paul wheaton wrote:I've been asked to say something here.
I have not read every spec of everything written here, but I did skim through it. This reminds me of a long conversation I had with the rather brilliant Peter van den Berg (who appears to already be on this thread).
The conversation goes like this: it is possible to design a rocket system with a lot of metal and have it be okay. It must be a very small system so that the temperatures will probably never exceed 1400 degrees F. Of course, at such low temperatures, it is difficult to get a non-smoky burn - but it can be done.
These forums have a parade of people that went down metal road and made systems that were amazing. For a while. And when the system failed in the middle of winter, they had massive heartbreak.
Rob's system comes to mind:
There has been so much heartache and so many disasters, it seems that there is a very strong push to discourage anybody from using metal in the burn tunnel and/or riser.
I know that we had a lot of people here at my place that were seriously loving on the pocket rockets for a few months - but after they all, universally, burned out their very thick metal tubes - we had to ban the use of this design here. After all: where does that burned metal go? The toxicity put into the atmosphere makes me very uncomfortable.
I think the people here are very much wishing to embrace another enthusiast. And, at the same time, direct your energy to a place that will have a strong positive long term result and not go through all the pain that so many others have gone through.
And, in the upcoming dvds, peter and i have a rather long discussion about this.
I think that the most important thing here is to keep in mind that steels are liquid at 2600 degrees, but they start to erode at even lower temperatures. F Styles provided this rather excellent list:
Material frequently used in manufacturing. Due to its low resistance
to corrosion it should not be used in water, diluted acids or saline solutions. It may be used in some
alkalis and in some concentrated acids. Temperature limit 900° F (500° C).
Approximate Hardness: 90 to 120 HB.
STAINLESS STEEL AISI 304
Alloy with 18% Cr and 8% Ni is the material most used in the manufacturing of industry due
to its excellent resistance to corrosion, low cost and availability in the market. Its maximum operating
temperature is 1400° F (760° C). Due to Stress and Intergranular Corrosion, its continuous service
temperature is limited to 790° F (420° C).
Approximate Hardness: 160 HB.
STAINLESS STEEL AISI 304L
It has the same resistance to corrosion as the AISI 304. Since its Carbon content is limited to 0.03%,
it has less Intergranular Carbon precipitation and therefore less Intergranular Corrosion. Its
operational limit for continuous service is 1400° F (760° C). It is susceptible to Stress Corrosion.
Approximate Hardness: 160 HB and in some cases,140 HB.
STAINLESS STEEL AISI 316
This alloy with 18% Ni, 13% Cr and 2% Mo, offers excellent resistance to corrosion. It can have
carbonate precipitation at temperatures between 860° F (460° C) and 1650° F (900° C), under severe
corrosion conditions. Maximum recommended temperature for continuous service is 1400° F (760 C).
Approximate Hardness: 160 HB.
STAINLESS STEEL AISI 316L
It has the same chemical composition as the AISI 316 but its Carbon content is limited to 0.03%,
which inhibits the Intergranular Carbon precipitation and consequently, the Intergranular Corrosion.
The maximum service temperature is 1400° F (760° C).
Approximate Hardness: 160 HB.
STAINLESS STEEL AISI 321
Austenitic stainless steel alloy with 18% Cr and 10% Ni stabilized with Ti, which reduces the
Intergranular Carbon precipitation and also the Intergranular Corrosion. It can be used in
temperatures up to 1500°F (815°C).
Approximate Hardness: 160 HB.
STAINLESS STEEL AISI 347
Alloy similar to the AISI 304 stabilized with Cb and Ta to reduce
carbonate precipitation and
Intergranular Corrosion. It is subject to
Stress Corrosion. Has good performance in high temperature
service. Maximum temperature: 1550° F (815° C).
Alloy with 67%
Ni and 30% Cu, it offers excellent resistance to the majority of acids and
except to extremely oxidant acids. Subject to stress corrosion
and therefore should not be used in the
presence of fluorine
and Mercury. In combination with PTFE, it is used frequently in spiral
wound gaskets for severe corrosion services, such as Hydrofluoric acid.
temperature: 1500° F (815° C).
Alloy with 99% Ni, offers
great resistance to caustic solutions, even thought it does not have the
global resistance of Monel. It is also used in spiral wound and
jacketed gaskets for special
applications. Maximum operating temperature:
1400 F (760° C).
Approximate Hardness: 110 HB.
Material often used in small dimension gaskets, where
the maximum seating stress is limited.
Maximum operating temperature: 500°
F (260° C).
Approximate Hardness: 80 HB.
Due to its excellent resistance to corrosion and easy
handling it is very often used in manufacturing
gaskets. Maximum service
temperature: 860° F (460° C).
Approximate Hardness: 35 HB.
Alloy with 77% Ni, 15% Cr and 7% Fe, it has excellent
corrosion resistance from cryogenic to high
limit: 2000° F (1100° C).
Approximate Hardness: 150 HB.
Metal with excellent corrosion properties in elevated
temperatures, oxidant service, Nitric acid and
Temperature limit: 2000° F (1100° C).
Approximate Hardness: 215
The anecdotal evidence is pretty huge: people that use metal for the burn tunnel and riser are experiencing failure in time.
At the same time, we are experiencing large positives when we super-insulate the burn tunnel and the riser and get even higher temperatures.
Further: the barrel is steel and the anecdotal evidence is that the barrel is fine. We think this is because the hot side is a very low oxygen environment and the cool side cools the metal very fast as is puts the heat into the room.
I guess I wanted to create this thread so that we have a place to cover this issue. And when people feel compelled to bring this up, we can post a link to this thread.
You see this thing? It's hydraulic tubing About 6.5 or 7mm thick. More than 1/4"
By the end of the total 32 hours of burning, it was like puff pastry in the elbow. The metal was about 1 inch thick inside, because of the spalling, and the cyclonic rocket was unusable. The gas bottle started sagging under it's own weight. That's because metal was heated to the white. You obviously see the spalling. That's not on the oxygen rich side, which creates the spalling usualy. If you don't trust me. Carry on. But please, don't tell me afterwards that your contraption works wonders and such. I know that if you reach the proper temps for a rocket stove of any kind to work properly, steel won't hold.
I can even tell you that i'm killing refractories too. I've had the bricks and refractory tubing in another stove, glowing orange. That's more than 900C°, yep celcius, not farenheit. How long do you think steel could survive at that kind of temp. Seemingly, a rocket can reach 1200C° on a regular basis.
I should have cut that elbow on the cyclonic rocket, to show the world what a rocket can do to metal. But i threw it in the skip before thinking about it.
People build rockets with metal fireboxes, heat risers etc. And as usually, they haven't read the fine prints, or even bothered to take notice of the caution messages. And then suddenly they heat a home with a metal rocket. In the middle of winter, the thing fails, and fills the house with smoke, during the night. As obviously, they are used to use the thing like a normal wood stove, and they fill it before going to bed. Left the doors of the bedrooms open, because the heat can go there. And what do you find the next morning, or three weeks later, Two dead babies and their parents. Who died in their sleep.
Please, tinkerers, mad welders, diyers. Use sense. Don't encourage the less gifted than you to take this risk. If for prototyping purpose, you do a metal rocket. Don't post it here or on youtube. It encourages people to take risks which they might not fully comprehend. And waste their time.
Once the prototype is done, and out of this world results are achieved. You might post here. But please, take it further, and do your prototype with refractories, and then show us the proper thing!
Use common sense.
This reminds me that it is time for me to write one of my long, boring articles about "common sense" - with the point being that there is no such thing. After all, if it were "common" then everybody would already know it and, therefore, there is no need to have the phrase.
Please, tinkerers, mad welders, diyers. [...] Don't encourage the less gifted than you to take this risk.
That one is gold. I like that.
One that I just thought of that might add a wee bit to this topic: I think a lot of people are used to doing welding with conventional wood stoves. Of course, the conventional design is to start taking oodles of heat out right away. So the temps never get very high. So, steel is great. Of course, "mother earth news" knows all too well that there can still be problems as their steel drum designs quickly developed a LOT of leaks since there was still a lot of oxygen inside the initial burn space. Therefore, a thicker steel will last quite a long while, although it will, eventually, fail (in 20 to 60 years?). And it will, put metals into the atmosphere.
Having metal in a space where the temps are below 1200, rapidly cooling, and it is low oxygen environment, is okay. Like where the barrel is. Or where the exhaust duct is.
This is when we had to replace the nice thick metal heat riser with cob after a couple of months of use. Granted, the metal wasn't any special high-tech metal, it was just the best thickest steel pipe we could get locally.
" Doughnut shaped " duplicate pieces that can be stacked one on top of each other to build a RMHs Heat Riser ? This has been proposed here, but I have
never seen a report from anyone who tried it ! This will be interesting to follow, how long has this been running now ? For the Crafts ! Big AL
Late note ; Past trial and error tells us that the wildly deformed part of your metal Heat Riser pipe was actually at the bottom where the hot exhaust gases
leave the Burn Tunnel and flow into the Heat Riser, can you confirm that that is what happened to that pipe ? A.L.
oops posted wrong link... fixed now. thanks mods
Big Al: Actually those small sections were intended as you say, but in the end we had to make a single vertical piece of straw-mixed cob. The first one also cracked and crumbled fairly quickly but then some of the guys rebuilt it again, and the current one seems to have been working for a year without cracking and looks really good, but we've got ash cleanout and exhaust problems so we'll probably have to rebuild yet again. Since it's all in part of "experiential education" I guess it's okay .
We use our two rocket stoves for cooking, not heating. The pots are huge, since we're cooking for 60 - 100 people most of the time. We have ash insulation packed outside the cob-straw riser, filling a standard oil drum barrel. The top of the barrel is cut away, and we have a flat steel ring reducer that we can put on top if the pot is smaller than the top of the barrel, or we put the pot straight over the flame if the pot is big enough to sit on top of the barrel.
Earlier we had a wood-fired oven that was a steel box with a fire underneath and the hot smoke going up around three sides of it before exiting from the top chimney. So we thought if we put the rocket stove exhaust under the oven where the fire used to be, we could bake bread while cooking on the rocket stove top, but it turns out the exhaust is not hot enough. And the old opening for the fire under the oven is not well sealed, leading to blowback, so we'll be going at the whole thing all over again. Because we miss our bread!
Here's a quote from donkey's forum.
A Quick Update.
This Little fire is just a wonderful performer.
I have noticed a few things others might find useful
1. It must be lit from close to the primary air for fastest smoke free condition (15minutes)
2. filling the batch box 80% achieves hottest flame and best burn time
3. Chet style portal is starting to deform from heat but is functioning extremely well and certainly acts as intended
4. The riser is 1200mm 10mm wall hydraulic pipe, it has suffered spalling for 800mm of it's length, which hot spots from 100mm to 400mm particularly affected
5. Stainless steel rack that i use to seperate the ash/small coal from the burning wood, melts and spalls away after 30 hours.
6. Opening the side door to refill the batch box once the batch has burned 40-50% leaves sufficient draft to make sure no smoke exists the box and is still hot enough to ensure no smoke leaves due to new added wood.
7. the higher density the wood the smaller the pieces must be for full combustion. i.e. 600kg/m3 i can get away with 10-15cm cross section, but the 1200kg/m3 i have most access to must be 5-10cm cross section to burn completely without smoke. larger pieces can only be added once there is a VERY significant coal bed.
From this thread
Not so high temps, mind you!
I have cleaned this thread from not so on topic comments.
Please refrain to post if it doesn't have anything to do with metal heat risers failures, and metal burn tunnels failures.
If you have a technical paper, master thesis etc, relating the use or failure of metal in high heat and corrosive environments. This is a good place to share the link to it.
If you have a successful metal heat riser or burn tunnel build. Please start your own thread. And the staff most certainly will add a link here, with a description. If the state of the art changes drastically after some new discoveries. Someone will edit or add on to this thread accordingly.
Thanks for your help ladies and gentlemen.
we need more pics.
Chris Burge wrote:First of all, it only reflects a certain amount of heat back into the system where you need it the most, while it conducts and transfers the rest of it elsewhere. Enough of the right kind of insulation will let you achieve the higher temps, but when you do, the other problem rears its ugly head: reduction and spalling.
Here's a picture I took of the inside of a 4.5 inch system after it been in constant use for 6 weeks...
The feed tube, burn tunnel, and the beginning of the riser are refractory brick, but the rest of the riser is a piece of schedule 40 steel pipe. The riser is insulated with 2 inches of a tightly packed refractory compound made up of fireclay, perlite, silicon dioxide sand, and alumina fibers. You can see the spalling of the steel sticking out like a bunch of steel cornflakes, creating a ton of drag on the system. The stuff that looks the same thing going on on the firebrick, is just excess fireclay slip from the assembly of the core. As you can see in this photo, it knocks off easily and reveals smooth firebrick...
The metal spalling, however, does not come off so easily, and merely continues throughout use. Here's a picture taken from the top of the riser after about 4 months of constant use...
As you can see, the insulation worked quite well, because i was able to get hot enough temps at the top of the riser to blister and spall the steel.
Metal seems like a quicker, sensible option at first, but in the long run, it will fail.
What I want to make clear is this: don't focus too much on temperature, there're other reasons why steel could survive provided it is done in a proper way. Too low oxygen as a whole isn't good because it will create a dirty burn. Too much air to keep the material below a certain threshold will cool down the fire and lowers the efficiency coupled to a dirty burn again.
And frankly, I don't believe in building an excellent burning appliance just by sheer accident.
So is the better question: Do we want a Rocket Stove or a wood stove? and is the rocket stove section or the wood stove thread section?
If we want a Rocket Stove then we want the temps to be high enough in the burn chamber and heat riser to burn as close to 100% of the fuel and gasses with out leaving any avenues of air or cooled walls to allow escape of unburnt gasses combustible material.
The electrical, magnetic and structural properties of metals can be changed through heat.....Metals are comprised of a symmetrical structure of atoms known as an allotrope. Heating the metal will displace atoms from their position and the displaced atoms form a new structure. This process is known as allotropic phase transformation. Allotropic phase transformation alters the hardness, strength and ductility of the metal....Annealing is frequently used to soften metals including iron, steel, copper, brass and silver. The process involves heating the metal to a specific temperature then allowing it to cool slowly at a controlled rate. Annealing alters the physical and chemical properties of the metal to increase ductility and reduce hardness.
When iron is heated past 1,674 degrees Fahrenheit it is able to absorb more carbon but will weaken if allowed to anneal or cool inside slowly like a controlled area inside the core.
Heat alone may not corrode metal but higher than operating temperature does weaken its molecular structure. The only obvious way to keep a burn chamber or heat riser constructed with lower temperature metals from changing its molecular structure or deteriorating is to cool it passively or actively and that may extend its life but comes at a price many Rocket Stove builders desire not to give up and one of the many reasons we seek a proper Rocket Stoves and that is burning the fuel completely as much as possible and by lowering the burn chamber and riser wall and air temperature to cooler than needed to protect the inner wall from damage hinders its potential to fully burn the combustible gasses and will allow those unconsumed gases to escape through the exhaust and degrade efficiency. The entire idea of a Proper rocket stove is to create a super heated burn tunnel where gasses can not touch a cooled metal side or ride in the injected cooled air for an extended period to escape combustion. If carbon steel is not getting hot enough to weaken that means the side are being cooled enough to allow gasses to cool and escape.
Peter and others are correct... when they say metal can be used in a stove... but a Rocket stove is a different animal and how I have understood it to be defined a J tube passive air drawn and cooled metal stove that allows unburnt gasses and material to pass through unconsumed does not fit in the Rocket Stove category and may be an awesome wood stove... Just not a Rocket Stove and should not carry or confuse the name we work so hard to keep and define.
The Peter Channel does a brilliant job of injecting air only in front to mix with inferno hot air and does not interfere with higher temps because of insulation and proper burn chamber and heat riser wall temps. cooling the side walls of the heat riser excessively to save a low temp metal will defeat the whole process and the reason we use refractory material and high insulation. super heated and insulated inferno walls are needed and help define a Rocket stove.
Rocket stoves are available online, and are not mass heaters, not as efficient as mass heaters, often metal and portable and often for outdoors cooking, hence the name STOVE
I think you have rocket stove confused with rocket mass heater (RMH) FStyles...
Although your mag stove concept is kind of a unique hybrid with a steel propane tank sealed combustion area, so how's your metal propane rocket mag stove holding up FStyles? It will be fine I'm sure, as mine
you have always missed the point with my argument and until a definition is fully written you will continue to call the very old idea of the J or L tube design a "Rocket"
If Satmax is in the room please share the old L or J design they did not call a Rocket.
the Dakota fire pit is built like a J or L tube design but yet it is not called a Rocket.
the M in RMH is because of the mass heat storage and not the size or material of a Rocket stove. you can have a Tungsten or titanium metal insulated stove that can handle the heat and it would be Called a Rocket Stove if its burning all the gasses and called a Rocket Mass Stove/heater if its inline with a mass battery storing heat.
My propane tank is doing fine because not only do i have refractory cement on the bottom of it, its not in the high heat burn chamber or heat riser area. the question here is not whether we can cool our low temp metal to the point it will not corrode it is will it perform to burn all the fuel as close to 100% as we can. wood stoves burn wood....Rocket stoves burn wood as complete as possible.
using the term Rocket on a wood stove regardless how fancy looking or professional it may be constructed dilutes the definition of our goals and our goal at permies is to be efficient, earth friendly, sustainable and you can not do all that by not burning almost all the fuel and gasses. Permies members seek efficient and environmentally built Rocket stoves. Allowing unconsumed fuel escape out the exhaust defeats those desires and goals.
Satamax Antone wrote:The fumivore, not a rocket!
the L and J wood burning core concept has been around a very long time and was never called a "Rocket" anything.
To burn all sorts of Wood in the middle of a Room without making any Smoak, is a thing so extraordinary, that all those that have heard speak of it, as well Philosophers as others, have asserted it impossible: but Mr. Dalesme Enginier, prosecuting his discoveries, has found out a Machine, which tho’ very little and portable, consumes all the Smoak of all sorts of Wood whatsoever, and that so, that the most curious eye can¬not discover it in the Room, nor the nicest Nose smell it, altho’ the Fire be perfectly open. This has given such satisfaction to all that have seen it, and to the King himself, that he has caused the Experiment to be made fever al times before Him.
This Engine is made after the manner represented in Fig. 1. and is composed of several hoops of hammer'd Iron of about 4 or 5 Inches diameter, which shut one into the other: It sands upright in the middle of the Room, upon a sort of Trevet made on purpose. A is the place where the Fire is made, where if you put little pieces of Wood, it will not make the least smoak, neither at A or B, over which you cannot hold your hand with¬in half a foot, there comes out so great a heat : If you take one of these pieces of Wood, out of the Fire at A, it smoaks presently, but ceases Immediately so soon as it is cast in the Fire again.
The most foetid things, as a Coal steept in Cats-piss, which stinks abominably when taken out of the Fire, notwithstanding in this Engine makes not the least ill scent. The same did Red-Herrings broiled thereon; on the other side all the perfumes are lost in it and Encense makes no smell at all, when burnt there¬in. We have since learnt that this is not shown, but when the Fire at A is well kindled, and the Tunnel B D very hot so that the Air that feeds the Fire cannot come that way but must all press in upon the open Fire; whereby the Smoak and Flame is all forced inwards, and must pass through the heap of burning Coals in the Furnace A, in which passage the parts thereof are so dispersed and refined, that they become inoffensive both to the Eye and Nose.
I have a 10" "J" style rocket stove that I want to boil sap or large amounts of water on. I insulated it my first year but the metal got so hot it melted it and it slumped.
Well it might be time for an update.
This all metal build has needed some serious repair work.
after approximately 500hours of burn time the wall thickness of the riser and chet portal ports have diminished the point of imninent failure (less than 0.75mm down from 8mm and 5mm) I've been keeping tabs on thickness via an ultrasound probe.
i've replaced the riser and portal. but this really is just a waste and i will have to start in ernest to build a RMH in the correct style.
Metal batch failure at Donkey's.
Satamax Antone wrote:The fumivore, not a rocket!
I was very intrigued by this article about Dalesme's work in the 1680's! I was especially interested in the idea that this invention was a "reverse siphon"! A siphon seems to defy gravity by forcing water or other fluid to flow uphill and then horizontally. A siphon is an upside down "J" tube. A rocket stove works on the same principle but seems to defy the laws of buoyancy! Flames and hot smoke should always rise but the "J' design of a rocket stove forces them to descend and then travel horizontally. But just as the driving force behind the siphon is a longer tube filled with fluid pulled downward by gravity, so the driving force of a rocket is a longer tube filled with hot gases pulling upward. (Increasing the length of the gravity driven siphon and increasing the length of the buoyancy driven rocket will dramatically increase the draw of either!)
I don't speak or read French so used a French-English dictionary to do a very amateur and painstaking translation to get the gist of it. Here is what I came up with.... CHAPTER TWO
Simple Smoke-eating Furnaces
Dalesme’s smoke-eating fireplace
There had long been a need to construct furnaces in which the owners could consume their smoke cleanly and eliminate the obnoxious (fumes) in the vicinity of foundries, breweries and mills.
French mechanic/machinist, Dalesme, appears to be the first to attempt to consume smoke. He demonstrated his procedure at the Saint-Germain fair in 1685.
His furnace of reversing flame was actually an upside down siphon in which one of the branches served as a chimney and was much longer than the other. Since the inside of the longer branch was superheated, a current formed by the air entering the shorter branch drove the flame back in the furnace causing it to pass underneath the grate resulting in the burning of the smoke.
Lahire, in 1689, conducted some experiments on this process about which he presented a report of his discoveries in the Journals of the Scientific Academy.
Satamax, as you live in France, I'm assuming you are fluent in French and would do a much better job at this than an Anglophone who is not bilingual! Could you translate the article for us clearly? Thanks a heap!
Satamax Antone wrote:Bruce, i'm French, and speak a smidge of English. So i think i could tackle this. But there's soooo many things to do before. I still take time to come here and try to help others. But i'm affraid, this one will have to wait. Besides my woodworking day job, i've tackled the building of my flat, and of a sawmill, lately. Working on a 1944 80kw generator for the moment.
You're a busy guy! Take your time. Thanks. Bruce
Glenn Herbert wrote:I did a translation of this in another thread a few months ago, and yours is pretty accurate. It gets all the important information right.