Flue Gasses - In's & Out's

[Teach]

Hey all,

Rocket heaters are always emphasized as requiring the same size opening all the way through them from start to finish .........8 inch feed tube through to the riser exit at the top or what ever size your system is it should be the same size all the way through.

I have an 8 inch chimney and I am wondering if anyone has done any experimenting with doing things like building a 10" rocket system to get more heat output and then having an 8 inch exit. The idea I had was to build a 10 inch rocket within a bell like Peterberg's first rocket bell and then exiting through an 8 inch pipe from the bottom of the bell.

Has anyone done any experimenting with mix matching pipe sizes?

I thought that if the rocket was the same size throughout it, it would step down in size at the exit where gasses are cooler. Thoughts?

Teach....

[canyon]

My thoughts are (unless someone else has experience pointing otherwise) to build it and share your experience! We need more real rocket bell experience!

[peterberg]

Feb 25, 2009 11:56:17 GMT -8 Teach said:

Has anyone done any experimenting with mix matching pipe sizes?

I did. The first rocket/bell combination was a 7" system size with a 6" chimney pipe.
A bastard to get going, but once heated up the thing was running very smoothly.

[Donkey]

Try it!
Just try it in a non-permanent way so it can be removed or re-worked if there are problems.

Check out ekw's post on the Gas volume with temperature change? thread.. It might help.

[Teach]

LOL Donkey, thats where I got the idea actually. I read that and then wondered whether a 10 inch system feeding a large bell ....if the cooling affect of the bell would be great enough so that an 8 inch exit would be large enough.

I'd hate to build this even in a temperary manner to discover it did not work. I prefer to do my homework first. I'd have to tear apart the bell too just to get to the rocket to resize it.

I had no sooner thought of building the wall of the bell across the combustion chamber so that the feed tube would be outside the bell and the riser tube inside the bell and Peter posted it suggesting the very same thing I had been thinking. I'm pretty sure this is how I am going to build my heater. I had considered building a fully contained rocket complete with barrel that would then feed a bell but have since ruled that out. I just want the ease of the outside vertical feed tube.

I'm also going to try to add secondary air into the mix but at the bottom of the heat riser at the end of the combustion chamber. I want to try to direct the secondary air into the middle of the stream so as to mix it up more and work it out and away from the middle just long enough to thoroughly introduce and mix it with the combustion gasses. I will probably use two 1 inch black iron pipes for the air tubes. I will bring them in via a similar fashion as Peter did.

One other idea I had was to add a cleanout slash second primary air supply purpendicular to and at the bottom of the feed tube forming a "T" like junction at the bottom of the feed tube. The air path when open would travel from the cools side of the fuel through it feeding the flames and straight on through to the combustion chamber.

My thoughts on this were that when a person were to really fill up the feed tube that you would really be limiting the amount of primary air that would be able to be pulled down the feed tube to feed the fire. You could then place a lid over the top of the feed tube reducing air from above and in turn open the horizontal air flow from the cleanout slash air supply giving more oxygen to the fire and at the bottom of the sticks where they are burning rather than trying to pull air all the way down from the top of the feed tube. If it does nothing to aid in the opperation of the heater it will still provide an easier access to remove ash that would allow direct access to the combustion chamber rather than trying to get a tool or your hand in around that 90 degree bend.

I'm still trying to work things out with the limitations of my basement and the cold external chimney I have but I am definately making headway. Once I get an answer back from Peter with regard to the clearances of the riser to the underside of the pavers I will be able to finalize my design plans. Fingers X'ed.

Teach

[Donkey]

A ten inch rocket stove is some serious business.. You have the potential for producing some real heat there. Maybe even more than you need/bargained for..

Rocket Stoves (the down feed type) seem to burn best stuffed full. This suggests that they don't actually need the full throated input of air. Peterberg has experienced this and noted it in his stoves as well. It would be nice to pin it down exactly, though it seems somewhere between 1/3 and 1/2... If memory serves (without searching), peterburg said something like 40%.. ??

You will notice, when you bring air in from below and cap the feed that smoke will be drawn into the feed and swirl around there a bit before finding it's way into the burn (when closed). It's not really a problem per se, though the area may get a little gummy with creosote and when you lift off the lid to feed it, it will smoke into the room a bit. You may need to provide a way to block the bottom air temporarily as you feed it to avoid this.

[Teach]

Donkey;

I have roughly 2600 sqare feet to heat so serious heat is what I need. I never had realistic expectations of heating the whole area just from the rocket but merely as a means of supplimentation........the greater the suppliment the better.

Great info on the way the rocket behaves when stuffed full! Makes me then wonder whether or not it would be worth while or not to add the secondary air I wanted to add to the bottom of the riser tube...?...?

Peter;

Earlier you stated that the single bell set up was hard to light. I got the impression from earlier posts in the Rocket Bell thread that it was only hard to light initially when the heater had not entirely dried. Was it always hard to light or just initially?

You also said that once fired and warm that it performed very well.

Could the cold firing problem be overcome by the addition of a bypass like you incorporated into the double bell system.......where by you fired it up with the bypass wide open so all the gasses go straight up the chimney and then once the chimney and bell were hot .......close the bypass to then draw off the gasses as intended from below for the single bell?

[peterberg]

Mar 1, 2009 9:42:00 GMT -8 Teach said:

Was it always hard to light or just initially?

You are right, later on the difficulties vanished for the largest part.

Could the cold firing problem be overcome by the addition of a bypass like you incorporated into the double bell system...

Not necessary with a pure rocket heater inside a single bell, I would say.

I am not convinced about the necessity of a secondary inlet in such a system. On top of that, the air feed shouldn't be in the bottom of the heat riser. Just before the entrance of the burn tunnel would be better, but the primary air from above already cater for that. The air feed from above is something you can't avoid, you need it to keep the fire down at the bottom of the feed tube.

You are aiming for a very large system, I am very curious how it will perform!

[Teach]

Well Peter sometimes my imagination gets the better of my common sense Although I have a large area to heat it has only ever been my intent to supplement that need for heat with the rocket stove only because I did not think it was capable of heating such a large area all by itself with only an 8 inch system.

I have spent so much time researching and experimenting with wood gasifying stoves that I can't seem to get out of my head the need for secondary air. Yet every rocket system I have seen or for that matter even heard about burns very very cleanly and virtually no smoke is expelled from any of them. So I'm also questioning the need for a secondary air supply at the end of the burn tube where it meets the bottom of the riser tube. I am suspecting I might be trying to fix a non-existant problem so I am going to drop that idea too.

With my first floor framing just 8 feet above the basement concrete floor I have worked out the dimentions for a 40 cubic foot bell that will only be just over 6 feet in height when I place the pavers on top. With 40 feet being the Max you would recommend Peter I'm hoping there will still be enough heat left in the gasses to get up and out the chimney when it gets really cold. I should just barely have enough clearance inside the bell from the riser top to the underside of the pavers to get that 3 foot min.

I priced out bricks today...............I almost fell on my keester! Even standard red clay fireplace bricks are between 70 and 99 cents a brick! Yowzers!!! Cement bricks that are solid/not hollow in the jumbo size which is 4x4x8 is the same price almost as the 2x4x8 clay bricks. Twice as much brick but only 15 cents more making them 95 cents each.

Peter, on your bells..........even though in your cad they are illustrated as red brick...........in your actual construction from your photos it looks like you used this same kind of concrete brick that I mention above. For a permenant type installation, what type of mortar would be best for these concrete bricks? Around here they are used or were used a lot in the construction of flue lined chimneys and it looks like they used a plain cement mortar. The building codes have cracked down on the use of fireclay/sand mix for fireplaces and as a result most of the building supply outlets don't stock it anymore and only sell the pre-mixed fireclay mortar which is what I will be using for the rocket itself but I need advice as to the type of brick and the type of mortar to use for the bell. Anyone?

I'm so close to buying material it's starting to get exciting.... ;D

[peterberg]

Mar 2, 2009 16:03:18 GMT -8 Teach said:

Peter, on your bells..........even though in your cad they are illustrated as red brick...........in your actual construction from your photos it looks like you used this same kind of concrete brick that I mention above.

The colour of the actual construction is misleading. It IS red brick, and I have sponged the outside with clay/sand to fill up every crevice.
I am not able to advice about mortar. Maybe normal cement mortar would be sufficient, but I am not sure about that.

[Teach]

Is doing a mock up firing with uncovered pipe fairly accurate in determining how long your run can be without condensation or will cob covered pipe give up heat more rapidly?

[Donkey]

Cob covered will give up it's heat quicker..
You could try shoveling dirt (or better yet sand) over the pipe as a rough equivalent.