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Post by philip on Sept 29, 2012 16:11:21 GMT -8
Hi, I'm Philip, a Brit living in Japan. I have been closely watching this experiment from afar and am going to start preparing the parts to make my own batch rocket heater next week. Thank you to Canyon and Donkey for sowing the seed of the project, Peter for his meticulous experimentation and reporting of his findings, Max for posing the questions on a real case scenario and all other contributors.
I have some questions to ask Peter about the final conclusions he came to regarding the dimensions of the stove. I want to summarise your result findings before starting my build. Please forgive and correct any mistakes I have made.
The csa of the fire box is 3.5 times that of the riser. The firebox is at a width:height ratio of 2:3. The tunnel csa is 70% that of the riser, and is 23cm high by 5cm wide on your stove.
There are two systems which you have reported with good results. One system has a fully open front and no p-channel. This is the method which Max shows on his video.
The other system has a restricted front, 1:4 gap ratio of the riser csa (about 25cmsq in your system) with a p-channel. The p-channel is 6.7% csa of the riser and only as wide as the tunnel (5cm on your stove). The p-channel hangs down into the firebox so its opening lip is just above tunnel top. The distance between the ceiling of the firebox and the top of the tunnel you are calling the 'active channel'. The riser height, starting from the top of the tunnel, should be at least 6 times the length of the active channel. The riser should be uniform in shape and not switch from a square to a round, especially at the bottom end. There should be a gap between the firewood and the tunnel entrance.
I now have a few questions.
What are the main operational/ burn differences between the fully open front system and the restricted front system with the p-channel?
How did you calculate the firebox csa 3.5 times that of the riser, how important is this?
I don't know how long you made the firebox, is the length important and what is the ratio between firebox length and riser height?
Once again, thank you for sharing the detailed reporting of your stove.
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Post by satamax on Sept 29, 2012 17:31:44 GMT -8
Philip. If i may say. Canyon and Donkey have used a bigger firebox than 3.5 times the riser CSA, that's what i went for. As i had the "box" already made. And as i have understood, the P chanel hangs down a smidge in front of the box to heat riser opening. 8 to 12mm iirc. You can find this in small scale developement or the other rocket bell discution. Can't remember which. Mine won't be either closed front nor open. But "gridded" Might even have another grid for "brochettes"  |
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Post by peterberg on Sept 30, 2012 3:57:07 GMT -8
The csa of the fire box is 3.5 times that of the riser. The firebox is at a width:height ratio of 2:3. The tunnel csa is 70% that of the riser, and is 23cm high by 5cm wide on your stove. Hi Philip, welcome to the board. You've got the right picture, but not the exact data. Firebox opening is 4 times the riser csa, minus the sloped sides. That will come down to 3.6 times the riser. Ratio w:h is correct. Tunnel (or port) csa is 70 % of the riser, yes, but 23 by 5.5 cm. Slight difference there, because I've narrowed the space above the port down to 10 cm maximum. There are two systems which you have reported with good results. One system has a fully open front and no p-channel. This is the method which Max shows on his video. The other system has a restricted front, 1:4 gap ratio of the riser csa (about 25cmsq in your system) with a p-channel. The p-channel is 6.7% csa of the riser and only as wide as the tunnel (5cm on your stove). Right, but the data aren't exact again. The ratio I've been using is 1:7. Last p-channel is a duct of 20x60x2 mm, coming down to 5% riser csa. All these are quite close, but there's an important difference between the first and the last incarnation. The last one runs hotter and yield a higher efficiency rate. The open version do need a good chimney, otherwise it will smoke back now and then. The fuel need to be free from the firebox ceiling by as much as 5 cm. The remark Max is making about the p-channel overhang is important as well. This should be about as much as the gap. In my latest test bed that's 16 mm. In fact, I've cut out the back of the duct over 16 mm and let this be flush with the top of the port. What are the main operational/ burn differences between the fully open front system and the restricted front system with the p-channel? The restricted one is running hotter among other factors, as mentioned above. How did you calculate the firebox csa 3.5 times that of the riser, how important is this? I didn't calculate this as such, but it's been proved empirical. I've started with a firebox opening 2.5 times as much and increasing now and then. The ratio of 4 was a bit too much. I don't know how long you made the firebox, is the length important and what is the ratio between firebox length and riser height? No ratio at the moment, the firebox has been between 30 and 40 cm long. Maybe it could be longer, this hasn't been tried yet. The 40 cm looks like being the optimum, given the fact that the fuel is burning all at the same time at some point. Making the firebox longer implies it could hold more fuel, possibly causing an overload. At the moment I'm using the ratio between width, height and depth as 2:3:4. With excellent results, I have to add. These are all the latest numbers together. |
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Post by peterberg on Sept 30, 2012 12:46:56 GMT -8
Do you think i can put the heat riser in the water tank as far back as possible, i mean, have the heat riser against the wall of the water tank? That way, that would hold it. As for the joint between the firebox and water tank, i'm still unsure. I might start to cut that tank today! We'll see. Max, It can be done in a steel tank, certainly not in a ceramic chimney element of any kind. Beware of the back of the tank, this can get awfully hot because of the close proximity of the riser end, maybe up to glowing red. That's because of that blue/purple flame we could see shooting out of the riser. A flame of that power would eat the tank eventually, could be gone in two winters. |
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Post by philip on Sept 30, 2012 14:59:59 GMT -8
Hi Max and Peter
Thank you both for your clarification of all the dimensions of the fire box and riser elements. I hope that your summary can also help others who are thinking of building a batch fed system. Max, I can almost smell the Brochettes sizzling on the extra grid!
Now that I've got my head around the theory, time to get my hands dirty!
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jham
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Post by jham on Sept 30, 2012 20:13:54 GMT -8
Hello, my name is Jim, I live in southern Virginia and I have also been watching this discussion very carefully for a while. I enjoyed watching the process of refining these batch boxes so much that I had to try and build one myself. I have attempted to understand the principles and ratios and Philip's questions and Peter's clarification helped me see that I was on the right track. About two weeks ago I started to build a scaled down model with the c.s.a of a 4 inch stove pipe as the base unit. There is a soapstone quarry near here where the owners allow you to go through their scrap-heap for a small fee so I built the box out of soapstone with the heat riser out of galvanized 4 inch vent pipe. I insulated the riser with some ceramic fiber blanket that a potter friend of mine was getting rid of. Here are some pictures: This is the soapstone base on some left-over ceramic board from a pizza oven:  This is the heat riser (after being used a few times) made of sheet metal bolted to a formerly galvanized 4" vent pipe.  This is the soapstone back wall slipped over the sheet metal burn tunnel and the tension frame to hold the walls in place:  Here is the complete box with the insulated heat riser in place:  The process of getting a good burn took some observation on my part and then Peter's tips on how to do it really helped. My first burn was powerful but smoky with flame jetting out the top of the riser and a ram's horn pattern developing in the first 5 minutes. The power was a little scary, the bottom of the heat riser glowed red through the two inches of ceramic fiber blanket. I took the stove apart to move it and to see if the soapstone cracked and the fire had left very interesting patterns on the galvanized vent pipe. I took a few days off and put everything back together but this time I got lazy and did not seal connections with clay. Big mistake, very smoky burns, weak ram's horn and no glowing stove pipe. Took a few days off again and this morning (inspired by all the new info on the thread) I put everything together again (this time on a rolling shelf unit) sealed connections with clay and built a quick little door out of fiber-cement board with small low opening (about 4 sq. inches.) I also burned as Peter suggested, no cross pieces, pieces short enough to not block the burn tunnel. Enormous difference, over 4 burns 3 or 4 very brief intervals of smoke and in each instance a piece of wood was blocking the burn tunnel. It burned crazy-hot, in mid burn the whole heat riser was glowing like a pumpkin through the insulation. Here is a video down the heat riser early in the last burn: There is still no p-channel on this stove so that is one of my next jobs. The goal for this week is to find some free materials to extract the heat coming out of the heat riser. From Peter's experiments it seems like some barrels are the way to go. My ultimate goal is to understand the principles with this make-shift stove and then make a more lasting one out of a combination of soapstone and cast refractory. Thanks again for all the info and inspiration. |
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Post by peterberg on Oct 2, 2012 9:58:21 GMT -8
Hi Jim, welcome to the boards.
Thank you for the report, You've seen for yourself what a potent stove this dwarf can be. Moreover, your stove is a scaled down version of my own experiments, making it a kind of undersized dwarf. There ought to be a more catching paraphrase to describe this?
Are you able to sum up the dimensions of what you have? The riser is a 4" stove pipe, you've mentioned that, but what are the sizes of height, width and depth? And the length of the riser?
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JBW
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Post by JBW on Oct 2, 2012 14:33:09 GMT -8
And another thank you to Peter and Max. I've just about got my head wrapped around this enough to start a build. Jim, I'm possibly not too far from you, about 20 miles south of the Martinsville track. Could you let me know where the soapstone quarry is? I can think of a few uses for some.  |
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jham
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Post by jham on Oct 2, 2012 18:53:30 GMT -8
Hi Peter, Thanks for sharing your results so thoroughly and for the welcome. I hope you don't mind, I am going to go through the measurements of my stove and how I derived them from your Sketchup drawing so that any errors I am making come to light. The cheapest and most easily available pipe to make a heat riser was 4 inch galvanized gas appliance vent pipe. Because I had to cut soapstone for the box and did not have to rely on previously dimensioned manufactured materials, I converted 4 inches to millimeters (roughly 100) so I could work in metric. I got the cross-sectional area of the pipe in square millimeters as 7580. Because the ratio between heat riser c.s.a and fuel box opening on your model is 1:4 , I used (3x)(2x)=4(7580) to get a basic unit of length for the model of 72mm. So the box opening (interior dimension) is 144mm wide by 216mm high by 252mm deep or 2 units by 3 units by 3.5 units. From “measuring” your brick Sketchup I got an aproximate height of 2 units by ½ a unit for the burn tunnel so my burn tunnel dimensions are 144mm high by 36mm wide. The height of the heat riser is the only place where I really deviate from the ratios in your model. I got the height of your heat riser in your Sketchup model as 11 units or (for my model) 792mm from the bottom of the burn tunnel. I had a three foot (914mm) section of 4 inch pipe at my house (from making a version of your small scale j-tube/tripwire/p-channel stove) and I just went ahead and used that so I didn't have to shop. Besides the easy availability of the pipe, what you said on the small scale development thread about needing a longer heat riser on a small c.s.a stove to deal with laminar flow issues made sense to me. So that is the stove. It is interesting to me that you are now using 4 units for the depth of the firebox (if I have that correct) because my burns have been shorter than I would like and “develop” very fast. Last night the vent pipe heat riser was glowing orange in under 5 minutes from a cold start. It seems like more depth to the firebox will extend the burns and slow the beast down a little. When everything in the box catches all at once that is one of the circumstances that makes the stove smoke. I heat my house with a little Jotul stove and when I want it to crank, I load it with wood in a grid so there is plenty of air space between all pieces of wood. This is definitely not the way to load this little stove unless I want to impress my wife with the size of the torch shooting out of the heat riser and the copious clouds of smoke. From your comments and my own observation the way to load the stove is longitudinal (parallel to the long dimension) without much room between the pieces of wood. When I have done this with pieces of well seasoned oak that are 1 inch by 2 inch by ¾ the length of the firebox the stove burns clean and hot and the longest. I have not weighed a fuel charge yet. I am off work tomorrow and a friend is coming by and we are going to run the stove body inside various rusty tanks as bells and see what volumes it can push. We are also going to try and run the exhaust down vertically from the bottom of the bells and sideways to see if we could vent the stove down through the floor of his little hermit hut and sideways. I hope to weigh some fuel charges tomorrow and will report them. My application goal now is a stove for the above mentioned hut. I hope to generate some sketchup models/ideas with what we learn tomorrow. I would really like to post info and ideas for the hut stove to get feedback from the users on this board. Should I start a new thread to avoid hijacking this one or continue on this one? Also... hey JBW, I am up in Blacksburg and the quarry is a little south of Charlottesville, so would the drive be about 3 hours for you? Here is the link soapstoneventures.com/ The scrap heap is inconsistent but I have called ahead several times to ask if the pickings are good and when they have told me there is good stuff in there, they have been right. They charge 50 bucks for a as much as you think your car can safely hold and 100 for a pickup truckload. The pieces are usually small, I am excited if I get something bigger than 12" x 12". That said, it is beautiful stuff, cuts like a dream with a $70 wet saw, I have been making floors for pizza ovens out of it for a couple of years. Jim |
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Post by peterberg on Oct 3, 2012 8:03:48 GMT -8
Jim,
Could you please elaborate about the ratio formula? I am able to read it, it's algebra I would say. When I'd fully understand this, it's very easy to scale up or down. So I'm very anxious to learn how it's been done.
As far as I know, there isn't a practical limit of pushing heat into a bell. Keep in mind the bell does need to be a great deal wider compared to the riser csa, in order to act like a true bell construction.
About starting a new thread or not: I would think it's best to do that. Maybe you could copy the text and pictures about your stove to the new thread.
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Post by satamax on Oct 3, 2012 10:19:34 GMT -8
Hey guys! All right? Peter, i meant to ask you something, but can't remember what. Jim, there's a soapstone quary over here, within 50 miles or so. Abandoned for heaps of years. And the local geology club doesn't want to tell me where it is! This gets me mad!  Huh, peter, it came back to me. Any idea about how much mass i should add? It gonna be bench under the sofa i think. |
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Post by peterberg on Oct 3, 2012 12:18:57 GMT -8
Any idea about how much mass i should add? It gonna be bench under the sofa i think. As much as you dare to install in your apartment. Which wouldn't be a lot, not like 2000 or 3000 kilograms (6600 lbs), but 500 would make a start. |
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docbb
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Post by docbb on Oct 8, 2012 23:15:02 GMT -8
Peter,
do you see any mean to enhance the end of the burning ?
air channel ?
crate ?
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Post by satamax on Oct 16, 2012 13:16:32 GMT -8
Not fast, but i did a smidge of progress today!  |
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docbb
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Post by docbb on Oct 17, 2012 19:35:26 GMT -8
a new design ?
what are you building this time?
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