Rocket oven with thermal mass.

I have an idea that I am trying to work out the details of how to finish.


I envision a J -tube rocket stove  going into a 55 gal barrel and this barrel being inside a cob oven, and this cob oven being covered with air crete for insulation then sealed with cement sealer to protect against rain.

I know cob is often used for the thermal mass in RMH, but I don't have clay, I am thinking about using broken concrete from roadways as the thermal mass, is this a bad idea?


My goals are:

1) Using a batch box design, heat the thermal mass to the point I can cook for over  2 hours of time with even heat with the thermal mass of the cob oven.

2) Know how much thermal mass is the right fit for this as if it is too great then you have days of time,  or too small I would not be storing the heat and I would not have a long cook time.

1st pizza!

Hi Mart,

So how hot did it get inside your oven when you were cooking the pizza?

Sincerely,

Ralph

Nice looking pie!
It looks like your rocket heats the bottom of the  inner drum directly?
What does the other end of the oven look like?

Hi Mart,

Is that air Crete squirted around your oven for insulation?

Did you buy a foam gun from Darwin or maker your own?

I am making my own as I like to do everything pretty much myself at least making one or two till I get it "perfected". I hope to have it done and making foam in a couple days when the last parts get here.

I am thinking of trying rapid set with foam, then maybe rapid set mixed with diluted sodium silicate and foam and run some temp tests.  

I will keep you appraised.  I have not bought anything from Darwin so at the moment I am not under advent sort of non-disclosure.

By the way good looking pizza, but where's the beef.  Just kidding!  I like vegetarian stuff too, but not exclusively.

Sincerely,

Ralph

Good job on the oven project. Looks like it works well.

Mart Hale wrote:I have an idea that I am trying to work out the details of how to finish.


I envision a J -tube rocket stove  going into a 55 gal barrel and this barrel being inside a cob oven, and this cob oven being covered with air crete for insulation then sealed with cement sealer to protect against rain.

I know cob is often used for the thermal mass in RMH, but I don't have clay, I am thinking about using broken concrete from roadways as the thermal mass, is this a bad idea?


My goals are:

1) Using a batch box design, heat the thermal mass to the point I can cook for over  2 hours of time with even heat with the thermal mass of the cob oven.

2) Know how much thermal mass is the right fit for this as if it is too great then you have days of time,  or too small I would not be storing the heat and I would not have a long cook time.

Hi Mart,

I am curious as to your design goals.  What is your desired cooking temp and how close do you want the oven to maintain that temp during your 2 hour window.   How often will you be opening the oven during the cooking window?  I think you may be able to achieve your goals with a much simpler design than you are proposing.  Is this for pizza cooking? Is it for a commercial venture or home use?  

Ralph Kettell wrote:Hi Mart,

So how hot did it get inside your oven when you were cooking the pizza?

Sincerely,

Ralph

I have seen temps from 325 -  450 degrees in the oven, it seems to spike then drop off.

I have been making make changes to my oven design so the temps have been a wide variance,   I have to adjust to each stove type and how to feed it.    

One thing I have noticed is that having the pipe right under the barrel it really affects the rate of the rocket  ( it slows it down )   I am slowly learning how to adapt to this.

William Bronson wrote:Nice looking pie!
It looks like your rocket heats the bottom of the  inner drum directly?
What does the other end of the oven look like?

I purchased the video from the kickstarter on rocket ovens from here on permies, that gave me the ideas I used for my setup, but I put in a few twists of my own.    Yes I heat the drum directly with a rack on the inside of the barrel,   barrels act like a giant heat sync and do spread the heat evenly.

>> Is that air Crete squirted around your oven for insulation?

No that is fiberglass batting,


>> Did you buy a foam gun from Darwin or maker your own?

Actually I bought their plans for the small gun, then after watching their other video of how to assemble the large one, I just scaled up, and I made myself a large one.




>I am making my own as I like to do everything pretty much myself at least making one or two till I get it "perfected". I hope to have it done and making foam in a couple days when the last parts get >here.


It was torture for me to wait for the parts ;-)



>>I am thinking of trying rapid set with foam, then maybe rapid set mixed with diluted sodium silicate and foam and run some temp tests.  

I have just made a chimney out of "Starlite"   I am looking forward to doing tests as well.

I am thinking that what I want to do is make the outer structure with air crete, then have tubes in the inside with refractory mix /  ceramic fiber / rock wool that deal with the heat.

I am also thinking about making interlocking blocks with the aircrete so that we could design / take apart and design again any shape we want, and reuse the bricks.




>> I will keep you appraised.  I have not bought anything from Darwin so at the moment I am not under advent sort of non-disclosure.

Kool, I am also looking at open source version of this, I think that I why I went with the ceramic fiber as it is simple and easy to replicate.    
I want to move toward something that is free for everyone to use and replicate and improve on.

>> By the way good looking pizza, but where's the beef.  Just kidding!  I like vegetarian stuff too, but not exclusively.

LOL  I adore my roasted veggies, it is what I have been fighting for to grow my own toppings for my pizza :-)



Cheers,

Mart Hale wrote:

I have seen temps from 325 -  450 degrees in the oven, it seems to spike then drop off.

I have been making make changes to my oven design so the temps have been a wide variance,   I have to adjust to each stove type and how to feed it.

So just to clarify, is that degrees C?

As I can't tell from the photos, but from what you said, I am guessing it is a black oven as opposed to the kickstarter white oven.

I do understand about the torture waiting for parts!

Sincerely,

Ralph

Ralph Kettell wrote:

Mart Hale wrote:I have an idea that I am trying to work out the details of how to finish.


I envision a J -tube rocket stove  going into a 55 gal barrel and this barrel being inside a cob oven, and this cob oven being covered with air crete for insulation then sealed with cement sealer to protect against rain.

I know cob is often used for the thermal mass in RMH, but I don't have clay, I am thinking about using broken concrete from roadways as the thermal mass, is this a bad idea?


My goals are:

1) Using a batch box design, heat the thermal mass to the point I can cook for over  2 hours of time with even heat with the thermal mass of the cob oven.

2) Know how much thermal mass is the right fit for this as if it is too great then you have days of time,  or too small I would not be storing the heat and I would not have a long cook time.

Hi Mart,

I am curious as to your design goals.  What is your desired cooking temp and how close do you want the oven to maintain that temp during your 2 hour window.   How often will you be opening the oven during the cooking window?  I think you may be able to achieve your goals with a much simpler design than you are proposing.  Is this for pizza cooking? Is it for a commercial venture or home use?  

>>What is your desired cooking temp and how close do you want the oven to maintain that temp during your 2 hour window.  

I am starting to learn that if you have an oven at 450 degrees, you don't have to have the food in the oven very long to cook it.

In fact if you pick the right time on the curve of cooking, you can put your food in the oven as the fire burns out and simply walk away and come back as the food will cook and be warm as the oven works its way down to 200 degrees.

I am testing different ways to store heat, I have put a chunk of rail road iron in the oven and I heated it on the hotest part of the oven.     I think that if I made a thermal cooker  out of air crete I could drop that heated steel in that box and cook my food on top of that.       I have been considering putting the chunk of steel directly into the J tube and then seeing how hot it gets.

I picked the 2 hour window thinking that is the time needed to get a thermo mass up to temp  ( provided the mass was well insulated, and the right amount ).     I have tons of sand, I have considered instead of using fiberglass insulation, I could use sand instead,  but that is very heavy.




>> How often will you be opening the oven during the cooking window?

I want to set it and come back to food that is done.      I have found that with experience you can do this, so I would not be opening the door except to take it out ideally.



>> I think you may be able to achieve your goals with a much simpler design than you are proposing.

I am finding after I built the 2 barrel rocket stove that there is much to like.     Indeed I agree I could reach the goals with less complexity, but I guess I am looking at efficiency I ask myself the question how could I design this so that I use the least amount of wood and get the longest burn time?        The thing is, the rocket stove by itself is very very efficient,  you can be wasteful with it and still get good results.   But in doing so, you will be cutting/ gathering more wood.

I have been thinking about how could one simplify the reloading of the stove or even automate it.      I was thinking about heating the burn chamber with propane to help stop the smoking at the start, and to help light the wood, after it is started then shutting it off.      

Once the stove is going and you are up to temperature it is very easy to maintain so it pays to cook multiple meals at the same time one after another rather than doing them on separate days.


>> Is this for pizza cooking? Is it for a commercial venture or home use?

I cook moringa bread from scratch, I have been using solar cookers, but I wanted to target a stove that I could use on cloudy days.

For a commercial venture, I think I would want to dehydrate the food so that it would have the longest shelf life.      I find it hard to beat my Excalibur dehydrator for that task.

Ralph Kettell wrote:
I am thinking of trying rapid set with foam, then maybe rapid set mixed with diluted sodium silicate and foam and run some temp tests.  

I'm dying to know how this goes,  please,  keep us posted!


BTW,  before I saw Darwin and his foam gun,  I saw the someone else market a foam gun called "The Little Dragon"
It's much more expensive, and there are no DIY plans offered.
Did Darwin get inspiration from them?

William Bronson wrote:

Ralph Kettell wrote:
I am thinking of trying rapid set with foam, then maybe rapid set mixed with diluted sodium silicate and foam and run some temp tests.  

I'm dying to know how this goes,  please,  keep us posted!


BTW,  before I saw Darwin and his foam gun,  I saw the someone else market a foam gun called "The Little Dragon"
It's much more expensive, and there are no DIY plans offered.
Did Darwin get inspiration from them?

Ralph,

Oh yeah, what type of Rapid set are you using, I looked up rapid set on home depot and I saw a couple different types...

https://www.homedepot.com/s/rapid%2520set?NCNI-5

I believe CementAll has the best ratio of ingredients for uses as a refractory.
The SDS list it at 40-60% Calcium Sulfoaluminate Cement more than the other Home Depot RapidSet products.

Ralph Kettell wrote:

Mart Hale wrote:

I have seen temps from 325 -  450 degrees in the oven, it seems to spike then drop off.

I have been making make changes to my oven design so the temps have been a wide variance,   I have to adjust to each stove type and how to feed it.

So just to clarify, is that degrees C?

As I can't tell from the photos, but from what you said, I am guessing it is a black oven as opposed to the kickstarter white oven.

I do understand about the torture waiting for parts!

Sincerely,

Ralph

It is a white oven.    I cut one barrel down the side, after that from that line  at the bottom I cut back on both sides till there was only 20 inches still attached to the barrel.       Next I opened up this barrel and put  a second barrel inside of this.        

Degrees F.

Hi Mart,

I can see sort of how you are feeding the heat into the oven, but between this thread and your rye bread thread, I do not see where you are exhausting the "spent" gasses.

Please give a few more pictures.  Thanks.  I have made several mods to my oven relative to the kickstarter exhaust, I was just curious how you are handling it.  I shared some of them in the existing photos of my oven in the rocket ovens forum.

Sincerely,

Ralph

The point of these barrel rocket stoves is to rapidly reach cooking temperature versus waiting 1.5 hours to get a mass up to heat. If you're adding a lot of mass that's probably why you're not reaching high temperatures. A barrel rocket stove can reach 980 deg F.

Ralph Kettell wrote:Hi Mart,

I can see sort of how you are feeding the heat into the oven, but between this thread and your rye bread thread, I do not see where you are exhausting the "spent" gasses.

Please give a few more pictures.  Thanks.  I have made several mods to my oven relative to the kickstarter exhaust, I was just curious how you are handling it.  I shared some of them in the existing photos of my oven in the rocket ovens forum.

Sincerely,

Ralph

Simple answer is, I don't exaught the gases  :-)    I create sorta a bell for the heat to go to the top of the oven, and then it comes out the bottom where ever it wants to.      This rocket burns cleanly so that I don't see smoke after the initial burn so since this is not inside I just let the gases go where they want to.

Graham Chiu wrote:The point of these barrel rocket stoves is to rapidly reach cooking temperature versus waiting 1.5 hours to get a mass up to heat. If you're adding a lot of mass that's probably why you're not reaching high temperatures. A barrel rocket stove can reach 980 deg F.

I agree, properly insulated these stoves can not help but get blazing hot.     I have not insulated the very bottom of the barrel, If I did I am sure I would see another 50 - 100 degrees.

I have been testing different methods of heating the barrels, and I am learning which designs work best for this setup for me.

I am happy with it now, and I am just enjoying that I am this far with this setup.      The bread and pizza is awesome, so it fits my needs very well.

Mart

Just for comparison I've uploaded a very brief video of my pizza cooking tonight.

https://youtu.be/_IWehtDdp8Q

However, it took my batch rocket stove about 2 hours of burning to get it to this stage, and the pizza still required 10 minutes of cooking.

I then used the residual heat to roast potatoes (wood was down to coals so combustion was still occuring).

Graham Chiu wrote:Just for comparison I've uploaded a very brief video of my pizza cooking tonight.

https://youtu.be/_IWehtDdp8Q

However, it took my batch rocket stove about 2 hours of burning to get it to this stage, and the pizza still required 10 minutes of cooking.

I then used the residual heat to roast potatoes (wood was down to coals so combustion was still occuring).

Looks good to me.     Good amount of thermal mass there.

I had an idea of screening the coals and putting them directly into the fire box in a pan to get more heat directly...    I am not sure how good of an idea that would be...

Mart Hale wrote:

Simple answer is, I don't exaught the gases  :-)    I create sorta a bell for the heat to go to the top of the oven, and then it comes out the bottom where ever it wants to.      This rocket burns cleanly so that I don't see smoke after the initial burn so since this is not inside I just let the gases go where they want to.

That is what I thought you were doing.  This is why your oven is not getting any hotter than it does.  If you wanted it to reach 1000 degrees or thereabouts you will need to do something like they have done in the kick starter.

Here is why.  A bell makes a great way to capture latent heat and be very efficient about it, but....  With a rocket oven we are not shooting for efficiency of turning every bit of the energy into cooking power.  Yes we want a very efficient, clean, and hot burn, but once we have that we want to funnel as much of those clean hot gases past the oven when they are their hottest to heat the oven up as quick and hot as we can.  Ultimate efficiency and quick maximum heating are an engineering trade-off.  You pick one or the other or somethng in the middle but never both.

Heat transfer occurs much quicker and more efficiently when the hot gases are at their hottest and moving past the surface to be heated.  This prevents stratification and stagnation.

I hope this is helpful.

Sincerely,

Ralph

Graham Chiu wrote:Just for comparison I've uploaded a very brief video of my pizza cooking tonight.

https://youtu.be/_IWehtDdp8Q

However, it took my batch rocket stove about 2 hours of burning to get it to this stage, and the pizza still required 10 minutes of cooking.

I then used the residual heat to roast potatoes (wood was down to coals so combustion was still occuring).

Very cool Graham.   How hot does it get in that beast?  Your oven is leaning more to the cob style of oven in terms of mass and heating times.  Not a critique, simply an observation.  Each style has its benefits.

Sincerely,

Ralph

Hi Ralph

I'm essentially using the top box of a DSR2 for cooking.  Nothing is mortared in yet but I do have a ceramic fiber blanket lining the "riser", and the floor of the combustion chamber.

After 2 hours I get to 250-300 deg C on the floor of the oven, the 300 deg being closer to where the riser opens into the top shoe box.  It could get hotter if I didn't have an ill fitting oven door with large gaps!
I'm going to make a door with plywood instead of just hardwood protected by aluminum, and the hot face protected with CFB painted with rigidiser (Matrikote).  The rigidiser turns the CBF into a heat reflective shield vs just insulation.

There's a trade off when using CFB.  You do insulate but lose mass as a result of the insulation.

Hi Graham,

You said the CFB was in the riser and the floor off the burn chamber.  You do not want thermal mass in those areas.  Thermal mass in those areas will steal heat from your heat source.  You want your thermal mass in the areas where you will be cooking only, mainly in the floor area of the cooking chamber.  

You might also try insulating your beast.  Try aircrete.  For the outer insulation you do not need a fancy mix. If you had a foam machine, you could make the aircrete you need for $20 or less of portland cement.

Happy experimenting and cooking.

Sincerely,

Ralph

Hi Ralph

As I understand it insulation just makes it quicker to bring up to temperature.  When I didn't have any CFB in my riser, it still reached cooking temperatures in the floor of the oven but I guess it just took a bit longer to get there.  My riser is all fire brick and at one point once it becomes heat saturated it then insulates.

The ovens that retain the most heat for prolonged cooking seem to have multiple levels of insulation.  So, I've seen one earth oven build on Motherearth that had about 3 layers of alternating cob and CFB so that heat retention was close to 24 hours.  I guess that's easy enough to do with a barrel oven.  If you covered the metal with CFB that might prevent the cob cracking?

My oven is a little inconvenient in loading the fire wood as it loads from the side.  It would be better to redesign it so that loading the firewood and oven are both on the same side. But I have heaps of CFB that I could use to encase the thing, and then cover it with cob or aerated concrete though that would require learning how to make air crete.

Graham Chiu wrote:Hi Ralph

As I understand it insulation just makes it quicker to bring up to temperature.  When I didn't have any CFB in my riser, it still reached cooking temperatures in the floor of the oven but I guess it just took a bit longer to get there.  My riser is all fire brick and at one point once it becomes heat saturated it then insulates.

It does not insulate per se, it simply heats up to its max temperature.  If it is exposed to the outside air on one side, it will be radiating from that side.  Thus the max temperature is a balance between the amount of surface area exposed to the fire and the surface area exposed to cooler temps.  By definition if it absorbs heat then it also radiates heat.  That was why I suggested air crete to you as a coating around everything you have shown.

The less thermal mass you have in your burn tunnel and riser, the higher the ultimate temp erature you can achieve. The faster it gets hotter, the sooner you can start to heat up your thermal mass where you will be using it.  Wasting wood heating up thermal mass in your burn chamber simply dissipates into the surroundings after you stop burning.  This is known as entropy.

=====

The ovens that retain the most heat for prolonged cooking seem to have multiple levels of insulation.  So, I've seen one earth oven build on Motherearth that had about 3 layers of alternating cob and CFB so that heat retention was close to 24 hours.  I guess that's easy enough to do with a barrel oven.  If you covered the metal with CFB that might prevent the cob cracking?

The cob absorbs heat and is insulated by the CFB.  I think it would work just as well or better if you put a very thick layer of rock wool surrounded by cob to seal it for air movement.  That is possibly the only benefit of a layer cake approach is that you eliminate any potential sneak paths for heat.

=====

My oven is a little inconvenient in loading the fire wood as it loads from the side.  It would be better to redesign it so that loading the firewood and oven are both on the same side. But I have heaps of CFB that I could use to encase the thing, and then cover it with cob or aerated concrete though that would require learning how to make air crete.

My only comment about using cfb for everything is that it is a bit expensive to use in areas which do not see 2000+ degree temps.  Rock wool is cheaper and in areas which are only 800 to 1000 degress F I think a cheap mix of aircrete would be a very inexpensive and easy way to add insulation to your hearts desire.

I hope this is helpful.  Again think aircrete.  I finished my foam generator last night and will be sharing it over the next few days as I have the time to start testing it.

Sincerely,

Ralph

Ralph Kettell wrote:

Mart Hale wrote:

That is what I thought you were doing.  This is why your oven is not getting any hotter than it does.  If you wanted it to reach 1000 degrees or thereabouts you will need to do something like they have done in the kick starter.

Here is why.  A bell makes a great way to capture latent heat and be very efficient about it, but....  With a rocket oven we are not shooting for efficiency of turning every bit of the energy into cooking power.  Yes we want a very efficient, clean, and hot burn, but once we have that we want to funnel as much of those clean hot gases past the oven when they are their hottest to heat the oven up as quick and hot as we can.  Ultimate efficiency and quick maximum heating are an engineering trade-off.  You pick one or the other or somethng in the middle but never both.

Heat transfer occurs much quicker and more efficiently when the hot gases are at their hottest and moving past the surface to be heated.  This prevents stratification and stagnation.

I hope this is helpful.

Sincerely,

Ralph

I thought a bit about what you said, and today I opened up vents on both sides of the the oven.    about a foot down.    The oven got hotter than it had before,  I also moved the chimney down about 3 inches under the barrel.    I got a much better burn than before,  and the pizza browned the cheese and the crust was very crispy.


I am have now achieved fully cooking on one load of wood, thank you for your insight again.    The bell works for me by allowing vents about 3 inches on either side of the barrel.


It is such a joy to cook with this now :-)

Mart Hale wrote:

I thought a bit about what you said, and today I opened up vents on both sides of the the oven.    about a foot down.    The oven got hotter than it had before,  I also moved the chimney down about 3 inches under the barrel.    I got a much better burn than before,  and the pizza browned the cheese and the crust was very crispy.

I am have now achieved fully cooking on one load of wood, thank you for your insight again.    The bell works for me by allowing vents about 3 inches on either side of the barrel.

It is such a joy to cook with this now :-)

Hi Mart,

I am so glad for you.  I was concerned about why you were having to burn so much wood and not getting very high temps in the oven. It is great that it is easier and quicker to use and more efficiently using your wood!

I have made some mods to my kickstarter style oven in the areas of insulation and exhaust gas routing and have another one or two in process.  Unfortunately my honey do projects have taken precedence for the past month or so, but they are finally starting to wind down.   I am really hoping to get my version of the kickstarter oven up over 1000 degrees, not that I want to run it that hot, but I would like to load it with some mass to even out the cooking temps and by having the capacity to heat it that much it will make preheating it that much quicker and the better insulation will make it so it does not lose heat unnecessarily quickly.

We should work together to try and improve awareness of aircrete in these type of projects and we can even give Darwin some free advertising in the process for his stuff.  I am not looking to get rich with this stuff, just learning and helping others.  I have gotten rich and unrich a couple of times already and frankly I don't need the stress.

Got any videos of the cooking process and of the final product?  I tried doing an open door video of pizza cooking but the phone stopped at 1:44 due to lack of storage!  I'll do another one once I clear some storage and get some more pizza dough.  The oven floor temperature does drop with the door open which kind of surprised me.

https://youtu.be/gc5Wf03mzE0

Ralph Kettell wrote:

My only comment about using cfb for everything is that it is a bit expensive to use in areas which do not see 2000+ degree temps.  Rock wool is cheaper and in areas which are only 800 to 1000 degress F I think a cheap mix of aircrete would be a very inexpensive and easy way to add insulation to your hearts desire.

I have seen several times rolls of CFB 7 m lengths for $1 on a NZ auction site.  So, I've currently got 70 m of the stuff. There isn't any competition when bidding for this stuff since its use is in kilns etc.

Graham Chiu wrote:

Ralph Kettell wrote:

My only comment about using cfb for everything is that it is a bit expensive to use in areas which do not see 2000+ degree temps.  Rock wool is cheaper and in areas which are only 800 to 1000 degress F I think a cheap mix of aircrete would be a very inexpensive and easy way to add insulation to your hearts desire.

I have seen several times rolls of CFB 7 m lengths for $1 on a NZ auction site.  So, I've currently got 70 m of the stuff. There isn't any competition when bidding for this stuff since its use is in kilns etc.

Scratch that argument.  Lol.  I don't think you can get rock wool that cheap.

https://www.ebay.com/itm/Kaowool-Ceramic-Fiber-Insulating-Blanket-Roll-1-2-x-24-x-50-8-Morgan-Thermal/262669915836?hash=item3d2858d6bc:g:5sMAAOxy-j9SQt3K:rk:32:pf:0

Kaowool Ceramic Fiber Insulating Blanket Roll 1/2"x 24"x 50' 8# -Morgan Thermal

USD140 + shipping.

Is that expensive?

Short video of rocket stove.    Today I added a small fan, and WOW.    Well over 450 degrees.    

This guy has the design I had in mind.  

Graham Chiu wrote:https://www.ebay.com/itm/Kaowool-Ceramic-Fiber-Insulating-Blanket-Roll-1-2-x-24-x-50-8-Morgan-Thermal/262669915836?hash=item3d2858d6bc:g:5sMAAOxy-j9SQt3K:rk:32:pf:0

Kaowool Ceramic Fiber Insulating Blanket Roll 1/2"x 24"x 50' 8# -Morgan Thermal

USD140 + shipping.

Is that expensive?

I think I got the Kaowool for a bit less than that, but I am not certain.  What I said to you in an earlier post was that for some of the apps we were discussing rock wool would also work in those apps and it is much cheaper.  A pack of rockwool with 5 pieces 4' by 15" by 3" thick costs less than $20.  To compare the two, rock wool is 6.25 cu ft or about $3.20 per cu foot.  The Kaowool is $33.60 per cubic foot.  We know that you cannot compare these two products except for the lower temp applications where either will work.  For  1000 degree +/- rockwool is the winner, but Kaowool is without compare in 2000 plus degree applications.

Sincerely,

Ralph