So far my research seems to indicate that a fire brick heat riser will last longer in a more permanent installation. A few questions: Do you need to insulate the riser if made out of fire brick? If so, how do you do that? Will it still fit in a standard oil drum?
I just got my fire brick heat riser stove going a couple of days ago. But for example, this morning it's been running for 4 hours at about 500 degrees f at the lid (with some tweaks I am sure I can get it to go higher, it's been running fine, but i am using the full 4.5 width of the brick as the thickness of the riser, as opposed to some people who stack them on edge. Still, the insulation needs only allow the fire to remain hot while rising. I would think that even the 1.25 thin fire bricks on side would contain enough heat to keep the fire hot enough while rising. I would think that in no circumstance, with fire brick as a heat riser can you get enough heat bleed through to not allow the sides of the barrel to cool down that which comes out of the riser sufficiently so that it will contract in volume. This is a video of my prototype which I used, essentially unchanged in my current stove. It's 13 bricks high and I put a barrel over top, cutting a rectangular hole to allow the barrel to slip onover top of the heat riser and fire tunnel (that's what I call it anyway). In the video you will see that the half sawn bricks when laid up into an octagon form a star pattern. It is really quite pleasing to look at, with the star pattern reversing each course. But I am not so sure that the tips of the stars sticking out like that aren't flowing the down draft between the rise and barrel walls. It is relatively such a slight flow, I wouldn't think so, but if i had it to do over (and the time) when I cut the bricks in half, I would have also set up a jig to cut those tips off. Also, I would like to see the stove just a bit bore rockety, so when i get a chance, if we happen to get a warm spell, I will add another 12 inches to the barrel height which will allow me to stack the riser another 5 courses higher.
The tick to sawing the bricks in half is EXACTITUDE. I used a table saw with a 10 inch diamond blade from home depot (about 20 bucks) do not buy the really expensive ones) Set up a "sled" that you can build a jig on to hold the brick exactly in place. make the jig so that you can make minute adjustments as to angle and the exact spot where the cut begins. Try it out by first using a wood blade on the say, cutting 4.5 x 9 pieces of OSB (flake board) and then switch over to to the diamond blade using bricks. You are going to waste several of them getting the cut just right. (save the pieces though) when propoerly cut, you should be able to put either half of one brick with either half of another, and it will form an EXACTLY straight brick. Also ytou should be able to take a half of one brick and put it on top of the other half of the smae brick and they will be EXACTLY alike. Also, you shoud be able to form the halves oif the same brick into a perfect 90 degree angle using a metal square with no gap at all between the square and the brick when measuring either the insider or outside corners. It seems like a lot of extra work, but you will thank yourself when stacking up the bricks, if you use the octagon design, that all of the brick surfaces come together with just a slight amount of adjustment. (I forgot to mention, make absolutely sure that your table saw blade is set EXACTLY perpendicular to the table saw surface)
Ordinary firebrick, the kind you need a diamond saw for, is not insulating; a larger thickness means mostly more mass to suck heat from the fire. Eventually it will reach equilibrium, but that means radiating a lot of heat out to the barrel sides. You will have better results with thin splits and some insulation wrapped around it. If you can get insulating firebrick, that would contain the heat very well (and be much easier to cut - a hacksaw works fine). I'm sure you don't want to scrap all the work you have done so far, so if you do raise the barrel and the riser, I would suggest making the addition of splits on edge. I'm sure you can figure out how to cut them for an octagon pattern (setting the blade at 45 degrees and the bricks square to the table would probably be easiest). Then wrap the splits with rockwool or something.
Dirk Esterline : It is easy to refer to fire brick as insulative, This is a partial characteristic of fire brick as we commonly use it. We can get in trouble if we trust that this
Partial characteristic will always do what we want !
It is the presence of many other heating devices that allow us easy and relatively cheap access to good fire brick, generally they come in two kinds, a light Firebrick and
a heavy and much denser Fire brick
The lighter and fairly easy to cut Fire Brick has three major characteristics, The slow rate of diffusion of heat energy Through itself, or rate of Thermal conduction, and
a modest Specific heat, or how much heat that the material will hold ! Mass is the usually dominate factor here, so light weight means not a lot of total heat energy
The third characteristic, is fire bricks ability of Reflect or bounce-back heat from its surface back into the heart of the Fire ! This is also called its Refractive-ness, This
heat is not radiated through the brick to heat its far side. The true amount of insulating effect then is in that amount of energy that is Refracted and not ether stored or
radiated through its mass !
Dense fire brick was created initially for use in kilns, where its ability to store Large amounts of heat energy Its specific heat, and radiate that heat slowly (its thermal- -conductivity ) is prized in locations where it is important to protect those objects heated within the kiln from the hot/cold stresses of too rapid cooling !
Again there is a noticeably higher Rate of Heat Energy Re-radiation or Refractory~ness. That has the final effect of being insulating when comparing the flame impinged
Surface temperature to the Surface temperature of the Firebricks far side !
It is this re-radiation of heat energy back into the fire (plasma would be a more accurate term) that gives us the freaky high Temperatures that produce near total
combustion and a clean burn !
An overwrap of a Ceramic refectory material* that helps separate and concentrate the Temperature differences between the Hot burning pyrolyzed wood gases inside
the Heat Riser, and the cooling denser/heavier Exhaust Gases Vertically failing down the inside of the barrel to the Transitional Area Is what creates the Push me, pull
you magic ( Delta T ) that allows the Hot exhaust gases to flow 50+ ft through the thermal mass!
I am in awe of David Searles heat riser#, and want to build one like it some day! Even with this craftsmanship, on extremely long burns the temperature of the two gas
streams can potentially Equalize and cause the System to stall !
* a 1'' layer of a high grade of Ceramic refractory batting or blanket like material will help ether the 'Soft' or 'Hard' brick to come up to its operating temperatures
slightly faster, also possible is to construct a Heat Riser of between 2.5'' an 4'' thick, from a mix of clay slip and ether Perlite, or Zonolite, an expanded mica product!
# The Searles Rocket Mass HeaterRMH, appears to be a sub 5'' system, increasing the height of the Heat Riser will improve overall performance, but it is already close
to its maximum performance. ether building a bigger Rocket system or converting his J-bend to a Batch Loaded horizontal burn type RMH are certainly the next steps !
I hope this was both clear, and useful, and timely ! For the Good of the Craft ! Big AL
Success has a Thousand Fathers , Failure is an Orphan
I think the quick answer, is that insulating the riser is preferred, even when the riser is made of fire brick. As noted above, I too recall it being said that if using the insulating kiln brick (can be cut with a hacksaw), in that case one can get away without using insulation around the fire riser.
As for an insulated fire brick fire riser fitting inside a 55-gallon barrel, I'd say the answer is yes. Some back-of-a-napkin geometry can answer this for sure.
I'm imaging a ring of full size fire brick, on edge, butting one to the next in such a way as to make a 7 x 7 inch opening. That's going to be a square, and the outside edges of that square will be 9" plus 2.25" = 11.25 inches; add to this 1" of insulation on both ends, and that's up to 13.25 inches, measured flat to flat. But what's the hypotenuse?
a^2 + b^2 = c^2, so
(13.25)^2 + (13.25)^2 = c^2
351.125 = c^2
18.74 = c
So, it will fit inside a 23-inch inside diameter barrel.
Call that 18-3/4 inch, or even round up to 19" if the insulating wrap is a little loose. So on the long corner-to-corner measurement, the 7x7 inch opening should still have about 2" opening between the wall of the barrel and the corners of the insulated fire riser.
On the flat sides there will be even more room. Feels like enough to me.
Area wise, the answer starts by finding the area of the interior dimension of the barrel, and subtracting the area of the insulated fire riser (call it 13.5 x 13.5 to allow for a little looseness in the rockwool wrapping).
Area of 23-inch circle = pi * r * r = pi * 11.125 * 11.125 = 388.821 sq. in. (Note: I made the radius a little too small, for a margin of error.)
Area of 13.5 x 13.5 square = 182.25 sq. in.
So, (area of circle) - (area of square) = area of the open space between the sides of the barrel and the insulated fire riser =
388.82 - 182.25 = 206.57 sq. inches of open space (area) around the barrel.
Is that enough open space? Depends on the system size, right?
CSA of 6-inch system = 28.27
206 / 28.27 = 7.29
So the open area is 7 times the CSA of the 6-inch system.
CSA of 8-inch system = 50.27
206 / 50.27 = 4.10
So the open area is 4 times the CSA of the 8-inch system.
I think having 4 times more open space to move air between the barrel and fire riser will work well. And 7 times as much ought flow very easily.
Bear in mind, I do make mistakes scratching on napkins! I may be mistaken, but that's my take. I think you'll be fine.
$3 per sq. foot. If anyone knows a lower price for the same or better material, I'd sure like to have the link!
I figured I get 8 sq. feet for my build. That should leave some extra material for stuffing around various places I want to stuff insulation or have an air/expansion gap. After shipping the cost estimate was about $65 (Nov. 2014).
Then you just wrap that around the fire riser and use some wire to tie it in place. I've read all kinds of wire lasts a long time on the outside of the fire riser. Someone even used chicken wire, which is pretty light weight stuff. I'd think hardware wire would work great, and lots of folks have done that. Bailing wire might work well too.
The other idea is more work. Buy a tube big enough to fit over your fire riser. Then mix up perlite and fire clay, using as little clay as allows it to coat and stick the perlite together (you don't really want to breathe in small particles of perlite, they're not healthy for the lungs, so I'd suggest wearing a respirator).
Then you light pack and tamp the perlite-clay into the form.
My brick supply store sells fire clay (50 pound bag) and 4 cu. ft. bags of perlite. Each are something less than $20 USD after tax (Nov. 2014).
Plus, this is one of the recommended ways of insulating the rest of the fire box, and other areas of the RMH needing insulation, so you may already plan on having 12 or so cu. ft. of perlite on site anyway. So maybe it isn't such a big deal to mix it up if you're doing a bunch more anyway??
I'm sure there are other options too, but those are the two I like best.
Don't confuse rockwool and refractory blanket insulation. Rockwool is cheap enough to be an alternative to fiberglass for house construction, and is heat resistant but not durable enough for direct fire exposure. Refractory blanket is rated for around 2000 degrees or more depending on the type, and can be used to build portable pottery kilns. It is ungodly expensive, but worth it if you need the capacity.
Rockwool has been used as the insulation wrapper for firebrick heat risers, and may be less durable, but will last long enough to be worth using and is easily replaceable.
Ceramic refractory blanket is the best product for where super heat resistance is required... it is overkill elsewhere.
it seems like a whole lot of well intentioned technical mumbo jumbo.
I am using a heat riser with 4.5 inches of soft fire brick for its walls.
As I write this my first build stove with the soft fire brick is putting out a constant 580 degrees F at the top of the barrel, with no vertical chimney vent, and this when it is 50 degrees outside and it is raining, so there is plenty of moisture in the intake air that has to be burned off.
why would I go through the trouble of lining the heat riser with 1" of hard fire brick? Especially when the soft fire brick so easily lends itself to building the heat riser into a self supporting stable octagon design which seems to be far far more conducive to a hot burn than a rectangular system?
Sorry, I never got that you had used soft (insulating) firebrick. That puts the structure and insulation in one piece, and the low-mass hot face will heat up as fast as possible. So well done! With that information, I would say that you not only have an obviously effective setup, but one that cannot really be improved on.
I know I am responding to an old post, but it's a goodie. I'm very impressed with the insulative soft refractory brick stack you concocted for your heat riser. Outstanding! Could you give me some idea of how that has stood the test of time. I noticed one of the other commentators mentioned that it was an idea that might not be able to be improved upon. I agree to some extent. My only contribution might be to drill holes in the bricks and drop some kind of heat resistant rods down inside the full height as a means of stabilizing the entire structure as the constant heating and cooling might cause the bricks to migrate away from each other. Another thought might be to use stainless steel hose/duct clamps to gently compress the structure together. Then again, maybe this is not an issue. Anyway. Very impressive. I will be attempting this on my build. Would love to hear what you have learned in the meantime.
If you are concerned about the bricks drifting, it wouldn't take a strong compressive force, merely a heat-resistant collar. Chicken wire or other wire wrapped around the bricks would work fine and last a long time in the oxygen-poor environment around the riser.
Thanks for the reply Glenn. I don't suppose the bricks would move much, if at all. Just something I considered worth looking into. A question I'm very interested in is just how hot does the rocket mass bench/bed get? I suppose it's a matter of how much fuel is burned and for how long. Has anyone ever injured themselves by sitting or sleeping on a cob bench that is too hot?