Need second opinions...

[roaringembers]

Okay so I came up with a design for a rocket furnace to heat my greenhouse, this is a rough sketch:


The whole purpose of this furnace is: I want to be able to fill the feeding tube up in the evening and not have to get up in the middle of the night to add wood, the stove would feed itself through gravity.

Now as we all know, some things are easier said than done...I built it but quickly ran into some problems, after all this is my 1st time building a rocket stove.

Right off the bat, 3 things were seriously wrong. Diameter of my chimney was waaay too small, so it wasn't drawing enough air at all. The feeding cylinder wanted to act as a chimney so bad, I had to seal every last little hole to prevent that. Lastly, the whole thing wasn't sealed and air was drawn from all over the place instead to where the fuel is.

Current state of this furnace is: I sealed everything and the air intake is left of the burn chamber in the picture, I've yet to add a wider chimney, change the air intake to how it's like in the pic, and add the additional ash tube. But before I do that...I need to know I'm not wasting money and time here.

Okay, so the plan is:
-replace the 10cm diameter 170cm tall exhaust with 20cmx20cm x 2m rectangular one, increasing total volume by more than 5 times (according to my errr..math). Is this enough? The exhaust would still be less in diameter and volume than feed cylinder.

-Would the additional tube that leads to ash box create any unforeseen problems?

-My main worry is that the fuel will create so much heat that it will start pyrolyzing the wood above it, creating too much smoke that will be unable to burn and will choke the fire, not to mention reducing efficiency. So will it?

-Feed cylinder is the same height as exhaust, but is capped. Still, is that a problem?


This project has given be a headache. Please help 

[belgiangulch]

Hi Roar;
This whole design is not much like a rmh at all, its more like a rocket stove.
I suggest you read a copy of rmh builders guide.
A J tube RMH requires specific dimension's to function. I'm afraid yours is not correct.

We heat our greenhouse with a rmh all winter long. No fire at night at all.
In northern Montana it is 40-50 in the morning in a plastic uninsulated building with tall ceilings.

[roaringembers]

I uhh...was impatient, the whole thing started to haunt my dreams, I had to finish it and test it.

Welp, I was so excited to see it actually work!!!


The air comes through cuts I made above the burn chamber so the fire is directed downward and than continues to the chimney. The fuel cylinder only heats up somewhat, not enough to cause any problems. The cap at the top is not even sealed but it doesn't smoke. When first starting, I open the little hatch on the left, the fuel tube smokes at the top somewhat but once the exhaust heats up and I close the hatch, it stops completely. There is a small amount of smoke coming out of exhaust, but it's not yet isolated, this is going underground, where the soil around it will serve as a heat battery, heating the greenhouse even if the fire goes out. Exhaust aka chimney will be isolated with a bunch of sand, so should heat up considerably, burning everything up.



The "smoke" you might see rising near the end is water vapor because it's standing on wet concrete.

It's a bit strong than I intended but the soil around it should soak up excess heat nicely. I'll report further progress if anyone is interested 

[belgiangulch]

Hi Roar; Your stove burns well at this point.
When you lengthen the exhaust, you may have issues.
But please keep us posted on your progress.

[pigbuttons]

How big is the green house you're trying to heat? That feed tube looks like about 20 hours worth at normal rocket stove rates. Good for you to get it working. One thing to remember is that the difference in height between the chimney and the feed tube will affect the draft more than just about anything else. It's that difference that promotes the vacuum that directs the flow of air.



Edit: After watching the video it is clear that your primary air is coming in through the air perforations just above the firebox, so disregard my comment on feed tube height.

And if your stove is built from thin gauge metal as it looks like in the video i would guess that it will be gone in a handful of fires. I started there too and quickly learned that there is no metal, repeat no metal that can survive the 980c (1700f) temps. Depending on where you live you might want to source ceramic fiber board for the top, ceramic fiber blanket for the riser, and fire brick for the floor and sides of the firebox.

[roaringembers]

pigbuttons The GH is about 250 cubic meters, judging by how much heat is coming from the furnace when I'm near it, I'd say it puts out about 7 to 10kw which is about what I want, I don't want it to be too strong. The GH doesn't need to be hot, 10-15C would be ideal. I've yet to work on GH insulation.

Yeah I'm worried about stuff melting, particularly where the air is drawn, it gets red-hot. The tube is made out of 1.5mm thick sheet metal, I might cut that portion of where the perforations are out and replace it with something more resistant. Exhaust is made from 4mm thick metal pipe, so should last imo.
It smokes quite a bit when starting but once it heats up it runs very good. In fact it seems the longer it runs the better it runs, probably because the exhaust gets heated up.
We'll see how it performs once I add stuff to actually extract that heat before the exhaust gasses leave the GH

[Solomon]

I don't think melting is the real issue. The steel is burning. It's only a matter of time before it burns through. The more efficient you make it, the greater likelihood that you'll be burning through all the metal parts in hot areas.

[roaringembers]

It's been through a couple of fires and so far there isn't any noticeable degradation or melting going on. I think the air is drawn through the holes at the bottom at a sufficient speed to cool it down a bit...we'll see, I'm planning on running it whole nights in February and March.



Now on to another little...problem. So, I've lowered the furnace down into the hole I dug up in GH for it. It works great once it heats up. I know rocket stoves are known for making a sound, but with this thing it seems the cylinder and the hole itself serves to amplify the sound to godly levels, if I get down it the hole while it's roaring I can feel my entire rib cage vibrating.


The vibrations carry through the ground and windows in my house were shaking...lol!?

The sound louder and frequency of it is lower than my phone camera car record it seems, but just have a listen:




In second video the phone can't even record the sound properly because it's so strong.

I've yet to come up with a solution to this lol

[peterberg]

I've heard this sound before, about 9 years ago. I was fiddling around with a small J-tube rocket, when the thing got hot enough it picked up a pulse like that. Fuel was eaten away at a racing pace, producing a low rumble at the same time like yours. I filmed it with the camera of a crappy Nokia mobile phone so it's a very back-then video. It isn't visable in your video but I bet the lid of the fuel magazine is dancing up and down with the same frequency.



It all boiled down to the acoustically resonant properties of a pulse yet, like the V1 in WW2. Even down to the frequency, about 40 pulsings per second. Have a look at this real V1, mine and probably yours as well are not-too-distant cousins, albeit valveless.

[pigbuttons]

Sometimes we just stumble onto science. In the second video I can see the flames trying to dart out of the air slots at the frequency of the noise. It is an odd balance of fuel and air so that the fire gulps for air and when it gets the gulp it surges only to run out of air, which makes it gulp again. Just my opinion but some changes may be necessary, just say'n.

[pigbuttons]

Jan 18, 2021 9:06:51 GMT -8 peterberg said:

I've heard this sound before, about 9 years ago. I was fiddling around with a small J-tube rocket, when the thing got hot enough it picked up a pulse like that. Fuel was eaten away at a racing pace, producing a low rumble at the same time like yours. I filmed it with the camera of a crappy Nokia mobile phone so it's a very back-then video. It isn't visable in your video but I bet the lid of the fuel magazine is dancing up and down with the same frequency.



It all boiled down to the acoustically resonant properties of a pulse yet, like the V1 in WW2. Even down to the frequency, about 40 pulsings per second. Have a look at this real V1, mine and probably yours as well are not-too-distant cousins, albeit valveless.

Now THAT is what I call a ROCKET stove. Thanks for sharing that peterberg.

.

[roaringembers]

Now I'm thinking...this noise might not be such a bad thing. I can think of several benefits to this noise:

-I can hear it in my room so I can always know if the fire is burning
-The sound may drive away rodents which sometimes cause problems in my GH
-It may annoy neighbors (which are themselves, annoying)

Hmmm I think I'd like to just dampen the sound a bit, it's too strong. I'm thinking some kind of amortization with rubber to absorb vibrations. This has to be because of the fuel cylinder, it's capped and it's acting exactly like a pulse jet.

[pigbuttons]

Jan 18, 2021 10:26:15 GMT -8 roaringembers said:

Now I'm thinking...this noise might not be such a bad thing. I can think of several benefits to this noise:

-I can hear it in my room so I can always know if the fire is burning
-The sound may drive away rodents which sometimes cause problems in my GH
-It may annoy neighbors (which are themselves, annoying)

Hmmm I think I'd like to just dampen the sound a bit, it's too strong. I'm thinking some kind of amortization with rubber to absorb vibrations. This has to be because of the fuel cylinder, it's capped and it's acting exactly like a pulse jet.

I don't know where you are located, but an air rifle of even low power can solve your rodent problem. Use green or red lights, which the rodents cannot see but humans can, and you then bait them to a good location for elimination.
Neighbors are the same wherever you live.

Do you have the entire system built? It may happen that adding more drag in the form of an extend heat extraction device will dampen the noise and reduce the effect of the pulse. One or two 90 degree elbows in the exhaust flow will almost certainly stop it all together.

[roaringembers]

You know on whistles how you have a small opening through which air is forced at great speed to produce a sound? Well basically that's exactly what's happening, the entire furnace is a giant whistle.

Luckily the only thing i needed to do to fix this is open the air hatch a bit so more air comes through.
Only thing left to make tubes that lead exhaust gasses out, I'll post when everything is finnished

[roaringembers]

I was just thinking about a modification that might improve my furnace, and the idea is to replace the enclosed feed cylinder with a cylinder cage, that way it doesn't try to act as a chimney:




My fear is that the the wood would just start burning up until all of it is on fire which would be catastrophic to say the least. 
The idea behind an enclosed cylinder is that the fuel is denied access to oxygen so it can't burn, but it comes with some aforementioned problems. I've seen rocket stoves where wood is inserted vertically and even though it's exposed to air it burns only where there's air draw, and flames are sucked in upside down.

Any thoughts on the cage idea anyone?