Vortex Stove

[Vortex]

Dec 20, 2022 22:54:36 GMT -8 awan said:

When you say 4" or 6" stove, 4" refer to what?

Diameter of the chimney pipe the stove is designed for use with. For example 4" / 100mm or 6" / 150mm pipe.

[Vortex]

Dec 20, 2022 11:55:46 GMT -8 victorlt said:

First of all thank you for sharing your idea and knowledge - it is really great and nice idea. If shortly, I have spent almost a year from I found this forum last winter till the stove design I'm running now. Probably like most folks here I also started from the test 125mm core which I made from vermiculite boards. Following your measures the core alone without heat exchanger run perfectly from the first run. But when I started tests with 250kg stones load in heat exchanger, it started to fail. Soot, overfueling, full chamber of charcoal - it's just a few well known issues I faced with trying to heat up this mass of stones. At some point I was giving up with decision that this type of burning is not suitable for such sauna stove design I'm trying to build. But day after day making small changes I finally started to get nice runs. After intensive autumn session (1-2 burns a day each day), finally I found balance between burning core and heat exchanger design which works exceeding my expectations. The main changes I made to the burning core comparing with yours are:

1. Higher firebox with flat floor (without V-shaped floor). Because of sauna stove specifics, till I reach 250kg of stones mass till the glowing red stage, I need to run stove 2-3 hours adding 4-5 loads of wood (20-30 kg of wood) . So if the firebox is V-shaped and square, after 3rd load all firebox is full of charcoal and there is not enough space to add more than 3-4 small pieces of wood. It leads to long stones heating time, frequent wood loading, overfueling. Also when the sauna stones are ready and gasses burn stage is over, charcoal from firebox is then removed to prevent any possibility to carbon monoxide poisoning.
2. Air supply from firebox sides. Biggest part of air I give from firebox bottom left and right sides and just a small part from the front (over holes in firebox door and floor near to door, to prevent glass from getting in soot when starting the stove). This air supply design doesn't allow air "jump" over the fuel pile and reach the port in a short way. I also noticed that such air supply also reduces the charcoal burning time.

3. More primary air. If to use original Peters and your configuration (20-25% of CSA primary air and 25-30% in total with secondary) means that stove can take ~ 1 wood load per hour. Despite very effective burning, converting this wood mass to the calories which I need to heat up stones in such conditions leads to a very long stones heating time - 4-5 hours, which is out of my target 2-3 hours time. The box full of stones seams to be very effective heat exchanger and can take more calories during the same time than 180 mm system can offer running it in conservative manner. So I decided to try speed up things a little and increased primary air up to 35% with possibility to close 5% on the firebox doors inlet. You already know consequences of that increase, but this was breaking point of my system design. More primary air in many cases destroy vortex and causes overfueling – this is what I got at first. Solution follows in next points.

4. Secondary air inlet from the port left ant right sides, 8 % of CSA. Secondary air in my design seams to be essential. After 1-1.5 hour of burning, when adding bunch of would in a glowing firebox it starting to generate amazing amount of gasses and first 10-15 seconds just a flickering gasses flow is seen in the secondary chamber. When closing the door, temperature in firebox rises up and air flows finds its way to the secondary air channels, vortex appears again, it clearly seen that secondary air holes ignites the gasses coming from firebox while middle of the port is fully taken by the gasses flow.

5. As you see, I’m using around ~43% of CSA air in total (35% primary and 8% secondary). This amount speeds up burning process in firebox, but because of ballast gasses can destroy burning efectivenes and kill the vortex at all. So this point brought thought – why not try to get rid of ballast gasses instead off letting them to pass over the port? After few test burns I finally stayed at the 10% of CSA in total leaving two vertical holes at the firebox end in the bottom trough which ballast gasses directly enters exhaust duct. This configuration in conjunction with points No 3 and 4 gives very stable and powerful burn even feeding stove with not ideally dry wood, vortex remains stable even when I add wood with remnants of snow and ice from wood bunch outside. Stones in the heat exchanger starting to glow after 2 hours of burn and stove is ready to serve gourmet “Löyly” for sauna visitors 😊

p.s. All conclusions above are based on my subjective opinion and may not work for you due to different circumstances, stove materials and overall configuration. I don’t have any analyzer like “testo”, and all tests I done are evaluated by myself empirically and visually. Hope this will useful for someone or will ignite some nice ideas 😉

drive.google.com/file/d/1nxjFENmpm_jsPb_YIBeyP8abzKzD1TZY/view?usp=share_linkdrive.google.com/file/d/1nuFRdzb3XFz20At5XcQgx5Uk_nl7AP6C/view?usp=share_link

Thanks again, that's really great information. I see now you're driving the stove very fast to get the heat into the sauna stones - turbo super charged! That would need a lot more primary and secondary. I noticed the air streaming in from the bottom front in the first video, it seems to be the best place for it in this design, as you say it can then not jump over the fuel straight to the port.

So this point brought thought – why not try to get rid of ballast gasses instead off letting them to pass over the port? After few test burns I finally stayed at the 10% of CSA in total leaving two vertical holes at the firebox end in the bottom trough which ballast gasses directly enters exhaust duct.

This is not clear to me - can you please explain a bit more? You have 10% CSA of holes from the firebox directly into the exhaust stream bypassing the afterburner? Front or back of firebox? Mostly air or a mix of air and woodgases? How does this help balance the system?

[victorlt]

Dec 21, 2022 5:00:26 GMT -8 Vortex said:

Dec 20, 2022 11:55:46 GMT -8 victorlt said:

First of all thank you for sharing your idea and knowledge - it is really great and nice idea. If shortly, I have spent almost a year from I found this forum last winter till the stove design I'm running now. Probably like most folks here I also started from the test 125mm core which I made from vermiculite boards. Following your measures the core alone without heat exchanger run perfectly from the first run. But when I started tests with 250kg stones load in heat exchanger, it started to fail. Soot, overfueling, full chamber of charcoal - it's just a few well known issues I faced with trying to heat up this mass of stones. At some point I was giving up with decision that this type of burning is not suitable for such sauna stove design I'm trying to build. But day after day making small changes I finally started to get nice runs. After intensive autumn session (1-2 burns a day each day), finally I found balance between burning core and heat exchanger design which works exceeding my expectations. The main changes I made to the burning core comparing with yours are:

1. Higher firebox with flat floor (without V-shaped floor). Because of sauna stove specifics, till I reach 250kg of stones mass till the glowing red stage, I need to run stove 2-3 hours adding 4-5 loads of wood (20-30 kg of wood) . So if the firebox is V-shaped and square, after 3rd load all firebox is full of charcoal and there is not enough space to add more than 3-4 small pieces of wood. It leads to long stones heating time, frequent wood loading, overfueling. Also when the sauna stones are ready and gasses burn stage is over, charcoal from firebox is then removed to prevent any possibility to carbon monoxide poisoning.
2. Air supply from firebox sides. Biggest part of air I give from firebox bottom left and right sides and just a small part from the front (over holes in firebox door and floor near to door, to prevent glass from getting in soot when starting the stove). This air supply design doesn't allow air "jump" over the fuel pile and reach the port in a short way. I also noticed that such air supply also reduces the charcoal burning time.

3. More primary air. If to use original Peters and your configuration (20-25% of CSA primary air and 25-30% in total with secondary) means that stove can take ~ 1 wood load per hour. Despite very effective burning, converting this wood mass to the calories which I need to heat up stones in such conditions leads to a very long stones heating time - 4-5 hours, which is out of my target 2-3 hours time. The box full of stones seams to be very effective heat exchanger and can take more calories during the same time than 180 mm system can offer running it in conservative manner. So I decided to try speed up things a little and increased primary air up to 35% with possibility to close 5% on the firebox doors inlet. You already know consequences of that increase, but this was breaking point of my system design. More primary air in many cases destroy vortex and causes overfueling – this is what I got at first. Solution follows in next points.

4. Secondary air inlet from the port left ant right sides, 8 % of CSA. Secondary air in my design seams to be essential. After 1-1.5 hour of burning, when adding bunch of would in a glowing firebox it starting to generate amazing amount of gasses and first 10-15 seconds just a flickering gasses flow is seen in the secondary chamber. When closing the door, temperature in firebox rises up and air flows finds its way to the secondary air channels, vortex appears again, it clearly seen that secondary air holes ignites the gasses coming from firebox while middle of the port is fully taken by the gasses flow.

5. As you see, I’m using around ~43% of CSA air in total (35% primary and 8% secondary). This amount speeds up burning process in firebox, but because of ballast gasses can destroy burning efectivenes and kill the vortex at all. So this point brought thought – why not try to get rid of ballast gasses instead off letting them to pass over the port? After few test burns I finally stayed at the 10% of CSA in total leaving two vertical holes at the firebox end in the bottom trough which ballast gasses directly enters exhaust duct. This configuration in conjunction with points No 3 and 4 gives very stable and powerful burn even feeding stove with not ideally dry wood, vortex remains stable even when I add wood with remnants of snow and ice from wood bunch outside. Stones in the heat exchanger starting to glow after 2 hours of burn and stove is ready to serve gourmet “Löyly” for sauna visitors 😊

p.s. All conclusions above are based on my subjective opinion and may not work for you due to different circumstances, stove materials and overall configuration. I don’t have any analyzer like “testo”, and all tests I done are evaluated by myself empirically and visually. Hope this will useful for someone or will ignite some nice ideas 😉

drive.google.com/file/d/1nxjFENmpm_jsPb_YIBeyP8abzKzD1TZY/view?usp=share_linkdrive.google.com/file/d/1nuFRdzb3XFz20At5XcQgx5Uk_nl7AP6C/view?usp=share_link

Thanks again, that's really great information. I see now you're driving the stove very fast to get the heat into the sauna stones - turbo super charged! That would need a lot more primary and secondary. I noticed the air streaming in from the bottom front in the first video, it seems to be the best place for it in this design, as you say it can then not jump over the fuel straight to the port.

So this point brought thought – why not try to get rid of ballast gasses instead off letting them to pass over the port? After few test burns I finally stayed at the 10% of CSA in total leaving two vertical holes at the firebox end in the bottom trough which ballast gasses directly enters exhaust duct.

This is not clear to me - can you please explain a bit more? You have 10% CSA of holes from the firebox directly into the exhaust stream bypassing the afterburner? Front or back of firebox? Mostly air or a mix of air and woodgases? How does this help balance the system?

Hi Trev,

Yes, I have two vertical holes (10% CSA in total) from the firebox directly into exhaust stream bypassing the afterburner. Reading this forum I was wondering that nobody are not using this theory in rocket stoves. Actually it's not my idea - I took it from the theory of Kuznetsov, which is named: "the principle of free gas movement". These holes helps to separate ballast gases from the hot ones and let them exit system in a short way without pushing them with hot gasses over the afterburner. This allows to keep very high temperature in firebox and afterburner even after you are adding cold bunch of wood. As I mentioned, all my wood I'm feeding the stove now is outside, under the sky and not in a shelter with roof. Wood has remnants of snow and ice and when I adding it to the firebox, I can see how this snow and ice melting and squealing till becomes water vapor. But despite that Vortex in afterburner recovers very fast after the fresh load, because water vapor and other ballast gasses have easier path to leave the system and allows to keep high temperature in afterburner. These holes have a name "dry seam". They are located in the back of firebox left and right sides in the bottom - each hole is around 1,5 cm wide and around 1/3-1/4 of firebox height. In Kuznetsov design stoves these holes goes vertically over all firebox height, but this design was not suitable in my case because through the holes gasses appears directly in to the exhaust channel where chimney traction is very high. So when I tried to make them full firebox height, I saw that big part of flames was simply sucked through these holes and high chimney temperature indicated that with ballast gasses I throwing out and some hot ones. You can find more details about Kuznetsov theory here:
eng.stove.ru/stati/chto_takoe_pechi_ivkuznetsova_sokraschenno

[ankitg]

Hi Trevor, 

Thanks for the confirmation. My stove build has been delayed and now I am getting the required time to continue the work.

I shall trim the firebricks to achieve the designed height of 75mm as you've clarfied. 

Will try and share my progress in due course.

Thanks,
Ankit

Nov 24, 2022 1:29:18 GMT -8 Vortex said:

The dimensions are important, the afterburner will not form the vortex properly if it's that high, and the surface area of the top outer channels create the back pressure that maintains the vortex in the afterburner. Being made of dense firebrick splits just means the afterburner will take longer to get up to temperature.

[ankitg]

Hi Trevor,

I have built the stove.

Did a couple of test burns. I haven't got any glass to actually see the vortex formation but based on dry runs I presume that vortex is formed. I could hear a faint rocketing noise in the firebox which gives me an indication of things going in correct direction. Although it's not as dramatic as the blow torch that you've observed.

No secondary air at the moment and no fixed firebox door just yet, am yet to finish welding it.

Firebox dimensions are 12" deep by 9" high by 8" inches wide. Not different to your original 4" design drawing, only difference being 1" higher but that will be mitigated when I put slopes on the floor of the firebox. Chinmey is 4" dia with length of 4 feet. Afterburner chamber exactly as per your drawing.

In trial runs I controlled the primary air by propping firebricks in the front and allowing only a 3/4" gap towards one side because I saw with anything more than that the air flow was too great and draft in the chimney increased significantly.

I feel that with the top chamber setup of contra-flow/split flow setup the back pressure may not be enough as this stove smokes easily with just 3-4 small wood pieces added at reload time. My 4" inch L type stove which only has 15" riser would not do this, because the vortex is so strong it wouldn't allow any unburnt gases to escape. Also, that little stove vortex core reaches 700°C+ easily.

Also, the vortex chamber remains sooty after the burns, it shouldn't. I think the vortex may not be strong enough to rise the temperature to achieve total combustion. Top temperature observed on my 8mm thick cast iron plate is 385°C. I think this stove can do better. Please share your thoughts.

A short video of dry run here youtu.be/yAOyj11dLHM


Thanks,
Ankit

Dec 21, 2022 22:46:36 GMT -8 ankitg said:

Hi Trevor, 

Thanks for the confirmation. My stove build has been delayed and now I am getting the required time to continue the work.

I shall trim the firebricks to achieve the designed height of 75mm as you've clarfied. 

Will try and share my progress in due course.

Thanks,
Ankit

Nov 24, 2022 1:29:18 GMT -8 Vortex said:

The dimensions are important, the afterburner will not form the vortex properly if it's that high, and the surface area of the top outer channels create the back pressure that maintains the vortex in the afterburner. Being made of dense firebrick splits just means the afterburner will take longer to get up to temperature.

[Vortex]

From your description and the short video I would say it's a combination of too many air leaks and too much cold mass.

The primary air is meant to be across the bottom not vertically up the front, that will cause most of the air to jump over the fuel straight out the port.

That lazy vortex looks like too much air and/or not enough fuel with too much cold mass around it. The fire will never overcome the cold mass of that concrete floor under it.

[ankitg]

Hi Trevor,

Thanks for the response.

The video is from when I did a dry run - when the gaps were not sealed so your comments about that are accurate.

However, since then I have built it with fire cement and waterglass and there are no more leaks in the firebox or in the top/vortex chamber - at least none that I can spot after thorough checks.

Also, I could see your point regarding primary air orientation. As I said earlier, I am half way through making the door for the firebox - the frame is done, need to finish the hinged door with air control and then it'll be built into the firebox.

Remembering that the lazy vortex in the video is from the dry run which can be understood. But I am trying to understand your comment on the "cold mass of the concrete floor" - are you referring to floor of the firebox or the floor of the vortex chamber??

For ref:
Firebox made of 3" thick firebricks of size 9" x 4.5"
Vortex chamber floor made of 3/4" thick firebricks of size 9" x 4.5".
Vortex chamber walls too are made of the 3/4" thick firebricks all round but these are trimmed to 75mm in width as per your dimensions of the vortex/top chamber.

I have played a fair-bit with my L-type 4" vortex stove. It's fabricated from 2mm stainless steel wrapped in 1" glass-wool so has no real mass as such. After various experimentation I managed to tune the vortex size such that it works very efficiently even with a 15" long riser. As I mentioned the vortex core easily reaches 700+ deg C and provides smoke-free high temperature (350-400 deg C) cooking heat at the top of the riser. Not bad for a such a "toy" I'd say and use of wood is so minimal.

I realize that with brick mass it can take some time to achieve optimum vortex formation and thus longer to achieve higher temperatures. But I don't believe that with the bricks I am using it would never reach desired optimum conditions - I think its a matter of fine tuning and finding what works best in terms of vortex geometry and air control. Peter's well tested batch box design too utilizes firebricks and does work with such a mass - I did a dry run of 150mm (6") size based on the SketchUp drawing and even with square riser (no rounding) the double vortex was very well formed - no smoke with riser at 27 inches only (6 layers of 4.5" firebricks).

I will continue with my experiments and will share the outcomes. Meanwhile your further inputs would be most welcomed as always!

Sorry for the long reply


Thanks,
Ankit

[Vortex]

Hi Ankit,

The floor of the firebox in your video appears to be the concrete floor of the building. I was saying how can the fire ever overcome that cold mass. If you notice in my pictures the outside test stoves are always made on wooden pallets so they are insulated from the ground. The more mass there is around the firebox the longer and more fuel it will take to get it up to operating temperature.

Trev

[ankitg]

Hi Trevor,

Thanks for the clarification regarding the concrete floor - after sending my earlier reply I thought probably that's what you referred.

Here the ground stays quite warm as you'd expect in the Indian subcontinent. The bottom of the firebox is not directly built onto the concrete floor but has a layer of firebricks underneath it. So I have 3+ inches of firebrick insulation on the bottom of the firebox. I can appreciate for colder climates this would need optimizing but should be OK here I believe.

As I said before, my experiments will continue, I will try and see how best I can recreate your stove setup. It is early to say but may be by not having a vertical riser is making it little challenging to achieve smoke free combustion? I don't know - need to play with it more.


Regards and thanks,
Ankit

[martyn]

Hi Ankit, all the bricks look like hard fire brick, they are not insulating anything if that is the case?

You seem to have deviated away from Trevors design, you need insulation and high heat throughout the system.

The 4” only has a small fire box and it will take forever to saturate those bricks, you need to hold the heat inside the stove not let it sink into the bricks and then into the air.

I think you have built the upper vortex chamber from hard fire brick, I dont know if this has been tried before and it may not work?

Can you find insulating brick or vermiculite where you live?

[ankitg]

Hi Martyn,

The issue is sourcing of vermiculite boards here. Apparently a lot is manufactured in India but from what I gather it appears to be made mainly for export purposes - I couldn't source it in my region.

Insulating bricks are readily available. I have that too but didn't use that purely because I thought such high insulation would not be needed but I am starting to think I could have been wrong there.

We use both firebricks and soft insulating bricks in our furnaces. Incidentally our high temperature salt bath furnace has 4" ceramic wool (1200 deg graded) + 4.5" firebrick lining. Together with high grade refractory plaster this makes around 9" of insulation. This is sufficient to keep the external steel wall around 90 deg C when the internal inferno is around 1200 deg C.

So firebricks do provide insulation but your line of thinking is very logical - firebricks like to absorb heat before reflecting enough back. In these stoves better insulation do make a major difference.

My vortex chamber is made up of 3/4" firebricks as I thought that would achieve what Trevor achieved using 20mm vermiculite boards - but my trial runs suggest that it may not be enough. When running for 30 mins the vortex chamber outer walls (which too are made of up 3/4" inch firebricks) heats around 80-90 degC which is on the higher side especially when the internal temperatures are not very high. So I am inclined to try out something else.

Floor of my vortex chamber too is make of 3/4" firebricks. I will have to dismantle the setup and replace the floor and the walls of the vortex chamber with insulating bricks. Firebox is made of firebricks which I wouldn't like to swap for softer insulating bricks due to the wear and tear. Also, Trevor's original test setup uses firebricks for the firebox.

Hopefully I will have some success by swapping insulation. Will keep you guys posted.

Once again, thanks for the inputs - much appreciated.

Regards & thanks,
Ankit

PS: I'd like to leave the link to a video I did sometime back - youtube.com/shorts/OKGQFu5xAZ4 (sorry for the background noise, its an industrial environment )
I was playing with Batch Box Design a while ago and this was a dry stack-up with a very small riser. Mind you all firebricks here and vortex was quite profound from the get go! This just goes to show that how easy it was with vertical riser, even without any air control in the primary and all leaks and cracks as it is, it just worked.

[martyn]

OK if you have soft fire brick I would start by laying down a base on top of the concrete floor and build on top of that.
If you have enough soft fire brick you could build a complete outer box and line the inside of the fire box with ‘split’ hard fire brick.
You can try full fire brick for the fire box, if it is backed with insulating fire brick it should work but might take a few loads to get up to temperature.
Make the top chamber using only soft fire brick.
On my own 4” stove the vortex chamber walls are running at around 7-800c during peak burn but it has an insulated roof.
I would guess the top of your hot plate (the outside roof of your vortex chamber) will get to around 600c while exposed to the air.
Good luck.

[Vortex]

Thanks Martyn, I've been in bed with the flu for the last week and don't have much patience at the moment. Feeling better today so hopefully on the mend now.
Trev

[ankitg]

Dear Martyn,

Thanks for the suggestions regarding layouts of firebox and the top chamber. I'll have to give it a go.

Will keep you guys posted.


Dear Trevor - get well soon!


Regards and thanks,
Ankit

[davejohn2u]

Hello, Vortex Stove builders! Life got in the way for several years, I am back at building a masonry heater based on the original moulds from Trev. that I built and poured from castable refractory several years ago. I am contemplating using the bottom, sides and back as originally cast, with an. ash trap under it. I am wondering about using 2 pieces of 30 mm kiln shelf with the throat parallel to the sides so I can see the vortex from the front with a piece of ceramic glass. Any thoughts about that? It is going to be built in front of my 1960's masonry fireplace with an insulated 6" s.s. flue liner that I will install first. The slab it will sit on is uninsulated, what would you recommend to insulate it from the floor? Is alumina kiln shelf workable for building the afterburner sides, shelf and top with? What are the pros/cons of using alumina kiln shelf vs vermiculite board?