variables to play with.  I can also
easily lift it higher.

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.

I moved the stove today from my workshop (which is 50 miles away from where
I now live)

When loading it into my truck I took the doors off.

>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.

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.

Here is a photo of a stove similar in layout to yours, I think, (top view)

Description: cid:image003.jpg at 01CCCEB4.5E3860D0

And here is a photo of the peephole, also top view

Description: cid:image004.png at 01CCCEB2.BAE93D10

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.

Description: cid:image006.png at 01CCCEB3.DF0DE780Description:
cid:image007.jpg at 01CCCEB4.5E3860D0

>
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.

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’.

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. 

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.

Regards

Crispin

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