Stove for a housebus (revisited)

[jgreen]

Hey Ya'll. I'm trying to get started on a rocket stove for my housebus (I have prior rocket building experience) and am trying to put together the right combination of materials that I have already gathered for free, design that meets my heating needs and will fit in the space I have for it in the bus (allowing room for plenty of fireproof insulating materials in the appropriate places). I'm stuck in two different areas.

First, I'm trying to figure out if I can incorporate my current cast-iron stove into the design of the combustion chamber or if it makes more sense to start from scratch with firebrick and build it up like the rocket design in the original Rocket Mass heaters booklet. I've tried to embed some pictures of my current stove to help with my explanation. Should be able to click them to get them bigger. I really dig the way donkey bulk feeds the rocket in his house via the back "ash drop + cleanout". I don't fully understand why it works so well, because I always thought (from reading the dimentia + proportia section of the rocket booklet) that it was critial that the horizontal burn tunnel be the "tightest part of the intestinal system". It seems to me that donkey's cleanout-turned-feed tube has a much larger cross sectional area than the heat riser it connects to. So maybe someone who understands that could explain that part to me. But assuming you can indeed connect a long burn tunnel to a smaller (in area) heat riser, then could I use brick and/or clay/sand refractory to line the inside of my cast iron stove (pictured is a quick version with bricks) and use the entire length of the stove as the feed tube + burn chamber.

Then build the heat riser on top connecting to where the stove usually has a 6" flue pipe attached. What I have pictured is a 3' length of 6" thick walled steel pipe.

I would insulate this as in any other rocket stove. The cast-iron stove has a sliding air-intake adjuster built into the front under the door. I figured that this could serve the same function as donkey's homemade sheet metal insert. I imagine I would need to create the right shape with clay/sand mix to transition from the box shaped tunnel to the 6" round heat riser. I'm not sure what that transition would look like, but it seems important at the very least to protect the cast iron that supports the pipe from quick deterioration. I would insulate around the body of the cast-iron stove in all directions. Any opinions on if this seems like a good idea?


I do have close to enough firebrick to build a more conventional downdraft feed tube and burn tunnel, however it seems like it would take a lot more work on my part to create a good space for insulation between the bottom of the combustion chamber and my plywood subfloor in the bus. I was envisioning putting down a layer of fire-resistant cement board (hardiboard) with firebrick on top on edge spaced apart to make channels that would be filled with perlite. Then another layer of hardiboard, then a course of bricks flat and then starting with bricks on edge to form the sides of the tunnel. Whereas the cast-iron stove has 4 legs that give me an 8" gap that I can fill with loose perlite and have a lot of insulation pretty quickly and easily.


My other big question mark deals with how I will utilize the heat stream after it leaves the combustion unit. I have a salvaged 17-ish Gallon electric water heater that I would like to heat and after much consideration, it seems the simplest and most straight-forward to just incorporate the tank into the flue so that hot gasses move over and around it. It seems best to put this as close to the hottest exhaust straight out of the heat-riser as possible. If I had more vertical clearance I might try to put it right over the heat riser, but as is I'll need to have it sitting to the side. So I'll need to reverse the direction of the rising exhaust and the easy way I know how to do that is to use a big upside-down barrel. But then I have a much bigger radiator than I need or want at that point in the flow! Is it effective to surround the barrel with insulation in order to keep the temperature nice and high or should I be thinking up a heavy duty 180 degree elbow that can attach to the top of my 6" steel heat riser?

I'm also wondering what the likely hood is of creating a dangerous pressure/steam build up in that water heater tank over the course of a longer burn. I dont really have a sense of how quickly a stove in this configuration can heat up 17 gallons of water. However my bus does not have a pressurized water supply and I'm not sure I'm going in that direction, so I'll need a way to fill the tank by hand anyways. I was thinking I would cut the top off of another old water heater I have and flip it upside down to make a big, open metal pot. One of the pipes coming out of the bottom would connect into the cold of my 17 gal. tank. Another pipe, located higher up inside this open pot would Tee into the hot connection of the 17 gal tank (the other part of the Tee would be my spigot for using the water). This would create a way to fill the tank (via manually filling the open topped pot) and also a backup pressure release in the form of a thermosiphon. Any reason this wouldn't thermosiphon if typical guidelines were followed (big enough piping, big enough vertical distance between tanks)?


One last thing. Does anyone know about what temperature galvanized sheet steel starts to release toxic gases? I have a bunch of pretty heavy gal. sheet that I would love to use up and incorporate into the stove, but of course my priority is to have a safe stove. My thinking was along the lines of using it to make barriers to hold loose perlite in place around the stove innards. I'm sure the wisdom of using this involves a lot of "it depends..."

Sorry for the lengthy post. This is a weeks worth of thinking about this to myself all coming out at the same time, in the same place. And I only have internet access on a couple days of the week. I would apprecate any and all feedback. Anything that comes to mind. Much thanks.

-Jay.

[jgreen]

A little update: I rolled her out onto the back porch and fired her up this morning (anxious to see if this idea is at all viable). So far, so good. Started a small fire right under the heat riser and then layered a bunch of wood to fill up the stove. A little bit of a smokey start, but it cleared up pretty fast and its been just heat waves coming out since then. It is lit in this picture..






It doesn't have a very strong draw right now though. Doesn't seem like enough draw to push the gasses through many directional changes. I'm hoping I can put that to the fact that the heat riser is not insulated, and also a good heat conductor (so as to be quickly loosing heat all the time to the cold outside air). I was trying to avoid insulating around the riser and taking it outside for another test because its going to be a bit of rigging to get that balanced on the stove... but I might have to do that before I commit to this route.

[swizzlenutz]

I would think that you need some consistency through your design. I can't see the inside of your stove so I don't know how you have it set up. I'm wondering if an 8" square burn tunnel of fire brick would be the right size for an 8" round tube. I would think the square would be bigger and maybe slow down your design a little. Being closed inside a stove like that doesn't really mean its air tight either. Maybe if you stuck a flue in there it would be more air tight or have fewer cracks to invade anyways. I like the idea myself because I have a small barrel stove I'd like to use. I'm just not allowed to have a wood stove in the house. Landlord issue. If I had a small one I could hide it and have the chimney running out th back window. something I would only burn at night to extend our fuel supply. Swizzle

[jgreen]

swizzlenutz -

i'm not sure i fully understand your reply... but i want to. can you clarify what you mean by "consistency of design through the design"? also, are you suggesting the 8" square and round parts because you think a 6" system is too small? i wasn't sure why you mentioned those dimensions. thanks for the response.

[swizzlenutz]

I was just spitting out numbers. If you have a 6 inch system then just go with that number. My comment about consistency probably isn't the best thought out idea I've had. Keep in mind I'm just the new guy here. My thought was that if you're going from a 6 inch square burn tunnel to a 6 inch round riser that the dimensions may be a little off and causing some inconsistencies. I'm just trying to guesstimate and help out. Sorry about the confusion. Swizzle

[Donkey]

Jgreen.. I wrote a big complicated response to your original post the other night... Unfortunately, moments before I was to post it, me finger slipped, smacked the "go back" key and I lost it all..
Anyhoo.. I think this idea is something worth pursuing for sure.. The question has come up several times lately, and I'm glad that SOMEONE has decided to take it on.

Your feed box IS quite a bit like me using the back, clean out side of my stove, I think you saw that already, so good hit.
I think what makes it work so well is in the way that I feed it. It's a "top down" fire, or in this case a front to back fire. Flame in front, the smoke (fuel) is forced to pass through the fire where is has to burn. As long as there is the right amount of air, it should burn pretty clean. If I stuffed the thing full then lit it from the back, I'm pretty sure it would burn VERY dirty.
Yes indeed, my "clean out" tunnel IS quite a bit larger than system size, though when it's stuffed full of wood, that size is effectively reduced. It's good to remember that even with a regulation (book designed) stove, it runs best crammed full. The reality is that what's needed is an air intake that is quite a bit LESS than system size. Seems somewhere between 1/4 and 1/2 system size.
When you fiddle with the air intake on your potbellied rocket, you will find a "sweet spot" in air intake size. When you find that sweet spot, carefully measure it and post it here, I'm particularly interested in that proportion.

As to feed-riser transition, it should be pretty rough. What you DON'T want is a smooth transition, which will cause laminar flow conditions. You want a lot of turbulence for fuel/air mixing, so make the transition into the riser abrupt and sudden. Perhaps even some kind of "turbulator" in the bottom of the riser could help here.

Insulation will indeed help. Insulate everything as much as possible. Perhaps, even use a more insulative material cast into the potbelly stove to take up the extra space, instead of bricks. Though light weight castings tend to be fragile, so some kind of balance might be found, bricks on bottom, cast all round the top or something like that.

As to galvanized metal, the stuff is toxic and I'm sure it's toxic at pretty low temps. Though you could probably build a fire outside, toss the galvie sheet on to the fire and burn off the galvanization before you use it. I really don't know what will be left over or how hot you need to get it to burn it all off.. Probably best to use something else, like a cut up 15 gallon drum or something.
When you use pearlite, you should mix it with some clay slip before putting it in place. Pearlite by itself has a way of finding the smallest cracks, getting out and blowing around the stove. Clay slip will help hold it together.

Using water for heat storage in this design (I think) is good idea. Steam explosion is a VERY real danger, so don't skimp on safety here.
If I understand what you were saying, as long as the pipes are set up properly, AND the water levels are ALWAYS above both pipes, it should thermosyphon. Also, with the top tank essentially open to the world, if you get steam flash, it will just vent out the top. This will avoid an explosive situation but you don't want to be standing next to it or you may (might) get showered with boiling hot water and steam. Again, don't skimp on this one.

I hope this was clear enough and helpful..
I'm looking forward to your results, so keep posting.

[grizbach]

This sounds like a fun project! I believe the melting point of zinc (galvanize) is about 400 deg f. We are flirting with this temp. as I have a galvanized pipe comming directly off my barrel. I'm getting about 400 deg vapors right at this point, the metal is slightly lower of course. I need to try and stoke this sucker up high enough to get rid of the toxins before it's inside.

My only concern is with the water heat. I think your going to have too much moisture in the air. Maybee if you were to run your open pot through a water trap such as under a sink. Then the surface area of the water exposed to your living enviroment would be only be about 2sq. in. I suppose you could just place some sheet metal over the pot also.

[jgreen]

swizzle - no problem about the confusion. i appreciated getting the response and the encouragement. I think I mostly needed the encouragement of other rocket stove people to push me toward giving this thing a try.

donkey - thanks for writing that up for me. i was hoping to get your perspective especially because I was inspired by the operation of your stove. i've written long emails/posts and accidentally lost them before. just gotta let go and start over (if you care enough to do it again).

reading the thread about your stove and now reading what you've written here has helped me see rocket stoves in a different way. I had only really pictured stoves as a brick combustion unit and heat riser with an inverted barrel over it. but I think I'm starting to realize that the J shape isn't strictly necessary to the efficiency of the combustion. It seems to me now that its function is to create a gravity fed fuel feed. and it seems to me now that the main reason for sizing the burn tunnel of a "regulation" stove to match the area of the heat riser is because its not designed to hold the burning/unburnt fuel. I'm not sure if that all makes sense or is entirely correct, but my point is that my sense of design is breaking free of my old rocket stove conventions. its cool.

I'm feeling good about putting out the energy to try this out for real. Your input about the stoking of the stove and air intake was really helpful. I'll be sure to post the dimensions of the optimal air intake and also pictures of the project when I get it working. I'm going to experiment with a homemade clay/sand mix for lining the sides of the burn/feed tunnel, leaving the surface textured roughly and as I get close to the transition into the riser, sculpting little ridges that spiral up toward the riser (an attempt at creating a vortex). We'll see what happens. I figure that if its homemade refractory, I can take parts out and reshape things later to improve it if necessary.

The water tank will have a dedicated (and new from the store) Pressure/Temperature release valve installed, in addition to whatever my backup thermosyphon turns out to be.


grizbach - thanks for the info on the galv. metal. after reading that and rethinking my design a bit more, I think I've come up with a way to use other materials where I would have used the galv. stuff. It seems simpler to avoid it in this application and put that stuff to another project. Your idea about the P trap in the hotwater system is helpful. Thanks!

For anyone who is curious... I got started on this idea because I'm working on the interior of my bus and its cold out these days. So I dragged in this old cast-iron stove that I had and lit it up as a temporary heat source until I built my 'real stove' which would of course be a rocket stove. I had routed the stove pipe through a metal collar (insulated) and out one of the bus windows. But on the first burn I smelled burning plastic and it turns out that the stove pipe was so hot it melted some plastic parts of my window, even though it was running through an insulated collar and had a lot of air space around it. So I was going to need to put the stove pipe out the roof or do something else more involved and I thought that I might be able to quickly throw a rocket stove together somehow using the stove and capture enough heat from the exhaust that I could safely run the flue pipe out the window. Then I saw donkey's post on his stove at home and it seemed like a way to do this. And now I'm just stoked on the idea of it...

[Donkey]

Good point.
It's important to remember what the purpose of any given design is. The purpose of the design in the book was to create a model that we knew would work, that most people could build without too much difficulty and that had the best compromise of convenience, power and usefulness.
Our experimentation at that time was all about keeping it simple and finding a model that we could reasonably say always worked. The design is conservative and doesn't take a lot of chances. In fact, some of the stuff that we discovered during that experimentation time, Leslie and Ianto purposefully left out of the book, for good reason..

In my way of looking at this thing, the rocket stove itself, the part that actually does the work, the ONE component that can't be taken away is the insulated heat riser. Everything down-stream of the riser is for convenience of feeding the thing, everything up-stream, for finding uses of the heat produced, this leaves us with a lot of options and possibilities to hang on either side of the thing..
The limits being our imagination and the laws of physics.

[canyon]

I have done this in building my "tester" rocket. I took a similar stove to yours (although made of 3/8") steel plate and welded on a 6" schedule 40 pipe and insulated with kaowool and a protective thin stainless cover. Then I welded a stainless keg on top of the stove over the whole thing (the regular barrel idea only welded for gas tight). I too wanted hot water so I welded another stainless keg on top of the previously topless "barrel" so the gasses hit the bottom of it like the cooking surface on a book version. It is of course tall. I think the tank on the side is better for most aplications. I would use two butt weld schedule 40 90 degree fittings to get it heading down myself in your situation but I am a welder and happy to get this stuff gas tight. I , like you are thinking of doing, then used fire brick to line the entire burn tunnel. The stove works great when burning dry wood, smaller splits, and top down/back front burning without smoke except start up. It is totally portable (with some help and ingenuity). I learned that the surface area of the "cooking area" is not very much although temps are highest. Having gasses pass around the water tank will do much better. Make sure you oversize your thermosiphon loop and provide a settling capability in your open head tank to particulate doesn't build up in the pipes. I manually fill mine and gravity empty it and although a pain, I only have to do that once or twice in a typical firing. Be careful about insulation too much on the outside of steel without firebrick to take the real heat. I recommended to a friend to insulate his fire box on the outside and the thing melted/burned away in a month. Whoops!
Anyway, kind of a lot in one post I can't remember all the questions!
Donkey posted a picture of my "tester" some time ago maybe under "water heater by canyon" ?? or something like that. Check out the old threads if interested.

[Donkey]

Here's the link to Canyon's thread..
The search function on these boards leaves somewhat to be desired.. It can't use words under 4 characters long... So "keg" is a useless search term.. Ugh!

[Donkey]

Canyon,
With your water heater, there's probably a similar air control to what jgreen intends to use.
So - I'm sure there's a "sweet spot" in opening/air intake size when you use it...
Have you measured the size? And if you haven't will you, and will you post it here?

[jgreen]

Yea... I'm thinking outside of the J-shape and barrel now...

Cool stove canyon. Very slick with the gas-tight welds. I have decent welding technique on steel with an arc welder but I don't have access to any real welding equipment at the moment... so I'm working with a gasless flux-cored thingie from sears (that I wish I never bought). It sticks things together and occasionally the beads look decent but it would drive me crazy to try to get it sealed up tight with welds. So I'm just welding things on for support and to hold everything in place.

For inquiring minds this is the update: I got the flue-gas / hot-water heat exchanger roughly put together today. I was able to utilize materials that I had in my junk pile and so did not need to go out and find/buy anything. I also layed out all of the parts of the stove in their approximate location to help me see how it will all flow. I decided to photograph this and put it in here for other people to see or comment on. I'm a visual kind of learner so I made a quick ms-paint diagram of the components and the flow path. It looks kind of like a kids crayon drawing. hopefully it is self-explanatory. I had to go get clay + sand from a friend's property this evening so tomorrow my goal is to insulate the riser, close it all up with cob and see what happens.

i'm also planning an experiment with my own idea for a 'vortexer'. more will be said later once I decide its a good idea or bad idea...

[woodman]

It appears to me this type of wood loading does away with the burn tunnel at the start of the fire, and doesn't come into effect till half the wood is gone, then the burn tunnel becomes larger in size with less wood taking up space. Was wondering if that changes things?
But with controlled air intake on a J type stove I wood think the wood feed could be any size to accommodate large splits.

[Donkey]

Nov 11, 2009 4:39:00 GMT -8 woodman said:

It appears to me this type of wood loading does away with the burn tunnel at the start of the fire, and doesn't come into effect till half the wood is gone, then the burn tunnel becomes larger in size with less wood taking up space. Was wondering if that changes things?

I don't really understand what you're asking here.. Could you re-phrase to clarify?

But with controlled air intake on a J type stove I wood think the wood feed could be any size to accommodate large splits.

Seems that controlled air changes things quite a bit. Yeah, it likely will make it more suitable for larger stuff. I'm pretty sure peterberg has some knowledge about this. His stove forced him to work out some of this.