First build of big RMH - first test problems - need ideas and feedback

Hi. I am new here, and want to share the project I and a friend has going on. It is primarily experimental but intended to be used to heat a specific indoor area in the future. The build is not complete, but complete enough for testing.

The material we built it with is as follows: Common bricks for the outer shell , vermiculite as insulation, fireproof brick for the feed/fire chamber, stainless steel tube for burn tunnel and inside of heat riser, vermiculite/clay mixture forr heat riser insulation with refractory mortar as a rounded top layer. Stainless steel tube the first two meters in the actual heat mass, the rest is galvanized duct tubing. The actual heat mass and mortar for the bricks consists of clay from the nearby ground, sand and water apart from several big rocks to reduce the amount of mixing needed. The end of the duct piping concists of insulated duct pipe going 4 meters up and through the wall.

We still have about 10 cm of heat mass we still need to put on the top of the heat bed and the final cosmetic plastering and so forth is still to be done. That's where we are at now.

While we built the construction we put the ducts together and insulated the with mineral wool just to try it out. It worked pretty good and I remember we could, after some trixing and smoking-in incidents, have the fire going for hours and hours.

The session we had after that we put in the heat storage material around the duct pipes, waited for a couple of months for it to dry a bit before we decided to test it out thoroughly. We have a construction with an access door used both for cleaning/inspection and priming. After priming with some wooden sticks and paper there we get the mass heater going pretty good. It's not doing any intense rocket sounds o anything but burns calmly and stable. But, after 20-30 minutes approximately it starts to smoke in. There is no way of getting the stove to burn in the right direction again without priming again in the priming chamber.

I really would like some experienced RMH bulders ideas of why that is and how it might be fixed. Below follows a diagram with measurements and some photos.











I held the camera in the chamber after the heat riser pointing back. what you see is the lower part of the heat riser and the inside of the barrel.

I'm sure people with more experience will chime in, but here's a couple of thoughts...
Do you have an external vertical chimney? Is it long enough? If not, back drafting could be caused by something in the house using your rocket stove as a place to draw in makeup air (like a dryer or bathroom fan or something).
Also, it sounds like you might be having an issue with your heat riser...if it's not insulated well enough, the temperature will equalize between the inside and outside of it, at which point it will stall. Also, is it tall enough? If you're not getting very good velocity through the feed tube, you can also cover it partly with a brick to speed it up, but if your heat riser is the issue that won't be much help. Also wondering if the transition out of the barrel is big enough...it might be choking the whole system.
Anyway, if it will run for a while and then stall, your heat riser is probably the first place I'd look...once a draft through the system is established, it should only get stronger as the core (and more importantly the vertical vent) get hotter.
Anyone else?

Ronald; Where to begin... I see many issues with measurements on your build. First question do you have a copy of ianto evans book rocket mass heaters ? If not you should see about getting a copy ,it will help you in conversing here at permies. After converting your measurements to inches ,you have a feed tube apx. 5 1/8 " thats larger than your burn tunnel of 4 23/32", You seem to have a very deep ash pit. You measured your burn tunnel (D)at 16 17/32 " long but i would add the 13 cm of your feed tube to that making that measurement 55 cm or a 21 21/32" long burn tunnel and your riser is 29 17/32" tall ,you have a 1 31/32" top gap on your barrel but you have only 63/64" side gap and all of that is going to an outlet of 4 21/64" I am not surprised that you need to prime this to make it start and that it stalls out after a few hours , in fact i'm surprised that it works as well as it is. What i see as issues are as follows. You are building the smallest size system in that your outlet is barely 5" that is a hard size to try for a first build. (1) Your feed tube is to big , it should match your burn tunnel.(2) your burn tunnel is very long, should be shorter (3) Your riser is to short (4) Your top gap is good but your side gap is WAY too small. You do have a very nice looking build and with some modifications i'm sure you can get a dragon roaring in your future.

Ronald McLaughing :Welcome to Permies.com / Richsoil.com, and a Big Welcome to the Rocket and Wood Stoves and Cob Forum threads !

Insulating the chimney was ideal, however if the core of your thermal mass is not dry, and it probably isn't, then your exhaust gas temperatures prior to Your
Final vertical chimney are to low to help you with the draft !

I can understand your desire for multiple clean outs, though at first I was hoping that a loose clean out might be part of your problem! Check them carefully,
re-sealing them if necessary.

It is a little difficult for me to translate metric into Imperial (inches feet) measurements ! But yes, you have a few problems -or my calculations are
seriously out of order !

You have a sub 6'' system, actually sub 5'',and for a 1st build. That is a build best attempted after a successful Rocket Mass Heater build of larger size with more
predicable results !

And then your entire vertical run of 960 cm / 2.5 = 384'' /12 = 32' of horizontal ducting, 30' of straight run without elbows is near the maximum for a well working
6'' system.

Generally we figure that for every 90º Elbow we should subtract 5' off of the final horizontal run, and you have four (4) of them !

For the size of the system you were trying to create the gap between the top of the Heat Riser and the Underside of the Barrel is fine, however I would have built
my heat riser thinner and gave myself some more room between the outside of the Heat Riser and the inside of the barrel.

The Transitional Area just below the Barrel where the vertically falling hot exhaust gases turn 90º to flow horizontally into the Thermal mass is unnecessarily rough!

This area needs to be sculpted smoother with more gently flowing curves.

The exact spot that you have marked as ''H'' should be a bell shaped funnel gently collecting and channeling those hot exhaust gases and reducing turbulence as
the gases flow into the Thermal mass piping !

Think like Fire! Flow like Gas! don't be a Marshmallow! Your questions and comments are solicited and Welcome! For the good of the Crafts ! Big AL

Ronald; After rereading your post I noticed that you have used stainless steel for a burn tunnel and for a riser. I'm sorry to tell you that they are going to fail if you ever get your rocket up to temperature. The metal even stainless can't take the heat, also round burn tunnel do not burn as well as square burn tunnel.

Hey all.

Thank you so much for the reflections and explanations on this.

It seems to be a necessity to rebuild the whole head of this dragon. Maybe we will get time to do this during the coming spring. I will update this thread when we reach the next step.


Thanks for your time

/Ronald

Ok. In a few weeks it's probably time to rebuild the burn section of this stove. I have studied ianto evans book a little bit and contemplated a bit about the situation here.

We have a system of 4 23/32 inch heat transfer tubing over 31 ft long with four 90 degree bends, two of them probably very turbulatory (is that a word?, you know what i mean though...). Ideally, from a functional standpoint, we should dig up the heat transfer tubing and replace it with 8", but the work and cost of doing that leads me into the following idea:

First, scaling down the 8" example from ianto's book. Even though we have a 4 23/32 inch (120mm) heat transfer system in place, to be a bit safer because couplings can possibly have bit smaller cross-section area and so forth, build as if it were 4 21/64 inch (110 mm). That means almost halfing the sizes of the example of the book.

The burn tunnel would be fire brick, 90x90 mm (3 35/64 x 3 35/64 inch), the heat riser slightly bigger along with the rest of the cross section areas which are important. However, the book says that the taller the heat riser, the more drag. So my idea is to make the heat riser as tall as possible to create enough force to push through the hundreds of miles of duct pipes. How tall can I make it without creating other problems?

Also, The barrel would be a standard 55 gallon so there is plenty of space for insulation around the new fire brick heat riser. Could that cause any functional problems?

Furthermore, from any people having experience with any kind of similar build, would you tweak the dimensions of the feed tube and burn tunnel lengths otherwise than scaling it from ianto's 8" to 4 21/64" ?

Thanks.

Nothing you can do at the combustion zone will overcome the fact that you have over 30' of approximately 4" duct in the mass. This is just too small to work effectively. If you don't want to replace that, I think the best thing you could do is change the combustion system to a batch box, as that style is said to give half again as much heat per hour for a given size as the J-tube style. Look for "peterberg batch box" at http://donkey32.proboards.com for details and dimensions.

Another thing that is relevant is use conditions. You haven't described anything about the space or what it will be for, but it looks garage-like or shop-like from the pictures. Presumably you will be insulating the walls. What is your climate like? How warm do you want to be able to make the space? How large is the space?

There is other problems than just the flue.

The biggish square feed tube, compared to the round burn tunnel.



That's no good, you do either square or round, but don't change from one to the other along the way. Even more if there's a slight reduction between feed tube and burn tunnel.


Secondly, the barrel gap and transition area seems a bit cramped.




Usualy we say that the transition area between barrel gap to flue should be 3x the CSA.


And finaly, you could do a long dead end bell, instead of your flue.

You could use barrels, in the same fashion as this, to make your bell.

http://api.viglink.com/api/click?format=go&jsonp=vglnk_14263240702356&key=bbb516d91daee20498798694a42dd559&libId=i78sig55010004m6000DAicszslgt&loc=http%3A%2F%2Fdonkey32.proboards.com%2Fthread%2F609%2Fheated-seating-nyc-restaurant&v=1&out=http%3A%2F%2Fs65.photobucket.com%2Falbums%2Fh228%2Fmremine%2FNYC%2520Rocket%2520Stove%2520Build%2F&ref=http%3A%2F%2Fdonkey32.proboards.com%2Fboard%2F5%2Ffinishing-finished&title=Heated%20Seating%20for%20NYC%20Restaurant%20%7C%20Rocket%20Stoves..%20Experimenters%20corner..%20Answers%20questioned!&txt=s65.photobucket.com%2Falbums%2Fh228%2Fmremine%2FNYC%2520Rocket%2520Stove%2520Build%2FMatt Walker's heated seating in NY.

"scaling down the 8" example from ianto's book... build as if it were 4 21/64 inch (110 mm). That means almost halfing the sizes of the example of the book."

Something you may not realize about scaling: cutting the diameter in half means the cross-sectional area is cut to 1/4, so the potential output is 1/4 or less of the original. Add to that the fact that there are surface drag effects that don't scale down, and the effective area goes something like 7" diam. (38.5 in2) to 3" diam. (7 in2), less than 1/5 (this is not the exact factor, just an example). There may not be enough heating capacity in a system of this size to heat your space, again depending on your space/climate details.

-C- should be the smallest 'opening' the whole system
-D- should be as short as possible .. (and ofcourse can not be shorter that the point where the edge heat riser comes)

-B- should be about 2,5 to 3 times the hight of E/the heat riser.
If your ashpit (B) is that deep as it shows on the image it will create very strange currents on the draft) Erica and Ernie Wisner even recomment to leave the ashpit out totally ( [youtube]https://www.youtube.com/watch?v=3mZQdu2wNi4)[/youtube]

I build my last one recently with these dimensions, so I talk out of experience) and it works like a charm