Rocket Mass Heater - deteriorating performance

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?

I'm not an expert on these stoves, so please excuse my ignorance if this is an obvious suggestion, but I am thinking
you are not getting an optimum convection effect because it appears from the pictures that your riser is not made
of steel. The hotter that steel gets, the greater the convection, no?

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

Are you 100% positive that the firewood you are using has been appropriately seasoned? (http://en.wikipedia.org/wiki/Wood_drying)

It just occurred to me that this may be the problem...? Sometimes its the simplest things that get overlooked..

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

Jonny,

I'm not sure I understand your "extra air" slots and where they are located. If your are drawing air in somewhere else other than the top of the feed tube, then smoke might escape there. Note on an 8" system, a 14" barrel with the bung hole open is enough air to keep these burning ok. That hole is not large at all.

Also why a 10 pipe? Does it have any height outside? How much?

A chimney that is too big diameter and too short will create little draft. A little gust of wind may be all that is needed to create blow back out your feed tube.

MP

Is it me or is your duct system going "down hill"? Flat is ok, and anything up is more ideal, down, not so good.

MP

Jonny Clothier wrote:...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...

Jonny,
The reason behind this: less mass in the stove will result in quicker coming up to working temperature. The surface of this insulating board is almost as hot as the gases passing through, more mass will do the opposite, extracting heat from it.

Apart from the differences in c.s.a. through your system I've spotted one obvious and one possible fault.
The extra air inlets at lower level in the feed tube are no good at all, please close these as permanent as doable.

The transition from barrel side gap to the flue is the main culprit which can hamper a stove badly. It's not easy to explain, but there's a difference between c.s.a. of a circle where the gases are streaming straight through and a hole in the side where the gases streaming from the sides.

In short, take the circumference of the flue and multiply this with the width of the side gap. This is the real opening where the gases stream through, called the stream profile. This profile should be at least as large as the c.s.a. of the riser. A more comprehensive explanation can be found on this webpage.

To conclude this: the feed tube too narrow isn't a problem, usually. Too short a chimney is almost always causing trouble, especially where the house is a leaky place, being a better chimney than the chimney itself. If not done already, raise the end of the chimney slightly above the highest point of the roof. A system which is 10 inch c.s.a. throughout should be a killer when running properly!

Where's the spelling checker of this website? I'm still unsure about the readability of everything I produce.

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

Jonny Clothier wrote: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.

Jonny,
You are definitely on the right track.
#1. Correct.
#2. Correct.
#3. Highly doubtful. Leave this modification out and see how it will perform first.
#4. Correct.
#5. Correct.

Jonny Clothier wrote: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...

What I can see of your riser it looks like it's made of fire brick. That's a better solution than steel, because the brick will reflect part of the heat. The riser slightly wider than the tunnel is not really important. It's obvious, this stove hasn't been in good working order before so it will not burn the soot off the inside walls.

Jonny Clothier wrote: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...

Don't care about the top gap being too large for half an inch or half a yard. Too small is bad, it will hamper the running of the stove.

And at the end of all this: when running properly, you could try the p-channel as described in "Small scale development" to tune the stove up to latest standard.

This tent is huge! Probably too far from my home...

Best of luck.

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?

Jonny Clothier wrote: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.

That's even better than fire bricks, more of the heat will be reflected this way. The soot will burn off once the stove is running properly, don't worry. The surface of those bricks will be very much hotter than the inside. My vermiculite board riser has always been clean within one run, no mass to heat up will create a very hot, fast streaming core inside the riser.

Are you familiar with the phenomenon of laminar streaming? The gases around the hot core are lower in temperature and running slower. Built up in layers, slower and cooler when closer to the walls. And at the end, the last micron is at a standstill against the wall, this is called the boundary layer. When you are interested, here's a "Crash course in aerodynamics and turbulence".

Oh yes, further down that thread you'll find the point in time this secundairy air feature has been given a proper name.

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!

What a lovely home!

If I were to make only one change, I would install temporarily a length of 7" metal lining (within the heat riser), cut & flange the tube at the top & bottom to neatly secure/adapt/reduce the inside of your existing heat riser. I would not seal with mud or fill the void, just a suggestion to boost heat and most importantly air flow.

james beam

Jonny Clothier wrote:Or am I talking rubbish?!

Yes, I am afraid you are.
Most of the burning will happen in the tunnel, adding air elsewhere in the system will restrict the main inlet. That's why a loosely stacked prototype will perform sub standard, because of all the crevices. Making smoke back as a plausible result.

Ha ha!!! Well said!

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