[Stoves] Heat / cook stove - proposed design

Mon Jan 9 14:26:44 CST 2012

Hello Crispin,

I was using some reclaimed construction timber, pine/conifer, 2"x2" and 
cutting it to short lengths.

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.

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.

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.

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.

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.

Best

Darren

On 09/01/2012 14:52, Crispin Pemberton-Pigott wrote:
>
> 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: 
> imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.2&filename=image001.png
>
> 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 1/40^th 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: 
> imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.5&filename=image003.jpg
>
> And here is a photo of the peephole, also top view
>
> Description: 
> imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.3&filename=image004.png
>
> 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: 
> imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.4&filename=image006.pngDescription: 
> imap://mail@mail.vegburner.co.uk:143/fetch%3EUID%3E.INBOX.Stoves%3E851?header=quotebody&part=1.1.6&filename=image007.jpg
>
> >...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
>
>
>
> _______________________________________________
> Stoves mailing list
>
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
>
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
>
> for more Biomass Cooking Stoves,  News and Information see our web site:
> http://www.bioenergylists.org/
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20120109/0a748d74/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: image/png
Size: 117129 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20120109/0a748d74/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: image/jpeg
Size: 11722 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20120109/0a748d74/attachment.jpe>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: image/png
Size: 85115 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20120109/0a748d74/attachment-0001.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: image/png
Size: 31509 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20120109/0a748d74/attachment-0002.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: image/jpeg
Size: 2311 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20120109/0a748d74/attachment-0001.jpe>