rocket mass heater questions

Hello, I am new here and have some questions and thoughts on rocket stoves. A little background to ease into this post: I build industrial equipment for a living and own a small machine shop, fabrication shop and foundry. One of the areas that I work in is the design and construction of high temperature equipment like foundry furnaces, heat treat ovens, heat exchangers, and some boiler work. I want to build for our next house, green house and shop a hot water heating system that uses multiple heat sources, solar, wood and a back up fuel. I am a tried and true scrounger and material re-user and have been collecting parts and materials for this system. I like the idea of using a rocket stove, I use this term because the thermal mass I want to heat is water, and bring in some other ideas. First; having a background in using refractory, I am considering making molds to form the feed chute and burn tunnel in a hard dense castable refractory that will be much like fire brick when done. I thought of using a lower density castable for the vertical combustion chamber that would heat up much faster than the denser castable. I have wondered about a few things; would there be an advantage to introduce pre-heated combustion air into the combustions chamber to get better secondary combustion? This could be moulded into the refractory of the burn tunnel. Also, above the combustion stack is the horizontal surface as in the drums that I see being use, does this horizontal surface add to more complete combustion or is this just used as heat transfer? Is there a optimum vertical height to diameter for the combustion chamber? Has anyone used chipped wood or saw dust for a fuel source or do the smaller sized pieces get sucked up the combustion chamber? My plans for the heat exchanger is a horizontal three pass fire tube boiler. I am still debating whether I want to run the system at low pressure, 15 psi or at atmospheric pressure. I like the safety feature of 0 pressure, but low pressure has some positive design features too. I would appreciate any thoughts and suggestion on this. I have enjoyed reading the posts here and have gleaned a lot of information, but if I can save reinventing something or some of the trial and error that would be great.
kent

The flat plate is used to heat a kettle. I don't think it's a necessary part of the design.

Pre-heated air should help a lot. I think standard designs have the fuel intake & air intake as the exact same aperture, and accomplish re-burn just by more-thorough mixing, for simplicity's sake. It might get difficult to design well, if the primary burn were somehow choked down and an auxiliary air intake happened after that. Pre-heated air injected right at the site of combustion might blow the flame back through the fuel loading aperture, which would not be good.

The air currents are probably too strong for sawdust in standard designs. Maybe a wide (i.e., slow-flowing) chamber before the re-burn would allow that to work? I worry it might explode, also. Twigs, perhaps de-barked by goats, are the standard fuel. There's a continuous spectrum running from sawdust to shavings, chips, and twigs; I think coarse chips would be good, except for the effort needed to chip them.

A low-density refractory would be an excellent riser, in my opinion.

Part of my thoughts on using chipped wood is that it is an abundant waste stream around a lot of urban to semi urban areas. Around here the counties will gladly load your truck with chipped wood to keep it out of land fills.
kent

I used some of that free chipped wood on a mulching job today, as it so happens.

A good third of it was twigs of the size rocket stoves typically run on, and some of it was too coarse to go in a rocket stove. There are lots of compelling uses for the fraction of this resource that has been cut too fine, naturally.

feeding the chips in at the correct rate is most often the hard part. until you build an RMH please dont tinker with it. build one in the back yard and get a feel for the air flows it already uses then start tinkering. by all means innovate but start from the basic hands on stove. We spend enough time fixing installations that someone had tinkered with before building the first one to spec. I would like to see some data of heats in various parts of the basic stove so i can move on in the design process myself.

the air intake is through the fire box because the wood meters the amount of air taken in. turbulence from the sudden direction changes and vortices on the brick edges enhance the air mixing and the limit of available O2 in the return part of the stove makes it almost impossible to get CO because the proper heat and mix of gasses is assured. Adding more or less O2 in critical areas might tip it into producing CO or cooling the re-burn so it does not use all the fuel.  This is just my caution as a researcher speaking; i want to stoves to do the best job possible with as little effort/human tending as we can get. so i personally rely on passive systems  to enhance the stove rather than active systems that need extra energy or care to function.

Ernie, thanks for the reply. I guess that I have the newbie excitement over a new idea. I will try some experiments this winter when I have some more time. rocket stove concepts seems to answer some questions I have had on a long term project. I am a big scrounger of materials and have been putting together a system for heating for a couple of years. I like the idea of a rocket heater as a complement to an active solar heating system. I gather that you are in the Portland area. I lived there back in the 70's while at PSU and have fond memories of the area. Thanks for the advice and your work in this area.
kent

No problem there the enthusiasm of new folks is what keeps us going. you see things we do not and that makes us think beyond what we know. it also lets us have perspective to request that folks use caution in tinkering. we have gone off in ways that failed so we try to help and support anyone willing to do this work. sometimes with warning but mostly with encouragement if we can. thanks for the wishes and we would like to see any data you would share. the data will help us all with our understanding of these stoves.