New rocket stove problems

Today I attempted my first rocket stove.  It didn't go so well.

I used 54 refractory bricks to make the J.  I didn't use mortar or any kind of sealant.  I just stacked them.  Then I put a 55 gallon steel drum on top of that.  The stack inside the barrel is 32 inches high, with the barrel at 34 inches high.  So I should have a 2 inch gap between the top of the chimney and the barrel.  The horizontal burn chamber is 4.5 inches by 4.5 inches.  And the chimney is a little bigger, maybe 4.5 inches by 6 inches.

I was able to start a fire and sort of keep it running.  According to a laser thermometer, I the temperature hovered between 180 degrees F and 260 degrees F measured at the center of the barrel right above the chimney.  How hot should this be getting?  Seems like I should be expecting 300 to 400 degrees.  What do you think?

I was expecting not to see any smoke coming out the exhaust.  But there was a lot.  I'm thinking the cause was that the fire wasn't hot enough to re-burn the smoke particles.

I never heard the rocket stove sound.

So here are some possible causes...

I didn't put a long exhaust pipe on it yet.  So the smoke just exits at the bottom of the barrel near the ground in the usual location.  I'm sort of thinking this isn't the cause because rocket stoves work even without the barrel.

It was really windy.  On a few occasions, the wind blew into the exhaust and made smoke come out the feed area.  But when the gust died down a couple seconds later, it would revert to the right direction easily.  Later, I arranged some concrete blocks to make a 4 foot long horizontal exhaust pipe.  But it didn't matter.

The wood I was using for the fire wasn't the greatest.  It's basically whatever I could find.  And there's a good chance it wasn't totally dry.

So what does everyone think?  I could disassemble it to see what happened.  Is there anything I can look for to know for sure what went wrong?  Did I make an obvious mistake somewhere?

brian

Well, no wonders.

Bricks stacked, barrel gap of 2 inches. No insulation i bet?  Nor a vertical chimney?

Horizontal exhaust without a vertical chimney doesn't work. Except in very rare occasions.

Try first without a barrel. Seal the bricks with mud. Insulate your core. Then, when the barrel goes on, the vertical chimney goes up at the same time. Even if you have your bench in the middle. Otherwise, it will never work.

On top of the points Max made, you describe a 4 1/2" square burn tunnel. That is a very small system relatively speaking; the usual sizes are 6" or 8". Smaller systems are inherently harder to get right, and put out much less heat too, even when they are running right. So rearrange your bricks to make at least a 6" x 6" core, feed tube, burn tunnel, and heat riser, all the same size as far as possible. What kind and size of bricks do you have?

Satamax Antone wrote:Well, no wonders.

Bricks stacked, barrel gap of 2 inches. No insulation i bet?  Nor a vertical chimney?

The chimney is vertical...  to be clear, I mean the stack of bricks inside the barrel.  Are you talking about the metal stove pipe that ends up in thermal mass?

I didn't insulate.  But I was under the impression that this wasn't necessary with 2" thick refractory bricks.  

Horizontal exhaust without a vertical chimney doesn't work. Except in very rare occasions.

Try first without a barrel. Seal the bricks with mud. Insulate your core. Then, when the barrel goes on, the vertical chimney goes up at the same time. Even if you have your bench in the middle. Otherwise, it will never work.

If the temperature of the gases coming out of the end of the bench aren't hot at all, I don't see why having a vertical pipe after that would matter, since there's no heat to rise.  (Assuming that's what you mean)

brian

Glenn Herbert wrote:On top of the points Max made, you describe a 4 1/2" square burn tunnel. That is a very small system relatively speaking; the usual sizes are 6" or 8". Smaller systems are inherently harder to get right, and put out much less heat too, even when they are running right. So rearrange your bricks to make at least a 6" x 6" core, feed tube, burn tunnel, and heat riser, all the same size as far as possible. What kind and size of bricks do you have?

The bricks are about  4.5" x 2" x 10".    They're some goofy metric size.  It looks like I might have to break out the tile saw and cut some down.  If I stack them 2-high, I'll get 9" high which seems like too much.  There are others available that are 1" thick instead of 2" thick.

How long would you say the burn tunnel needs to be from one end to the other, from the far end of the feed tube to the near end of the heat riser?

I forgot to ask in my response to Max.  I thought the gap between the top of the heat riser and the barrel needed to be 1.5" to 2".  Is that not right?

brian

since heat rises, you need to do a test with a vertical chimney after the bell, or as mentioned before, remove the bell and test the core and riser first.

since expansion happens in the hottest areas of the stove, your burn chamber needs to be a good size, at least the same CSA as the riser and feed.   heat wants to rise so if your final exhaust is going downward you will not get a good drawl or flow.  in fact the contraction of the exhaust gasses cooling as they exit upward vertically will cause the "pull" on the exhaust side.   the "push" is at the riser.  you will also get poor results with an unsealed system.  did you specify what size your system/exhaust is?   will it use 6" exhaust?  a 4.5" burn chamber is far too small for a 6" system....    with 10" FIREbricks 2" thick you should be able to make a nice 6" or 8" core...

i personally always test everything outside in safe conditions on my personal projects, starting with the core to ensure proper functionality.  if you are not careful you could cause damage to yourself or your property.  forum members will not be accountable for your actions, so always be safe, do proper research, and preliminary testing before you install or use any DIY wood burning stove or heater.  

Brian, yes a chimney, not the heat riser inside the barrel.


Well, the gap can be all right at 2", 1.5 i would never advise.   Basically, what happens, you have very hot gases, hitting at speed a metal plate, being forced to turn a sharp 90°, then back down against their tendency to rise. Plus, going through a 90° bend creates turbulences, which are no good for the flow.

With a 4.5"² You have 15.9"²  You have with that 4.5 riser, about 18 inches of perimeter. X 1.5 for the gap for example so that's 27"², at 2 inches, you have 36"²  32 should be enough. But those 36 are better. The more you increase your barrel gap, the less friction you have, at the detriment of cooking abilities.

But, even with lukewarm gases, they rise. It's nearly impossible to make a RMH work with an horizontal flue. This, with dry stacked bricks.  A tight barrel gap. Plus, i bet your transition area between barrel and flue is not that good either.  To reassure you. We've all done mistakes. I started with metal rockets. You can't get it right the first time. Except if you follow a plan tightly.

Satamax Antone wrote:Brian, yes a chimney, not the heat riser inside the barrel.


Well, the gap can be all right at 2", 1.5 i would never advise.   Basically, what happens, you have very hot gases, hitting at speed a metal plate, being forced to turn a sharp 90°, then back down against their tendency to rise. Plus, going through a 90° bend creates turbulences, which are no good for the flow.

With a 4.5"² You have 15.9"²  You have with that 4.5 riser, about 18 inches of perimeter. X 1.5 for the gap for example so that's 27"², at 2 inches, you have 36"²  32 should be enough. But those 36 are better. The more you increase your barrel gap, the less friction you have, at the detriment of cooking abilities.

But, even with lukewarm gases, they rise. It's nearly impossible to make a RMH work with an horizontal flue. This, with dry stacked bricks.  A tight barrel gap. Plus, i bet your transition area between barrel and flue is not that good either.  To reassure you. We've all done mistakes. I started with metal rockets. You can't get it right the first time. Except if you follow a plan tightly.

Thanks for the info.

Do you know what the temperature range will be on the cook top?  If it's normally really hot, it might be better for cooking to open the gap a little to bring the temperature down a little.

The transition from barrel to flue was just a gap in the concrete bricks supporting the barrel.  I can cut a proper hole in the barrel and attach a flue pipe.

Should I try to insulate the heat riser?  Or was I right that fire bricks don't need to be insulated?  Maybe I should use something else for the heat riser so I can insulate?  I have some thick steel pipe, like maybe 1mm thick.  I could use that.  But I was worried about it burning out.

brian

some firebrick is insulative, some is dense.  the light weight ones are more insulative.   most people here will tell you that using steel is bad.  my small systems use 2-3" gaps and the cook tops range from 400-800F typically.   cook top temp depends on many things , burn chamber lenght, riser height, system CSA, etc.

John McDoodle wrote:some firebrick is insulative, some is dense.  the light weight ones are more insulative.   most people here will tell you that using steel is bad.  my small systems use 2-3" gaps and the cook tops range from 400-800F typically.   cook top temp depends on many things , burn chamber lenght, riser height, system CSA, etc.

How do you control the temperature for cooking?  I was thinking I could cut up some porcelain tiles I have lying around and use them as spacers to get the pots and pans up off the cook top.  Mabye a metal rack with some kind of height adjustment would be better though.

yeah i used a metal charcoal BBQ rack and lid when i did my apple pie episodes and the pizza as well.   that way the food sits on the rack rather than the top of the stove.  however you would not require any rack or height when boiling water or perhaps cooking in a pot.   depending on the size of the barrel top, there will be intense heat in the center of the barrel top, but for low heat simply move your cooking pot off to the side.  its hard to tell unless you have a functioning stove and temps to read...   because almost every HSMH/RMH is different or custom made, even identical units may have different elevations or different chimneys, flow rates etc.

Brian Lanning wrote:
How do you control the temperature for cooking?  I was thinking I could cut up some porcelain tiles I have lying around and use them as spacers to get the pots and pans up off the cook top.  Mabye a metal rack with some kind of height adjustment would be better though.

I use a cast iron trivet to raise the pans a little higher.  I also frequently adapt my cooking style to suit what heat source is available.  A good cast iron pan will heat up well and evenly on the stove top, then can be put in something like a hay-box to finish cooking slowly when the stove is out.  

Brian, the top of the barrel can reach a good 220C° with a 6 incher. Even higher temps have been recorded.

Bricks need to be insulated imho. First of all, stack these on edge, not flat. You will have less mass in the heat riser, which means clean burning will happen quicker. And yes, you need a vertical chimney. You need a fair gap, like 4 or 5 inches between heat riser's outer surface, and the flue exit, if cut in the side of the barrel. And the flue transition there, should be at least 150% of the CSA of the riser, so 6 inch diameter for a 4" riser.

Brian; My barrel top can reach 1100 F, it has an 8" glowing orange circle over the riser at this temp , normal running temps are 600-800 F , 8" system ,  all temps taken with a digital temp gun.

Satamax Antone wrote:Brian, the top of the barrel can reach a good 220C° with a 6 incher. Even higher temps have been recorded.

Bricks need to be insulated imho. First of all, stack these on edge, not flat. You will have less mass in the heat riser, which means clean burning will happen quicker. And yes, you need a vertical chimney. You need a fair gap, like 4 or 5 inches between heat riser's outer surface, and the flue exit, if cut in the side of the barrel. And the flue transition there, should be at least 150% of the CSA of the riser, so 6 inch diameter for a 4" riser.

I stacked my bricks on edge.  I could probably track down two 5 gallon metal cans to wrap around the bricks with sand filling the gap.

Everyone is saying I need a 6" to 8" feed tube and burn tunnel, so the heat riser would need to be the same right?   CSA=cross sectional area?    So a 8" burn tunnel would need a vertical stove pipe (chimney) of 10" in diameter or so.  Does this sound right?  Sounds too big to me.

If I insulate the heat riser, this might make a somewhat tight fit inside the barrel.  I'm thinking the only way to get that 4" to 5" between the flue and the heat riser is to have the heat riser off center in the barrel.  Is this ok?

ok, so:
8" feed tube, burn tunnel, and heat riser
insulate the heat riser
4 to 5 inches between the flue and heat riser
mortar the bricks together
10" chimney exiting from the side of the barrel at the bottom

anything else?

thomas rubino wrote:Brian; My barrel top can reach 1100 F, it has an 8" glowing orange circle over the riser at this temp , normal running temps are 600-800 F , 8" system ,  all temps taken with a digital temp gun.

What size flue pipe do you have?

8" system with 8" flue pipes. Straight from my transition area it goes 11' then a 180 turn and 9' back to my vertical rise. Vertical rise is another 14' all indoors , then thru the roof a foot or so with a coolie cap.

A square cross section will have about the same drag as a circle of the same diameter, so for a 6" x 6" burn tunnel you would use a 6" diameter riser.