jonasp
Junior Member
Posts: 102
|
Post by jonasp on Feb 19, 2023 4:40:59 GMT -8
Probably a good idea to insulate the bottom there after the exit. If you use CF blanket you could always remove it through the cleanout If necessary. Is there a reason your afterburner top chamber exit is a bit less than 1 csa? (97 x 247 = 23959 90 x 90 x 3.142 = 25450 23959 / 25450 = 0.94% csa) Having it long and thin and that far forward tends to make it pull more on one side of the vortex. A squarer exit near the back works better, that's why I have that recessed area in the rear of the top chamber so I can make the exit farther back equalling the pull and maintaining the important surface area to volume ratio of the top chamber. I actually measured everything on 175mm firebox so it's 1 csa aha yes that makes a lot of sense i was afraid it could have that effect. I've reduced 1 shelf board to decrease the overall height of everything so the exit height is already lower than your version. If i make it the same as your system i would have 12.1 x 19.9 Do you think this would be enough to equalize the vortex or should i just increase the height of the top chamber by another 2.5cm?  Thanks! |
|
|
|
Post by Vortex on Feb 19, 2023 5:17:40 GMT -8
Ah OK the csa's good then, I thought you were doing a 180mm system like in the thread title, I missed where you changed it. I wouldn't raise the top chamber as that will mean you'll lose the surface area to volume ratio. You might need to cut the top layer back a bit more so there is no where it's less than 1 csa. Here's a picture of how I did that on mine:  |
|
jonasp
Junior Member
Posts: 102
|
Post by jonasp on Feb 20, 2023 0:57:42 GMT -8
Ah OK the csa's good then, I thought you were doing a 180mm system like in the thread title, I missed where you changed it. I wouldn't raise the top chamber as that will mean you'll lose the surface area to volume ratio. You might need to cut the top layer back a bit more so there is no where it's less than 1 csa. Here's a picture of how I did that on mine: If i copy that design and also do the same with the ceiling it would be 14.5cm height x 16.5cm wide. So that would be very close to a square, thanks for pointing it out |
|
jonasp
Junior Member
Posts: 102
|
Post by jonasp on Feb 27, 2023 2:24:42 GMT -8
|
|
jonasp
Junior Member
Posts: 102
|
Post by jonasp on Mar 21, 2023 9:00:56 GMT -8
slowly getting there.. the chimney pipe is fully installed with bypass.  I've measured and cut all the vermiculite boards into the correct sizes I've been reading that vermiculite will expand and might crack when bigger pieces are fixed together. Now I'm wondering how I should tackle fixing these boards together? My thought are having the sides of the shelf fixed together and the shelf itself just put on top. The sides of the afterburner I'm a bit lost what the best plan of action would be. Just fix the sides together and let the roof rest on top or fix the roof onto the sides. I'm using a vitcas board adhesion to fix the boards together.  |
|
|
|
Post by martyn on Mar 21, 2023 9:20:31 GMT -8
It doesn't look like you meed to fix the boards together, which is good!
|
|
jonasp
Junior Member
Posts: 102
|
Post by jonasp on Mar 21, 2023 9:52:39 GMT -8
It doesn't look like you meed to fix the boards together, which is good! allright then i'll keep it unfixed. it'll all be hold together when i place the oven on top of it (not fixed on the vermiculite) thanks for the input! |
|
|
|
Post by Vortex on Mar 21, 2023 10:09:46 GMT -8
Some of the manufacturers say the board can shrink very slightly when first fired due to the binder burning off, I've never noticed that or any expansion.
None of mine are fixed or glued in any way.
|
|
|
|
Post by martyn on Mar 21, 2023 11:23:36 GMT -8
It definitely moves around, my fire box pieces (12”x8”) looks perfect while cold but show numerous cracks that open up when heated.
In fact I have just built a new fire box one today, I have loosely screwed it together but used smaller pieces (8x6”) and drilled clearance holes in one side.
As a side note, the zircon coating I used is starting to peel away from the vermiculite….
|
|
|
|
Post by masonryrocketstove on Mar 21, 2023 11:52:00 GMT -8
It definitely moves around, my fire box pieces (12”x8”) looks perfect while cold but show numerous cracks that open up when heated. In fact I have just built a new fire box one today, I have loosely screwed it together but used smaller pieces (8x6”) and drilled clearance holes in one side. As a side note, the zircon coating I used is starting to peel away from the vermiculite…. Pretty sure that has more to do with the vermiculite movement with heat cycling and moisture absorption than it does with the zircon coating. |
|
|
|
Post by Vortex on Mar 22, 2023 10:11:13 GMT -8
My original outside test model vermiculite pieces cracked up after being fired damp. The only other pieces I've ever had crack are the 2 bits I put inside against the sides of the firebox walls to make the firebox a bit smaller 2 years ago. The bottom of them is held behind the V shaped floor sections, they both cracked vertically up the middle but are still fine otherwise. I wonder if there is a chemical reaction with the wood ash, as the afterburner vermiculite pieces have never cracked and they glow orange hot every time it's lit.
|
|
|
|
Post by martyn on Mar 22, 2023 10:55:05 GMT -8
Same with my afterburner pieces, they are fine too.
I have never been able to find out exactly what binders are used in vermiculite board, it seems the general opinion is just clay.
I use Skamol board and their glue states it is made from clay and waterglass, so perhaps there is some used in the board construction.
However when I cut it there is no smell of anything chemical, so perhaps it is just crushed vermiculite and clay.
|
|
|
|
Post by Karl L on Mar 22, 2023 12:03:06 GMT -8
I've used the same bits of vermiculite board for each new design of firebox for several years. They have been in contact with red hot ashes daily and they've never cracked.
I think it might be because I cut them small enough so the thermal stresses aren't so big. (I cut them to the same size as a standard fire brick split, and mount them vertically.)
|
|
|
|
Post by pianomark on Mar 22, 2023 15:44:51 GMT -8
Vortex,
From your desription--
"The only other pieces I've ever had crack are the 2 bits I put inside against the sides of the firebox walls to make the firebox a bit smaller 2 years ago. The bottom of them is held behind the V shaped floor sections, they both cracked vertically up the middle but are still fine otherwise."
--I'm guessing that uneven heating is causing the vertical cracks, since the lower part is shielded from sudden startup heat by the V-shaped pieces. Top part heats faster than the part behind the V sections, expanding til it cracks to relieve stress. Just a thought, for those still designing, it's probably best to avoid temp differences across larger pieces.
And thx for all your contributions to this forum over all these years! Even though I finished my stove several years ago, I still enjoy following all the latest experiments/developments.
|
|
|
|
Post by masonryrocketstove on Mar 22, 2023 21:47:05 GMT -8
Vermiculite is one of the most absorbent and expansive clays that exists. It doesn't have to be left outside or in the rain to absorb atmospheric humidity, even in your house. Every change in relative humidity will cause tiny volume expansion/shrinkage in vermiculite. That's not normally a problem with standard stove boards.. but when you coat an expansive clay like that with something that doesn't expand and shrink with moisture, you're gonna get peeling between layers.
|
|
