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garn modulation observations

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by TCaldwell, Feb 23, 2008.

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  1. TCaldwell

    TCaldwell Minister of Fire

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    i changed the static 3450rpm single phase motor with a direct bolt 2 hp 3 phase motor controlled by a variable speed drive,0rpm to 4600 rpm. Originally i was expecting that a higher rpm would increase the secondary burn temp, wrong , just the opposite, lower burn temps and higher stack temps, i guess the physical characteristics of the garn burn chamber is not meant for more air. With the stock motor the secondary burn temps 1800/ 2200deg and the stack temps 400/500deg, when i lowered the rpm of the new motor at full burn to 2300rpm,40hz the burn chamber temp stayed the same and the stack temp dropped to 350deg, i guess lowering the cfm increased the retention time of the flue gas to be absorbed by the hx tube, with still maintaining enough combustion air. There becomes a point when the burn temp lowers and the target flue temp cant be maintained with modulating lower i was able slowly lower the flue temp. with the stock setup the flue temp would drop off faster and to a lower temp. QUESTIONS, if the burn chamber temp is above gasification and you maintain a flue stack temp above condensing temp does this constitute effiency? I was thinking of a flue temp target of 350deg to be maintained by modulation until that can no longer be maintained due to low fire then a flue temp to be lowered incrementally by modulation until a low flue temp set point was reached and the vsd would shut off? does this sound logical, i would appreciate any feed back . thanks

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  2. Gooserider

    Gooserider Mod Emeritus

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    I would think the max efficiency should be at the point where you have the greatest difference between the combustion chamber temp and the flue temp - presumably any difference is being absorbed by the water (where else would it go) so the more delta, the more absorbtion.

    Thus I would look for what combination gave me the highest burn temp and the lowest flue. If I'm understanding you correctly that sounds like around the 2300 rpm point you mentioned. I don't think I'd target a particular flue temp, rather I'd try to maintain the largest delta I could while keeping the flue temp above the creosote making stage. I'd then shut off the air once your combustion temp reached a point where you were no longer gasifying or didn't appear to be producing meaningful heat - maybe when flue temps approached your boiler water temps?

    Gooserider
  3. brad068

    brad068 Feeling the Heat

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    I am a little disappointed at the new motor/vfd setup. I was expecting, well not exactly sure as to what I was expecting but I thought it would work better than stock. One theory I am thinking in regards to more btu like in High E's boiler is that he is getting more secondary combustion right in the firebox and not so much in the secondary chamber. His boiler will pulsate a long time were as mine will mellow out and just burn smooth. But in these style boilers the more turbulent the air the better the mix thus better burn. And when that boiler is pulsating the air flow is not steady through the flues thus giving it more retention time and maybe that is how he is getting more btu/hr.

    On the other hand, I took up some of my wood for him to burn and his boiler spit it right out! It couldn't even get over 600*f on his first temp probe! :lol:

    There is another SS Minot in my neighborhood now and we should be able to get some direct comparison between a Royal owb and a Garn style, 2722gal. worth! The new owner of the boiler, aka... thinking of a new name for him....., anyway, he is a tinkerer to so hopefully he will be doing some experimenting too.
  4. TCaldwell

    TCaldwell Minister of Fire

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    I believe i am in unchartered water with this vsd, effiency thing , i have some dimensional dry oak planking that i can weigh and be pretty certain of consistency to do some test firings. since i dont have data logging capabilities, i will use start temp, end temp, time and beginning,ending water temps isolating the boiler from demand. first firing will be at 60 hz for 2.5 hrs, second test will be modulating fan speed to target flue temp of 350, at end of burn modulating to attain highest flue temp down to 200 ending test, test 3 i hope to try a turbulator in the last hx pass and base it on test2 also try it on test 1. please advise if other criteria should be considered ,would welcome all opinions.
  5. slowzuki

    slowzuki Feeling the Heat

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    I think the problem you'll find is they probably sized the fan nicely. Here the scenarios:

    1) You add more air to a fire that already had enough, so excess air - combustion temps drop and stack temps drop due to shorter dwell time in HX offset by lower initial temp. The stack temp could rise depending on the HX design.

    2) Fire was running fuel rich and you add air to bring it to neutral - combustion temps rise and stack temps rise due to shorter dwell time.

    So if you're adding more air, you will probably like to use more finely split wood or softwoods that gasify more readily. If using large chunks with less surface area, reducing the air may be appropriate.

    Also, during the later part of a cycle, there are less volatiles left in the wood and the air consumed drops as the carbon is burned, so less air is needed.
  6. Nofossil

    Nofossil Moderator Emeritus

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    I don't understand the details of the Garn design, but here are my thoughts on the gasification process as it relates to this thread. I'm making this up - don't place too much faith in it.

    Any gasifier has two separate air inlets - primary and secondary. Increasing primary air will generate more wood gas to a point, but beyond that it will just create more complete combustion in the primary chamber. I don't understand the physics exactly, but I think that the available surface area limits the maximum wood gas generation. More surface area, more available gasification capacity, and higher potential output given enough air.

    The primary air is intended to drive gasification. If there's plenty of fuel surface area, the mix going into the secondary nozzle will be oxygen starved and quite flammable. If it's mixed with enough secondary air, it will burn cleanly and with great enthusiasm. Not enough secondary air and you'll get smoke and odor out the chimney. Too much and you'll start dropping combustion and flue temps, and wasting heat by pushing excess air through the system and up the chimney.

    The problem is that early in the fire, there's plenty of gasifiable fuel and you need a lot more secondary air to burn it. As the fire progresses, you're burning a higher percentage of charcoal in the primary chamber. Charcoal does not generate wood gas - it burns completely in the primary chamber. The more charcoal, the less secondary air is needed. Towards the end, you could shut secondary air off completely. Remember that this is speculation without hard data, but I think the ideal ratio between primary and secondary changes a lot over the course of a fire. It also varies depending on wood size, species, and perhaps moisture content.

    If you maintain the desired primary/secondary ratio, then changing the total amount of air should act as a throttle, allowing you to modulate the total heat output. Unfortunately, whatever mechanisms control the ratio are probably not linear in their effect, so changing total air may change the ratio as well - that appears to be the case for the EKO.

    Without exhaust gas analysis, we're shooting in the dark. I'm trying to set up a wideband O2 sensor to get a better handle on this.
  7. Eric Johnson

    Eric Johnson Mod Emeritus

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    I got a good look at a Garn at the NY Farm Show, and it's an interesting (and quite simple) design. There's no secondary air to speak of, just two ports to get air into the firebox. From there (the firebox) the wood gas passes through a ceramic refractory tube, where the secondary combustion presumably takes place. It then rises through a 5-pass heat exchanger (just a series of pipes with 5 horizontal runs going through the water vessel, and out the exhaust. I'm not sure how you'd clean that tube out, given that it makes a few 90-degree turns (actually 180 on at least two occasions). Maybe you don't have to. I think on bigger marine boilers, they clean the tubes out with compressed air.

    Anyway, there's not much to it, but by all accounts, the design works really well. Looking at Garnification's fabrication pics, it looks to me like he has smaller firetubes and more than 5 passes between the firebox and the exhaust.

    I think your description is correct, nofossil. Towards the end of the burn cycle, when there's nothing but charcoal in the primary combustion chamber, there's no more smoke to be had--just hot gas. That's also true with a conventional wood-fired boiler: when you've got a firebox full of coals, you don't get any smoke (about the only time you don't get any smoke).
  8. slowzuki

    slowzuki Feeling the Heat

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    The garn is very much like my Jetstream, which operates almost always at excess air and relies on turbulent mixing of the excess air in the refractory tunnel at temps above autoignition to burn the gases. In the jetstream, wood slides down the tube so a fairly constant surface area is in the turbulent combustion zone, the rest is up in the relatively cold loading tube waiting its turn. This type of setup is sensitive to surface area of the fuel but simply cranking up the air flow messes with the hx and the combustion tunnel.
  9. TCaldwell

    TCaldwell Minister of Fire

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    I will give that the garn is very basic,and as slowzuki statad the high end fan speed is probabaly pretty well dialed in, i will do the burn tests hopefully this weekend, yesterday i ordered a turbulator for the last pass, should be in next week, i know that there is room for end of burn / low modulation heat savings , this weekend will tell. it takes a little time to do these tests, alot of water to heat up and then disapate, i appreciate and welcome the advice
  10. TCaldwell

    TCaldwell Minister of Fire

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    i spoke to red eye combostion today about oxygen trim, they hope to have a low end price unit to market in about 1,5 years that would be wood compatible and interface with 4/20 milliamp drive, they feel the market is changing from very large , 10,000,000btu boilers to 1,000,000btu boilers and under.
  11. brad068

    brad068 Feeling the Heat

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    Hey Tom,

    I was tossing around another idea that High E is maybe incorporating. He is not moving as much air as I am but we have the same fan motor and wheel and I was wondering if because of that he is creating more of a negative/low pressure in the firebox in turn releasing more gas out of the wood. The wood chunks are releasing more gas under vacuum than a free flowing firebox. I'm thinking of restricting my air intakes alittle and seeing what happens. I know that when I was dialing in my boiler I had smaller nozzle opening and it pulsated terribly so I opened them up and smoothed out the burn, maybe that was the wrong way!
  12. slowzuki

    slowzuki Feeling the Heat

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    Usually the pulsing is from areas of wood gas and air that are present encountering an ignition source. In the Jetstream this happens with fine split wood, the gas will accumulate in the dead area of the fuel loading then enough air mixes and a slug of the mixture makes it to the flames, burns rapidly and this gets into a cycle. There are other ways to setup a cycle too.

    I suspect opening up your nozzle increased the excess air and turbulence enough to break up the cycle.

    The frequency of the cycle is probably a function of the fire box size and air flow, I can make it happen in my wood stove for example with highly volatile material like paper. Burns uses up all the oxygen, goes out, gas builds up oxygen mixes burns and repeat. A nice whooooomp whoooomp whoooop type noise.
  13. TCaldwell

    TCaldwell Minister of Fire

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    garnification/slowzuki, the puffing does happen in the first 1/2 hr and especialy if the wood is a little long and starts to burn at the front of the chamber closest to the loading door, if you have the room to push the wood closer to the rear of the chamber it will help. if you look at the largest garn it has a larger primary chamber and a 36'' long secondary burn chamber 950,000btu/hr, i wonder if it puffs. a combustion engineer i showed the garn schematic to wondered why it even worked, he said the secondary chamber should be larger and secondary air introduced through nozzles at the secondary chamber forced by a fan, basically the stock chamber does not mix the gasses long enough, claims this would mitigate puffing. ok garnification start building, or modifying . what do you think slowzuki?
  14. TCaldwell

    TCaldwell Minister of Fire

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    oh i forgot, the big garn uses the same motor /fan as the on garnification's boiler
  15. slowzuki

    slowzuki Feeling the Heat

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    Traditionally the secondary air is done like your friend says but then you get problems that the "other" gasifiers have. There isn't a big need to have secondary air like that unless you are trying to burn a load of wood slower. The garn and jetstream burn the wood in the burn zone at full tilt, they aren't trying to gasify at a slow rate and burn the gases, the are trying to gasify at the max rate for the surface area of the wood and burn the gases.

    When the front catches it likely uses up the excess air available and steals air away from the rear, somewhat extinguishing the material further back. I suspect your CO emissions from the stack climb when it starts puffing.

    I'm not convinced the secondary should be larger, I haven't seen a garn in person but the chamber on mine is intended to force the excess air, unburnt gases, and hot gases through a tight enough space for them to mix and burn. The length is based on the incandescence period for soot particles, basically how long they take to burn, the refractory path on mine is about 24 inches I'd guess?
  16. TCaldwell

    TCaldwell Minister of Fire

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    slowzuki, sounds like the jetstream , and garn share symptoms, your description is pretty much as it hapens, what is the website to view the jetstream, if you go to garn boiler and look at the effiency thread there is a schematic of gas flow, ect. thanks tom
  17. slowzuki

    slowzuki Feeling the Heat

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    Best I've got is my website, www.onthefarm.ca and my wikipedia entrance under jetstream furnace. The Jetstream was designed over 30 years ago, mine is a 1981 model.

    In reality, the air for combustion is preheated before the hx tubes. The burn path is actually reentrant around the burn tube, ie the gases just about hit hte back the unit then take a 180 to get back to the hx tubes. The system is not actually open either.

    Here is a simplified picture of the guts:

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  18. Eric Johnson

    Eric Johnson Mod Emeritus

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    What are you doing for a controller, Ken? I was pretty impressed with that wall of switching gear that came with the one you picked up in Rome. When it occurred to me that the controller in my EKO basically replaces all that (I think), I realized how far we've come where electronic controls are concerned over the past couple of decades. Think about what a small business computer looked line in 1981 and it becomes even clearer.

    Did you ever explore using an EKO controller instead? The new models are around $250, but I bet you could pick up an older version free or cheap from someone who is upgrading. Or are you going to build your own?
  19. slowzuki

    slowzuki Feeling the Heat

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    Eric I started looking over that monster and it is interesting. It is more than the EKO controller, since it is meant to run the boiler, storage, an oil boiler and a pump zoned or valve zoned system. I won't be using it, as the cheapest honeywell outdoor reset controller with a couple of extra relays/timers will handle it all. The old eko controller is interesting for the boiler itself but I'm not sure the fan modulation will work well with the Jetstream.

    I was running the Jetstream fan a bit on the weekend, all my boiler wiring is done, just need circulator and hvac wiring now. Something is wrong with my fan motor, it is supposed to pull 7 amps and is drawing 40 and kicking out the breaker. The capacitor is still good, its all wired right. I'm wondering if the dry oil cups could really add that much load. I pulled to old dry belt off as the thing was hard to spin by hand, will try again.

    Climbed into my milk tank too and have figured out how to solder my coils. Wife is not happy that I will be soldering inside a tank but I'm going to have ventilation.
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