Jonny Clothier

Ha ha!!! Well said!

I'll get on with fixing this one.,.

Ha ha. Yes. I did do a bit of fluid mechanics at A-level. Many, many years ago - school Physics etc.. Might have been in the back of my mind...

I'll make the mods next week and report back.

One thing about secondary air. We were messing about with wood gas cookers last year trying to come up with a design for a collapsible one. It always amazes me to see the way the secondary air makes such a difference. We also have some Westfire stoves with secondary air holes in the fire bricks at the back. The vents only open when the trigger gets to the right temperature and expands to open the vents.

Just wondering if that would work at the base of - or some way up - the feed tunnel. A little injection of extra air a little way up the tunnel when it heats up. If the vents/jets were angled it might also provide the elusive turbulence that you're all looking for in the riser... i.e .rather than fins of some sort.. could be as simple as a metal sleeve on the outside of the riser but inside the insulation. It would be capped above the vents and open to the outside at the bottom somewhere. The vents themselves could be drilled into the blocks at an angle with a simple drill (the blocks are super soft).

Here's a little sketch.

In a wood gas stove the air is forced up the outer chamber until it hits a dead end where the vents are. Looks like it would work exactly like that here.

Or am I talking rubbish?!

Best

Jonny

PS Not that far away. we're only in England!

Brilliant. Thanks Peter.

However the riser is nto made of firebricks. They are fireproof insulating blocks - so they don't heat up like normal firebricks. Quite the opposite in fact.

Here's a pic of it being built.

Hence the soot possibly!

And the idea of a steel insert.


Jonny

ps when you say "P channel" in that article (you don't refer to is as such there) do you mean the secondary air plate in the front of the feed tube?

Ahah! Peter your readability is just fine. And your observations are most appreciated.

The bit I haven't told you is that this is in the middle of a 24ft diameter, canvas covered, geodesic dome (attached). The flue goes all the way round inside the ground level bench in the centre and then exits under the rest of the floor at the far side of the dome. The dome is completely sealed at the top so airflow inside is not really upwards. Around the outside it gets a bit weird though. The lower edges of the tent are not well fixed (yet) and quite draughty (probably why I haven't died of CO poising yet!) .

The outlet of the exhaust (just outside the far side of the tent in the picture) is also at quite a low level, so your chimney/house theory may be a factor. We'll stick a big tall tube on it and see what happens...

As for the other points.

Barrel-side-gap is definitely suspect, so we can trim the outside of the riser down to get the gap CSA to match the others.

Junction to flue - you are right - the stream profile is definitely out. It was interesting writing the original post, it made me consider everything in a more orderly manner and so started seeing the holes. Looking at the way I measured it, the profile could be as much as 20"sq too small! So, we'll make that section much larger.

Interior of the riser. Should that be clean or sooty? We have a woodfired pizza oven here and that clears the soot at around 350-400 degrees celsius. As the internal face of the RMH riser is still sooty it can't be getting to that temperature. Should it? If we line it with steel then it will get hotter maybe and draw better.

So my list of to dos is...

1. Block holes in feed tube.
2. Leave tunnel as it is.
3. Possibly add steel liner to the inside of the riser.
4. Shave outside of riser to standardise riser-barrel gap.
5. increase manifold size to increase the stream profile there.
6. Lift outlet to a decent level.

The only one of those I'm not sure about is the lining the riser. Do you think the difference in mass between mine and yours makes that worthwhile.? The resulting, slightly reduced riser csa would certainly make the line of csa's more consistent...

Last point. Is the barrel-to-top-of-riser gap too much? If so a new steel riser-liner would take care of that if it pokes up half an inch...

Many thanks, to you all, for helping so far.


Jonny

Oh Yes. 100%. We burn a lot of firewood here. So there's no variance there. Thanks for the thought though.

Hi Gary,

Thanks for replying.

Interesting point.

I'm not sure! Steel would certainly heat up more. However, on the thread about small RMHs "Peterberg" - who clearly does a lot of this - seems to be using a square section tube for the riser made of insulating board...

Interestingly you can see from the two internal photos of our setup, that the tunnel and riser are made of two different bricks. Firebricks for the surface of the feed tube and tunnel. But insulating blocks from there on up to the top of the riser.

The first section has no soot so is clearly getting up to temperature and burning it all off. But as you can see the riser internals are all lined with a, albeit thin, layer of soot. So maybe it's not that hot in there after all...

I would have assumed that a riser made of only insulating material would not, itself, gain any heat but the internal temperature would be constant and reflect, exactly, the temperature of the gases contained within it.

but then again...?


Jonny

Hi Guys. I'm new here. Although I've been watching and reading for ages...

So, I bought the book and built a 10" system rocket stove following all the advice there and on this site earlier in the year and originally it seemed to work fine. lots of heat, no smoke, very little in the exhaust. Now it just billows smoke and flames out of the feed tube if it s full (and not so full) and the exhaust is a great plume of dark smoke. I've cleaned out everything and there are no obvious obstructions or anything. Sometimes it works (although, no big noise that you all talk about) but other times, well it just doesn't and it sets all the CO alarms off... (there are three!)

I've taken it apart and remeasured everything too. So here's the list starting with the feed. ...

Flue 10" commercial flexible s/s liner.

Feed tube 5.5"x 7.25" (csa 39.9 sq") 12" high
This also has 2 brick vents at the back where we left some slits out in the brickwork which add about 10" to the air intake.

Tunnel 7.25 x 7.1" (csa 51.5"sq)
Riser
Height from base of barrel 29.5"
Height from base of tunnel 39.25"
internal 8.1" x 9.25" (74.9"sq)
external

Barrel
dia 22.2"
height 31.6"

Perimeter of riser at top 34.5"
gap between riser top and barrel 2 1/2" (csa 85"sq)

gap between outside of riser and inside of barrel is 3" at the front and only 1" at the back. In total is seems to add up to about 75" sq .

Flue is 10" (csa 78.6"sq)

Internal Riser height is 3.25 times internal feed tube height...


So, in summary, following the path of the gas, starting at the feed tube end.
The feed is small (40") but it has extra vents at low level to allow another 10" (secondary?) air in.

It then gets bigger in the tunnel (52") then bigger again inside the riser (75") bigger again in the top barrel gap (85) slightly smaller on the way down (70-75") and then bigger on the flue (78")

So. Do I have two problems here? the top gap is too much and the side gap is not enough. I could add a quarter inch to the top of the riser and then shave half an inch off the outside of the riser...

Or is the problem further back?

Does the 20" mismatch in the size of the tunnel and riser reduce the updraft or should there be some help from Mr Venturi in the tunnel which would even things up? Do they really have to be exactly the same or can you allow the riser csa to grow a little...

I guess I thought that as long as the internal size of the riser was bigger than the csa of the tunnel it would work fine. Better than the other way round? Right?

Anyway, thanks for your time anyone... any suggestions would be eagerly absorbed!

Cheers



Jonny
p.s. I did try to attach a pic so you'd know what it looks like... but it doesn't seem to want to upload them. Is there a size limit?