I came up with an idea for making a neat heat riser from diagonally cut firebricks. Since I have learned so much from everyone I wanted to share it. It's been a year and I STILL haven't finished with my other chores so I could build the whole daned thing - hopefully I will get to that befoire the end of this month (or next month, or the one after that : - )
The trick is to get a cheap diamond blade (I got a 10" blade fro under $20 at the box store, and made a "sled" for my small table saw to be able to adjust the cut precisely so that the 45 degree angle cut goes through the exact middle of the brick. Look up on you tube how to make sleds, but design yours so that it can be adjusted ever so slightly for the correct anle and location of the cut. You should be able to take both halves from one brick and put the together flat and they should perfectly match one another. This is important so that the bricks face together perfectly. Look in the video at the star shape they form. The arms of the stars all go in one direction on the same tier of bricks and then are reversed for the next tier. I built this outside on the ground. I had a hard time getting the ground perfectly level so I used the "close enough" method by putting down a mound of sand and took a piece of OSB (flake board) and moved it arond on the sand so that it was perfectly level in all directions (very important for stability) and stacked the bricks on the board. (For a permanent build I would make a small slab by pouring a bag of DRY concrete mix and spend as much time as you need to spread it out perfectly flat. (This part is sooo important, that if it took you an hour to do just this, it would not be a lot of time.) And after you stacked up the bricks, put a could two three handful's of gravel on the burn floor, which probaly would need to be renewed from time to time. (The gravel would be to protect the dust from the dry concrete mix from getting vacuumed up during cleaning, In the video, I spent the time getting the base flat, but once I got it, I was very impatient to lay up the bricks. If you look really close you will see some very small spaces between the bricks because of my impatience, but the presence of thise spaces didn't seem to affect the burn at all.
This was the absolute first burn. You will see that during the video I put my hand over the stack to get an idea how hot the fire was. The last time I did it, wow - that was hot. I didn't get a second degree burn on my tough skin, but I could feel that thing for a week.
I didn't really have a plan for stacking the bricks for the entrance tunnel, but as I recall the tunnel entrance ended up being offset just a bit which I think began the fire spiral you can see down the heat riser when my son pointed the camera down there..
The diameter of the octagon formed by the stacking oif the bricks is just about 5.5 inches. The stack was absolutely stable, because the bases was perfectly level and the way that the bricks were stacked in the alternating tiers. I am quite soue I could have stacked it with another 4 or 5 tiers of bricks and it still wouldn't have made a difference.
One thing I would do if I was going to buld a lot of them, would be to get a table saw with an attachment on it that you could hook up a really strong shop vac to cut down on the dust, which really wasn't all that bad considering how many bricks I had to cut.
I do curse on a regular basis in the video, as I really hadn't thought that it would work so well on the first burn, or i would have moderated my language some, so you might not to have kids present when you watch it.
David Searles : W.O.W., David, that is amazing !Welcome to Permies.com, our sister site Richsoil.com, and the Rocket and Wood stoves forums!
This is an eye opener, I have seen open lattice wooden wishing wells made that way, but I never made the connection ! Have you measured your inside space
to determine exactly what your inside diameter is ?
A 4'' Rocket Mass Heater that can be built with a high degree of success by a 1st time Rocketeer has been a hypothetical idea, with your heat riser I KNOW we
are a lot closer, if not already there ! Some insulation of the heat riser will be needed, but I think it will still be possible to go to a slightly smaller barrel, remind-
ing myself that regardless of the size of the barrel, the same amount of BTUs of heat energy must be radiated off of the barrel. Counter-intuitively, the smaller
barrel with a smaller surface area will radiate that heat energy off at a higher temperature !
This is why when your 3-4 year-old wants up in your lap, they are such a good cuddle, with less surface area to mass they radiate their heat off at a higher temp
A 4'' RMH, with a J-bend, top loading, Batch Box, should easily be able to draft effectively enough to match a more conventional 6'' system !
Have you considered making a cut to the single side bricks to allow for a Better bridging of your bricks ?!
Anyway I cant wait to see what you come up with next! For the good of the Craft ! Big AL
Success has a Thousand Fathers , Failure is an Orphan
LOOK AT THE " SIMILAR THREADS " BELOW !
posted 6 years ago
Looking back at my post, I should have said that the 45 degree diagonal cut had to go through the exact center of the brick. A 45 degree cut yields an octagon inner surface. As I said the inner diameter ends up being about 5.5 inches. I suppose you could make a significantly larger diameter with a nonogon or even a decagon design, but you will sacrifice the stability of the stack in doing so.
Increasing the number of sides would decrease the angle at which you cut which would increase the surface of the brick that faces the fire. For a brick that is 9" long to begin with, using a 45 degree cut would result in the length of the exposed surface being a hair under 2.25" Multiply that times 8 and you get a rough circumference of the burn tube of 18" Dividing by Pi gives a rough diameter of 5.7". Decreasing the angle of the cut to 40 degrees would yeild a nonogon with sides of roughly 3". Multiply that times 9 gives a rough circumference of 27". Dividing by Pi would give a rough diameter of 8.75", or a cross sectional area that would be well over twice as big as that of the octogon. I am a certified oil burner technician. It's all about the quality of the flame and that, in the case of solid fuel is mostly controlled by the air flow. Again I have almost zero hands on experience with controlling a small wood fire, as in a rocket stove, but it would seem that in a smaller system you are gong to get far better consistency in the burn than in a larger system especially considering the non-uniform fuel shapes, sizes and compositions inherent in the burning of very small amounts of burning wood.
Increasing it to a nonogon design I think would also begin to compromise the stability of the stack, but I'd have to try it to see, and since the octogon works so well, and time and money being in short supply I won't bother with it.
If you did want to get the octogon design down to a rough 4" diameter you could simply, after cutting the 45 degree angle, cut off the end of the brick to make the exposed brick face 1 5/8 ", but at the diameter the octagon might not allow for such a regular fire spiral. In that case you might want to consider going to a nonogon (40 degree cut) with 1.4" exposed surfaces, or even a decagon (36 degree cuts) with exposed sides of a bit over 1 1/8", but again, I'd have to try it to see what the extra sies and decreased brick lenghs would do to the stability (and the tightness) of the stack.
Richard Wood : If you look at the Thread I posted in the Rocket Mass Heaters Forum :::-->
'' Fake Fire brick ''
I fell into a similar trap, part semantics,and part a cary-over of terms from The days when Potters/Ceramists were naming
and deciding the definition of the terms used.
Because I freely admit the fact that my Thread contains errors, let me direct you to the Thread Extension posted by Erica Wisner, whose answer is the clearest one posted yet . Her Thread Extension was posted Dec 20 2013, near the bottom
Actually there are Two types of fire brick,* with similar but different jobs and abilities they can bring to the same task !
We have to talk about all Fire bricks ability to be Refractory ! That is- to reflect a high percentage of Radiant Heat Energy back
into the Combustion Zone of the RMHs Burn Tunnel and Heat Riser ! This ability, shared by both types of Fire brick that creates
the High Combustion Temperatures (time, temperature and turbulence) that give us our great efficiencies.
Of the two types of F.B. The lower density 'insulating' F.B. will Reach its Maximum Refractory or reflective state a little sooner
and thus sooner create the High temps and High Efficiency state!
While the denser firebrick does Have a higher thermal capacity/will hold more heat, and Conducts far more heat through itself,
it's ability to provide much more uniform constant combustion Temperatures for the entire burn cycle, and even for hours after
helps all of us maintain the clean burn state ( but especially New RMH users ).
Here to clarify my point (I Hope) I want to talk about a potters kiln after firing By carefully controlling the temperatures
of the Kiln -long after the fire is out, the Potter hopes to have very little or no 'Thermal Shock' breakage!
(S)he lines the inside of the kiln with dense firebrick to allow the whole thermal mass , f.b. and pottery to slowly cool down. The
dense f.b. are only doing 1/2 of the Kilns job and there is an outer shell of some type of lighter but also refractory insulation
to slow the cooling of the kiln !
So- it is the Refractory-ness of the two Fire brick types that we prize, Dense fire brick still hot and refractory from an earlier
firing will be the easiest to relight ! The low density Fire brick will light nearly as easily and reach a refractory state sooner than
cold fire brick.
In a well made RMH system with plenty of insulation in its outer shell- the difference in time to reach refractiveness will be One
minute or less, this seems to tip the scales towards dense fire brick !
It is also very important to remember that part of the job of the insulation is to insulate and protect the Cob, and the surrounding
exposures ! For the good of the Crafts !
Think like fire, flow like a gas, don't be the Marshmallow ! As always, your comments and Questions are solicited and Welcome!
Pyromatically Big AL
* Available as a reasonable Substitute for 1st time RMH builders/experimenters, and those on a budget who want good, acceptable
results, are old dead soft Building bricks, the kind that can be used like 'sidewalk chalk' and leave a clear red make behind ! These
take the longest to perform at maximum efficiencies, seeming to need to get hot before becoming Refractory !
Success has a Thousand Fathers , Failure is an Orphan
LOOK AT THE " SIMILAR THREADS " BELOW !
posted 6 years ago
Thank you allan. The hard fire bricks are denser, therefore conduct heat faster than the soft?
The are denser and therefore hold more heat?
And therefore take a longer period of time to cool down to ambient?
This last one, I'd have to see. If both bricks were heated to say 500 degrees F, through and through, I would think that the one with the most internal resistance to heat flow would take longer to cool off.
While the amount of heat that the brick can hold may be important in a pottery kiln, it would seem that as long as the temperature of the brick was even 30 degrees above ambient, it would be enough to create enough of a draft to very easily relight the stove.
I went with the soft bricks for several important reasons -
1.) the stack that I made took 4 bricks per tier, 18 tiers, perhaps, and then extra bricks for the base and tunnel, maybe 90 bricks total. I got all of those bricks for maybe $130, which I am pretty sure is a lot cheaper than the hard bricks would be;
2. because they are lighter, a heck of a lot easier, putting them on the trailer and taking them off, and all of the dozen or so times moving the bricks before putting them into their final resting place;
3.) They cut easily, and most importantly for this application, precisely, and without a terrible amount of dust using a simple dry table saw with a very inexpensive blade (10 inch diamond);
4.) They all lay perfectly flat against each other without the slight wobble between any of them.
5.) My local lumber yards sell the soft brick and doesn't even have access to the hard ones.
this list is a tremendous resource, thanks.
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
David Searles : Yes, The hard dense fire bricks rate higher as Thermal conductors, though other things like crystalline structure can also affect
a given materials ability to conduct heat
Yes Here we are talking about a given materials ability to hold heat, also called its specific heat Soapstone inserts on a fire box are rated high,
Air is much more insulative and is a damned poor conductor of heat !
The next two lines of questioning shows me it time to send you to the experts ! However, it is a fact that After firing two identical RMHs the one with
the lighter brick will have a cooler surface Temperature.
The important thing is the More insulating brick will also sooner reach its Refractory state where The high temps are maintained by the Reflection of
heat energy back into the RMHs combustion core !
The more dense brick will take slightly longer to reach its Refractory temperature, because it absorbs More heat energy, specific heat and will
also transfer heat through itself well as a Thermal conductor !
The RMH will hold enough heat to burn an uninitiated Rocket owner 12 hours after a firing !
It is not a matter of the temperature of the heat stored within a given material, it is the the Total Heat Calories or BTUs that are stored that determine
the starting point and the thermal conductivity that determine how fast something cools down
Draft is important in starting the RMH, The higher the Difference between the combustion core and the final vertical chimneys Exit determines how strong
the draft is !
Also important is the fact that hot fire bricks will come up to their respective Refractory temperature than will cold fire bricks !
1) I agree the light bricks are cheaper
2) Yes they are easier to handle
3) and cut, cheaper there too
4)If all the bricks come out of the same batch irregularity should not be a problem
5) Hard are the only kind home depot and Lowes carries, Hard splits are all you can get at Central Tractor ( as of recently)
This could be covered under (4) but even as nice as the brick fit together in a stack, your heat riser will work more efficiently if there is even more
insulation wrapped around the heat riser/tower! For that reason dipping your heat riser in Clay Slip or using a cob mortar to bind the bricks together
is nearly mandatory This too would take care of any irregularity in the Bricks !
In my attempts to detail the difference between the two Firebrick types I neglected to say only the one i made with Ernie Wisner has firebrick in it.
The other two were made after and contain 100 year old dead soft red/orange red house brick ! I am a notorious tightwad when my better half cant
tell the difference !
I am not proselytizing for any one brick except that different conditions Sometimes will favor one over the other ! Change lets us all grow !
Finally to allow you to check up on me, please see these two articles in the Engineering Toolbox! A right click should at least get you a google search