Rocket Stove progressing.

Hi All,
I have finally got around to a test burn on my rocket stove. Pretty pleased, good flame, not too much smoke. I did get a bit of flame and smoke blowing back out of the fuel/draught opening. Would this be because the chimney is too low, about 2.2m, or is the fuel/draught hole too short?
Does the principle of the rocket stove mean that as everything gets hotter it works more efficiently?
I have attached a picture. Once I have finalised the design I will "pretty" it up.
Any comments, suggestions really welcome.

Hi Wow !! looks really good and exactly what I'm going to build for an outdoor kitchen this year

Yes the rocket works best once its up to temperature - but that doesn't take but a few minutes really.

based on my limited experience with my RMH I'd say the heat riser is probably is a bit short and the other point is, have you made sure you have a constant cross section area on the feed tube, burn tunnel and upriser ?? if you don't it'll smoke whatever you do !

Best Wisner advice was change one thing at a time - wish he'd said it sooner but hey he's oddly taciturn for a yank !!

keep up the topic need more pics as you go along looks like it'll be great when you're done

Roger

I'm sorry, but I gotta say this looks more like a Lorena stove than a Rocket stove. A taller heat riser may help, as well as a taller exhaust. Everything I have read says Lorena stoves need very tall exhausts to have adequate draft.

[quote=Craig Moore]I'm sorry, but I gotta say this looks more like a Lorena stove than a Rocket stove. A taller heat riser may help, as well as a taller exhaust. Everything I have read says Lorena stoves need very tall exhausts to have adequate draft.[/quote]

I'd have to agree with Craig - this is an interesting adaptation of the principles behind the rocket stove (cookstove) and rocket mass heater, more like a lorena stove or an ancient furnace. It does look like you have a reasonably tall, insulated chimney, but you probably won't get the efficiency or clean burn that you could expect from the more common types of rocket stove / mass heater.

With a mass heater or rocket stove, you include a chimney immediately below the cooking area to provide good draft and efficient heat. This raises the cooking area up higher than the fuel feed, usually by a few feet or more. There can be a second, final chimney for the exhaust (as on a rocket mass heater), or you can just release the exhaust around/behind the griddle for an outdoor cookstove.
With a lorena stove, you use the final chimney to draw heat horizontally under a series of cooking areas, much like the way a conventional metal cookstove uses baffles to direct the fire around and heat a griddle or oven on its way to the stovepipe exhaust.
Because the chimney is at the end, it doesn't get as hot as an early chimney, and you lose some of the benefits of draft and clean burn.

People are commenting on your heat riser being a little too short - I don't see that you have a heat riser at all, am I missing an earlier post?

-Erica W

Looks like a Rocket Stove to me, but the brick is hiding a lot. A Lorena stove can be a rocket stove if it uses a rocket combustion chamber, among other modifications. It is not a RMH so don't get overly distracted with comments about the height of risers, etc. Are you using an insulating liner? Iirc, one major key to getting and maintaining clean combustion and good draft in a rocket stove is to use an insulating liner in the combustion chamber and flue -- but there are Rocket stoves without insulation (mine), though they are not (may not be) as efficient. You do need to make sure that the area of the flue is pretty much equal from the combustion chamber opening to the chimney. In your setup, it might get a little tricky as the flue gases pass under the grills. What reference were you working from to build your stove?

If you add more height to your chimney, make sure you don't increase the draft so much that you start pulling flame off the fuel (like blowing out a candle) or moving heat through the system too fast.

In my experience with my little uninsulated backyard one-pot rocket stove/barbecue, it heated up the bricks/pavers quite a bit before they were mortared, but once sealed, I can cook for an hour or so and not get much heat through the bricks. Rocket stoves are finicky beasts, so you will have some trial and error to go through in order to get things to where you are comfortable. I keep a propane torch handy to help things along when needed.

Your fuel is a major variable. It can be be too thick or too thin, too wet or too dry, split or unsplit (split is almost always better), hard wood, or soft wood, too many volatiles, etc. Ambient air temperature, barometric pressure, humidity and wind must also be considered. You will have to make adjustments for all these variables. Practice makes perfect.

A grate at the end of your fuel shelf can make things work better by keeping the coals where they will do the most good. Ideally, the coals stay on the tips of the wood until they turn to ash, but in practice, they often will drop off, which sometimes snuffs out the flame.

You could make it a J-tube Rocket stove so you could feed the fuel vertically, but you will definitely need to insulate the combustion chamber and flue riser to get the most heat to your cooking surfaces.

A very ambitious project. It looks pretty good, but you can probably tweak it some more. Good luck.

Regarding chimney/flue riser height for a rocket cook stove, I copied the following from a rocket stove design guide by Peter Scott:

"...the ideal relationship between the fuel magazine (or feed chamber) and the rocket chimney is 1:1.5. Which means that if the feed magazine were 100 mm in diameter then the Rocket chimney would be 150 mm (note: the rocket chimney is the vertical part of the combustion chamber). If it was 115 then the
Rocket chimney would be 172. Again these are ideals that might not be practical in the real world."

"When deciding on the height of the combustion chamber, remember:

Taller elbows produce less smoke but are slightly less efficient - due to the greater distance between the pot and the radiant heat of the coals and the higher losses into the stove body.

Shorter elbows produce more smoke but have greater heat transfer due to the closer proximity of the pot to the radiant coals."

A different design manual at www.rocketstove.org suggests a chimney height of 3 times the diameter, or width, of the fuel feed opening. From the photos, it appears that Nick is definitely in the ballpark with his stove height. The soot stains above the fuel feed opening show that he is having some draft problems. Too many possible variables hidden behind the brick in the photos to know what might be the problem. Flue cross-section, insulation and chimney height (at the end of the stove) are all reasonable suspects. It is an outdoor stove so wind might be causing some smoking problems as well.

Hi further to the other responses - yes it's sort of a lorena stove but - I assume - a rocket adaptation .......... that was what I was planning anyway

Looking at the photos the burn tunnel is approx 10" or 11" wide by 5" high - which is huge !! for comparison I was planning a 6" j tube version.

Apart from the basic problem of cross sectional area within the burn tunnel and riser - it surely doesn't matter if the "flue" cross section is way too big, as it pretty much must be in this case to heat the 2nd hot plate.

I imagined a primary hot plate above the heat riser (as per usual practice) and then a 2nd warming plate set into an otherwise insulated flue. In this case the only areas you're looking to heat up are the hot plates so every other surface is insulated with vermiculite cob. The flue would be a wide shallow channel larger than the CSA of the rocket. Does that make sense ??

The height of the chimney is immaterial With a rocket its a vent not a chimney different beastie all together ??

Roger,

The conversion of the Lorena stove to a Justa style rocket stove required using a L-rocket as the combustion chamber, carefully maintaining a constant CSA from fuel feed to the chimney, sizing the plancha or grill (or conversely, the combustion chamber/chimney) so that there was approximately 1 inch of space underneath it for the flue gases to pass through, and insulating the combustion chamber and flue to minimize heat loss in the system, improving combustion and draft.

If you don't want to build one, you can buy an Ecostove. There are various manufacturers worldwide. I don't know what the availability or shipping costs would be for your area.

There are a few design manuals available on the internet. www.rocketstove.org has a couple, also Aprovecho as well as bioenergylists.org have some.

If you want a barbecue with direct flame, you will need to have a separate rocket stove for it. I have one and it works alright. It needs to have a bigger combustion chamber to increase the btu's. Sometimes, I need to finish the meat in the oven.

Hi thanks for the link - agree re the insulation - am surprised by the constant CSA for the flue ! I'd have guessed that a wide narrow flue would decrease airflow because of increase drag - I'd step up the CSA in the flue to avoid this ? No ?

one main question why an L tube ? tbh I've always though that system missed the point of a rocket and requires tedious constant fiddling - whats the benefit or whats the problem with a J

I can cook on top of my RMH which is a 5" system heating big raised beds in a greenhouse so a 6" just running 2 hot plates should be overkill

What am I missing

Hi Guys,
Thanks for all your input. I'll try and clarify what I have built and answer various questions at the same time. For the last few years I have been cooking outside over an open fire with a grill over the top of it. Whilst this is great fun, the food tends to be a bit charred, there is too much smoke and you get through loads of wood.
I wanted to use wood as my fuel, not charcoal, reduce smoke as much as possible, have more control and enjoy building my project.
initially I thought of a lo trau, but I don't have access to a suitable fuel.
When I read about rocket stoves, RMH, lorena stoves, etc, I thought I could probably combine a bit of all these beasts and end up with something approaching what I want.
My stove is built from standard 100mm thermalite block, this is an insulation block that we use in the UK. The cross section of the feed tube is 10" x 7" split between fuel and air at 5" so the fuel area is 10" x 5" and the air 10" x 2". the fuel/air tube is 15 1/2" to the start of the burn chamber. The burn chamber and heat riser is 7 1/2" x 9 1/2" in cross section. from the bottem to the top of the fuel/air tube 7" and then the heat rise is a further 15" giving an overall height of the burn tube/heat riser of 22". The cross section of the area under the second hot plate is 5" x 6" and a total of 34" long. I can easily make cross sections smaller if it will improve things, I'm not quite sure about all the technical stuff. I sort of assumed that if it was slightly larger it would be bigger and hotter. This seems not necessarily to be the case. Much of my dimensions were dictated by available materials,
I have attached some more pictures so you can see more deatil. I have removed the hot plates as I am hoping to get a purpose made griddle or plancha to fit the top.
Pic 1 is looking along under where the hotplates would be if I hadn't removed them.
Pic 2 is looking into the feed/air tube.
Pic 3 is looking down from where the first hotplate would be looking down the heat riser into the burn chamber.
The chimney is about 6' tall and 5" in diameter.
As always I am grateful for our comments and suggestions. Putting it basically if i cut down the cs of the whole unit to that of the channel running under the hot plate, ie 5" x 6", would I improve my stove?
Cheers
Nick

Nick Sellick wrote:Putting it basically if i cut down the cs of the whole unit to that of the channel running under the hot plate, ie 5" x 6", would I improve my stove?

Basically yes, if you also make the chimney 6" in dia. Right now the burn chamber is 70 sq in. The heat riser is 71 sq in. and the area under the griddles is 30 sq in. The 5" chimney is only 20 sq in. That is a recipe for smokeback. Better for cooking would be to have the area under the griddles shallow and wide (10x3 or 15x2) for more surface area in contact with the heat.

Roger, they have been building these cook stoves for a long time with general success. A wide flattish cross-section under the cooking surface does not slow down the flow enough to be a problem. You want the hot flue gases to scrape against the cooking surface to release their heat.

Nick, your story is similar to mine. I started with a simple fire pit with a grate made made by a previous owner from an oven rack. It used up prodigious amounts of wood, but it was fun and I needed to burn about 400 feet of 6 ft cedar fence that had blown down and a pile of rotting firewood that came with the house.

Your deep flue channel under your griddles is moving all that heat right past without giving much away.

If you want to get all scientific, you could build the stove around the width of your griddles. If they were, say, 16" wide, you want a 1" gap for the flue gases so your combustion chamber would need to be 4" x 4". Your 5" x 6" is probably close enough. as you also need to provide enough btu's to get the heat you want and the 1" gap probably has some wiggle room.

Check out the Justa and Ecostove designs to get some ideas about transitions and gaps. Here is one design manual:

http://www.green-trust.org/freebooks/justastoveplans.pdf

Andrew, Sorry, I'm probably being a bit dim here, but what do you mean by the 1" gap for the flu gases?
If it ever stops raining here I am going to make the whole thing smaller and see what happens. I can always make it bigger again if it's no good.

Nick, right now, you route the flue gases through a 5" x 6" channel under the grill/plancha. You need to flatten out that channel to the width of your grill/plancha and around 1" in height. To keep the cross-section area of the flue constant with a 5" x 6" combustion chamber, you would need to make the gap under the grill/plancha about 2" in height, assuming your grill width is around 16". Just to confuse things a little more, because you use an external chimney, you can make the flue under the grill/plancha closer to 1" high, because it provides additional draft.

Now, since someone mentioned you are using 5" dia. pipe for your chimney, you would use a 4.5" x 4.5" (4.43" x 4.43" to be precise) square combustion chamber. That would give a 1.25" (1.23") gap with a 16" wide grill/plancha, according to the design manuals. If that seems a little small for your application, you might want to go with a 5" x 5" or stick with your 5" x 6". As far as strength of draft, the diameter of the external chimney may be compensated for by its height. Some Ecostoves' external chimneys are smaller in cross-section than the internal combustion chamber/flue.

To see examples of the Ecostove, go to http://www.english.ecofogao.com.br/index.html also search for ecostove or justa at www.bioenergylists.org

Hi

Sorry - keep butting in on this thread but its all good stuff !

Andrew it all makes sense, from my previous (albeit limited) experience with rockets and I'm very happy to use a system that I know is going to work straight out of the box

but .....

One of the things I found accidentally while building my RMH is that a short increase in CSA in the flue slows the flow rate and you get a greater transfer of heat in that section ( I put a 4' length of 6" ducting in an otherwise 5" flue) This means I can concentrate heat in one area which for my purposes was useful. Wouldn't the same be true in this set up ? A slight increase in csa under the hotplates to maximise heat transfer would be useful wouldn't it ?

Also have you found a chimney necessary ? I mean as a chimney rather than as an extravagant vent ......... are the gases still hot enough for a chimney to work and have you found the extra draw provided by a chimney necessary for the rocket to work ?? Again on my RMH (obviously a different thing altogether but still) adding a chimney slowed the flow rate - vent temperatures are very low so there's no chimney affect.

One last question ! - why not a J tube ?? I'm not the best cook in the world and fiddling around poking wood into an L tube seems like an extra level of hassle I could do without

sorry again for all the brain picking

Roger

Hi Roger,
Don't apologise, I'm loving all this interest in my humble project.
I haven't tried it without a chimney, but I am particulalry keen to get any smoke that I do produce up and out of the way. Given the pictures that you have seen, how would you suggest I change to a J tuibe. You are absolutely right, it is a hassle feeding fuel in.
I guess that I would also need to change the fuel/air feed tube in some way.
I get the fact that the more hot air that touches the undersidde of my hotplate the better. Can I slow it down more with baffles or something without affecting the airflow?
Thanks again.
Nick

Roger and Nick,

The RMH is a different animal to a cook stove. There are some valid comparisons, but not many. Much experimentation has already been done with the rocket cook stoves. If you want something that has worked, stick with the proven design criteria. There is always room for innovation so playing with the CSA is certainly an option. I, for one, consistently disregard instructions in favor of experimentation, when I have the time and the budget. It is a great way to learn.

In the rocket cook stoves, the gap next to the cooking surface is key to rapidly transferring large amounts of heat from the flue gases. You want it to be as narrow as possible and still maintain sufficient draft in the system. An external chimney at the end of the system is key to maintaining sufficient draft.

The original rocket design was a J-tube. The L-tube, or rocket elbow, was used because it allowed simpler construction, was simpler to get started and make adjustments, and cooks were accustomed to feeding fires from the side. Another benefit of the L-tube is that it puts the fire beneath the pot, so it can receive radiant heat from the flames and there is less convective heat transfer to the stove body (insulation just slows it down) thereby increasing heat transfer efficiency at the cooking surface (In the RMH this is not an issue, as you want to extract heat throughout the system.).

The Ecofagao uses a system of baffles in the gap under the cooking surface to improve heat transfer. I think that the details of the baffles is probably the patentable item in their system.

thanks Nick Thanks to Andrew too for putting up with being quizzed !

ok really interesting re the Chimney - utterly different from an RMH where there is insufficient heat left in the exhaust for a chimney to work. Nick absolutely get your point re smoke but on an RMH after 2 or 3 minutes its producing CO2 and steam no smoke at all............ bit baffled as to why this would produce smoke.....

I get the idea of the narrow flue - glad Nick posted this because I wouldn't have built it that narrow - I agree, never read instructions they're for wimps !! but I reckon picking brains is different

The very simple baffles in a lorena stove work in an obvious "Ohh yeah" kind of way when you see them in action may be worth an experiment with something similar.

If the weather ever improves and I'm not too busy collecting 2 of every animal and building a boat, I'll set to work soon and start putting something together - expect more questions then Andrew !!

Roger