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rocket mass heaters

portable rocket mass heater

paul wheaton

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posted 13 years ago

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I spent way too much time yesterday trying to figure out how to get the rocket radiator to work and .... as I am trying to mash the solution into the problem space .... and weighing Erica's words ... I came up with a possible alternative.

Assuming that one owns a shop vac (which I do) then loose materials could be moved via paper sack.

I'm thinking that this would be a six inch system.

If one were to take 2 1x4's that are 6 feet long and attach them to 2 1x4's that are 22 inches long, you could make a shallow box that is 72 inches by 23.5 inches and 3.5 inches tall. I choose to call this "a frame".

A stack of five frames is then lined on the inside sides with brick (I have lots). Total height is 17.5 inches. Lay down foil on the bottom and coming up the sides. Put in two inches of perlite on the bottom.

And then the brick is laid in for the burn tunnel, wood feed.

The combustion chamber is six inch duct with ten inch duct and perlite and clay. Just like in the video. This piece will stay contiguous during the move.

The bench will have four pipes in it and filled with sand (maybe a mix of sand and ash). Since there is so MUCH pipe, then the whole bench becomes lighter - so weight issues are mitigated. (and the downside of less weight is less mass - but part of the design is to work in the space of a typical floor in a typical home)

The area around the combustion chamber and 30 gallon barrel and the like would be filled in with perlite and wood ash.

The transition from the barrel to the duct will be a little tricky. So far I am imagining that it will be framed in brick to make a 12 inch deep chamber with two bricks over the top and lots of sand/ash over the whole mess. Cleanout would involve setting the sand ash from the top aside (maybe with the help of the shop vac) and pulling the top bricks to get access to this chamber. The beginning of the duct would rest just inside the chamber.

The top of the bench would be 2x4's with a quarter inch gap between each stick. And 2 1x4 sticks that would hold them all together and keep them in place just within the top of the bench. The gaps would allow the heat to pass through.

I suppose one could add a flexible dryer duct to carry the exhaust away.

It could be built in a garage or on a deck. Or patio. It just needs to stay dry. And then the dryer duct could be laid anywhere away from the group.

And, it could be put inside and the duct run out a hole in the wall, or through a window. And since this is portable - it becomes a sort of hand made appliance - I'm not sure what the laws are for that sort of thing. Probably a little beyond burning 50 candles at the same time - or running a dozen propane torches, or having a party with smokers - only the exhaust is routed outside.

In transport, there would be a barrel, lots of bricks, the combustion chamber, lots of duct, wood frames, the wood bench top, foil and paper sacks full of perlite, sand, ash, or a mix of two or more of these.

After it has all been built one time, maybe setting it up would take .... 90 minutes? And taking it all apart would take ... 60 minutes?

So ..... as with many of my crazy ideas, I later learn why the idea is lame - what have I overlooked?

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paul wheaton

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posted 13 years ago

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My attempts at drawing this.

The first is without the loose fill, the siding, the bricks in the thermal mass or the top. The second is the same but with the siding.

rmh_mini_1.jpg

rmh_mini_2.jpg

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Ernie Wisner

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posted 13 years ago

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I like it.
couple things.

1. dont use the dryer flexihose it slows the exhaust down to much.
2. aluminum tape is your friend tape the joints and make sure they are sealed.
3. i think you will need to morter your bricks and plaster the out side of the burn tunnel.
4. test it outside first

Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info

paul wheaton

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posted 13 years ago

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1) okie dokie

2 and 3) I'm kinda shooting for something that will be easy to take apart and put back together. I think tape, mortar and plaster might be contrary to this goal.

I guess I could try to tape all of the pipes together and keep them as one big gob. They would then just be rather light and bulky.

I was hoping that dry stacking the bricks, combined with a lot of ash would do the trick. I guess I was leaning heavy on the ash plugging any gaps anywhere. If ash won't work, might there be something else that would work? I could attempt to wrap things in foil. Foil is cheap and I can re-use a fair bit of it.

4) How about a garage with the door open? That way my work space is nice and dry.

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Ernie Wisner

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posted 13 years ago

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naa the aluminum tape is easy and the morter and plaster is just clay sand morter. no big deal it does not stick it just fills the gaps up. garage is fine as long as you do it in the door and not way deep in the building.

Erica Wisner

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paul wheaton wrote:
My attempts at drawing this.

The first is without the loose fill, the siding, the bricks in the thermal mass or the top. The second is the same but with the siding.

Looks nice - both safer and more efficient than the open-air radiator, to my eye.

According to one conversation I had with the inspectors, the code definition of "appliance" is "anything used for heating or cooking" regardless of whether it's site-built or portable.
I suppose it could be called a "physics of combustion demonstration device."

Do be sure to give any thru-wall connections proper clearances or insulation. The smaller thermal mass and shorter heat-exchange will mean the exhaust is hotter. Probably not as hot as woodstove exhaust, but I'd aim for that level of protection for combustible walls.

And I suspect you only need the siding around the heat-exchange bench; you might even be able to get away with a washtub sitting on an insulated "sandbox."

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paul wheaton

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posted 13 years ago

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I think I'm ready to move forward and start gathering materials.

I think the wood all of the way around not only lends stability to the outer layer of bricks, but I think it also is an opportunity for a look that some might find more appealing. At least, I think it might open options.

After all - has anyone tried doing a thermal mass in a wooden box before?

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Ernie Wisner

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ive not had the time since you mentioned it to me. so i would ask that you document every thing you do and spend.

paul wheaton

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posted 13 years ago

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Oh great. Now you made it feel like homework. Or a job.

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Ernie Wisner

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all you got to do is play with fire and keep track of stuff. what do you think i would be doing?

paul wheaton

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All I gotta do is take lots of pictures/video as I go and track the stuff I pay for. Are you worried that if you're not there I won't take any pictures?

If you're so worried, you can come up to montana and help!

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paul wheaton

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posted 13 years ago

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Here is a list of what I think I need so far:

buckets with lids and handles (for hauling sand, ash, clay, etc.). I
have four, but could use about six more.

6" duct: at least 30 feet. Plus eight elbows.

10" or 12" duct: three feet.

newspaper: about three inches thick of stacked newspaper

120 lb grease can (I'll try getting this from the local jiffy lube)

red bricks: about a hundred. I might be able to get 50.

wood ash: about four buckets worth - this is something that cannot
really be bought, but should be able to be found somewhere

clay: one bucket. It would be cool if we got it from a field or
something. It doesn't even have to be pure, geologically accurate
clay. It just needs to be clay-ish.

sand: once we have the buckets, getting the sand should be easy.

perlite or vermiculite: they used to sell this by the huge sack that
was, I think, three cubic feet. Three cubic feet would be good, but
.... where is a good source to get a lot of it really cheap?

1x4 or 2x4. 12 eight foot sticks. Or 2x4 or 2x6 would work too.

fuel wood: dry branches or regular dry firewood. I'll need a
barrel load so that I can burn the paint off the barrel. And then
another half barrel load so that we can test the RMH.

Did I leave anything out?

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paul wheaton

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posted 13 years ago

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I built it.

It sucks.

I think it might be too much pipe with too many turns.

I bypassed most of the pipe and it worked much better, but the combustion didn't seem as rockety as I remember. And the exhaust seemed rather smoky.

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tel jetson

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posted 13 years ago

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any plans to fiddle with it, maybe find out for sure what's fouling it up?

either way, I'm glad you gave it a shot.

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so put up some darn Pics and lets see if we can fix it. I have some suspicions but i want to see it before i make any noises.

Max Kennedy

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I like the idea and have been thinking of the same portability concept. My thoughts were more along the line of modularity. Break a regular bench into sections, all about the same size, about as wide as a fridge. This is not a random size, appliance carts for handling fridges are standard and getting one up and down stairs not too difficult. The parts that stick up would also separate. Seals between the sections would be high temperature gaskets, and the sections might be clamped together using threaded rod and pressure distribution plates on the end. More complicated to build the 1st time than Paul's but retains the massive thermal battery properties of the original and comes apart/goes back together faster. Once moved to a semi-permanent site the joints could be filled with clay slip and the butt warmer approach the aesthetics of the monolithic original. The design would have a minimum of 4 or possibly 5 standard parts. These would be a lower fire chamber section an upper fire drum section, 1 or more standard bench sections and an bench end piece. Optionally you could also design a corner module so the bench could take the form of a corner couch. I've been considering this because I would like one now but will be moving, I hope, soon to a property where we can develop a permaculture style B&B/pet boarding business with sides of beekeeping, green building, wild foods etc.

It can be done!

paul wheaton

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posted 13 years ago

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I took lots and lots of video. Not much in the way of pics. But I should be able to take a pic today.

Afterward I felt mighty sore all over. It wasn't until the next day I knew why: I was getting sick. I'm still sick .... but how stressing can it be to take a pic? I'll try.

I have one idea of what might be causing it. .... maybe I'll draw a picture ....

burn_tunnel.gif

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paul wheaton

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posted 13 years ago

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So .... a six inch duct has a cross of (pi*r*r) -> (pi*9): 28.26 square inches

I think the burn tunnel is 7.1 inches high and 4.1 inches wide.

The intersection between the two is .... less. Maybe 20% less. I suppose that could be problematic, but I didn't think it would be this problematic.

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Ernie Wisner

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Critical dimensions are in a word "Critical" Cross sectional area cannot be reduced by 20%. the air volume is to low and the laminar flow coefficient is too high.

Other considerations if the system was build to the drawings is the inside loop; the Apex is a hot gas trap. unless you get the rest of the system working nominally you cant push through the block. with no mass on the system the surface cooling is sufficient to lower temps and give you lots of help but as soon as you get mass around it the temp wont drop fast enough to allow for the system to run correctly.

all this is hemming and hawing from the system i have in my head so just take it for what its worth. other thoughts are that the sand while damp is a good thermal mass but when it is powder dry its a good insulation might cause problems down the road. Keep at it Ah ha moments dont come with perfectly functioning systems they only happen in ones that dont work right.

paul wheaton

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So, today's lesson is that when you stack bricks, they often don't make 5.3 x 5.3 square. So if you gotta do a rectangle, it is best to do something that is wide and not tall.

Of course, the problem with a wider rectangle is to have the brick that spans the burn tunnel.

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paul wheaton

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posted 13 years ago

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Ernie,

If I fix the burn tunnel to combustion chamber point, will the four ducts in the bench be okay, or should I drop that to two?

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Ernie Wisner

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I would think the four runs would be fine but you might want to drop only one if needed at all. a burn tunnel that is high and not wide is fine as long as the CSA is right. cutting the size down with a huge decrease in CSA at the heat riser is just not good. you dont need much to make the square work with the bricks; the bridge bricks just need to lap about 1/8th over the sides.

some orientation of bricks will do at least this.

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So ..... maybe I can just whittle the existing bricks a wee bit to allow the transition from ractangular-town to tubular-town ....

??

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tel jetson

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paul wheaton wrote:
So ..... maybe I can just whittle the existing bricks a wee bit to allow the transition from ractangular-town to tubular-town ....

would arranging the bricks underneath the combustion chamber in a more circular pattern to match the shape of the combustion chamber work?

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paul wheaton

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So here is what I think I'm gonna try

burn_tunnel.gif

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tel jetson

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that's roughly what I was trying to suggest.

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paul wheaton

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tel wrote:
that's roughly what I was trying to suggest.

Yup!

I guess I'm trying to wave it in ernie's face to see if he'll say "that's stupid because ....." or "yeah, that's what I would do."

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Ernie Wisner

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Well you should know by now i am not going to give you either of those answers

as per usual i am going to say..............spoilerspace................... try it and see

Mom's here and we spent the last couple days unpacking her moving van so i have been a little absent.

I think this part is only a little bit of the problem.

paul wheaton

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Ernie wrote:
I think this part is only a little bit of the problem.

What is the rest of the problem?

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Ernie Wisner

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I think the piping runs might be a larger factor.

but working out one part at a time is the way to make a system work.
I would love for this one to work so i am thinking it through step by step. I dont want to change to many variables at once.

paul wheaton

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piping runs? You mean the duct in the bench? Four is too many?

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Mike Paulus

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You lose significant velocity every time you go through a 90 degree bend. Three 180 degree bends before you route the gasses out is a tall order. You could probably just force it through by increasing the potential draw enough. The easiest way would be to make a taller heat riser. The thing is you still have a bottleneck in your mass with all those bends so the velocity is not likely to go up much.

Getting into CAD to work on gas flow is way beyond my pay grade, but you might be able to find someone who could help you with it since you live in a college town. The easy bet is to reduce the bends in the bench.

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Yes i think so, but i would suggest you change one variable at a time. the burn tunnel has already been identified so change it first, the ducting is a likely problem and as i suggested before taking a run off might be a good option. just dont try to fix it all at one time; think of it like programming. if you go in and make a lot of changes you have no idea of what worked.

tel jetson

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jumping ahead: is splitting the duct into several shorter runs a workable solution to too many turns?

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Ernie Wisner

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Splitting the ducting into smaller diameter is a very tricky thing to do. each split off handles a different amount of flow unless it is done just right and the flow is totally constant where you split it off. laminar flow can and will bite you if anything is slightly off.

not generally something i suggest even at the best of times. I would not do it unless i had a really really good reason for it; introduces to many variables. pulling one run off the ducting would work better and be much simpler to figure out.

paul wheaton

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So I am going to go completely contrary to ernie's suggestion: rather than fiddle with one thing at a time, I'm going to fiddle with several things at a time.

I have now fully removed the burn tunnel and am going to rebuild it. The changes will be:

1) The burn tunnel will be square instead of rectangular. According to my calculations it should be 5.317 inches square. So, a hair bigger than 5.25 inches.

2) the tunnel will be widened where the heat riser meets the burn tunnel. Similar to what was drawn earlier.

3) I've already shaved about a quarter of an inch off of the top of the heat riser. The previous gap was a little more than 1.5 inches. It is now a little less than 2 inches.

4) Where the exhaust just inside the barrel merged with the first duct was goofy shaped, it is possible that it may have become too small. It will definitely be larger.

5) I want to speed up the build process. I think I am going to create a cardboard form for the burn tunnel. The idea is that the form will represent the final shape of the burn tunnel plus a layer of bricks at the bottom.

burn_tunnel_form_1.gif

burn_tunnel_form_2.gif

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paul wheaton

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Another idea pops into my head.

Rather than putting a heat riser on a stack of bricks, what about a heat riser that goes all of the way down. It seems like seals between the riser, bricks and the barrel would be easier. Plus, the insulated portion would be taller (thus, better?).

heat_riser_1.gif

heat_riser_2.gif

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FWIW: the shop vac did an excellent job with moving the sand.

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paul wheaton

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The first video of the work in progress:

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Max Kennedy

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Nice video. Did you take any measurements? eg temperature of exhaust, temperature near the wood, % retained heat. You mentioned fixing the smokeback. Would be interested to hear how?

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