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Post by Solomon on Jan 25, 2021 12:43:34 GMT -8
I've been playing with some ideas (as you all know newbies like to do).
One of them is experimenting with just exactly what is the contribution to the draw between different components of the system.
Like many of you, I grew up with a wood stove. It was our only heat source. And since I was the firebug, it fell to me to light many many thousands of fires. And I still enjoy lighting fires, one of the main reasons I'm here. Anyway, like you all, my stove was powered by the draft in the chimney.
And I've seen Matt Walker's riserless core designs. So obviously, a riser is not required, and in fact, if one has enough draw (read heat being wasted up the chimney) one should not need a core at all.
So I am thinking of just playing around with mine, when I get it built, because I'm using a CFB riser, I can just pop the riser out very easily. And in fact, it will be in two pieces, so I will be able to play with different lengths.
To science this up properly, one of the ideas I've been playing with is manometers. Water powered manometers are super easy to play with, you really only need some vinyl hose (with metal ends if they're meant to be hot) to get started. Has anyone done this? Would be interesting to better know the dynamics of pressures within the system.
My woodgas generator has a built in manometer which is how you monitor how things are going (along with type k thermocouples). I would love to play with the thermocouples at some point also, I already have the reader. Though it seems Type-Ks are limited to about 2300 degrees which may or may not be suitable for certain points in our systems. Electronic pressure sensors would be great too, if I ever get to that point.
Tinkering is good, don't get me wrong, I'm a tinkerer too, but I'd like to see more engineering in the RMH world. Maybe I could come up with something interesting, maybe a second master's. ;-)
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Post by Karl L on Jan 25, 2021 14:44:57 GMT -8
I built myself an electronic manometer because I was interested in what flue pressure I was getting in an old unlined stone chimney. It's just a 'lash-up' at the moment, but works well enough.
It has 0.4Pa resolution which is plenty good enough to measure the flue pressure range wood stoves operate in.
It's based on a Honeywell sensor with a 0.5 PSI range, and an 18-bit DAC. There's no need for signal conditioning - you can just digitise the sensor output directly.
One thing I think I've learnt by using it is that a fire doesn't reach the point of being self-sustaining (when the door is closed) because of the increasing draft in the flue from hot exhaust, but because the fire box has become hot enough to keep the fuel gassifying sufficiently. There can be plenty of draft (say due to a breeze outside) but the fire still won't take off until the fire box gets hot enough.
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Post by Solomon on Jan 26, 2021 8:31:09 GMT -8
Interesting. Can you post a link to the sensor or a model number or something?
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Post by Karl L on Jan 26, 2021 9:49:06 GMT -8
Here's a link to the part: cpc.farnell.com/honeywell/24pcefa6d/pressure-sensor-0-0-5psid/dp/SN36174?CMP=i-ddd7-00001003I previously used this type of sensor in my work (control systems design). They work best driven with constant current, but in my lash up I'm just using the 3v3 supply rail. That means the output 'drifts' with temperature, but it's quite easy to zero it. I also happened to have a digital manometer from work that I used to calibrate my lash up. The manometer doesn't have the sensitivey to measure flue pressure levels, but it was very useful for calibrating.
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Post by Solomon on Jan 26, 2021 18:42:12 GMT -8
Thank you. Very interesting bit of kit.
I'm thinking it would be good to have a pressure sensor at the base of the chimney, at the mouth of the wood feed just inside, at the bottom of the barrel before the mass.
Temperature sensors in the same places and also at the top of the riser.
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Post by Karl L on Jan 27, 2021 8:18:54 GMT -8
I guess it will be interesting to measure the temperature and pressure at those points.
I am measuring pressure at the bottom of my bell, where gas flow is very low. I am assuming that it is the pressure differential between this point and the air pressure in the room that is pulling air/flames, etc through my DSR2 core.
I don't know how the venturi effect will affect the pressure measurements at points where there is a high gas velocity.
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Post by Solomon on Jan 30, 2021 21:16:38 GMT -8
I just remembered this: wiki.gekgasifier.com/w/page/6123727/Gasifier-Control-UnitIt's a control unit created for the GEK, gasifier experimenters kit. The kit eventually evolved into a 25kw wood gasifying generator that sells for tens of thousands of dollars and they kinda dropped all the open source talk and started making money. But one of these would be fantastic for monitoring/controlling one of our RMHs, including possible primary and secondary air control. Sadly, I don't know if it will be possible to get them.
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Post by Karl L on Jan 31, 2021 0:49:09 GMT -8
That board looks great, but very expensive.
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Post by Solomon on Jan 31, 2021 8:19:31 GMT -8
That board looks great, but very expensive. Yes, it is. And that was back when it was actually available. It is much cheaper if you're using something like an Arduino Uno with individual sensors and link it all up with a breadboard. I think we could get away with 4 pressure transducers and 4 thermocouples, plus an oxygen sensor, CO sensor, maybe some sort of PM2.5 sensor. And that would be way more than enough. That would be for experimenting. There are pretty simple units if you just want to monitor it, a good old fashioned slack tube manometer, though I don't know how many inches of water pressure we're working with, likely very few. Also, there are plenty of cheap Type K thermocouple readers if you just want to monitor the temperature at different points. They cost half as much as those stove thermometers and are much more readable and accurate. I bought a high temperature furnace thermocouple I'm going to put right in the burn path hopefully and figure out once and for all what temperatures we're looking at. Because I think some people's numbers are wildly optimistic. These stoves get hot, but the don't melt firebrick and they don't melt ceramic fiberboard and I haven't seen any evidence of melting steel. Steel is melts at 2600, CFB is rated for 2300, the thermocouple I ordered is rated for 2372, so it should outlast the CFB. So I think claims of 3000F plus are simply fanciful.
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Post by josephcrawley on Jan 31, 2021 16:35:31 GMT -8
I've been playing with some ideas (as you all know newbies like to do). One of them is experimenting with just exactly what is the contribution to the draw between different components of the system. Like many of you, I grew up with a wood stove. It was our only heat source. And since I was the firebug, it fell to me to light many many thousands of fires. And I still enjoy lighting fires, one of the main reasons I'm here. Anyway, like you all, my stove was powered by the draft in the chimney. And I've seen Matt Walker's riserless core designs. So obviously, a riser is not required, and in fact, if one has enough draw (read heat being wasted up the chimney) one should not need a core at all. So I am thinking of just playing around with mine, when I get it built, because I'm using a CFB riser, I can just pop the riser out very easily. And in fact, it will be in two pieces, so I will be able to play with different lengths. To science this up properly, one of the ideas I've been playing with is manometers. Water powered manometers are super easy to play with, you really only need some vinyl hose (with metal ends if they're meant to be hot) to get started. Has anyone done this? Would be interesting to better know the dynamics of pressures within the system. My woodgas generator has a built in manometer which is how you monitor how things are going (along with type k thermocouples). I would love to play with the thermocouples at some point also, I already have the reader. Though it seems Type-Ks are limited to about 2300 degrees which may or may not be suitable for certain points in our systems. Electronic pressure sensors would be great too, if I ever get to that point. Tinkering is good, don't get me wrong, I'm a tinkerer too, but I'd like to see more engineering in the RMH world. Maybe I could come up with something interesting, maybe a second master's. ;-) I would not characterize Matt's design as riserless despite it being in the title of his core it is just a riser that is not vertical. I built a stove that had a riser of about 60% of recommended height so I could fit an oven in the bell. It did not burn as well as I would have liked and compared to the full height riser stove I replaced it with I believe it was the shortened riser. It of course is hard to compare anything without a flue gas analyzer.
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Post by Solomon on Mar 25, 2021 8:15:20 GMT -8
Now that the core is installed and fully cobbed (bench not yet completed, but the core is), I have done a test by removing the upper 2 feet of the riser.
Results: Upon ignition, the system seems to heat up faster. Total peak temperatures not significantly changed. Slower burning with less heat out put possible.
But the concern: With full fueling, pulsation occurs. This leads me to believe that this configuration is somehow unstable. Without a solid consistent draft from the heat riser, pulsing happens because gas speeds and fuel burning interact at the right frequency. Pulsing is obviously bad as it demonstrates instability and poor design, but also because strong enough pulsation can blow flame and coals out of any wood stove, leading to fire danger.
I've seen old iron wood stoves do this, blowing flames out the intake as much as 4 inches, spraying cinders into the room. Not good.
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