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Post by invention1 on Oct 1, 2018 19:52:03 GMT -8
I learned two things about pellet feeders: A 2" square or round tube will feed pellets. 45 degrees seems to work OK, although steeper is better. [EDIT] Later I learned that a larger feeder tube works better, and pellets become sticky if they get hot. If there is any kind of bend or turn, they will clog after the first fire. Pellets will not travel, gravity fed, around any kind of bend. I built a pellet feeder with a 5 gallon bucket and a steel tube, with a little bend in the end of maybe 60 degrees. pellets won't feed through the angle in the tube. They really need a very straight tube. So I built another feeder out of a 5 gallon bucket and a straight steel tube about 14" long. Set this up so it feeds into the left side of the pellet feeder, an inch or so above the top of the pellet basket with the center of the tube about where I want the top of the pellet pile to be. This worked quite well. Then I cemented it into the stove, waited a couple of days, and fired it up. It filled the pellet basket nicely, covering up all the grates so that all the air needed to pass through the pellets, no short circuiting around. So with a single pile of pellets,no feeder, the fire would last about 20 minutes then would start coaling. Lots of white smoke after that. But with a feeder, it was different. This fire just kept going and going. After an hour there were still flames coming out the riser, and pellets still falling into the firebox. Smoke absolutely clear the whole time. Just a perfect burn. I created the ersatz dry-stacked pellet firebox mentioned above, left about a one inch gap in it for the port, and have no secondary air in the pellet firebox yet. Only secondary air in the system is in the conventional RMH firebox. I started up the pellets with a handful of alcohol-soaked pellets in a 2" section of pasteboard egg carton. Shut the removable brick (see previous diagram). 8 minutes later there was a nice ram's horn in the riser. 16 minutes later the temperature at the center of the top of the riser was 629 C. at 24 minutes I heard the first new pellets falling into the firebox from the pellet feeder. The fire would increase somewhat a bit after each new addition of pellets could be heard. Flames and glowing hot gases exiting top of riser. The burn was still as energetic after 30 minutes, 40 minutes, one hour. It just kept going at a constant roar. It had reached a steady state, with flames exiting the riser, absolutely clear smoke even when pellets fall, everything just at a constant temperature humming along. I finally shut it down after an hour by blocking the flow of pellets and shutting off the primary air supply. I expect the fire will be out there coaling for another hour or more. Temperatures inside the overhead P-channel, near the outlet reached 273 C. Temperature at the pellet feeder tube where it enters the firebox measured over 400C, maybe 428 (it was changing rapidly due to open door, not very accurate). I'm concerned that the feeder tube may burn out. If the end starts eroding, this will affect the pattern of pellet distribution and perhaps mess up the burn. Right outside the firebox, the pellet feeder tube was warm to the touch. No concerns about pellet tube overheating outside the stove. Always fresh unburnt pellets at the feeder tube, no concerns about burnback. Some coals in the ash drawer, but the gasket (just a foam gasket I had laying around) wasn't excessively hot. Might do well to put a real stove gasket here, but the rubber gasket seals really well, and I don't want any air leaks in the ash drawer, would make it into a burn chamber. Lessons here are: A simple gravity pellet feeder with a straight 2" pipe works really well. Control would be simple - a guillotine grate. Set it to half or zero. Also control by how many pellets you put in the pellet feeder hopper. So far no problems with burnback. Pellet hopper is airtight. Pellets can create a constant, long term fire that just goes and goes. My earlier theory that the pellets would be a smaller fire - that theory's out. Pellets can be an awesomely hot fire that will reach a steady state as hot as cordwood and burn far longer. I have no doubts that the pellet side of this stove will adequately heat the bell, in fact maybe it needs a larger one. I have no idea how to size the bell for this, using any physics, so for right now I'm sticking with my idea of building the bell and deadend bench sized for a 6" system and hope it is good enough. The small firebox around the pellet basket is really successful. I had the idea that I'd need secondary air in there. Now I am thinking that the secondary air from the conventional RMH (currently an overhead P-channel) is plenty sufficient. The small firebox is needed to get the pellets up to temperature, and create a fast, roaring fire. without this little firebox, the pellet fire in the big open firebox is lazy and cool. This was the best pellet burn yet. Came up to temperature fast, reached a steady state and just kept on burning, pellets feeding in by gravity whenever needed. I think I have the bugs worked out of this thing.
This is the pellet feeder that worked, simple. A pellet hopper and a 2" square pipe.
Another view
Pellet feeder about to be cemented in place. Bottom just above the top of the pellet basket, center of the tube about where the top of the pellet pile should be.
Pellet firebox just before the burn. Pile is higher on one side, but still covers the pellet grate pretty well. This is how the pellets naturally fell into the firebox, I didn't stir or encourage. Just dumped a bag into the hopper.
Fire reached this happy steady state and just held there for an hour. Either actual flames or red hot gases were exiting the riser for the last half hour.
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Post by invention1 on Oct 7, 2018 15:23:21 GMT -8
It's raining, so I'm going to skip the test burn I was planning today (prototype stove is still outside) Since it is raining, I made an improved pellet basket. The old pellet basket, with the nichrome reinforced ceramic rods, has held up surprisingly well, however every ceramic rod has cracked in the middle. They are all still protecting the metal, however they are a bit floppy now and some pellets fall through. But I've had several successful burns with the rods in this condition, so the nichrome is still holding up fine. Some pellets fall through, but the spaces between them weren't really very precise to begin with. To improve it, I created a steel frame, and obtained 9.5" x 1/2" OD x 0.300" ID ceramic rods from a fancy barbecue grille (DCS brand 245398). As I've mentioned these are widely available, cheap, and quite durable compared to the other rods I used. I special ordered 9/32" steel rods (7MM also works well) to fit snug but not tight inside the ceramics. The pellet basket is designed so there is very little steel up next to the burning pellets. Most of the strength of the frame is way back in the primary air stream where it should stay cool. The rods are simply welded on to the frame (not replacable - I decided that was just making things complicated) and were spaced precisely with a piece of steel exactly 0.185" thick. Pellets are 0.250" diameter, so they won't fall through but ash will. A crossbar of ceramic protected steel protects the ends, which would otherwise be exposed. I believe that this should be a pellet basket that won't burn out, because essentially no steel is exposed to flame, and what is exposed has parts well out of the heat. Photos tell the tale better than text:
Steel spacer 0.185" thick keeps the rods lined up:
Pellet Basket, from the hot or fire side. Welds at the end protected with ceramic rods.
Detail of corner. Most of the steel is well away from the hottest areas.
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serg247
Junior Member
The mountain can not be conquered, it can allow it to ascend...
Posts: 111
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Post by serg247 on Oct 7, 2018 21:48:07 GMT -8
One option is to make a grid of pipes in which to preheat the secondary air.
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Post by invention1 on Oct 8, 2018 17:33:38 GMT -8
Added a bell, made of an old hot water heater tank: The green Menards bucket is the quick-and-dirty pellet hopper for now. Other crap in the background includes an odd solar panel, more water heaters and experimental wood stoves which you should ignore.
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Post by invention1 on Oct 8, 2018 17:58:58 GMT -8
Several disadvantages of a water heater bell are becoming apparent: They are lined with plastic, and I could smell that cooking off during the burn. The outside has some kind of organic coating, glue or something, from the foam liner. This didn't come off with a wire brush on a grinder, and doesn't really come off when heated and brushed either. I fear this coating will slowly burn off in my house, poisoning us. It is likely to be polyisocyanurate foam, which breaks down into cyanide compounds as it burns. That is really bad. The water heater tank might get nixed if I can't clean it down to bare steel. Added the new pellet grate pictured above. I think it is taller than the previous one, and affects pellet flow by cutting down on the pellet heap, leaving most of it on one side. Overhead pellet feeding is definitely better, and works like a champ in most other DIY stoves. Now I'm wondering about the over-and-under firebox idea (wood firebox on top, pellets below) and thinking again about the side by side firebox theory. Pellets can feed straight down into the pellet basket. A sidewinder port would take the fire into the main (cordwood) firebox for additional mixing with secondary air. Right now that's just an idea. Did a burn today with pellets. Started at 5:19 PM, burn was still going at 8 PM when I shut off the pellet supply. Once lit, this pellet burner will go continuously until the pellet fuel runs out. That's a good point. Much of the run had a lazy, tepid fire with thick white smoke. That's bad. The Bell has definitely cut down on the draft, negatively affecting the burn. It was rocketing pretty well without the bell on it. The burn improved (smoke more clear, flames roaring and filling the firebox) twice when I stirred the pellets, smoothing them out with a poker until they covered the pellet grate. This is telling me that I don't have enough fuel for the airflow and firebox I am using. Spreading the pellets out and making the pellet pile deeper improved the burn. Peak temperatures on the bell: 215C on the very top of the steel bell, right above the riser. Bottom of steel bell up to 69C. Bottom of masonry bell (at the floor) 29C on a 25C ambient day. Top of chimney (this is the conventional chimney, not the riser which is now inaccessible) 35-45C. Early in the burn there was creosote running down the chimney pipe, a sure sign of incomplete combustion and cool temperatures. Dangerous too. This machine isn't ready to come inside the house if it is going to have creosote running out of every crack in the chimney. So on the plus side, I proved that the pellet feeder can go continuously, forever until the pellets run out. Also showed that more fuel is better in this configuration - the 6" stove, firebox volume, riser diameter, all really want more intense fire from the fuel load than i"v been providing. When I get enough fuel in the basket, it roars and the smoke clears up. But mostly it was a pitiful, tepid and lazy fire that wasn't up to standards, with a bell that could pump out toxic fumes unless I get it cleaner. And creosote, because of poor combustion. Mostly bad news, in need of a lot of improvement.
Lazy flame coming out of pellet firebox, reaching the cordwood port, but just barely. This was about 15 minutes into the burn.
Grey smoke was about as good as it got until I stirred the pellets and got the thing roaring. I've seen it absolutely clear on pellets.
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Post by peterberg on Oct 8, 2018 19:11:33 GMT -8
I'd expect lengthening the exhaust pipe to well above the bell would help.
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Post by Dan (Upstate NY, USA) on Oct 9, 2018 16:54:18 GMT -8
What's your top gap on the bell??? Do you have a constriction somewhere???
How high is your exhaust pipe above the top of the riser???
Seems weird that it runs well without the bell then bad without. Points to a bell constriction rather than a firebox problem....
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Post by drooster on Oct 10, 2018 13:19:23 GMT -8
The outside of your steel bell/barrel will clean up great with a wire-cup-brush or flap-disc on an angle grinder. Is there a significant gap between your pellet basket and the port - allowing the flames to subside/cool? Is there a way to prevent your pellets from blocking all of the primary air? (You claim the feeble flame is lack of fuel, but I think [based on zero observation] it is lack of primary air.)
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Post by coastalrocketeer on Oct 14, 2018 14:36:46 GMT -8
As Wolf suggested I would look for constrictions with the bell in place
Top gap, and ESPECIALLY the manifold area where cooling/falling gasses transition to your chimney... This is often a place where free flow of gasses becomes constricted. 1.5x system CSA is reccommended.
The other would be having a chimney that goes from your manifold a good bit higher than the top of your bell, preferably insulated. (Relatively inexpensive rock-wool should work in this area.)
Can't see how you transitioned to your chimney, so share some pics and fhe other experimenters and experts may be able to comment.
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Post by invention1 on Oct 17, 2018 19:30:53 GMT -8
The outside of your steel bell/barrel will clean up great with a wire-cup-brush or flap-disc on an angle grinder. Is there a significant gap between your pellet basket and the port - allowing the flames to subside/cool? Is there a way to prevent your pellets from blocking all of the primary air? (You claim the feeble flame is lack of fuel, but I think [based on zero observation] it is lack of primary air.) As Wolf suggested I would look for constrictions with the bell in place Top gap, and ESPECIALLY the manifold area where cooling/falling gasses transition to your chimney... This is often a place where free flow of gasses becomes constricted. 1.5x system CSA is reccommended. The other would be having a chimney that goes from your manifold a good bit higher than the top of your bell, preferably insulated. (Relatively inexpensive rock-wool should work in this area.) Can't see how you transitioned to your chimney, so share some pics and fhe other experimenters and experts may be able to comment. Thanks guys, great points: First, check out this video, then howl about how I'm cheating: 1. The outside of your steel bell/barrel will clean up great with a wire-cup-brush Been there, done that, now it's gonna have to be something more aggressive, I'll ditch the 4" angle grinder and get out the 7". 80 grit sanding disk, the one I used to remove bark from trees. 2. Is there a significant gap between your pellet basket and the port - allowing the flames to subside/cool?
Is there a way to prevent your pellets from blocking all of the primary air?
(You claim the feeble flame is lack of fuel, but I think [based on zero observation] it is lack of primary air.)I've proved primary air is a problem. There is a port near the pellet basket. I'm actually going to have a port for the pellets (because they need a completely different size firebox) and another port for the wood. This is going to be a combo of two different size fireboxes, probably with a common riser but that remains to be seen. Yes, there is a port near the pellets. 3. As Wolf suggested I would look for constrictions with the bell in place Top gap, and ESPECIALLY the manifold area where cooling/falling gasses transition to your chimney.
There probably is a bit of a constriction right at the bottom of the 6" chimney. It is probably 6" X 5" at the manifold, and that's a problem. The rest of the bell is quite wide open, should be about 3X the diameter of the system. Top of bell is at least 36" above the top of the riser. Riser is 6" diameter, bell is about 20" diameter. I've stuck a camera in there and verified there's nothing wrong with the riser, I didn't knock it over placing the bell or something. So yes, the bell has a restriction at the manifold. So I was able to make a really good burn today
By cheating, see video link above. I added 14 ft of stovepipe (still bare uninsulated). Draw was better, but still some white smoke. I noticed that the system seemed to burn more vigorously with the door open, however never would burn clean. It starts pretty slow, and smokes out the P-Channel and every crack in the brickwork (which has been damaged by removal of a brick, all of it is slightly cracked now) so it is definitely going to need a bypass for starting. The steel bell just sucks out every BTU of heat and when it is cold, there isn't any heat to create draw. Once it got going there was a draw, the P-channel stopped smoking, but it never worked right. White smoke, pretty heavy. After a while on a whim I decided to go get a heat gun (industrial blow dryer), set it on no heat and stuck in in the primary air inlet. A turbocharger of sorts. Within 45 seconds the smoke was absolutely clear.
In the video you can see the heat gun, acting like a fan, the fire burning briskly (and the door smoking badly) and then at the top of the riser, the exhaust is totally, completely clear. Nothing but heat waves coming out the top. I left the turbo charger in the primary air inlet for a few minutes, then pulled it out. The stove remained burning perfectly with clear smoke for another 15 minutes or so, then started to put out white smoke again. I stirred the fire, which hadn't settled down inside so some logs were up in the air a bit, then it started burning better, but needed the turbocharger to really get running again. Once again after ten minutes I could turn the turbocharger off, and the system would run like than for ten or fifteen minutes. The system definitely ran better with 14 ft of stovepipe vs. 6 feet previously. The system definitely ran better with the door open or the ash drawer cracked open, acting like it wants more primary air. But without the turbocharger, it would not ever run right. White smoke. With the turbo, it was able to run perfectly. Invisible smoke. (You claim the feeble flame is lack of fuel, but I think [based on zero observation] it is lack of primary air.) Well both, actually. There is definitely a lack of primary air. But also, in one of the pellet runs, the pellet basket was not full, letting a lot of primary air bypass the fuel, and just cool off the whole system. Without sufficient fuel, covering the entire surface of the pellet basket, it just has no chance of working right. I've since adjusted the pellet feed system so that the pellet basket gets filled up completely. Is there a way to prevent your pellets from blocking all of the primary air? Pellets don't block airflow significantly. In fact in most pellet stoves, all of the primary air passes directly through the pellets. Without this, pellets do not burn, they smoulder. In some experiments earlier, blowing through piles of pellets in tubes, I found no perceptible difference in resistance by my crude measurements (breath) between an open tube and a tube full of pellets. Pellets are very porous. No, I am not proposing to add a fan (but I would be OK with that if that's what it takes). The turbocharger is a hack to diagnose symptoms. All this is pointing to lack of draw and lack of primary air. The primary air is sized right, but it is a long tube and the air must snake around a bit, plus go through the pellet basket. That is more resistance than a simple opening. Pellets probably do restrict primary air some, causing poor performance. I'd be OK needing a fan to run it on pellets, most pellet stoves need that. A small plastic computer fan would probably do it, there's no heat down there. But first I'll keep trying to solve the draw/primary airflow problem. I'll probably knock out some bricks to expand that manifold. An insulated chimney would sure help. When I measured it before the chimney was about 45C, I didn't measure it this time, but I could touch it without getting burned. Almost any insulation would work at 45C. I have proved that this stove can run perfectly with the bell in place.
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Post by peterberg on Oct 18, 2018 0:04:02 GMT -8
Have you checked the moisture content of the fuel? The way to do that: split a bigger piece and measure the fresh split surface. When moisture is more than 15% the burn will be inevitably very sluggish. Judging the video of your cheat (howl!) without knowing anything I'd think your fuel is too wet, lots of smoke inside the firebox.
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Post by coastalrocketeer on Oct 18, 2018 13:13:24 GMT -8
Another way to test the moisture content of the wood is to take a smaller split, weigh it, and bake it at 250F until it no longer loses weight, then compare before and after weights to know how much water was in it... could take a number of hours of baking (even a day or two) before the weight levels off, and you will need a scale accurate in the range of weight you are comparing...
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Post by travis on Oct 18, 2018 16:33:09 GMT -8
Do you need the fast heat of a large metal bell? In my experience that becomes an uncomfortable heat if needed for extended burns. Maybe I would consider a brick bell with a metal bit on top of just brick or another dense heat sync. The heat release becomes slower and more comfortable.
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Post by invention1 on Oct 18, 2018 16:38:08 GMT -8
Have you checked the moisture content of the fuel? The way to do that: split a bigger piece and measure the fresh split surface. When moisture is more than 15% the burn will be inevitably very sluggish. Judging the video of your cheat (howl!) without knowing anything I'd think your fuel is too wet, lots of smoke inside the firebox. Another way to test the moisture content of the wood is to take a smaller split, weigh it, and bake it at 250F until it no longer loses weight, then compare before and after weights to know how much water was in it... could take a number of hours of baking (even a day or two) before the weight levels off, and you will need a scale accurate in the range of weight you are comparing... Quite possible my cordwood isn't just perfectly dry. It was cut in june and stacked in a dry woodshed the rest of the summer. I haven't measured it. I have an expensive moisture meter bought for work, but I can't seem to get any readings out of it that make sense.
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Post by invention1 on Oct 18, 2018 16:40:26 GMT -8
Do you need the fast heat of a large metal bell? In my experience that becomes an uncomfortable heat if needed for extended burns. Maybe I would consider a brick bell with a metal bit on top of just brick or another dense heat sync. The heat release becomes slower and more comfortable. Yes, that's a consideration. The bell certainly is a large radiating surface. I have a relatively wide open house, kinda set up to circulate for wood heat, and the current wood stove (the old Fisher from the 1970's) is nice but has never overheated the space. Still, more masonry would certainly slow and delay the heat delivery. It's worth a try. First I got to get it running though.
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