Jacob Silver

It's been cca one month since I shortened my heat riser, I heat for three weeks, and can tell my heater works much better, draft seems stronger, sound is more even, and there's no 'pprrr...prrr' sound.
Thanks a lot! :)

Satamax Antone wrote:Jackob, i think your porit bricks, are not insulating firebricks, but air entrained concrete, which cracks everywhere, under intense heat.

The name 'porit' associates on aerated concrete, but it isn't. These bricks are for building industrial stoves, and company for refractory materials sells them.

Sorry for late answer, I couldn't reply for a weekend. Now everything is clear for me. I'm still searching for ceramic boards, for now there aren't on our market yet. We have ceramic wool and refractory insulated bricks - they call them 'porit' bricks. I believe that CF boards are much better insulator than porit bricks, so I'll try to purchase ceramic boards. I fI will demolish this and build new, I will put best materials.
Thanks a lot for your help, Fox and Thomas, I am a little bit out of RMH trends in material and building, this was fast response for my needs .

thomas rubino wrote:Hi Jacob;
When I had an 8"  cast J tube core and a cast riser , I regularly had barrel top temps of 1100F  But no higher.   1300 F sounds like a batchbox  design rather than a J tube.

I replaced my cast core after the first season, because (like yours) the feed tube was too fragile.  I was constantly trying to patch it.
I replaced it with an all heavy firebrick core . Now I can get +1000 F on the barrel top...only if I baby it with small wood. It regularly runs around 800 F.
My fireclay / perlite cast riser I have continued to use on my studio / greenhouse rmh.  I am planning on replacing it next season with a five minute riser made from stove pipe and ceramic fiber blanket.

As Fox suggested ceramic fiber board/blanket is the newest best product's to build your rmh from. The downside is that it costs a whole lot more than clay and perlite.

I am currently building a J tube with ceramic boards and a riser with ceramic blanket.  I have high expectations from these build materials, I expect the extra costs involved to be balanced out by the superior performance from them. It added over $300 to the build cost.

These products may not be available in your area....   If that is the case, then building your burn tunnel with insulated bricks is an excellent idea. If you have enough, then build your riser from insinuative bricks as well.  Use the heavy firebrick to build your feed tube. Split heavy brick would be even better.

Fox and Thomas, thank you a lot for answers! Now I feel better :). Here I can find ceramic wool, which us enormous expensive because it sells in 30m rolls. Maybe I can find some rests. There'no boards here, for now. Are these boards fragile? I mean -  can I build whole tunnel, or only the bridge? Now you gave me some more things to think of :). I can buy insulating refractory bricks.
One more question - do stovepipe - no matter which type of insulation - 'steal' some heat? Or, maybe it us the same if it gets very hot?

Cast core vs. Refractory brick core – and cast heat riser vs brick heat riser

Hello everyone, I have uncertainty which bothers me a lot .
After three RMH's I've built, I still didn't decide for myself – which one is better – cast (perlite-cement 1:10) core or standard refractory brick core? First I made 7'' brick core and it worked fine, two seasons. Then I started thinking that I have too much ash, and that I could have higher temperatures and better usability of fuel (wood) with cast core. First RMH brick core was isolated with dry perlite around and 8cm ytong(porocement) underneath. Second, cast core of 8'' is made od 2cm thick refractoy plates, and insulation is 5-6cm perlitcement, and around rockwool. Underneath is 8cm perlitcement. Stove works fine.
I haven't made measuring, I rely here on pure subjective experience. I've tried to measure it but barrel was shiny and we could't get consequent results.
I think temperature is little bit higher with new heater, amount of ashes is similar, but I don't feel there is drastic improvement. Also, as I read in books and hear some experiences  - people say it is enough only few hour per day of burning, and that they have some abnormal temperature on the barrel (1300° F of higher). I have two floors (heat goes upward through the staircase) little bit less than 40m2 (430sq feet), styrofoam insulation 8cm, 25cm on the attic, but windows aren't high quality (they are some 'insulation' but old, wooden).
Ah – first heat riser was steel tube with perlit around, now I have pure cast perlitcement heat riser.
I was somehow happier with classic brick core, I felt it better, wtih this plates I'm sometimes scared it will break (one is damaged), I have to be careful when I put wood inside, and result isn't so dratically better. And, of course, I have lost cca 300 kg of thrmal mass.
I have idea to make brick core with bridge of insulated firebrick (white ones), pure dry perlit around. And I'm not sure about the riser – is better cast riser as I have now, or should I make riser of 1'' refractory plates (as in book of E&E). Or, if square riser is better, is it maybe better to make cast square heat riser?
I'm in this uncertainty a lot, and don't have the answer. Here are picture of first and second unfinished cores.

Thanks,
Jacob

So -  we have been burning two more days, and temperature on the lid of the barrel now is 735°F (390°C), and we are satisfied with it. Obviously it was water.
We didn't catch up to cut riser, but we will.
Thanks a lot lot everyone for a help, I'll post when we make some changes!

Thanks Tomas, I'll make it these days :). Also have removable lid so it will be finished in minutes :).

thomas rubino wrote:Jacob;  I run mine at 2.5",   there is flex with the removable lid so gap can be 2.25" to 2.75"

I've watched videos. I think I'll widen the gap and on my own heater. It isn't problem because I have removable lid and is perlite-cement only. Maybe this is the cause that my own heater sometimes dones't draw as I wish :).

Satamax Antone wrote:Hi Jacob.

Well, that number has been devised using guesswork and experience.

I don't know if you are familiar with fluid dynamics.

But what happens when your gases are exiting your heat riser, is pretty much what happens against one half of this cube.

Plus a sharp direction change, squashing the gases.

You can see the boundary layer here

Here is the turbulence behind a port.

What happens at the top of the  heat riser is as turbulent as this.  Plus the top plate of the barrel squashes everything.

Well, i thought, i have a kind of real life example. Tho, with a 20cm gap.

See the radius of the gases, when these hit the barrel top? That's the reason you need a bigger top gap. If you are really set on keeping it at two inches. There is a trick,  forming the top of the riser, like a trumpet endbell. So your circumference is larger than the one of the heat riser, allowing your top gap to be smaller. But there isn't many people who have tried this. Two or three i am aware of.

I have searched a bit on permies and donkey's forums. I can't find the first instances of the 1.5 figure.

To check: 8'' system – P=r2x3,14  =50,24sqin (314cm2);  M=2x3,14rh  so  50,24=2x3,14x4xh  so 50,24/25,12=2''. I don't remember 1.5x factor from the book of Ianto. Is it something new? We were very precise by measuring this gap, it is 2''. It is no problem to expand it. W will do it in a few days. Just tell me please where 1,5 factor comes from, I'm interested. Now I checked, Ianto says it is something between 2'' and 3''.

Satamax Antone wrote:Jacob, trust me, your top gap is too small.

So it's round, then your CSA is 50.26 sqin.

8 inch round circumference, 25.13 inches. X2 for your top gap. 50.26 sqin again; So, considering the direction change, the turbulence created by the direction change, and the friction resulting of this, your top gap is too small.

Remember, the figure is 1.5 times the CSA for the top gap, at least.  So that's 75.39. /25.13  = 3, you need three inches top gap.

Thanks for answer, Satamax.
250°C we got barely by burning small chopped hornbeam which would at normal working RMH give 500-800°C. By burning normal pieces of wood (also hornbeam and pine, spruce) on the top of the barrel is 130°C, which is very low. Our heat riser is rounded - made of firebrick 20cm duct and perlite/cement. I'd rather wait for a week of burning, than I'll consider to raise a barrel.

Satamax Antone wrote:Jacob, two details.

First, 250C° is not that low a temp. Your fire, in a 8 inch is small. Compared to a box stove. And a barrel is 1.86m² of ISA (internal surface area)  This sheds a lot of heat. Plus your heater being wet. I don't think you should worry too much. What is interesting in a rocket. Is the amount of heat recovery. Compared to a normal box stove.  When you have the mass installed.

Then, about your top gap.

You have a CSA (cross sectional area) of the heat riser of about 64 sqin. If i understood well, you made a square heat riser. Then your top gap is  4x8" X2" which is 64 sqin again. Or, due to the boundary layer, your heat riser must be closer to 50 sqin. But still, you are under the recommended 1.5x csa usually accepted. You would be better off "freeing" your draft, i think. by raising your barrel a bit more. At 2.5 or 3 inches.