Add ons to rocket stoves

I work pretty hard on rocket stoves to make them the best possible passive system i can devise the input is wood and thats pretty much it after its built. However i do know that we need some things to push air around from time to time; or provide some other aspect of life that makes us more comfortable. I want to talk a bit about fans, pumps, diverters and restrictions.

Lets do restrictions first; Restrictors work to narrow the path that exhaust passes through or to narrow the space that feed air passes through. there are a coupe of things we can observe about these 1. momentary restriction like a flat 1/8th piece of steel with a 6 inch hole it it placed in the exhaust path. this will momentarily speed up the system in that place and over all slow the system behind it; we can use this when we need a little bit more dwell time for the smoke in the secondary burn. it allows the gas to flow slower and keeps the volatiles in the heat for a little longer allowing for total combustion.

now this can also be used to make the system pass more heat off to the thermal mass because it can be done very late in the system. the trade off is obvious you can extract a great amount of heat and make the stove to sluggish to work.

We have the other end of restriction in the feed tube. the more wood you put in the smaller the opening for air to enter the system however the hotter the fire the faster the system draws air. so we again have two things to consider. Air flowing into the stove very fast mixes very well and tends to hold the fire down on the fire wood. add a tone of small sticks to the spaces between the larger pieces you can actually choke the stove off and get CO instead of CO2. this happens because while the stove is pulling air in super fast its not actually got the room to pull enough air in. the other one is Erica and I's use of bricks on the feed tube. if we want we can close off the whole feed and put the fire out. We dont but we could we use the bricks to speed and funnel the air to where we want it. one function of the bricks is the thickness this means the air going between the bricks is at speed longer and has enough mass to hurl itself into the fire. A handy thing if you want to only burn one or two sticks at a time. There are a few other things you can do with restrictions that will come later..

Diverters: in the best of designs we actually have a couple places that we use diverters to get some really awesome effects. the first we already use of course the back of the burn tunnel this diverter breaks up a stream of gasses and diverts them into a roil at the back of the burn tunnel this is our second mixing place in the stove. You can change some things by changing the diversion at this point A few tweaks here can increase the heat of burn make the flow to fast to burn in the secondary burn zone Etc.

The third diversion is the bottom of the barrel itself. in this diversion the column of gasses slams into a flat surface that has space all around. Because of this the gasses make a torus by hitting the side of the barrel and curling back under the splash. this is another mixing place for the final burn the stuff that is to heavy like steam and CO2 drop out of the torus and cool. this space can also be tweaked to give you different effects; you can move the heat up and down the barrel by narrowing the gap between the heat riser and barrel bottom. you can twist the flow by putting a slanted plate here, you can split the flow with a ridge here, you can cup the gasses by welding a open can here. Each of these actions will have a reaction in the system. Some are good some are bad from our point of view.

one diversion that we dont show that I have been unconsciously doing for years is to have the second bridge brick a little lower than the first bridge brick. this leaves a ridge in the roof of the burn tunnel about 41/2" back from the feed tube. Some folks are calling this a trip wire. I call it good design because it makes a nice mixing disturbance here and makes for a cleaner burn. not to mention that the system gets a little hotter. No this is a very small ridge and bigger is not better.

I guess the other diversion that is very important to the stove is the surfaces of the bricks themselves these rough surfaces help in mixing air and pyrolitic gasses so we get a well mixed very flammable compound at the secondary burn zone.

the last diversion is the mechanical diversion that i use in very long cool systems that allows me to heat the chimney before running gasses into the cold mass. this diversion device is a flap worked into a T just out of the manifold. it allows me to choose to run hot exhaust through the bench or straight to the chimney. the diverter lets me make a decision of do i want just radiant heat or do i want the mass heated; or do i want to pre heat the chimney before i start trying to push hot air into a cold mass. Another use would be if i wanted to make an oven that cooks at a max of 400 degrees. I could choose to run that oven with out loosing heat to a mass.

Ok i covered two and will get to more later. please discuss this. I am not going to take to many questions here i would like folks t make statements. "Like i dont see how the diverter will work in a cool system" this is going to help me when Q&A time comes.

pumps; in the normal RMH build there are two places that act as pumps the hot stack that accelerates gasses as it heats; the hotter it gets the harder it draws. the bell or barrel that radiates heat and cools the gasses, the faster it rids itself of heat the harder it pushes. these two pumps are balanced to provide an over all push t the system. you can also shorten the system and have a draw from the chimney and balance the system with this in mind. you have to be careful when adding things i call pumps because if the balance is wrong you wont get a clean burn; the gasses will be moving to fast. this will also have an effect on how well the thermal mass absorbs heat; A balanced system will move heat into the system at 1 inch an hour, the heat moves through the mass at 1 inch an hour and reaches the surface a known number of hours later. speed the gas flow up to much and heat has no time to absorb much and it leaves the building Via the chimney.

Fans and blowers are active parts of the system hat you can use or not. some pull the cold air out of the system creating a suction in the system and aiding in maintaining a negative pressure on the system. this makes it easy to light and you wont worry about getting smoke in your house. the other places fans tend to be used is to force air around the bell to cool things faster and move heat around a house. While this is not efficient it is useful if you have rooms with headers over the doors. this can make things more like a regular wood stove and make folks who dont really understand RMH's a bit more comfortable.

All in all most of the work i do is with the passive systems that allow the stove to function without human inputs other than wood. however i can see the use of many active systems in the future or as adaptations to specific needs in home heating.

Ernie Wisner wrote:speed the gas flow up to much and heat has no time to absorb much and it leaves the building Via the chimney.

so if a stove moves gas too fast, can that be easily slowed down by restricting air flow into it? I'm considering adding more surface area to my bell for better heat exchange, but I fear it will also speed things up excessively since I've already got a tall riser and a short exhaust run.

nope adding some ducting would work. putting on a bigger bell wont speed it up to much if at all. it tends to slow it a little because it takes lots more gas to fill the space. one of those funny things i am looking at.

no room for more ducting at the moment. and I'm not planning to add a larger bell, but to weld more metal on to speed cooling of the stove/heating of the room.

from what you're saying, it sounds like this might be a bad idea.

I would go slow with it. I built the stove we vaned to handle the heat dissipation. so i would go slow with yours and do one or two at a time burn the stove and see how it affects the burn. it may not bother the stove at all but then again it might.

excellent. good advice. thanks.

what about restricting the exit? is reducing the exit diameter a reasonable way to slow down a too fast stove?

it is if you do it slow you are going to want to make sure you dont over do it. take your time and see how the stove reacts. burn it hard so you can see if you get flame creep and other indications. Dont do the fins and the restriction at the same time!

when you say 'flame creep', you're talking about creeping up and out of the feed chamber, yeah?

Another way to soak more heat into the mass is to break up the laminar flow conditions inside the bench pipe. Turbulence inside the pipe will cycle more heat into the pipe walls and by extension, the mass of your bench.
"trip wires" can be made, which are simply little projections from the top of the pipe, shaped like a V, with the pointy end facing downstream. They should be quite small, never large enough to restrict flow or reduce pipe size significantly.
They could possibly be made out of baling wire or cut from brick and glued to the pipe surface or built up out of JB Weld or something similar. They should form a little ramp, with the downstream side sharp edged and bluff-like.

Another method for simply slowing the flow could be occasional widenings in the pipe, sudden voids that have a larger volume than the rest. This will slow the flow pretty effectively in these spots. It should create hotter places right at the voids though, heating the bench unevenly.

good info for sure. in this particular instance, the stove is built and there is no bench. modifications will likely have to be limited to the intake, the exhaust exit, and the outside of the bell at this point in time. there will be a bench in the future, but not for several years.


edit:
so as not to further hijack this thread, we probably ought to continue talk about this stove on the dedicated thread instead of here.

hey Donkey!
hows that look? you got anything to add? (well other than you just did.)

next part is voids.

great Satamax what does a turbine generator have to do with a rocket mass heater? However: i will look at it.

Well, it's not the generator part which is intresting. You were talking about fans. And a turbocharger is a fan. I like his system, have known his site for ten years may be. They haven't progressed on the woodburning turbocharged stove/generator for a long time. But i thought, could it be used on a rocket stove? I mean, it's not easy, needs to be used on a closed system. Exhaust gasses are slow, and cold'ish. But the advantage of a turbocharger, is that it is self powered. May be two metalic fans linked by a shaft between flue and intake could be devised. As the same principle of a turbocharger. Well, that's the thing which came to mind. On another subject, related to theyr research, with a rocket, obviously the easiest way to produce electricity, imho, would be steam.

Sata let me read on it and think about it for a bit. Please remind me about it in a couple days. start another thread maybe.