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Post by invention1 on Aug 26, 2018 5:39:09 GMT -8
Hmmm... In order to run well in an 8" version the pellet basket should be closer to the port, I'd think. It's all a matter of scale, in the original test bed it made a markedly difference having the initial small start-up fire close to the door or close to the port. The latter option yielded a much better burn overall, not to mention an earlier switch to smoke-free running. Explanation is simple: the earlier the flames reach through the port the soonest under pressure and turbulence is created. Yeah I was also thinking this same thing - move the pellet basket closer to the port. I've been studying pellet baskets just by googling the images of them. All commercial pellet baskets (A.K.A. burn pots) that I found are made of stainless (occasionally cast iron). There was one company that made a ceramic burn pot, but then started making a replacement out of cast iron. Apparently the ceramic version was a failure. This tells me that better minds than I have gone down the road of trying ceramics, and also that I'm probably on a fool's errand with the ceramics. However I'm going to soldier on with them for a while. I'm seeing some cracking, but they are mostly holding up pretty good because of the nichrome wire reinforcement. I do have one that has spalled off a piece, and you can see the nichrome reinforcing wires exposed. This really looks like a serious failure, and may indicate that the ceramic rods just aren't going to make it. I'm not giving up on them yet though. Now a standard pellet basket for a particular pellet stove (Whitfield Advantage) has dimensions 5.75" X 3.25" X 3". That is 56 cubic inches of volume, and pellets weigh 40 LB/cubic foot or .023 lb/cubic inch, so such a basket would hold about 1.3 lbs of pellets. Pellet stoves (almost all of them) use 3" flues, with about 7 square inches flue area. An 8 inch rocket stove has a riser area of 50 square inches, 50/7 is about seven times larger. My homemade basket has a volume of about 280 cubic inches, or about 5X the volume of a standard pellet stove. I'm holding about 6.4 lbs/2.9 kg of pellet fuel. The batch load of an 8" stove is 14.2 KG/31 LBs of cordwood. So the pellet basket holds far less wood, and probably will not work in any kind of batch mode, but will be in a continuous mode. That's OK, as long as combustion is right. I'm using 5X the fuel in a stove that is 7 times as large as a commercial 3" pellet stove. This could explain why the flame from the pellet burn seemed a bit small. But you are right simply moving the basket closer to the port would help quite a lot. Also, cordwood is really meant to be used in batch mode, fire it twice a day and forget it the rest of the time. You get a long tail of heat out of the stove for hours. Pellet stoves are meant to be fueled continuously, and so a slightly smaller amount of pellet fuel, applied automatically in batches every so often, would ultimately yield a far larger amount of heat than a stove fired with a batch of cordwood. Pellet stoves with a 3" flue are rated up to 36,000 BTUh/10.5KW, several times what we are saying a batch fired 8" stove loaded every 12 hours would produce (4.4 KW average). There are 284000 BTU/83KWH of energy in a bag of pellets, and at 36,000 BTUH/10KW you'd burn a bag every 8 hours, approximately. This logic argues that I would be fine with a smaller stove - 6", I can batch load it every once in a while with cordwood, but when I'm older and feebler (that's coming soon) I can just pour in pellets and let the thing run. I haven't decided one way or the other yet. My current wood stove has an 8" flue, I have no idea what it is rated but it does the job (at 40% efficiency). Pellets are mostly air. I punched holes in the bottom of a plastic bottle, blew through it, filled it with pellets, and blew through it again as a test. I could not tell the difference between an empty and full bottle of pellets. This crude test tells me that just getting some air to the bottom of the pellet pile is enough to get them going. A deep pile of pellets isn't going to smother itself if air can get under it. (A pile of pellets without air to the bottom or sides will just smoulder.) Pellet baskets have every imaginable arrangement of holes, in the sides, in the bottom, straight sides, and sloping sides. I'm beginning to realize something else about pellet basket design - as long as air can get to the pile under the flames, it's good. Doesn't have to come in through the bottom, and in fact some of these burn pots just have a few larger holes on the sides - 3-4 of them. My current pellet basket is arranged with the pellets all sitting on top of rods, over a chamber that contains primary air. Lots of air can get in. But many of these commercial baskets have the air coming in from the sides. No holes in the bottom. What if I did that? Say the bottom of the stove had a pellet ramp, made of firebrick. The back of the ramp is a grate made of rods on a removable frame. The pellets sit in this ramp or trench, on firebrick, but the air is coming from the back of the trench. The rods/grate are easily removable for cleaning, but mostly the pellets pile up on top of the firebrick ramp. This gets the rods farther out of the way of banging cordwood, less subject to damage. If the rods are stainless, they are exposed to primary air and theoretically cooler, they are not in the most intense heat. The most intense heat and metal-eating flame is at the front of the pile, which is surrounded by firebrick. Maybe a stainless basket would last a while in this position before it died. In many homemade pellet baskets, the makers report that the front stainless basket rods, the ones in the most intense flame and farthest from the air inlet are the ones that fail first. I'm thinking about making a quickie small scale 4" prototype of this idea and testing it out with a quick and dirty 4" riser. I can build that just with loose stacked firebrick splits, without elaborate masorny work and still have a working prototype. Also for cleaning - lift out the pellet grate and sweep all the ashes down the ramp into the primary air/ash pit chamber where they can be scooped out every once in a while.
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Post by invention1 on Aug 26, 2018 15:40:23 GMT -8
I made a quick-and-dirty 4" rocket stove out of dry stacked firebrick splits and some sawed off pieces of firebrick and did a bunch of pellet basket experiments today. I built that little firebrick ramp/trench I mentioned above, and was just about to make up a quick steel pellet basket for the rear of it, when I happened upon a piece of ceramic fiber board that was about the right size for my experiment. I'm experimenting with ceramics because I want to make a pellet backet out of refractory materials. Stainless will die, and several DIY pellet stove makers come back a year later showing how many stainless parts had failed in a season. Well ceramic fiber board is a refractory material par excellance, and one that won't shatter into pieces when touched. It is, however, soft and friable. On the second try I hit on the idea of drilling holes in the ceramic fiber, and hard-facing it with thinned refractory mortar (later I plan use thinned castable refractory for this, all you mud dauber types will use some kind of clay slip.) This worked pretty well once I had adjusted it. Holes are 1/2" diameter and drilled about 1" on center. The 4" stove meets the standard rocket stove specifications with some slight errors - there is a port, a 4" riser, a primary and secondary air inlet. I found although I had the right amount of area drilled into the back of the ceramic fiber "brick" in 1/2" holes, I needed a little more primary air to get it to burn right. This, I surmise, is because a small hole has less effective free area than one would think just from the geometry - more air resistance in several small holes than in one big opening of the same open area. I ended up with eighteen 1/2" holes, a theoretical free area of 3.5 square inches. A 4" rocket stove needs 2.19 square inches of primary air area, so this is greater than that number. However I still had to crack a little gap in the bricks to get the pellet pile to take off and burn intensely. I think that the primary air inlet should be the correct theoretical area, however the pellet basket free open area should be at least twice the primary air area because of obstructions and buildup of ash and klinker. The ceramic fiber piece should be installed so it can be removed, and ashes/klinker swept down into the ash pit frequently. The best pellet baskets self clear (see this video for a good one) but most pellet stove require daily cleaning. If this is not a big chore it is no big deal - get up in the morning, pull out the pellet basket and sweep stuff down into the pit. Get out the ash vacuum once a week. Another theory I have is that the pellet basket should be as wide as the flue/riser, or even as wide as the firebox. This is certainly true in all the DIY 4" pellet stove builds (there are dozens of these) one finds. Most use 4" square steel tubing and a 4" basket, and there is even a commercial version of these. My first experiment (above) had a pellet basket more like 6" wide in a 12" wide firebox, fire was too small to fill the firebox and it seemed a lot happier (better burn) if the firebox dimensions were brought down to 9" and 6" flue. So That seems to support his theory. For an 8" stove (11.5" firebox width theoretically) I'd want more than 8"to 11" wide pellet basket, ideally the width of the firebox. A lot of pellet baskets are 3"to 4" deep. IS this because of the physics of pellets and fire, or because that's just a practical depth? Maybe I will stay with a pellet basket about 4" deep. If the backside of the pellet basket for an 8" stove is 11.5" wide, the business area is 4" tall, and the basket is in a trench with a 45 degree slope on the far side, I'm thinking about an 11.5" X 4" area of ceramic fiber board studded with holes. Theoretical primary air inlet on an 8" stove is 10 sq. inches. That's about 51 holes worth of area. I can put these holes on 1" centers, getting me 44 of them in an 11X4" area (with some extra area ont he sides) so maybe I need the business end to be 5" tall. 5X11=55 holes, plus some open free area so that the pellet basket isn't the restriction in the system. I'll make sure there is more open area than this, but try to get a lot of it to end up going through the pellets. For instance the bottom of my pellet basket insert should be scolloped so that air runs underneath it. Working the geometry I am getting the cross sectional area of my brick-and-ceramic fiber pellet basket to be 14.7 sq in, volume assuming it isn't heaped up about 170 cu.in, holding about 4 lbs of pellets. Pics next post
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Post by invention1 on Aug 26, 2018 16:02:20 GMT -8
Here's a few photos: 4" dry stacked rocket stove, just before the lid goes on: Detail from the top of the pellet basket area, this time made of firebrick, later of ceramic fiber: 4" dry stacked test stove, side veiw, 8" prototype in the background. Piece of ceramic fiber, size of a firebrick split, holes drilled for air, later coated with thin refractory cement for a hard face. Backside of the ceramic fiber pellet basket in use. There is a bit of an air gap at the bottom, this could be expanded. It needed a crack at the top (made by jiggling some bricks) to burn well. But as you can see there is flame all the way down to the bottom of the pellet pile. In some other experimetns, the pellets burned from the top and were unburnt at the bottom, so this shows we are at least getting some air down there. Reinforcing the back (cool) side of this item with some steel angle iron would help it stay whole, as well as hard facing it. I imagine this part would be trapped loose, and pulled out for frequent cleaning as it won't self clean. Overall this was a great experiment and I learned a huge amount of useful info.
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Post by invention1 on Aug 30, 2018 4:27:13 GMT -8
Working on a bell. My plan is to have about half the bell be steel. The reasoning is, my house itself is relatively massive (built for solar, wood stove gets us through those sunless winter weeks common around here). I want to put heat into the house and warm it up relatively quickly, then use the mass of the stove to keep things going for a while. Also, with the pellet feeder, I can put in a relatively constant amount of heat, in which case the mass isn't as useful to store heat after the fire is out. I've argued myself into building the 6" version - although we will definitely batch load it with cordwood, my wife is pretty excited about bags of pellets vs heavy cordwood that requires stacking and splitting and cutting. She can and has injured herself messing about with cordwood, pretty regularly in fact, so she's off woodsplitting duty. She fears if I'm gimped up, more and more likely as I age, the woodstove isn't getting any fuel. She also really likes sitting around a hot stove in the winter. I'm going to keep in mind that pellets may be the main fuel, so it should be optimized for that use if there is a choice. I live in the middle of pellet-making country, and they are cheap here at rural farm stores. Just went on sale in fact, I ordered 2000 lbs. = 50 bags. I have no idea how much I'll use. So I examined my collection of old water heaters and tanks, picked out the one with the largest diameter (about 19") and stripped off the coverings. Used to be, water heaters were insulated with fiberglass, peel that off you have a nice steel tank. Nowadays, they are insulated with polyurethane foam, which sticks to the tank like glue. I was able to manually peel, pry, and grind off the foam until I have a bare steel tank. Inside is a layer of corrosion protection, called "Glas-lined" but actually looks like polyethylene. This will burn off, so the first experiement needs to be outside. That's OK, I'm starting the third outdoor prototype now, trying to work out the final details before tearing it down and building it inside for real. Water Heater tank under construction: Grind 'er down: Bell ready to go. I welded up all the plumbing fittings, and welded two handles on the sides. Once the coatings are thoroughly cooked off, I plan to cover it with stove black. ... aaaand the first prototype is gone!
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Post by invention1 on Aug 30, 2018 5:23:18 GMT -8
What size of bricks for the firebox?
Locally I can source two brick sizes - "splits" which are 4.5"X9"x1.25" or full firebricks which are 4.5"X9"X2.5".
Although I've done masonry on and off for years, I am no skilled mason. I'm pretty much stretching my skills to the limit with this project. My first prototype, I built the core out of splits standing up on edge, which tended to be a wobbly and teetering wall. Although It held together for a few firings, I wouldn't have trusted it. I tended to lay a few brick, then dislodge the ones below that hadn't fully set yet. Finally I found that two layers of splits were required for me to actually pull it off. I had to lay them slowly over days, waiting for the layer below to cure a bit. It wasn't pretty.
Full size firebricks would be easier to lay one atop the other, more stable, not tend to wobble about while another brick is laid on top. However, that's twice as much mass inside the firebox, inside the layer of ceramic fiber insulation that will surround it.
I'm really thinking I could manage the fullsize brick much better. Would you recommend building the firebox out of a thinner (1.25") layer of split firebrick for low mass, or thicker layer of fullsize firebrick for longevity and less teetering as I add more on top?
Or does it make little difference?
[EDIT] well I am building it mostly of fullsize firebrick, as that helps me match my meager skill to the job. The floor and roof will be built of "split" firebrick.
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Post by invention1 on Sept 8, 2018 6:38:28 GMT -8
Well, I'm proceeding mostly using the fullsize firebrick, as I think that will make a more stable structure while the mortar is still wet. Those thin firebricks look pretty good, but balancing them on edge is beyond my meager skill as a mason. Here are a few progress photos:
Most of the metal parts. "Electrical disconnect" door has some low-temperature handles welded on, now has a gasket inside, and soon will have ceramic fiber insulation to keep the door cooler, cuz it gets really hot:
Here's a dry-stack mockup of the installation of the ash door:
Detail of primary air and P-Channel. Both will have a shutoff door (Just a plate that can flop down over a matching ground surface). This might be important if I want to shut a fire down, and in the summer to prevent smelly chimney-stench from backdrafting into my house whenever I use the exhaust fans. I'm kinda proud of this welded hinge, built from scratch out of tubing and a bolt.
Dry stack mockup of the brickwork, view from the door end. My "electrical disconnect" door plugs into the metal frame you can see through. There's some complex brickwork around the pellet basket area. Half the pellet basket is made of firebrick, the business end will be a removable piece (I'll photo that next post). The pellet basket empties into the ash pit, cleaning (which I expect will be once a day at least) will involve just sweeping stuff down the chute into the ash drawer. You can see the pellet feeder through this view, this arrangement is totally not gonna work, too much metal in the firebox. I'm going to completely reconfigure the pellet feeder as it is not working smoothly. It will probably just dump pellets from one side, which will result in an uneven pile but little metal int he firebox.
Another view of the dry stack mockup from the Riser/port end. The primary air and P-Channel come out the side of the stove. Usually P-Channels come out the door end, however the pellet basket is in the way and prevents this. I suppose I could do an overhead one but this is the direction I'm going. These openings will be more constructible at the side, with the sacrifice of possibly less preheating of secondary air.
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Post by peterberg on Sept 8, 2018 7:51:26 GMT -8
I think I spotted an error. The floor channel's parts shouldn't be the same csa. It's different from the overhead p-channel in that the horizontal part should be 1.5 to 2 times the csa as compared to the vertical part. At the time I tested this quite extensively and this ratio yielded the best results. Probably because of the vertical narrower part is acting as a temporarily restriction, ramping up air speed. Above that, the net result is a better cooled vertical part.
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Post by invention1 on Sept 8, 2018 15:31:54 GMT -8
I think I spotted an error. The floor channel's parts shouldn't be the same csa. It's different from the overhead p-channel in that the horizontal part should be 1.5 to 2 times the csa as compared to the vertical part. At the time I tested this quite extensively and this ratio yielded the best results. Probably because of the vertical narrower part is acting as a temporarily restriction, ramping up air speed. Above that, the net result is a better cooled vertical part. Aha! Yes, I used the vertical part out of some old 2"X2" tubing (1.75X1.75 inside) I had laying about, figuring once the air was restricted by the horizontal part, vertical size wouldn't matter. My part will result in the secondary air slowing down, but you indicate it should speed up! More speed = more turbulence. Fortunately I haven't built anything with this yet, I'll cut it up and reweld it. A 6" stove should P-Channel theoretically have a width of about 2.17", height of 0.65", CSA 1.41 sq in. My main horizontal tube is from some stock tubing that is 1.77"X 0.76" inside, 1.34 sq in CSA or 5% less for the actual part vs. ideal. [Edit] Reading more of Peters posts, I find I understood this backwards. The vertical part should be per the spreadsheet, 5% of riser CSA, in my case 1.77"X 0.76" inside is 4.76%, pretty close. The horizontal part should be 50% to 100% bigger. The horizontal part should be made of 1.5"x1.5" inside tubing (2.25sq inch CSA)
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Post by invention1 on Sept 8, 2018 15:38:16 GMT -8
Pellet Basket Photos:
Pellet basket weldment
Full of reinforced ceramic rods. Welded spacers hold them apart.
Added Ceramic Fiber to trap the ceramic rods, glued in with refractory mortar, and hardfaced with same. The rods are actually loosely trapped, they are not surrounded by mortar, so that they can expand and contract freely. The metal parts of the pellet basket weldment are not exposed to flame. We'll see if any of this can stand up.
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Post by invention1 on Sept 8, 2018 15:51:10 GMT -8
Well, as soon as I finished the pellet basket, some parts came in that may improve it. I've been using ceramic rods from Ozark Technical Ceramics. They are fragile, but hold up better when reinfoced with nichrome wire. They are 3/8"/9.5mm diameter, 8.5"/216mm long. If you whack an unreinforced one against your fingers, it will break. There is a commercial cooking grille that uses ceramic rods for some kinda radiant heat. These grills are very popular and there are a lot of people selling replacement rods. I ordered some of them. When these rods arrived, they are obviously a lot more durable. They are 9.5"/241mm long, 1/2"/12mm diameter, but have a 0.30"/7.6mm hole in them, just right to let them protect a steel rod. They pass the finger-whacking tests, I haven't tested them harder lest I break too many. Ceramics like this can be machined easily with woodworking tools, I may cut them to length and make a second pellet basket with them. They are bigger, meaning there will be less free area, but it is the only restriction, so hopefully it will work. Pellets need 3/16' gap in the basket to be retained but let ash fall through. Steel Rods would help them be better trapped by the steel weldment. This steel rods would help the ceramics be more rigid, and the better strength on these should hold up longer. Weldment can still be protected by ceramic materials against corrosion. 1/4" dia steel rods fit, but 9/32" or 7mm would fit better. That's an odd size, but there are some suppliers on ebay that can sell such rods in stainless steel, perfect for this purpose.
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Post by invention1 on Sept 8, 2018 16:03:38 GMT -8
5 minute Riser: Took me about an hour to make, I must be one of the slow kids. Using 8" galvanized sheet metal duct as the form, lined it with 1" ceramic blanket insulation. Cut out a hole for the port. Right now, my 5 minute riser is a bit over 48" long. Peter's spreadsheet says 43.4" I can cut it down later.
Blanket was sprayed with rigidizer. After I decided I could not see the clear stuff, I added some red food coloring so it would show up where I sprayed it.
Place a wood block cut to size in the port to keep it the right dimensions (about 2.25" wide) while the rigidizer sets up. Shoot a few sheet metal screws to hold it all together.
Insulation is pink, right? Top of the riser, with some red-dyed rigidizer sprayed on the insides. Measures about 6" diameter inside, with a few bumps.
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Post by invention1 on Sept 11, 2018 14:14:24 GMT -8
Phase III final prototype coming along well. Have the pellet basket worked out so the flame is either against ceramic or firebrick. No metal is exposed to flame. Firebox is moved back toward the port as earlier experiments showed it needed shorter distances here. Dry stacked everything, took it apart then put it together with refractory mortar and precise cuts.
Bottom of firebox plus port is complete.
Another view of the pellet basket area
Pellet basket easily removable, slides into place, easily replaceable if something doesn't work.
Ash drawer helps with cleanout, pellet basket sits over the top of ash drawer. Sheet metal Liner in ash drawer replaceable - there will be a few coals in here, eventually it will burn out, so can easily be replaced.
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Post by invention1 on Sept 12, 2018 9:57:19 GMT -8
Here is an innovation: a replaceable P-Tube. OK, people say the P-tube doesn't burn out in 3 years. How about 30? My old wood stove is 40 years old. There is a steel plate, a "smoke shelf" in this old stove that has sagged over the years even with the relatively cool fires in a normal woodstove. Will these metal P-Tubes really last decades in direct flame? Well, I don't want to find out, and then have to tear the hole stove down to replace it. So I'm designing the overhead P-Tube to be easily replaceable with no tools. There is a "Reciever hitch" at the door end of the stove, tapered a bit to lead the end of the P-Tube in, and will have a hole for a 3/8" stainless bolt to trap it there. At the port end, there will be a pin that rides over the top of the king brick above the port. Presuably the reciever hitch is in a part of the stove where metal survives, else we'd be replacing our doors all the time. Not much direct flame up here. Reciever hitch isn't replacable easily, but it is still easier to fix than a P-Channel embedded in masonry full length. I could see replacing this part without destroying the stove. The replaceable part of the P-Tube is fully inside the firebox. It should gain more heat that way, and create hotter air for the secondary combustion. Pics will make this clearer.
"Reciever Hitch" at the Door end of the stove (prototype)
Replacable part of the P-Tube. Note that I made a mistake and made the vertical (the part with the angle) too long, that will be fixed.
Installing the replaceable P-Tube is simple.
Replaceable P-Tube in place. As noted, the vertical part is too long, and there will be some sort of pin that engages the king brick over the port.
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Post by peterberg on Sept 12, 2018 11:23:21 GMT -8
I see what you are trying to achieve. True, it is a shortcoming of the overhead p-channel that it isn't easily replacable. But where's the opening at the end of the vertical part? You did a 45º angle there which isn't in the drawings. The opening should face the port with a slight overhang, that's all. Clearly depicted as such at the batchrocket site. Is there a reason why you want to do it differently?
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Post by invention1 on Sept 14, 2018 6:42:58 GMT -8
Not to worry, Peter, the detail you are mentioning doesn't show clearly in my photos, but it is there. And it does face the port! Here's a couple more photos. I am SO glad you are watching and catching my errors! This pic on batchrocket.eu seems to show just an opening, in an overhead channel, with a cutout on the port side. You are right, Peter, my little 45 cap doesn't look like this. This pic and this pic on batchrocket.eu seems to show an opening with a little 45 degree kick at the end, which is what I was imitating. These are floor channel details, I'm flipping them over to be on the ceiling. Do let me know if I've got this wrong. (Again. ) To my eye, the ends of the floor channels shown, with that 45 degree kick on it, should stir up more turbulence than a rectangular-ish opening. In the original design, floor channels are getting air from the same opening as primary air, behind the "transom". Overhead P-Channels can't really do this, and must vent outside the stove, so that's how this one works. Other than that, I'm imitating the floor channel details, just flipping them over, putting the secondary air opening about halfway up the port. Your original overhead channel, I believe, put its air just at the top of the port opening. I could certainly do it this way, but it seems like putting secondary air right at the middle of the port would be best. Is there a reason that the overhead P-Channel puts air at the top of the port, but floor channels put it at the middle? This system has an advantage of probably heating the secondary air more than a masonry embedded P-tube. PS It looks like this guy Radek was experimenting with replacable P-Channels in 2015, so this isn't new. His design is a helluva lot simpler too. Photos:
Detail of the port-end of the overhead P-Tube. This opening faces the port.
Assembled "replaceable" overhead P-Tube
P-Tube removed showing "Reciever hitch". opening to large inlet tube on both sides, mostly for structural integrity, not needed to be this large for airflow.
Another view, disassembled
Another view, assembled. I've tested this inside my dry-stacked prototype, and it is quite easy to assemble, one-handed, inside a cramped firebox.
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