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Post by peterberg on Nov 26, 2015 8:58:13 GMT -8
Yes, shortening of the p-channel is no problem at all. The 8" batch box thingy in Montana also sports a short one because of a glass top of the firebox. No disadvantages that I could detect, even the Testo was very happy with that one. See this link.
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Post by exoplasmic on Nov 26, 2015 13:21:47 GMT -8
Thanks for the link. That was interesting. Then it's pretty much settled: I am going to shorten the p-channel and leave some more room on top of the fire box for heating a kettle or something. On another note: the bricks around my 2nd bell are not heating up as hot as my first bell bricks and ceiling. I think that the opening to the second bell is too close to the opening of the chimney, so the hot gases are being pulled up the chimney befor they have enough time in contact with the bricks. I have the chimney inside the second bell and it goes to about a foot from the floor. It's about 16 inches from the opening between the bells. I dry stacked some bricks in a wall between the opening to the 2nd bell and the chimney. So the bricks slow the exhaust which has to go around them before it goes out the chimney. These bricks get hot. I'll have to remove them to clean the 2nd bell and the opening to the 1st bell. I think I should have made the 2nd bell longer so the exhaust rises to the top of the bell befor it's sucked out the chimney. Here's a picture is what I'm talking about. www.dropbox.com/s/iufzna60jm8eabp/2015-11-07%2018.11.20.jpg?dl=0The 2nd bell is incorrectly labeled as 64 inches high. It is 54 inches high. So it's less than the 85% internal surface area than was originally planned. I need to calculate the true surface area of the bell system. Before the shortening of the second bell it was 8 m2 (not including floor area). However, 8 m2 does not include the area of the down draft channel that transfers gases from the top of the heat riser to the bottom of bell 1. Plus this down draft channel has 1 surface that is not covered by brick ( it is VERY hot to the touch).
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Post by satamax on Nov 26, 2015 13:35:49 GMT -8
Exoplasmic, what are the temps of your second bell?
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Post by exoplasmic on Nov 26, 2015 19:01:21 GMT -8
Exoplasmic, what are the temps of your second bell? I don't know temp. I need a temperature probe or something. I know that when I open the clean out door and touch the bricks I put in front of the chimney, those bricks feel hot, but not scalding hot. Maybe 200F.
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Post by exoplasmic on Nov 27, 2015 16:13:19 GMT -8
Peter and the Donkey Gang,
I want to thank you all for sharing your rocket stove plans, the time to respond to questions, your tips and encouragement. I couldn't be happier with the results. I love to watch the fire spew into the heat riser, knowing that the secondary air via Peter's p-channel enables a very robust burn in the heat riser. The bells and refractory and theory were all provided free. Wow. I feel so lucky. Thanks!
I think I've added one new twist to the many great designs on this website: a metal firebox as "skin", that surrounds the refractory firebox core. I'm albsolutly sure that the metal firebox shell lined with firebricks can and does connect to the heat riser and its support structure with no stress; or so minimal that the bricks don't move. That's because I cut the back off the metal stove and left room around the sides and top between the bricks and metal. The metal can expand and contract separately from the bricks. Where any brick touches metal, both are able to move independently. Maybe I got lucky too. But I won't know for certain until the "metal and brick experiment" is replicated independently.
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Post by peterberg on Nov 28, 2015 4:55:23 GMT -8
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Post by exoplasmic on Dec 4, 2015 8:17:42 GMT -8
I Modified the second bell.
It is no longer a bell. I put a wall 50 inches high about 8 inches in front of door from the first bell. The chimney still goes down to about 8 inches from the floor and is still right in front of the clean-out door. Now the bricks heat up nicely in the 2nd "bell". So it's technically a single bell with an up-down flue.
I made this change because of a poorly designed 2nd bell. The door from the first bell was too close to the chimney so a lot of hot air went out without heating the bricks. I should have made the bell longer, which would have required a shorter bell (in order to keep the 85% of 1st bell surface area rule). Now those bricks heat up nicely, capturing more heat.
I haven't calculated the total inside surface area (ISA). There's an 8 m2 rule (don't count floor). I'll post my ISA when I get a chance. That's sort of a squishy number for my particular system because: do you count the area of the down-draft channel after the heat-riser? Half of that channel (made of 2.25 inch firebrick) is exposed to ambient room air, and the other half is inside the first bell. This flue gets VERY hot. When I made my calculations for ISA I didn't include that area. I also made my 2nd bell at 75% of first bell (because I wanted it to be the same height of 1st bell). It was a complicated algebra equation setting height of bell1=bell2. SoI fudged a little, and got burned. Conclusion: 2nd bell too small. In hindsight I should have made the 2nd bell the proper size. It would have saved time and effort. As well as this next problem...
The the first time I started a fire after I put the brick wall in "bell" #2, I started with cold bricks. There was a strong down draft when I opened the clean-out door in front the chimney. So I eventually had to start a small, but hot, fire under the chimney to get the draft going in the right direction. It took a bigger fire than I thought; a cardboard-only fire burned too fast. It would draw air up the chimney for a bit, then the reverse happened. This resulted in a significant amount of smoke in the basement workshop! After building a proper wood fire, I also opened the sliding door to the outside for about 5 minutes to help the draft go in the right direction. That got the air really going up and out. But not before my kids called down asking if everything was alright. I said, "Oops! Just a normal back-draft, no worries. It's working now." "Ok"
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Post by exoplasmic on Dec 10, 2015 15:06:17 GMT -8
I have small cracks between two different types of firebrick on the downdraft channel (from top of heat riser to bottom of bell). I was given some hard firebricks that were for very high temps. I put regular hard firebrick in between these blocks of high-temp bricks. Where they meet a crack formed. I patched the crack with clay, which failed. Then with refractory mortar which failed. Now I have a specialty "Caulk" on it. Everything else works great. All casted parts are crack free. No leaks elsewhere.
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Post by daniel on Dec 10, 2015 23:55:48 GMT -8
Hi,
Cracks like that are difficult to repair especially because of the mechanical stress that appeared due to different densities of the different bricks, If the cracks are many and leaking I would come up with another layer of something over it, they are hard to repair and will look less appealing .
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Post by exoplasmic on Dec 18, 2015 15:28:14 GMT -8
Hi, Cracks like that are difficult to repair especially because of the mechanical stress that appeared due to different densities of the different bricks, If the cracks are many and leaking I would come up with another layer of something over it, they are hard to repair and will look less appealing . Thanks Daniel. I've had no leaks since I caulked it inside and out. I took off the cover above the heat riser to get at the inside seams. I put a small thin nail in both cracks to hold then open before I applied a 1/2 - 3/4 inch (2 cm) refractory putty. I've only let it cool down twice in the last 7 days. So every morning and evening I have a roaring fire going. Still, I have a feeling I may have to build a shell around the cracks if it happens again. Lesson: use one type of brick for all hot surfaces if they form a continuous layer. If you use two types of bricks with different compositions that result in varying expansion and contraction rates, then expect cracks. Build in a mechanism for containg the escaping gases.
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Post by daniel on Dec 18, 2015 15:44:05 GMT -8
To me the easiest way for an extra layer would be a cover of clay mixture perhaps with some fiber and pigment for color. I am thinking at the moment to give my heater a plaster of clay. If applied while hot and expanded there should be no cracks and it will have a nice smooth look of any colour, first layer with some fiber and straw and the second for finishing.......
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Post by daniel on Dec 18, 2015 15:46:08 GMT -8
I forgot, plastering lets you ad some other things like inserts etc. I'd like some Ammonites to show through, it is also cheap
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Post by exoplasmic on Dec 21, 2015 19:53:04 GMT -8
I like that idea about clay plaster, possibly with fiber. I could use straw as long as it doesn't have any air pockets. I don't want to insulate anything. I've seen (on a website) shredded straw used with clay. I think they had to whip the straw with a weed whacker or drill attachment. I seem to recall it was done in a bucket with small batches of straw.
Also, I wish I would have made my 1st and 2nd bells bigger. 8m squared inside surface area is conservative, I think. 9 m would be ideal for my set up.
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Post by daniel on Dec 23, 2015 10:54:57 GMT -8
Hi Exo,
I have seen that the first layer is used with chopped straw which will char but will help dissipate cracks, then the second layer with smoother clay. I have seen a heater in which the clay was covered with some natural oil and some pigment and polished smooth. I think they also used some animal dung in it for the finer fibers it contains. If the first layer is applied with the heater hot being expanded it should not crack the second coat. The chopped straw is not that much to insulate and charred it becomes carbon.
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Post by exoplasmic on Dec 23, 2015 17:40:21 GMT -8
I'm definitely going to plaster the heater, but not right now. I plan to do it when the heater is hot/warm and "expanded", so it minimizes cracks.
There are lots of free horse manure piles around my area. Manure is mixed with straw. Sometimes it's partially composted when I pick it up. Great for the garden vegetables (after composting for at least 1 year so urine doesn't kill the seeds). I don't have a feel for how much manure to mix with clay/lime/straw for good "cob", or specialized cob for un-insulated layer around my heater. I may just use manure for garden if I get some.
I have lots of clay that I bought. The clay in the earth from around here has a lot of gypsum in it. That kind of clay absorbs water and swells. It is not good for ceramics. It explodes about a month after firing, at 1900F. Actually it just crumbles. Every potter around here knows about this problem with the local clay.
I have 150 pounds of real "fire" clay in bags that I bought for making bricks. I ended up buying or getting free bricks, so I never used it. I want to experiment making insulated fire brick by mixing sawdust with clay+sand and firing at high temp. The sawdust burns off leaving air pockets in the brick (after firing at 2300F). Insulated fire brick is light and airy. Perfect for insulating a kiln, which is my next project. I want to make pots and sculptures eventually. I'm interested in reduction firing (starve the oxygen the last hour of firing). You get much prettier color glazes with a reduction kiln.
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