Rick Frey wrote:I've been reading a bunch and watching videos (a couple of great ones here), I made a few brick rocket stoves with sheet metal flus and I'm finally ready to try out a design for a rocket stove mass heater for my greenhouse. The basic idea is to use really simple sheet metal pieces that I can get from a used building store near me for almost nothing. I'll build as much of a frame as I need to to support and contain everything, but here's the first version: (I guess see attachment
I use a 6" T for the feed and air supply. The 6" T connects to an 8" riser (no insulation). The 8" riser is surrounded by a 10" piece (it had a 8" reducer on the top for the cap) that works like the barrel most folks seem to use. The barrel piece is 32" tall. Then I've got 6" ducting coming out of the end of the 10" barrel. I have an elbow and a 6' riser that goes on the end that I used to test the system, but the longer term plan is to have about 24' of duct run through a box of sand to distribute the heat. The air in to the system will come from outside the greenhouse and the final vent off the last riser will go out the greenhouse. I'm planning on putting a 6" cannister fan near the top of the exit riser to help the system create draw when it's getting started.
So, 6" T into 8" riser surrounded by 10" barrel, all sheet metal duct pieces. Sound problematic? Reasonable? Good?
A couple of questions.
1. First, I heard I didn't need to insulate the riser in the barrel if I have a final riser that creates draw there. Is that true? Any problems with not having the internal riser insulated?
1a) If the Heat Riser is made of Firebrick, the firebrick Can be insulating and you can make a Type of lower performing RMH, Even with a Heat Riser Made With
Firebrick and 4''Thick, An additional wrap of insulation helps to create those freaky high temps that RMHs are Famous for
2. Any magic numbers for the ideal height of the internal riser and the height of the barrel?
2a)Unfortunately, there are competing Formulas and they all have to be understood independently, and in their entireties. I will direct you to help -directly
3. I've got about an inch or so around my riser for the gases to swirl and go down to the exhaust ducting. I have the barrel a bit off centered, so there's more room on the side where the duct starts. Does that seem like enough room or should I use a piece of 12" duct?
3a) Your System is too small for adequate gas flow, 12'' would better, but leaves you little room to make up the Transitional Area where the vertically falling
hot exhaust gases turn to flow horizontally through your Thermal mass ! Here your flow thru diameter should increase 300%
4. I want this to be somewhat modifiable/movable for at least the first few versions, so I'm planning on using sand, bricks and cement pavers in the framing box to help hold up the pieces and catch and hold the heat from the fire. I was hearing something about sand being too good of an insulator and that it doesn't work well for storing heat? Does that mean all the heat will stay in the ducts and I'll end up with hot exhaust coming out the final flu instead of having collected most of the heat along the way?
4a) The sand can be somewhat or a lot insulating, and may need to flow through 10% to 30% more sand and piping to flow into your Thermal Mass with
minor exceptions we normally say That with a 6'' system a bench made with cob needs a minimum length of 20' for sufficient heat energy flow to the
Thermal Mass !
5. If that's the case, any material better than sand?
5a) Cob Made out of Clay And Sand, with lots of heavy Dense rock in it !
6. For water heating, I'm thinking of using some high temperature rubber hose wrapped around the exhaust duct as it leaves the barrel. This is for an aquaponics system, so I can't use copper. I don't want/need the water to get too hot, so I could wrap the duct a few times with something to distribute the heat a bit first, or I could cover/surround the ducting with sand, maybe only an inch or two on top of the ducting and then coil the hose on top of the sand and cover it with more sand? The hose can handle temperatures up to 160 degrees, so I'm thinking I could try a few runs with the ducts packed in sand and check temperatures and see what I'm looking at. It might work fine to lay the hose on top of a few inches of sand or I might need to put six inches of sand under the hose otherwise it'll get too hot. But the idea makes sense to me, any problems anyone sees?
6a) Rubber is two insulating, Your 160ºƒ temperature is for air temperatures, and long periods of sporadic use, not buried in a higher temperature medium, and will fail
in the high temperatures it will be exposed too! You need Large Diameter 1.5 inch Stainless steel*, and your system has to be made so that it is thermo-
syphoning and unpressurized! The presence of valves that can be closed or pumps that can fail brings you perilously close to the" Boom - Squish'' Scenario, where
you are the Squish ! Think Boston Marathon Bombing with more Deaths, and more Full thickness, full body burns !
7. A buddy told me that the exhaust ducts need to be accessible for cleaning. Is that true? How do the folks who have these built into benches and covered with cob clean them? Is it just have a clean out and turn on a leaf blower and let the ash/dust go out the exhaust? Is there some other trick for cleaning?
7a) Hot off of the presses stuff, Common practice has been made to place stove pipe 'T's in the system with the unused leg closed off with a cap and positioned
horizontally towards the outside edge of the Thermal Mass Bench ! Attempting to place the Clean out leg facing vertically so that the clean out cap is on the top of the
Thermal Mass Bench creates a hot spot and appears to disturb the gas flow of the hottest gases at this point. Taking a clue from this more people are pointing their 'T's
downhill 45% off of the Horizontal where practical, which usually means well hidden !
Thanks so much for all the good information here, I'd greatly appreciate any thoughts or comments on this design. It seems like it's a pretty nice, easy way for folks to get started with easy to access pieces, I'd love to get some feedback on whether I need to go in a different direction or how to improve this design.
Thanks -- Rick
Satamax Antone wrote:Rick, gasifiers arent insulated. And, they can't output 2200F° for long period of time, which the rocket can.
But if you know better, please do it!
Rick Frey wrote:I'm not trying to build a gasifier, sorry for any confusion. There were just a few too many responses saying one couldn't use metal in a rmh design because it can't stand the conditions. I referred to a gasifier as a machine that operates under similar conditions that is made out of metal. Having now finished the Ianto book, they mention metal risers frequently, so I'm not sure what the concern was about a metal riser or about metal for any part of the system. I totally get that the cheap duct pieces I was thinking about using are insufficient for the burn tunnel and riser, but good quality, heavy gauge steel tubing gets used on all sorts of projects at extreme heats and has no problems--other than cost. If you can get bricks cheaply, that's great. I have better access to old tubing than I do to good bricks
Alan and Glen, I get the concern about the power going off and that shutting down the fan and messing up a system that depends on the fan for draw. My best sense is that I would still include a final exit flu, but just put the fan at the top of that. That way I can use the fan to control the beginning and end of burns, increase the flow rate if I want the fire to burn hotter, but turn off the fan once the system is hot and burning nicely. Otherwise, I get that power going out could be a big problem. When it's time for the fire to go out, that seemed to me to be a dirty time the few times I ran mine. The temps cool down but there are still some embers in the chamber that aren't easy to remove and that seemed to get smoky at the end.
Rick, we get that you are set on experimenting with iron and steel in your future builds. I for one keep coming back to counter your comments because others
following this thread extension in the future might be tempted to think you had proved your point ! Enjoy your research with materials that have failed again and
again and again, we will expect your future reporting to be just as straight forward and factual !
I am a little confused with your report of your system cooling down and running dirty, Any properly built J-bend Rocket mass heater should maintain high enough
temperatures to assure the complete burning of all fuels-especially charcoal ! Paul W.s brother put his hand inside the combustion chamber the following morning
after one of his RMHs first runs and came close to burning himself hour latter !
Perhaps your system has leaks and needs tightening up ! Regardless, this is exactly the kind of indicator of a problem that just should not be !
When I added my fan into the system, it let those last pieces burn up, exhaust outside the space then I turn off the fan.
And I'm not at all suggesting make a crappy rmh and just try to fix it all with a fan. The fan does seem to offer a few advantages, however, a smokeless, easy light, a smokeless, clean finish, and the ability to increase the heat of the burn by increasing the flow rate of oxygen into the fire. Those all seem like nice advantages and other than the work/cost of adding the fan somewhere near the final flu, I don't see the downside to it, other than the chance power goes off. I'm in the city, (Oakland, CA) and we haven't had the power go off once in 12 years, so I'm not too worried about it, but that makes a lot of sense if you live in an area where power outages happen with some regularity.
james beam wrote:You might try checking out this site for more info. on gasifiers http://driveonwood.com/sites/default/files/styles/verylarge/public/wood%20heat3.png?itok=JbmTjkvh