Dimensions and barrel temperature Question

I am a first time builder and have had my system up and running for a month or better.From the beginning the RMH has had good draw with no smoke back and a good rocket sound.My problem has been barrel temps are low.As far as I can measure with crude oven thermometer,its about 200 degrees ball park,on the top of the barrel.My dimensions are 6x6 feed at 13"deep,6x6 burn tunnel at 15" and 6x8 riser at 32".It is possible for me to close the riser size down with some major fire brick cutting,but if that doesn't sound like my problem I would rather not do all of that extra work.Being that the system has what I consider (good draw) this seems like, in my limited knowledge,the only thing I have wrong.If more information on my system is needed,ask away!Thanks

Ok I thought I replied to this but I can't see it so if this a repete than please excuse me. I am still a novice but have been running my 4in feed for the second year and it works almost perfectly. I believe tour burn tunnel is to long making it so not enough heat is left tore burn within the barrel or what I believe is called the manafold. The hight of the burn chamber probable is acting as a pump to pull air in but not in couris with the barrel so a drawl is made but no burn happens. One way to tell is to test the exuast and see if it is carbon rich. The exist should be mostly CO2 and water vapor. Could also be that the surrounding material is pulling to much heat from the tunnel. Long story short I think your demmensions are off, but without pics and other info not sure.

Mikael

Michael, describe you system a bit more. I.e. it appears to be close to a 6 inch system, and the 6x8 inch riser indicates that it is constructed of brick. If the performance hit has to do with the riser, it is usually because of lots of mass and little insulation, making it difficult to achieve temperatures high enough to burn the smoke. An indication of this will be lots of black deposits downstream from the riser. How much insulation do you have around and underneath the feed and burn tunnel?

What is your riser to barrel top clearance? Side clearance? It should not be less than 1.5 inches for a 6 inch system, from what I recall.

The other (and most common) performance trouble spot is the manifold area transitioning from the barrel into the horizontal ducting to the thermal mass bench. This is best made really large, several times system size, and smoothly tapering into the 6" bench ducting (assuming that's what you're using).

What is your chimney system like? A tall insulated vertical chimney is best, with the better part contained within the envelop of the building.

Dry wood makes for hotter fires than unseasoned wood. My rule of thumb is one year drying time for softwood, 2+ years for hardwood.

Pictures are worth a thousand words!

Mikael,I believe my measurements are within the 1:1.5:3 suggested dimensions except maybe the riser is a bit short ,but within the specs as far as I have read.Byron ,The j tube is made of full size fire brick with the riser being insulated about 1.5"and wrapped with sheet metal.3" gap at the top of riser.With the barrel being offset to allow for shorter burn tunnel.It then enters 8" galv. HVAC pipe all the way to the top off chimney.There does seem to be some black residue in the cleanout area near the vertical transition.

I will post some pictures from my phone asap

Pictures

The burn tunnel area is covered with brick but that is the only insulation there.

Looking pretty good there. But, in the manifold, the ducting is extending into the manifold space well past the barrel. That is one problem I see straight away, causing drag on the exhaust gas flow. I'd put a coupling "stub" of ducting at the barrel, so that the inner edge could be cut around the circumference forming lots of "tabs", and then bend those "tabs" back against the insides of the barrel. For the exhaust coupling "stub" you could even use something like a 10 inch to 8 inch reducer, 10 inch end mounted to the barrel exhaust port. How much horizontal space is there between the exhaust hole in the barrel over to the riser?

What's your riser insulated with? Also, it is really best to insulate the feed and burn tunnel all around. Tweaking both the manifold (as mentioned above) and insulating your feed and burn tunnel should bring your dragon up to temperature.

Is your thermal mass bench "fill" completely dry or not yet complete?

Byron,i can probably get in and make tabs and get them bent and clear.The riser is wrapped in fire rated insulation with the fire brick being on edge.Then wrapped in galv.sheet metal.The bench area is at this point only covered in busted up concrete block in sizes from sand to golf ball.Everything is deep freeze here and waiting on warmer weather to complete the mass in a less insulating fashion.

Oh yeah, if a recent inspection of the barrel's interior hasn't revealed any creosote coating the barrel or riser, then the riser is performing OK. The only deposits you'll have there should be fly-ash.

Fire rated insulation? If that is spun fiberglass it will not hold up long term and riser efficiency will suffer. I'd replace that with loose fill Perlite from your farmers coop or garden supply.

The pictures are from today and don't show any signs except fly ash.Do you think that the CSA of the riser is slowing down the flow of the system?

At this point I'm not worried about the Rockwool as I can change that if needed.I have ran this for many hours continuous without any loss in draft due to equalization.

Hi Michael I miss read your dimensions, I am a visual person, so the pis are great. Again I have only built the one unit with many prototypes before i built the final build with several modifications including the use of wood pellets as the main fuel mixed with other stuff, it can burn any thing but because of the small size 4in a dense fuel gets me up to temp fast with no fly ash. That said I am only making guesses. Is the RMH sitting on a concrete foundation?, Is there insulation under the burn tunnel? Both of these may being effecting the temp of the gasses entering the manafold? That's all I got I think others also gave good feed back which seem along the same thread. Good luck keep us posted.

Mikael

Mikael,The entire J tube sits on face brick on edge so that the holes show and allow for heat transfer away from the concrete floor.I was afraid it would spall the concrete if moisture was high.

Hmmmm, not sure but I bet your getting a air current through those holes? Are they covered? Or did you leave them open to vent the air. I am just guessing.?

M

Michael Gillingham wrote:At this point I'm not worried about the Rockwool as I can change that if needed.I have ran this for many hours continuous without any loss in draft due to equalization.

Ah, Rockwool is fine, being "stone wool". If you've got extra it can be used to insulate your burn tunnel and feed, and stuff the gaps in the edge bricks under the burn tunnel too.

While your working on the manifold, measure horizontally the distance between the barrel's exhaust port and the heat riser. I would want a minimum of 3" of spacing there. If it is less than 3 inches the manifold performance will most likely benefit by going to a much larger hole in the barrel, like using that ducting 10 inch to 8 inch reducer I mentioned earlier for fitting to a larger 10 inch exhaust hole in the barrel. Or maybe even use one of those HVAC rectangular vent to round pipe fittings.

could be over draft

Byron,I have 5" at the riser to barrel and then 3" as it goes around to the other 2 sides.The 4 th side is tight to riser.

OK, 5" horizontally between the barrel's exhaust port straight across to the riser, if I read you correctly. That is a bit much. To make the barrel heat up more, less gases shortcutting to the exhaust on that side of the barrel, I would shift the barrel back towards the feed to make that 5" gap less. When the manifold opening in the barrel is tight and restricting gas flow, offsetting the barrel is often needed. Without manifold restriction issues, a more centered riser is better.

From your measurements, you've got room to fully center the riser in the barrel, with the result being 2.5" of clearance all the way round. This would be my preference. Then if the system fails to draft strongly after centering the riser, I'd turn my attention to improving the manifold area, opening it up more.

On another performance point, the 32" riser height measurement, is that taken from the top of the riser to the floor of the burn tunnel?

You system appears to have burn tunnel CSA of 36", making it closer to being a 7" system than a 6" one. I.e. a 7" system has a CSA of 38.48", where as a 6" system's CSA is 28.27". For a 7" system I'd make the riser closer to 42" to 43" than the current 32" height, assuming that is the measurement. This could be done while repositioning the barrel to center the riser. Adding two more rows of firebrick on edge would yield a riser height of between 41" and 42".

Edit: This would of course require a bit of work, so consider it a last resort. That is, it would be easiest to accomplish by going to a half barrel manifold system like Ernie Wisner does on his builds. And it would require procuring a couple of new barrels, two that could be clamped together.

This may be a crazy question...what is the ratio of - gases in:gases out - in terms of volume? i think i've assumed a 1:1, but that's probably not right. heat and gasses are being released. You both have the addition of gasses released by the wood, and expansion of the gasses caused by gas. perhaps some simple volume math looking at a perfected rocket stove could figure out the ratio for us.

there are just so many variables to take in to actually do math. a mix of math and guess and check goes a long way, but there's probably always something unknown with being strictly mathematical. my dad was a math teacher and I was a nerdy kid that is currently getting a bit of the math bug while i design different stuff.

Ok,well to wrap this up,last night I made tabs and bent them over to the barrel (picture 3) and then I crudely insulated the feed tube/burn tunnel area with face brick.I also covered the holes in the base layer of brick that the system sits on.The draw improved slightly but the temperature increase was really good (400 +) with an oven therm.surrounded by brick.This is also with less than seasoned Hickory and Maple.So thanks everyone for the help! I believe that with better insulation around the core area and dryer wood,it will be even better yet.As far as my dimensions and barrel placement are concerned,I think the system will run well as is.This is a work in progress you could say ,and its looking like WE have the bugs mostly worked out now.Thanks again!!!

Michael, that's excellent news. You'll know if your system is close to being optimized in the end. It will easily reach 800° F. on the barrel's top when burning full tilt on well seasoned wood.