"On demand" Rocket boiler??

I am a newbie to this forum, so pardon me in advance if I make assumptions, or ask silly questions.

I have been designing a wood fired boiler. I started with a wood gasification unit, but my first proto-type was complicated, finicky, and simply did not work. The simplicity of the RMH is appealing to me.
I have a preliminary design which utilizes a nearly vertical feed magazine. This magazine has an air tight lid at the top, and is surrounded by water to keep it cool. My air intake is horizontal like those that I have seen on Rocket cook stoves. I plan to use a damper door controlled by a bi-metallic switch, to open and close the door, based on water temperature in the tank. The combustion chamber and heat riser are typical Rocket style components. My water tank is designed drop down over the heat riser. The feed magazine tube mentioned earlier, is an integral part of the tank. Lastly, I have 2 corkscrew shaped tubular heat exchanger tubes that allow the exhaust gasses to pass through the tank a second time as they assend. Above the tank the two heat exchanger tubes are combined in a 2 to 1 collector similar to those seen on truck exhaust systems. Now for the questions...

Has anyone tried running an RMH in an "on - off " fashion, by shutting off the inlet air for a period of time, then allowing air back in? Wood gas boiler manufacturers do this quite well. I have done this with a well insulated combustion chamber, and could successfully restart the wood after 2 to 3 hours. This leads me to believe that a properly sized system could run at 20 to 50 percent duty cycle dependably.

Has anyone had experience building a RMH with 4" ductwork? I am sizing mine for 20,000 BTU / hour at 50% duty cycle. I have read a lot about keeping the CSA constant through the combustion chamber and heat riser zone. It appears that the ductwork after the barrel is again the same CSA. How about the CSA relationship between the OD of the heat riser and the ID of the barrel? Is this to be a reduced velocity zone to enhance heat transfer?

Thanks so much for your patience, and I look forward to any replies.

hi
this is an old german/austrian system, like a kelly kettle.
http://www.google.at/images?hl=de&rlz=1T4MEDA_deAT291AT291&q=kohlebadeofen&um=1&ie=UTF-8&source=og&sa=N&tab=wi&biw=1198&bih=560

i think it would work rocketfired.

cu
auer

How will you restart the fire, once it has burned out due to a lack of oxygen?  Do you have a pilot light system in mind?

In a wood gasification boiler, the cumbustion chamber materials are castable refractories with very high insulating properties. The job of this material is to maintain very high temperature, well above the flash point of wood. Once oxygen is reintroduced, spontaneous combustion occurs. A well insulated combustion chamber can make it possible for restarts after 4 or 5 hours of dormancy.

I've attached an image of my preliminary design. It shows a cross section taken through the centerline of the assembly. A quick description, first the stuff you do NOT see. I have not yet modeled the insulation surrounding the combustion chamber and heat riser, but plan to use an insulating castable refractory. I have not yet modeled the airtight covers for the fuel magazine, or the ash cleanout shown on the lower left hand side of this view. On the lower right side is the air inlet. I have not yet modeled a "Plug / Grate" assembly which will support the wood a specific distance above the floor of the combustion chamber. It will also reduce the air inlet CSA to a slot across the bottom of the opening. The comb like fingers of the grate will be spaced about 1" apart, and will be inline with the air flow. There would be an air tight damper flap controlled by a bimetallic device which would close once the desired tank temp was reached. It would re-open on a 20 degree temperature drop.
What you DO see...
The tank with integral fuel magazine sits atop the base assembly. The feed tube is 8" Tube,
surrounded by water. This helps insure that only the bottom tips of the wood burn, minimizing smoke. The tank would of course have numerous ports for inlet, outlet, thermo / pressure pop off valves, drain, expansion chamber connections, etc. As designed the tank capacity is approx. 80 Gal. The heat riser is 4". I may consider using a spun ribbon styled turbulator in the riser to impart spin. The tube which defines the outside diameter of the heat riser is 9". The inside diameter of the barrel is 11.75". I have approx. 180 degreees of open area between the barrel and outer heat riser tube for exhaust gases to pass into the low velocity ash drop area. The gases would then be pushed up through the 4" dia. corkscrew shaped heat exchanger as they exit the boiler at the top.
I look forward to any input. Thanks again

Wouldn't you be robbing too much heat from the exhaust gases ?  I would have guessed the "barrel in the water jacket" concept would transfer as much heat to the water as one might need, without the added complication of the  corkscrew heat exchangers. But then again, I have never built (yet) one of these things, I am only theorizing at the moment.

Very nice and compact desing all the same. How do you plan to build it, if I may ask ?  I see lots of welding in your future   

Erica's formula shows that the height of the fuel feed tube + the distance of the horizontal burn chamber needs to  be less than the height of the heat riser.  (Stated another way is that the heat riser needs to be taller than the load chamber and horizontal burn tube combined). Also I believe that Erica and Ernie mentioned that the diameter needs to be the same (feed tube, horizontal burn and heat riser).  I recommend their books to save you all kinds of re-work later. 

Erica or Ernie Wisner posts have the link to their info.. just look for them in alt energy posts.

P.S. Dakman, That diagram looks amazing. I'm not sure i understand it, but it sure looks cool.

Thanks all, for the input so far.

Ronie, I have considered the guidelines for riser height to feed tube plus horizontal length. However, since I plan to seal the top of my fuel magazine, I should not need to worry about smoke backing up that direction. As for the size of my feed magazine and air inlet, versus the combustion chamber and heat riser dia. I'm hoping that the plug / grate assembly that I described in an earlier post (but have not yet modeled) would maintain the proper CSA throughout these critical areas.
Thanks for the compliment on the model. I have a lot of work to do yet.

Zoidberg, you may be right. I have no clue as to whether I need additional heat exchange area besides the barrel itself. I was kinda hoping to hear from someone who has tried something similar with a double walled barrel version. My plan is to build the lower base with the combustion chamber and heat riser first so I can do some preliminary testing with a single barrel over the top. If it looks promising, I'll build the tank, complete with the corkscrew heat exchanger. If the exhaust temps are too low I can cap off the heat exchanger and add a flue to the back wall above the ash cleanout.
I work as a design engineer for a metal fab company. We lazer, bend, and weld steel all day long. We also work with castable refractories. That may help to explain my rather unconventional design.

I have seen comments suggesting smaller rockets, such as 4" versions, are finicky and require a lot of tuning. I wonder why the Rocket strategy wouldn't be scaleable. I am hoping someone can enlighten me.
Thanks again

I believe I read somewhere that a closed feed magazine (i.e.: no air coming through it) becomes a smoke trap, which means smoke comes out of it every time you open the top to feed more wood.

Also, I wonder about proper mixing of gases in the combustion chamber (due to reduced turbulence) if air comes from the side. Just speculating here.

I would like to ask you what your thoughts are on water condensing inside the barrel and exhaust channels, since having water all around is certainly not the same as having air all around.

Maybe you know the place, I would suggest posting your design in http://donkey32.proboards.com, I bet they will happily comment on it.

I wonder if anyone has tried to add a vertical fuel magazine to an existing RMH. If one were to raise the bottom of the magazine an inch or two above the base.... using blocks or the like, this would allow air to be pulled in from the original elevation, and the wood above would remain cool and smoke free. If the magazine was a little smaller then the original feed opening, there would be no hang ups.

how will you clean out the spiral section of the heat exchanger?....i built a top loading downdraft boiler (not a rocket type) a few years back with a spiral heat exchanger/flue similar to what you show in your drawing and it worked really well EXCEPT that it was a major mission to clean out the spiral pipe when it needed done

whats the point of this thing?

Wouldn't a batch heater be simpler? I'm thinking how can I use the rocket system to quickly heat water for a shower for example when a solar unit would not be very effective as in winter. With an elevated hot water storage tank sized for purpose (shower quantity) and piped to a smaller rocket-fired boiler in the kitchen area. The rocket boiler on the lower section heats water which would hopefully rise up one pipe while cooler water comes down from the storage tank in another pipe to be heated. Once enough temp is reached stop feeding the rocket and hop in the shower. Anyone done something similar?

I'm searching this forum for tips but there is a lot here and someone may have already hoed this row...

yes it has been hoed and planted and harvested. but like most things that can kill you. it needs careful stewards. For instance have you ever worked with live steam? have you ever done plumbing (not the connect pipe A to pipe b in this book but actual work on systems that are pressurized and if you get it wrong you get to try and shut it down while its spewing fluid at pressures that saw through 2X4's?) That sort of thing. Do you under stand the concept of dont put a valve in a system unless it is absolutely necessary cause some on WILL come shut it off Etc.

I dont teach rocket water heaters to those who cannot answer those simple questions. And those who lie about it get caught in a hurry and i kick them out of the workshop. When we are talking about making hot water it goes way beyond the simple idea of a pan on top a stove. now we are talking about systems that need folks to be aware of the risks and know the way systems act in use. Sorry to those folks who feel insulted but i have taught several boiler makers and they all blanch at the thought of teaching this in an open class. All of the boiler makers have at least 20 years working boilers and live steam if they take it as a tough class then i feel it is justified to be wary of teaching the general permies.

A batch heater would be good its easy and if left to gravity (open tank) it works for most things. batch boilers are not very efficient however and more advanced designs need safety precautions. I will put it out here that anyone with the before mentioned skills are welcome to set up a work shop among other folks with the same skill set let me know and we can either do the workshop here at my house or at a site you all agree on. I am not adverse to teaching it just to teaching it with folks that do not have the back ground to understand some of the basic concepts.

I would love to teach about 12 other folks that can also teach or do installs. (preferable teach).

Ernie Wisner wrote:yes it has been hoed and planted and harvested. but like most things that can kill you. it needs careful stewards. For instance have you ever worked with live steam? have you ever done plumbing (not the connect pipe A to pipe b in this book but actual work on systems that are pressurized and if you get it wrong you get to try and shut it down while its spewing fluid at pressures that saw through 2X4's?) That sort of thing. Do you under stand the concept of dont put a valve in a system unless it is absolutely necessary cause some on WILL come shut it off Etc.

Well thanks or the info, I do have a bit of common sense but that doesn't mean I can't do something stupid now and then... I've seen some of the designs with a copper pipe around the exhaust riser and I believe the one in the aprovecho book was similar... I don't want a continuous system but just a small batch setup for an off-grid cabin. My main question is about the efficiency of using a smaller "tea kettle" boiler looped into a regular off-the-shelf water heater for the main holding tank... This tank would already come equipped with the standard blow off valve...

The two tank idea (I'd like to say came to me in a dream just to sound cool) is with the main tank in the rafters of the cabin, water is introduced into the system from an elevated cold water storage tank to generate the water pressure... Unless you turn on the water in the shower the water heater tank would remain full... I suppose it could be a single tank system since there is water pressure but then this means a larger "boiler" in the RS heat stream... Anyway the smaller "tea kettle" size boiler is catching the heat and the hot water would (hopefully) move up one pipe into the upper and larger hot water tank while colder water would come down another pipe to be heated... Once the desired water temperature +/- 130 degrees is achieved you stop feeding the rocket stove and take your shower... Since the shower uses a standard shower valve you can moderate the water temp to suit by mixing in cold water like a normal shower...

This idea is not for a community type setup but an individual cabin with persons of reasonable common sense... I have a different (crazy) idea for a steam boiler to power equipment but this would need to be constructed to handle much higher pressure and not for showers... Any thoughts on the efficiencies of using the smaller tea kettle boiler instead of a coiled copper pipe around the exhaust riser? This is akin to putting a pot of water on a rocket cook stove rather than circulating water around the exhaust riser... Guess I could make two side by side systems for testing and see which one uses the least fuel to get the water hot enough for a shower...

I look after oil burners for water heating, but I couldn't agree more with what you say about steam and solid fuel heaters. I have only once worked on the burner of an autoclave used for curing rubber. The guys in the factory did most of the steam side, but not the oil. However, I went home, downloaded the manuals, and researched a bit about steam systems. I wouldn't touch them now unless I did full time training in a company which did that type of work.

I see peoples steel box woodburner water heating systems and it makes me cringe. They often have a disaster waiting to happen, being fed by what is almost an uncontrolled heat source, with valves all over the place, pipes too small, and no heat dump..

Thank you for not taking it personal; unfortunately many folks are long on books and short on sense. Common or other wise.

the double boiler system is not very efficient but will work. providing you can get the circulation (I can so i figure folks smarter than me can since thats about everyone you should have no problems) if you put it on top of the barrel and insulate the tank and around where the tank sits on the barrel. Replace the popoff valves with new ones and get the good new ones not the crap they use on the tanks from the factory. set it up outside first to make sure nothing is going to go wrong. make sure the water tank in the rafters is secure you dont want several gallons of scalding water to suddenly fall. if you do put a valve in wire it open tape the wire and put a padlock on it with you as having the only key. I am serious about the valves Especially in a community setting, you might know how the system works but others wont and shutting off the valve is not good. with this system you have a lag time so it wont immediately blow. also make sure you have fresh water coming into the system it wont matter if its gravity or utility you just need to put cold into the system your lower tank must not run dry. OK thats enough for now. and you seem to have all your shit in a sack so you should be ok from there.

I do wish i was there cause i hate the butterflies i get when folks do hot water. No offense.

ya Roy I hear ya. I was trained in it and know how to handle it ; folks want hot water so i try to at least help them be safe.

What most dont get is that this is a topic that comes up all the time, CCC and aprovecho both put out very simple and inefficient designs that are somewhat safe. advancing beyond that point it gets real unsafe real quick.

I get butterflies every time someone brings it up. I very much wish to teach a group that has some solid skills in the area because i would like some development help to make safe systems.

the RMH world is not polished enough to handle a few deaths and keep on; I am sure the first time someone kills them selves the stove and myself will be blamed. thats all right for me but the stove is just a tool and doesn't deserve to be scapegoated.

any how thats all i got for folks.... dont die please!

Ernie,

Would an unpressurized system be safe enough? [Sorry, after reading the thread more thoroughly, I noticed this was discussed earlier. I hope you find the url's useful.] If you setup a heat exchanger with an unregulated output for immediate use or into an insulated tank, there should be no pressure buildup at any point in the system (anything is possible, of course, especially if a system is poorly maintained). You would still be dealing with water hot enough to scald, but no pressurized steam.

Below are descriptions of a successful kerosene inline water heater marketed in South Africa:

http://www.engineeringnews.co.za/article/alternative-hot-water-source-for-low-income-homes-2010-11-12

http://www.cbuying.co.za/index.php/paraffin-geyser
http://www.cbuying.co.za/index.php/paraffin-geyser/howit-works
http://www.cbuying.co.za/index.php/paraffin-geyser/typicalinstallations

http://www.slideshare.net/GreatNews/paraffin-water-geyser


You could use a similar tube-in-tube exchanger (they might use coiled monotube, but they aren't giving design details) and use a rocket, tlud, propane, kerosene, wax, or any other source for heated gas. There are other heat exchange designs that could be used, but a tube-in-tube is pretty simple.

I believe the CBS design was originally developed as a portable (using a vehicle or large pack animal) camp heater.

Working now on a 2-stage Rocket boiler -- about 85% complete. It is multi-biomass fueled, and will gravity-feed operate on 20" X 5" logs, wood scrap, briquettes, or wood pellets (with a special gravity-feed sleeve). Results so far have been amazing. Will produce steam and hot water, simultaneously, from two separate coils within. On startup, initial convection water flow begins within 3 minutes. Hot water (from "secondary" coil) flow begins within 5 minutes. Secondary chamber gets no hotter than 200 degrees (working to lower that), and stack temperature ( 5") hovers around 75 degrees! The initial rocket stack is built from 4" stainless steel pipe, and the combustion chamber/feed hopper are built from a combination of 6" "U-channel" and 3/16" plate steel. All this stuff came from my junk pile. The secondary chamber is built from a discarded 40 gal. well pressure tank. My mass heat storage will be achieved using a discarded 75 gal. water pressure tank. I plan to hook these two units up to an air handler built from a Dodge van radiator, and possibly powered by a steam-driven fan. Hope I can get all this done before Winter is over! BTW, does anyone know where I can get parts for an older "Atlas" 6" metal lathe?

POSTSCRIPT: : Am thinking about stacking on a gasification chamber, and for now, channeling the possible wood gas produced back in to the combustion chamber. Think it might work? Also, copper coils around the gasification unit will serve to reclaim its heat, and serve as a "pre-heater" for the boiler make-up water. Am going to instrument all of this eventually, with pressure and temperature gages. For now, am using a "Sears" multimeter with "IR" function and "K" remote sensors.

Roy, any photos of your set up so far?