|
|
Post by jgreen on Mar 11, 2009 12:40:44 GMT -8
Hi everyone. This is a general request for any information regarding rocket stoves used for space heating in bus conversions. The reason I bring it up is, I own a retired school bus and am moving in the direction of building a house bus within (inspired by some of these: www.mrsharkey.com/busbarn/busbarn.htm). A few months ago I had a conversation with Ernie and he mentioned a scaled down rocket stove in the bushome of a friend of his. I think it was something like a 4" unit. Hearing about that convinced me that it was possible... and maybe even a good idea. But, thats the only reference I've ever heard or seen about rockets in buses. So, hopefully putting this out there will bring out some more information, ideas, pictures, etc. This year I built a rocket stove and heated bench in a mobile home (for friends of mine), so I have an understanding of how they work. It seems like there will be a lot of design modifications to make it appropriate for use in a vehicle (and a relatively small area / volume of air to heat). It would be especially helpful if anyone has any experience or ideas about wood heating in a bus (or other small spaces) that use wood efficiently, heat adequetly without over heating. I've been living in a greyhound bus conversion for the past year and in anticipation of winter I installed an old style cast-iron heat/cook stove. The issues I've experienced are.. difficulty getting even heat distribution throughout the space (the radiant heat being too intense right around the stove when its burning hot enough to heat the rest of the bus to a comfortable level), and having no storage of heat so the space cools down unless the fire is stoked constantly (additionally its not a good quality of construction and the bus is very drafty). Also, this particular stove seems to need to burn a minimum amount of wood at a time (to stay lit and burn clean) and often, that creates more heat than necessary once the space is heated initially. So I'm imagining a rocket stove might be a partial solution to some of these issues. But there are still a lot of unanswered questions and I would ideally like to learn from other peoples experiences (or ideas) before I start designing and building. |
|
|
|
Post by Donkey on Mar 16, 2009 6:58:56 GMT -8
The challenge in building a rocket stove for a bus is going to be getting around the thermal mass.. You can't put the usual amount of mass in the bus, it's just too darn heavy.
It's the mass that tempers the heat coming off of a rocket stove. That heavy storage bleeds off the heat, storing it and releasing it slowly.. Without it, you would most likely have the age old too hot or too cold issue..
Ok.. So basically what I'd do is first figure out how heavy it CAN be and work from there.
What do you think is the max weight you would feel comfortable with?? 1000 pounds? Less? More? Then build a stove that is balanced to heat that mass without either wasting too much heat out the chimney or being undersized and not heating the space.
|
|
|
|
Post by jgreen on Mar 16, 2009 14:21:17 GMT -8
Donkey (and others)...
I'm not certain what is a reasonable amount of weight for the bus. I've thought about it a bit and figure that it must at a very minimum, be designed to carry 3 students per seat, at 100 lbs per student (and probably it is way over engineered). I think those are low figures, but to be safe and give me a baseline that I'm comfortable with being an assumption... that leaves me with a payload capacity of 11,000 lbs total. I was wanting to build a 6' bench (1.5 or 2' wide) and using my figures based on how many kids would be in that amount of space, that gives me around 900-1000 lbs to work with.
Is there any general estimatation for the weight of cob, or does it vary so much between individual mixes and ingredients, that I just need to mix up a batch with what I have and dry a cubic foot of material and weight it? I was imagining, a very slim bench design that maximized surface area (both with the flue pipe and with my body on the top) and minimized unnessessary volume (thereby decreasing total thermal mass). It would probably be elevated to comfortable sitting height with a sturdy wood platform, rather than building up the height from the floor with rock, urbanite or cob.
Does anyone have any advice on how to think about sizing the combustion unit to a fixed amount of mass storage? I imagine that I'm dealing with burn tunnel and heat riser area as well as heat riser height as my most important factors. But how do I know what will be underpowered and what will be overpowered? What are the downsides of having a combustion unit that is too big for the amount of thermal mass it heats... wasted heat + fuel (because it burns too hot?). I think I'd rather it be overpowered than underpowered, because it seems like the result of having a bigger combustion unit than mass storage would be shorter burns before the mass heated up completely.
|
|
Teach
Junior Member
Posts: 89
|
Post by Teach on Mar 16, 2009 17:11:57 GMT -8
To start with, like Donkey said about mass vs/heat output match will be all important and will also change depending on your heating needs. More mass = less peaks and valleys of temperature extremes. The mobility of the bus creates its own set of unique problems.
First thing that comes to mind would be to not use bricks and mortar that could be shaken apart or cracked apart from road vibrations just from normal driving much less hitting a pot hole. I think I would use steel pipe welded into your j shape and insulate around it in the usual way. You might want to consider something like a box rocket made entirely out of sheet metal except for the feed tube, burn tube and riser which would be steel pipe.
Now, what came to mind when I was reading your two posts is............you need mass storage of some sort in order to buffer the extremes of too hot/too cold right? And busses pretty much always carry their own water supply on board right? Why not up the capacity of what the bus can carry by installing something that doubles as water storage for domestic use and also serves as a heat sink for the rocket? It could use a thermosyphoning affect to circulate the water being heated. Just remember to build it as an open system so that pressure can not build up in the system. If you wrap between the inside of the barrel and the outside of the riser tube with soft copper tubing I personally would not use sizes of less than one inch in diameter to prevent steam generation within the tube as the flow of water will be greater carrying more heat to the storage vessel. Just some ideas.
Teach....
|
|
|
|
Post by Donkey on Mar 17, 2009 8:03:58 GMT -8
Right on Teach!!
That's just about exactly what I was thinking.. Though I would do away with the copper coils and feed the main exhaust through and/or around a water jacket of some kind..
Seems like an all-welded steel deal with space to pour pearlite for insulation and water hookups for mass/water heating would be an ideal thing-amabob!! Something ready-made like that would almost certainly sell like hotcakes.. Even in THIS economy!
|
|
|
|
Post by Donkey on Mar 17, 2009 12:29:09 GMT -8
Donkey (and others)... I'm not certain what is a reasonable amount of weight for the bus. I've thought about it a bit and figure that it must at a very minimum, be designed to carry 3 students per seat, at 100 lbs per student (and probably it is way over engineered). I think those are low figures, but to be safe and give me a baseline that I'm comfortable with being an assumption... that leaves me with a payload capacity of 11,000 lbs total. I was wanting to build a 6' bench (1.5 or 2' wide) and using my figures based on how many kids would be in that amount of space, that gives me around 900-1000 lbs to work with. I know I'm being redundant here, BUT.... Don't forget to take into consideration all the other modifications you plan to add to the bus.. If the images from your website are any indication, they should add up to quite a lot. Beautiful, by the way.. Very impressive. Yep.. That's EXACTLY what you gotta do! Even if you weigh a batch of your own cob, it's just a rough estimate.. Just to be on the safe side, add 10-15% on to your measurement. It seems to me that if you build it along one side of the bus, that you will need to add a similar weight to the opposite side just to balance the load.. Unless of course, you plan to build it across the back. I was, just the other day, following a motor home down the coast that was listing dangerously to starboard. Whatever they had going on in there, it was OBVIOUSLY way too heavy on the one side and slewed badly around right hand turns. Seems yer right about over-powered verses under. Though on account of inefficiencies of heat transfer to thermal mass, an overpowered stove will tend to throw a huge amount of heat out the chimney, which is what (with rocket stoves) we are attempting to remedy (among other issues). Unfortunately, no one has yet (as far as I know) pinned it all down. As a blind, shot in the dark type guess, I'd say that an eight inch system should serve. Output from a rocket stove is contingent on quite a few factors.. Insulation, the relationship between the length of the burn tunnel and the height of the heat riser, turbulence conditions, air input, the length of the thermal mass run, height/heat of the chimney, etc. For a bus conversion (and other uses) the all metal thought-form above seems like a good one to me. Though I'm sure the cob rocket stove concept could be made to work. |
|
Teach
Junior Member
Posts: 89
|
Post by Teach on Mar 17, 2009 19:38:53 GMT -8
Donkey, LOL..........I thought about the water jacket and thought in favor of the copper coils because of the increased surface area of the copper coil which would be far greater than a water jacket. Greater surface area = greater heat transfer. With that said, a multiple boiler tube carrying water with the flames rising up through those pipes would work as well and have more surface area than a water jacket as well.
With regard as to placement of the rocket.......my vote would be midships ie as close to the middle of the bus but being middle to the front and rear axles of the bus not the middle between the front and the back of the bus. The mid point of the bus between the front and rear wheels will minimise the felt road shock to rocket. Just pick a side of the bus to place it.
Something else to consider in this application................the pocket rocket! They have a much smaller footprint than a full sized rocket. When well built ie not as a disposable short lived stove, but one that is built and insulated well....pocket rockets can be an awsome little heater. Take a hot water tank and use the liner for the body of the stove, give yourself an access/feed door on the side, insulate a combustion chamber in the bottom half and have the top half act as a radiant portion, containing coils or water jacket inside and be very self contained. You can even have the primary air pre-heated by drawing air from around the exhaust from within a larger pipe that surrounds the inner smaller pipe of the exhaust and draw it straight down onto the fire acting as a forced air supply. 4 in air supply, 6 in exhaust, a stove body of approx 18 in dia and whatever height works for your situation. Just more ideas.
Teach...
|
|
|
|
Post by Donkey on Mar 18, 2009 7:08:33 GMT -8
A'course, that would depend on the shape of the jacket.. But, I see yer point.. I guess I like the idea of the water jacket simply because the whole mess can be welded up into one piece. The advantages of that are numerous...  |
|
Teach
Junior Member
Posts: 89
|
Post by Teach on Mar 18, 2009 9:38:50 GMT -8
Jgreen.........Well, there certainly should be enough ideas here to come up with something that works and each with its own advantage/disadvantage over the other. I guess what you will have to do is decide on which way you want to go based on your needs and also what you have laying around that can be used to meet your end need.
I suspect that the ideas donkey and I have suggested don't even begin to scratch the surface with what can be done but they will provide a point from which to start. Good luck! Let us know what you build and how it turned out.
Teach...
|
|
