8" batch rocket with bench

[invention1]

Basics:  8" stove, metal bell made from a 100 gallon water heater tank, flue has a bypass.  Door from a barrel stove kit.  Cleanouts in the bench and masonry bell two places.  Went through one winter without tile, very successful heating season.  Cleanout after heating season showed very little ash or buildup inside the bench and bell.  Bell sanded to bare metal and painted with Rustoleum Ultra High Heat "Aged copper" color.  Baked the metal bell out in the driveway for several hours to cure the paint by making a quick and dirty rocket stove out of loose firebricks under it for a while.  Still had problems with paint outgassing for the first few fires once the bell was installed permanently - it'll set off the fire alarms.  Bell is welded onto a flat flange, which is then bolted down to the masonry over 3/8" spacers and sealed with a 3/8" wide silicon rubber sealant joint.  

Theoretically the ISA should be 101 sq ft, this has about 97 sq ft, a bit shy but it works quite well.  Core is insulated with a layer of flexible and rigid ceramic fiber insulation, riser is a basic 5 minute riser made out of a 10" diameter steel duct lined with ceramic fiber to make an 8" diameter riser.  The bench is insulated and has a 2" air space between the bench and the wall, as well as 4" of masonry, for safety.  

Core is cast out of  Harbison Walker "Mizzou" castable refractory assembled with Blakbond refractory mortar.  The rest of the bell and bench is made of solid 8"X8"X16" concrete blocks, the tops of the masonry bell and bench are steel plates supported by angle iron, with two sheets of concrete tile board plastered on with thinset tile mortar and anchored to the masonry with counterset tapcon masonry screws.  "Mizzou" is actually mined about 30 miles from my house, but I had to drive 130 miles to pick it up!  

I have very little problem with the stove smoking when the door is opened, because the top of the door is designed to be below the top of the port.  This, with a strong draw in the chimney (about 25 feet tall) and the bypass makes it not smoke like many rocket stoves will.  

Bench is quite comfortably warm, we usually keep a thin blanket on it for a bit of padding and because it gets a little too warm at times.  

Carbon Monoxide sensor on the wall for safety, plus a USB charging outlet for the phone.  

[invention1]

Under Construction.  

I actually built this stove once then tore it down and built it again.  The first iteration was built of firebrick, using good high temperature mortar.  The joints all cracked and fell apart, you could literally pull the firebricks out with your fingers.  Firebrick is not a good thing to make rocket stoves with, even good refractory mortar will fail, I'm here to tell ya. 

[invention1]

Only one real innovation in this stove - a *removable* secondary air system with a Walker pre-port.  

The secondary air tube can be completely removed for cleaning or replacement.  Also the Walker pre-port is removable - just slip fits onto a collar.  I went through one heating season already, found no appreciable wear on any of the steel components but having them removable is a nice feature.  I simply cast a slot into the core that is just a smidge larger than the steel secondary air tube.  

[fuegos]

"Firebrick is not a good thing to make rocket stoves with" really cool build , glad it's working well but plenty of folks on this forum will disagree with your assertion about firebricks.The joints cracking isn't a problem as such only if it causes air leaks.A clay sand mix 1:3 is in my opinion better than refractory mortar.The idea that strength = rigidity is false .Any structure needs to be able to move or expand , to be elastic under load .

[martyn]

Lovely finish work on the first picture..

[Orange]

you're happy with the performance? How big is the space you're heating and how many fires a day?

[invention1]

Sept 7, 2020 10:59:14 GMT -8 fuegos said:

"Firebrick is not a good thing to make rocket stoves with" really cool build , glad it's working well but plenty of folks on this forum will disagree with your assertion about firebricks.The joints cracking isn't a problem as such only if it causes air leaks.A clay sand mix 1:3 is in my opinion better than refractory mortar.The idea that strength = rigidity is false .Any structure needs to be able to move or expand , to be elastic under load .

Fore some reason, when I posted about the firebrick failure the first time, the consensus was "Yeah, we know, a cast core is really much better than a firebrick core".  Joints in firebrick didn't just crack, it wasn't just a few air leaks, it was a complete structural failure, using very good quality materials.  The stove core literally fell apart in a few weeks and I tore it down because it was not safe.  People here seem to be all about cob, and that's great, but all my experiments with cob were failures as well, I'm sitting on a lot of clay but gave up trying to get anything out of it worth making more than a mud cake.  Blackbond Mortar is cheap and a 2 gallon bucket was enough to finish the build.  Cast core hasn't cracked or developed leaks.  I'm going with what worked and making sure I report failures so people can learn from them.  

[invention1]

Sept 10, 2020 12:34:06 GMT -8 Orange said:

you're happy with the performance? How big is the space you're heating and how many fires a day?

If anything it's too hot!  

We're heating 2400 square feet, an open plan house on two levels.  Upstairs, it's shorts and sandals all winter.  In mild weather we fire it once an evening.  We'll go to twice a day when the weather gets colder, when it's really bad we'll load a second charge of wood before the first is done burning for essentially three fires.  It's amazing to throw in a large log, and count to five before it's already on fire.  Backup heat is mini-split heat pumps.  

The bench is so nice!  Just laying on something really warm is a great luxury.  

[Solomon]

Can you share your dimensions? I don't see any pictures, do you have any? I'm very interested in this design. I have about 3700 square feet and am interested in this project.

[invention1]

Here's an update: 

Although my prototype stove in the back yard produced burns with no smoke whatsoever, indicative of very high combustion efficiency, I've never achieved that with the permanent rocket stove design.  


Cleaning out the bench this year, I find that the bottom of the bench is covered with a black, dry, granular material that is slightly, but not strongly, flammable.  It will hold a flame for a few seconds if I hit it with a propane torch.  This doesn't seem like ash, it is more like a creosote deposit although it is not greasy.  The chimney flue seems to be absolutely clean, I was able scoop out maybe a handful of deposits when I ran the chimney brush through it this year.  

At the cleanout doors, I am getting some dripping deposits of creosote. I think this may be a combination of combustion products and moisture in the smoke.   These doors are cooler than other areas of the bench, and seem to condense vapors from the stove smoke.  I have placed some superwool behind one of the cleanout doors, this seems to help with the mess, however these indications lead me to believe I am not getting very good combustion from my stove.  

Here are some basic dimensions:

Riser 8" inside diameter (Riser CSA 50.264)
Calculated base dimension 5.78"

Riser height 58"  (ideal is 10* base or 57.8)

Width of firebox 11.5"
Height of firebox 17.5"
Depth of firebox 28" (Ideal is 23" or 4 X base to 31" or 5 x base)

Port height 12.5 width 2.8 depth 2"

I'm using a walker pre-port design.  The horizontal part of the secondary air is 2.13" square or 4.5 sq inch which is about 9% of CSA
Vertical part of the secondary air inlet is a 2.5" Scheedule 40 steel pipe, inside diameter 2.47, area 4.79 sq inches or 9.5% CSA 
Primary and secondary air come into the firebox through the same opening.  Pri + secondary air opening is 3.1" square or 9.6 sq inches (19.1% of CSA) 

I'm concerned there might be a couple of pinch points in the airflow that might be affecting performance.  First, most of the flue is either 8" round stovepipe or 8" square masonry chimney pipe.  However there is one section where the metal stovepipe enters the square masonry chimney that it necks down to 7" diameter.  I wonder if this is affecting draw and maybe reducing performance.  

The second area is at the transition between the horizontal and vertical Walker pre-port.  This necks down so that the removable vertical pipe sits over a small section of smaller tube.  

I wish I had access to a Testo, but without that I can only speculate.  The indications of buildup of some creosote and deposits that are not ash when I clean out the bench, plus not achieving clear smoke lead me to believe the stove is not performing adequately.  I'd welcome any suggestions about the above parameters and could experiment with different dimensions.  

[Forsythe]

Oct 16, 2022 15:36:07 GMT -8 invention1 said:

Cleaning out the bench this year, I find that the bottom of the bench is covered with a black, dry, granular material that is slightly, but not strongly, flammable. It will hold a flame for a few seconds if I hit it with a propane torch. This doesn't seem like ash, it is more like a creosote deposit although it is not greasy.  The chimney flue seems to be absolutely clean, I was able scoop out maybe a handful of deposits when I ran the chimney brush through it this year.

At the cleanout doors, I am getting some dripping deposits of creosote. I think this may be a combination of combustion products and moisture in the smoke. These doors are cooler than other areas of the bench, and seem to condense vapors from the stove smoke.  I have placed some superwool behind one of the cleanout doors, this seems to help with the mess, however these indications lead me to believe I am not getting very good combustion from my stove.

I didn’t get to see any of original pictures, so just making a blind guess here based on the description and the problem at hand:

Have you checked the gap between the top of the riser and the interior top of the barrel/tank metal bell?

When that gap is set less than 2 inches it can cause smoking problems and soot deposits in the bench channels, because it places too much backpressure on the riser — even though the rocket appears to be drawing okay from the front-end of the system. I’ve seen a few cases where raising the barrel another inch fixed the issue.

The other common problem with that barrel/bell being a choke point is the “manifold” — specifically where the bottom part of the bell transitions to the bench flue pathway. The old specifications listed in some of the earlier editions of the builders’ guides didn’t place enough emphasis on making that transitional area wide enough to act like a funnel into the bench flue. Abutting the flue pipe right into the lower portion of the bell manifold (without a widened transition / funnel shape there) will also add a lot of backpressure and slow the draw of gasses through the riser. The circular footprint of a plain bell manifold causes a lot of turbulence and gas friction on that inner shoulder just before entering the bench. Opening that circular form under the barrel into more of a teardrop shape helps to avoid that choke point and greatly improves draft, too.

[peterberg]

Oct 16, 2022 15:36:07 GMT -8 invention1 said:

At the cleanout doors, I am getting some dripping deposits of creosote. I think this may be a combination of combustion products and moisture in the smoke.   These doors are cooler than other areas of the bench, and seem to condense vapors from the stove smoke.  I have placed some superwool behind one of the cleanout doors, this seems to help with the mess, however these indications lead me to believe I am not getting very good combustion from my stove.

You are talking about an 8" batchrocket, or so it seems.
Let's talk about moisture first. The stove has been built in 2020 and is still leaking condensation fluid from the cleanout doors. All moisture from the build is long gone by now, so what you are seeing is produced by the combustion proces itself. Wood combustion is very similar to natural gas, it'll produce heat, CO² and water. Vapor amount will be roughly 0.6 of the fuel's weight, so that's a lot of water. It's paramount that this will stay in vapor stage and be exhausted by the chimney.

That said, you aren't mentioning the chimney's internal temperature, measured in the middle of the gas stream. When this is below 50 ºC (122 ºF) for the entire burn the vapor will condensate either in the heater or in the chimney, whatever is the coolest. My conclusion: apart from possible pinch points like the ones Forsythe mentions, the total Internal Surface Area of your heater is too large. The pipe that necks down to 7" in the masonry chimney could do some harm. I'm inclined to think the deposit in the bench is from the heating season directly following the build. Wet bench, cool gases and condensate deposition.

By the way, the numbers given by Forsythe are valid for a J-tube. The top gap, the space between end of riser and top of bell should be at least equal to the riser's diameter for a batchrocket.

Please, buy a simple pen thermometer and stick that in the pipe. Note the maximum temperature for 5 burns in a row and, according to what you find, adjust the ISA of your bench or leave it as it is.

[Forsythe]

Oct 17, 2022 6:34:35 GMT -8 peterberg said:

By the way, the numbers given by Forsythe are valid for a J-tube. The top gap, the space between end of riser and top of bell should be at least equal to the riser's diameter for a batchrocket.

Oops, Sorry Peter — I totally missed that. My brain was picturing a traditional RMH. That’s a great point to note about the wider top gap needed on a batchrocket barrel.

I'm inclined to think the deposit in the bench is from the heating season directly following the build. Wet bench, cool gases and condensate deposition.

Oct 16, 2022 15:36:07 GMT -8 invention1 said:

At the cleanout doors, I am getting some dripping deposits of creosote.

…If this truly is creosote, then it’s not *just* an issue with condensing gasses due to an oversized bell. (Although that could very well be an aggravating factor compounding the problem.)

Creosote (wood tar) should not be present in any rocketstove’s exhaust. Peter’s right that the presence of condensed moisture could easily be from over-harvesting of the heat from a bench being too large… but if that’s solely the case, the [recent] condensate should not contain enough uncracked pyrolysis volatiles to form creosote… unless the front end of the system had developed a decreasingly efficient burn over time.

…Maybe I’m misreading or misunderstanding the sequence of events here, but it sounded [to me] like the first season post-build got an ash clean-out, and it did not have such an accumulation of creosote at that time:

Oct 16, 2022 15:36:07 GMT -8 invention1 said:

Went through one winter without tile, very successful heating season. Cleanout after heating season showed very little ash or buildup inside the bench and bell.

If this is something that arose after a full first heating season post-build and subsequent first clean-out, that’s what would lead me to believe it’s a choke point. Particularly if that choke point is in the barrel, then it would slow air intake direly enough to crash the riser’s secondary combustion, thereby dumping unburned wood tars and long-chain carbon compounds in the immediate vicinity. That type of thing (at least in J-tubes) causes an increasingly thick accumulation of creosote at the top gap of the barrel, which further blocks the passage of gasses, further slows the draft, and further exacerbates what becomes a creosote feedback loop… both as the effect of an early blockage — and as the cause of the worsening blockage…adding to the choke point with every burn cycle’s contribution to the growing creosote deposit’s thickness.

…Might also be a function of the hybridized preport/secondary air supply. I can’t quite visualize what’s being described there…but it sounded like a combo of P-channel along the top and yet also a walker preport along the bottom… Maybe? That might be a contributing factor, too. Hard to say without knowing what that actually looks like in operation.