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Gassifier Draft Control

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jebatty, Jan 25, 2011.

  1. pybyr

    pybyr Minister of Fire

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    I have a Bacharach MZF draft meter that I generally keep hooked up to my flue.

    Draft seems to vary tremendously depending on conditions, with flue temp being only one of the variables. Outside temperature and weather patterns seem to be the largest variable. Draft definitely increases after the boiler has been running, especially after the boiler has been running in a mode where the flue temp. is particularly high, but it does not seem to be a rapid/ direct response to flue gas temperature-- seemingly more related to the gradual heating of the entire mass of the chimney. The response may well be more rapid for those with a low-mass insulated stainless chimney.

    Even with my relatively low-tech gauge, the fly ash that's going up the flue tends to get in the gauge's intake hose and cause erroneous readings; I need to pull the gauge's intake out of the flue pipe at least daily and give the hose a good shaking to get the combustion particles out. If someone were going to try to have a solid-state sensor/ feedback arrangement, you'd really need to be sure to use materials and a design that aren't prone to fouling from the various forms of 'stuff' that's going to be in the flue.

    You'd also need the sensor to be relatively robust to deal with the occasional pressure spikes that can occur in the flue, such as if the firebox is reloaded with anything other than a tiny bed of remaining coals- or, for those whose units may idle at times, sometimes when the fan resumes operation after idle with a firebox full of nicely pyrolizing wood. The draft gauge really gives some interesting glimpses that chimney conditions are far from steady-state.

    Not trying to throw cold water on anyone's inventive ideas, just passing along what I've observed from monitoring draft.

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  2. TCaldwell

    TCaldwell Minister of Fire

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    Pybyr,as you hit apon, the thermal mass of different flue materials can create a flywheel effect to natural draft, unrelated at the time to actual combustion differential pressure in the firebox, this is one reason for the sensor in the firebox. As to the erratic differential readings due to disturbances in the firebox, most transducer control signals would be input to a pid controller with a filter to even out the signal to be useable. On the exhausto ebc 12 the pid algorithim is designed for this application.
  3. TCaldwell

    TCaldwell Minister of Fire

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    eliot, did ssr arive, curious how it works? hope this thread is not dead
  4. ewdudley

    ewdudley Minister of Fire

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    Got the SSR yesterday and wired it together on the bench (boiler is running so I can't mess with it tonight).

    Set it up as a chimney with a 75 watt bulb at the bottom with updraft controlled by the damper plate [see photo above]. Temperature sensor is suspended in outlet of chimney.

    (When installed a weight suspended by the clip at the end of the arm will close the damper by gravity, for this test the arm is closed by the pair of rubber bands.)

    Controls plus or minus two degF with the PID controller auto-tune parameters. Probably could be improved upon with some gain tweaks but is fine for proof-of-concept.

    SSR PWM is able to hold the Taco 555 heat motor steady at aribitrary control points in between full-open and full close. Many would describe it as an inelegant kludge, but I would point out that we humans often underestimate the effectiveness of the guileless Orcish technology.

    I'm pleased with these $40 PID controllers (JLD7100 a.k.a. TET7100 a.k.a. Auber SYL-1512A) . The one I'm using for boiler return temperature control works just great; tell it what the return temperature needs to be and that's what the gauge will read through the whole burn.

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  5. ewdudley

    ewdudley Minister of Fire

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    Connected to boiler and have got a couple burns' worth of experience.

    Couldn't control well at all until I added 25 ohms ballast in series with heat motor coil. Definitely could use some more but that's all I could scrounge up just yet that had enough wattage. Without ballast it opened all the way with only about 10% heater duty, now it still only takes less than 50% duty to open all the way, but it controls well.

    The PID controller auto-tune feature failed, which is not surprising. There can be a fair amount of positive feed-back when opening and negative feed-back on closing, kind of like the difficulty in trying to keep the power steady on a corner in the rain with a turbo car; during transition steady throttle doesn't equate to steady power. I backed off on the proportional and integral gains, and stayed somewhat aggressive on the derivative, so now it runs smooth in a nice fat error band of plus or minus a couple kelvins.

    So now I can burn hard and hot, but it will throttle back some on the small light super-dry fuel.

    The other thing is that the inlet damper is powered by the draft fan enable circuit, which itself is controlled by a separate alarm relay function of the PID controller. So when the fire gets down to coals and the flue temperature starts to fall despite the damper being full open, everything shuts down and the inlet damper closes tight, killing the fire without any smoldering to speak of.

    (Shutting down the boiler then enables boiler purge logic that runs the return injection pump whenever there is a call for heat from storage and the top of the boiler is hotter than the water returning to storage, yielding an hour or more of free heat scavenged from the hot boiler at the end of the burn.)

    --ewd

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  6. steam man

    steam man Minister of Fire

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    I like the concept of what your trying to do but here's something to think about. If the valve flow characteristics are not linear you may have a problem trying to tune the controller for stability. Old time pneumatic loops had some kind of cam that would ultimately adjust the final control element to give just that-where percent of rated travel is equal to the same percent of flow. I used to sit at a test bench custom making cams just to experiment with. I've calibrated enough vane actuators on boilers that had cams. Electronic controllers would just have a characterizing block built in to adjust the output for what ever flow charateristc you need. It seems to me you shouldn't have to open the valve all the way to get full flow. Just a thought. Good looking job though.
  7. ewdudley

    ewdudley Minister of Fire

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    Yes, good points.

    As a starting point, to sidestep the butterfly nonlinearity problem, I made the inlet tube diameter as large as I could and still have it fit practicably. Before proceding I taped a piece of tube to the boiler with a sheet metal disk positioned at about a thirty degree angle and verified that there was plenty of flow for full output of the boiler. Butterfly valves behave more linearly at small opening angles.

    The geometry of the linkage translates the full throw of the heat motor to fourty degrees rotation of the damper plate and that's all the further it will open. As it turns out it controls down around twenty degrees open.

    It's hardly a linear translation, and neither is the flow rate as a function of plate rotation. But achieving stability with any PID control is normally simply a matter of reducing performance and avoiding the temptation to waste time trying to eliminate error -- while failing to recognize that error is what makes it work.
  8. jebatty

    jebatty Minister of Fire

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    I'm glad, Eliot, you're taking the route of controlling flue temp through damper-draft control. As you get a little further, do you have a means to determine whether from equal weighed loads of wood your pid draft control results in more btu's into your system vs no draft control?

    My new pid controller arrived Friday, and I finished installing it yesterday. I'm using it "on/off" right now to simply turn the draft fan (Tarm has only one) on/off based on flue temperature. It worked very well to maintain temperature on a short burn yesterday: sv=215C, control +/- 2C with a fixed setting. I'm still waiting for the new 1-wire adapter to arrive so I can log data and compare equivalent burns with and without draft fan control. I can run my system with 0 load and full output to storage only, so I hope I will be able to see a measurable difference with and without draft fan control. If not, I still will have a fancy digital flue temp meter on my boiler control panel!
  9. ewdudley

    ewdudley Minister of Fire

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    I hope to have something fairly soon, but I'm running out of excuses for the hot tub not being online yet, so my priorities might get shuffled.

    I'm using an injection pump to meter system return water into the boiler recirc loop, so once I get digital probes on the boiler outlet and the system return temperature I should be able to record injection-pump-milliseconds*inlet-outlet-deltaT as a measurement of heat output, assuming injection pump delivery rate is not affected significantly by thermo siphon pressure. Then I was hoping to correlate the injection pump running-time integrals to total mixed-tank before-burn-temperature vs. after-burn-temperature so I can show how many btus are in an injection-pump-second-kelvin.

    It's so surprisingly nice how well the on-off works. I know of refrigeration units that use on-off of 1hp fan motors to control high-side pressure and they deliver correct reliable performance for years on end. I only went with the damper because it incorporates the inlet air shut-off function.

    Another thing to try might be to get a couple washing machine water valve solenoids and use them to control poppet valves into an intake manifold. One valve would provide a smallish base flow and the other valve would provide a largish on-off PID controlled flow. Then close both valves to shutdown the boiler.
    From what I've been reading it should only amount to 1% gain in efficiency per 20 kelvins reduction in flue gas temperature, or only about 3% improvement in going from 500 degF to 400 degF. But it sure doesn't seem like it standing next to a flue that's putting out 500 degF exhaust.
  10. jebatty

    jebatty Minister of Fire

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    I think your priorities are where they should be - get that hot tub on line before the winter is over!

    My reading was the same: 2-3% for 100F reduction in flue temp. I assume this is for equivalent loads of wood and burn times, meaning the extra 100F is captured to the system. I'm wondering though, in addition to dropping flue temp, I think I'm slowing the burn rate and getting longer burns at lower temp for the same load of wood. Any thought on that? I hope my time/temperature logging shows measurable changes.
  11. DaveBP

    DaveBP Minister of Fire

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    If you are slowing the rate of combustion by slowing the draft through the boiler (and I think that is a safe assumption), by forcing it through a nozzle whose size does not vary it seems likely that the temperature in the refractory afterburner chamber will be decreased. That may decrease the combustion efficiency. Longer burn times might not produce more total heat. Think of the stereotypical OWB.

    But the servo-controlled gassers (Froling, Effecta, and all) don't vary their nozzles. And yet they do modulate their output somewhat, presumably without sacrificing efficiency.

    Perhaps the control of primary/secondary air, as well as draft, is required to keep the afterburner temp up and so keep up the combustion efficiency.

    Closing down the nozzle somewhat to slow down the burn rate but maintaining that highest temperature torch might be one way to adapt. But why didn't the manufacturers do that to begin with?

    To answer my own question, I guess then they would have just made a lower BTU rate boiler with a larger firebox.

    And that sounds great to me.
  12. jebatty

    jebatty Minister of Fire

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    When the draft fan shuts down for a few seconds there is visible smoke out the stack, not much, but some. No visible smoke when the draft fan is "on." It's obvious that there is some incomplete combustion during the brief "off" periods. I don't know how problematical this may be, if at all. The smoke likely is somewhat akin to smoke produced during an idling cycle, except the "off" periods are very brief and the start-up again is very rapid. Also, the smoke is only during the very high burn stage. Once the burns settles down to under 420F, not an issue.
  13. ewdudley

    ewdudley Minister of Fire

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    The low-speed/high-speed variation I was running kept a blue turbulent jet running all the time, it might be worth a try for you. I used the PID controller to switch a double throw relay and fed low speed from a solid state variable speed fan control in on the NC terminal and the full 120 VAC in on the NO terminal and connected the common to the fan. So then the PID activated high speed when on and low speed when off.

    --ewd
  14. ewdudley

    ewdudley Minister of Fire

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    I believe it's necessarily true that decreasing flue temperature by slowing the fan or throttling air intake will lower the burn rate, assuming combustion remains complete, and assuming that excess oxygen isn't somehow increased.

    I'm seeing that my secondary flame jet stays blue, so I'm reasonably confident I'm still getting complete combustion, but as TCaldwell points out I don't know what it's doing to excess oxygen levels.

    And I've been assuming that the secondary flame temperature is not decreasing substantially, the reduction if flue temperature should be due to the fact that there's less combustion occurring and therefore the heat exchange surfaces are relatively larger and more effective, which is the whole purpose of limiting the draft to begin with.
    Exactly. The more affordable boiler I bought is perhaps a little weak in heat exchanger design so I was planning all along to somehow limit burn rate so as to improve efficiency. I've blocked off the middle half of the nozzle slot and it seems to run hot and steady at less than half the nominal output rating. So far so good, but like I say, I don't know if maybe I've got too much secondary air, and I also don't know if it's going to cause the nozzle to wear out faster.

    --ewd

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