rocket bell stove

[Donkey]

Peter..
I'm not exactly sure why.. But Rumford fireplace proportions popped into my head the other day and I'm having a difficult time shooting them down..
In particular, the 1 to 10 proportion between the throat and the front, open face of the stove..
How does your stove fit here?? What is the size difference between the fire door and the riser pipe??
I can't really explain why it would matter.. What do you think?

[peterberg]

Difference between the heatriser and the loading door is 1:6.7. True, when the heatriser and chimney are warmed up, the rocket keep running even with the loading door open. In fact, it's then generating an awful lot of noise.
But the analyzer do tell me the efficiency is quite low at the same time.

But I don't see any relevance to the proportions of the Rumford, sorry.

[Donkey]

I really don't see the relevance either.
Really, it's just my hind-brain tossing up junk that the fore-brain then tries to make sense of. Neither could I make sense of it, nor could I sweep it out.. So.......

What I'm seeing with the burn patterns on the inside of your stove is the tendency for smoke to swirl about in the low pressure areas at the top, depositing creosote and some soot before finding the burn tunnel. It's a similar pattern to what I've experienced here on my attempts to reduce fiddly-ness, etc.. I've not been quite satisfied with it here and am still looking for a better way.
Your stove seems to point in some very interesting directions. The bell, despite my earlier skepticism, looks as good as gold. I'm sold and WILL be using the idea where appropriate in the future. Thank you.

Have you tried bringing air in above the fire-door? Would intake air aimed at those upper corners change flow dynamic to remove those low pressure spots?
It seems to me that regardless of the rest, the air intake volume needs to be system sized.
And, how did the steel pipe trick do? Got results there yet?

[peterberg]

Donkey wrote among other things:
"Your stove seems to point in some very interesting directions. The bell, despite my earlier skepticism, looks as good as gold. I'm sold and WILL be using the idea where appropriate in the future. Thank you."

You're welcome. Beware however, don't make the bell too voluminous. Sometimes the bell I've been using threw up problems when the chimney/outside air temperature difference was too small. The volume is around 800 liters equivalent to 28.25 sq ft, with a 6" system inside.

The air intake is about half of system size. The best results were obtained with that.
Didn't try the above door air intake yet. Mainly because I've no welding equipment anymore.
The steel pipe trick didn't work at all. No difference with or without it could be measured.

I am heading down another route now. I've taken the bell apart, next weekend I am building again. The plan is to change the rocket to one that's 4" deeper. In fact, the stoke hole will be risen up to the ceiling. After that, the bell will be closed on another height, say 4' 4".
Three or four test runs later I'll open it again and change the rocket to one that's horizontal fed, still at ceiling level. Then several test runs later I 'll be able to compare the results, which one is best.
Both the same results? Then I'll go for the simplest solution, i.e. the horizontal or so-called side-feed rocket.
All for the sake of "keeping it simple".

[chronictom]

Could the problem with the startup and the bell, just be that it has to heat that volume up before it starts pulling? Having a primer box at the base of the chimney (after the stove) would fix that, no?

[peterberg]

It would fix the problem, yes. But the reason is the low exit temperature, not the warming up of the bell. The problem (sometimes) will arise after 10 to 15 minutes, because the flue gases are starting to condensate in the flue. And then after some time the draw is coming to a standstill. Warming up the flue with a paint stripper usually would fix the problem.

In short, the bell would work better with a short and insulated chimney. The one I am using is masonry, and very long.

[chronictom]

as in chimney blocks and clay flue liner?

That' what my back chimney is like, so I hope that's not what you mean... lol

[chronictom]

Peter, the point of you using the firebox was to reduce screwing around with the fire right?

Why not just stick with the J tube rocket, but instead of using sticks, cut the wood in chunks use it like a pellet stove type idea?

I can see a couple of issues with it... you'd spend more time prepping wood for one... making some type of jam free feed system is the other...

But then you would accomplish what you are after without the fire box.

Also, and maybe this is a better question for Donkey, do you have any idea how long wood takes to burn in a rocket? As in, if you put 2 foot pieces of a certain type of wood in, they would last x number of hours (or minutes?), 3 foot is this 4 foot is that sorta thing?

[Donkey]

Peter,
Just wondering..
Your air intake has been consistantly smaller than system size..
Why is that?

[peterberg]

Donkey,
The best results were obtained with an opening of half the system size or less. In my design so far, that is.

I've tried full system size air inlet, but it'd cool the fire down and gave a lower efficiency. This was even the case with the first iteration, a pure rocket stove inside a bell, remember? These test runs were not on the dirty side, surprisingly.

I do wonder, how a rocket stove with a bench system would perform when measured with a Testo gas analyzer. It would be very interesting to be able to compare the results of different systems.

[Donkey]

I do wonder, how a rocket stove with a bench system would perform when measured with a Testo gas analyzer. It would be very interesting to be able to compare the results of different systems.

Agreed!
Unfortunately, their price range is a bit out of my budget right now..
Perhaps later for me.

[peterberg]

Progress has been made!
I've spent three weekends to change the prototype 4 times. After the last update the stove is behaving itself. No spikes in the CO readings anymore, consequently high efficiency, expected end temp readings.

This is what I've done:
lifted the entire rocket and entrance of the rocket 4",
created a 2" space between the entrance and the ceiling of the primary combustion chamber,
fitted two copper pipes of 1 3/8" diameter to end left and right of the rocket entrance.
Incoming air is divided between under air via ash pit, over air via the loading door and tertiary-air via the copper pipes. All three openings are individually manageable and separated from each other.
Under air a very little bit, over air half of what I've been using before and tertiary air wide open. The pipes are running vertically through the combustion chamber in the far corners left and right.

Before, I've had the pipes feeding from the ash pit. That was a mistake, since feeding from outside the stove the results are so much better.
I've had the rocket 8" deep, 4" deep, and horizontally. All with the opening at ceiling level. The thing did behave horribly during test runs with that configuration. At various points during a test burn the flames where too dominant and the air couldn't enter the rocket anymore. With sky high CO readings and even a lot of smoke from the chimney as the results.
One change before last I've added the 2" space between ceiling and entrance, results where better but not excellent. Last change I've added the pipes feeding from outside, which did the final trick.

See the picture for current configuration.

And this link for a graph of one out of three similar test runs.
pberg0.home.xs4all.nl//pictures/large/test081006.gif

Efficiency is not very high, because the bell is a lot smaller than before. But the low CO line is very nice! This is a run with no refills, and one batch consisting of 8 pounds of softwood. The stove was not entirely cooled down at the start of the testrun.

Next phase: building a second bell on top of the first one, and try to get the most out of it.

[chronictom]

Peter, have you considered this? Move the firebox up even higher, have a steel grate on the bottom with the exit flue going straight down a few inches, turning 90 degrees, run back behind the firebox, come up through a rocket tube to the top of the bell. Have the air intake come in from the top of the firebox, so it is being drawn down through the wood, into the flue/rocket, carrying any ashes, gases and residue right through the highest temperature zones.

It may give the benefits of the firebox (larger loads/longer burns), as well as eliminate the ash and lower the emissions (not just the CO).

[peterberg]

I've thought it over. At first it looks good, has been done earlier. One commercial stove do use the principle, the Tempwood. It's a top-lit downdraft stove with the kindling on top.

As far as I can see, your idea has two serious drawbacks. Firstly, it's difficult to get enough air into the down fed rocket, because the pile of wood is on top of it. Secondly, all the ash and pieces of charcoal will go into the rocket. In that way it will clog up very fast.

About the CO readout: it's being used as a tell-tale sign of all other emissions. Especially during the height of a burn, not in the glowing phase, however.
CO can be very high at the end of a burn with particulate matter low at the same time.
My goal is to have the CO levels low over the entire burn.

[chronictom]

I'm not positive about this, but I think if you are getting top temperatures, what is left after burning isn't enough to clog anything, but even if it was, instead of the elbow under the fire box, a T with a cleanout, and a tall riser inside the bell would solve it.

The draw issue could be a serious bitch though, unless you had some type of bypass system which is getting back away from the point in the first place... having a primer box could solve it maybe...

In case you are wondering, after being out in the country again, and filling our current box stove every 25 minutes or so, made me appreciate the efforts to include a firebox... lol

We have lots of wood and I have some ideas for making a rocket run longer that I want to try out, but a firebox that you could put 5 or 6 hours worth of wood in would be really nice, especially over night.

If the inverted fire can't be made to work, then I think putting the rocket riser over top of the firebox, with the air coming in from under (but over the 'ashpan'), and burning straight up into the riser would be next. Leaves a place for ashes to drop to, gives lots of draw right away and can have almost as big of a box as you wanted... no?