Gassifier Draft Control

[DaveBP]

Wouldn't ya just love to see a flow chart of the algorithms used by some of the lambda controlled boilers?

On-Off control of the fan is going to be a hard technique to generalize about because the 'Off' portion of such a cycle is so dependent on the natural draft of the individual chimney involved. Huge variations there. Even with any one chimney, weather and being already 'warmed up' can make big a difference.

Might make a barometric draft control even more pertinent.

 

[ewdudley]

...
Also- rather than the expense of a VFD, I wonder if something approximating variable speed could be achieved by pulsing the motor at a fairly rapid rate (rather than letting it coast all the way to zero rpm), but varying the length of the pulses. Could that maybe even be roughly achieved via fine tuning the various parameter settings in a PID controller (I'm more at home with analog parameters so have not really become acquainted with all the ways those devices can work)?

I'm having excellent flue temperature control using a PID controller to switch between low speed fan and high speed fan, which avoids the stop-start problem. The only problem I'm experiencing is that in order for the low speed fan to be low enough to drive the flue temperature down, the initial air inlet damper setting has to be inside a fairly narrow critical range.

Which makes the on-off approach interesting to me since it's much more of a sure thing to drive flue temperature down by shutting off the fan entirely.

In the end though, I doubt I'll be happy until I can get closed-loop control of the inlet air damper.

--ewd

 

[jebatty]

Trevor: Also- rather than the expense of a VFD, I wonder if something approximating variable speed could be achieved by pulsing the motor at a fairly rapid rate (rather than letting it coast all the way to zero rpm), but varying the length of the pulses. Could that maybe even be roughly achieved via fine tuning the various parameter settings in a PID controller...?

The thought of pulsing the draft fan occurred to me, but I have not pursued it ... yet. Pulsing would have the effect of a net lower speed of the motor. How about some input on the long term effects of on-off of a capacitor motor, of pulsing a capacitor motor (which really would be rapid on-off), and if pulsing is an option, can this be done with a PID controller?

 

[TCaldwell]

check out the exhausto ebc 30, this is a diff pressure controller that will operate a vfd or damper, also i think it will also vary the speed of single phase motor up to 5 amps. most vfd's offer integrated pid and i/o control, and for a motor of under 1 hp they are not too pricey.

 

[jebatty]

Kiel: After reading this thread my mind is boggled by the way you guys are tweaking and controlling your boilers. I’m very impressed.

While perhaps not unique to Americans, there is something about us that always challenges almost everything, is very creative and inventive, and asks many questions, especially "why can't this be done better, faster, less expensively" or "what new something would address a need/desire that isn't now being satisfied?" President Obama talked about our creativity and inventiveness in his State of the Union address last night. We are not a satisfied people, we don't accept the status quo, and we believe anything is possible if only we put our head, bodies and hearts to the task.

While all of this has many benefits, it also carries the risk that at some critical, no turning back point, we may be wrong -- and what we thought was possible proves not to be, at great cost and loss to us and maybe civilization itself. Skol, cheers, prost, and chow!

 

[TCaldwell]

some available control technology in europe, fumis controller-- [email protected] and micatron mog 1000-02K -- www.micatrone.se

 

[Singed Eyebrows]

I've been wondering what to do on the Atmos & am leaning toward fan varied speed according to flue gas temps although as Nofossil says if there isn't much wood in the boiler increasing fan speed isn't going to help. I can pull air past the shut Samson on full rpm so I won't lose boiler power & when the draft falls with lower rpm the Samson will have full control over the boiler power. At least with my Atmos, I think this is the way to go or at least start, Randy

 

[jebatty]

for the atech.si site, see Fumis.

 

[TCaldwell]

jim, I enquired about the fumis 2 years ago, they at that time had no ul listing and could not sell in the usa, this is a prime example of honed fuzzy logic for wood, the homework has already been done but we cant get it! seems like the market share here is growing.

 

[jebatty]

For theoretical purposes, I've thought a DC blower with PWM control might be an option, and I've found a DC blower closely rated to the AC blower on my Tarm that likely would work. As a practical matter, CFM's needed to maintain good gasification seem to be over a fairly narrow range towards the high end of CFM output. Output on the low end of CFM seems relatively ineffectual.

Maybe my bang-bang experiment might actually be both an inexpensive and a reasonable way to accomplish draft fan "speed" control which substantially meets any need to control the draft fan and still let the boiler burn full out for all practical purposes.

 

[pybyr]

Along with data (once you're back up and running on that side of things) it will be interesting to hear how you find this modulation of the fan may (or may not) affect BTU/hr output of the boiler.

As to DC and PWM, it seems like the ultimate would be a permanent magnet ECM motor, such as is being implemented to improve circulator efficiency- super efficieent, super-controllable, but that'd take a manufacturer being able/ willing to dive into it. It's the sort of thing that wouldn't be all that costly if it were being produced in mass quantities. but would be infeasible for a one-off (unless you had amazing resources and skills), and probably prohibitively expensive even on a medium production scale.

 

[Singed Eyebrows]

There is also the Smart Fan Stratos that will take quite a few inputs & will control your standard single phase fan motor. It is a VFD PWM I believe. As to inputs that vary the speed, this is beyond me for the time being, Randy

 

[jebatty]

Along with data (once you’re back up and running on that side of things) it will be interesting to hear how you find this modulation of the fan may (or may not) affect BTU/hr output of the boiler.

I have a full datalog of boiler operations from Jan 1- 25. Yesterday I accidentally broke the 1-wire USB adapter connected to my laptop, so data-logging came to a stop. I also ordered two more adapters. Might as well have a spare on hand. But with the Jan data, and then with closely matching outdoor weather data in Feb, if possible, I might be able to get some data that may show something of interest. And then again, maybe the output change will be too small or there will be too many other variables that will not permit a reliable conclusion. Regardless, at this time I intend to go ahead with the bang-bang control as soon as the new controller arrives.

 

[Nofossil]

A really cheap alternative might be the Nimbus VS controller that I use with the NFCS. It's a glorified AC chopper, but it works with most fan, blower, and circulator motors, allowing them to be controlled as variable speed devices. They may not reduce power consumption as much as a VFD and they certainly provide less torque at lower speeds, but that's not typically an issue for blowers. I use them as slave devices with a 4-20ma input from my controller, but they are designed to be used stand-alone with temp sensor inputs. The manufacturer has a minimum buy, but I have a batch and I've been known to resell them at cost for a good cause.

The manual is here.

 

[jebatty]

I had previously installed a fan motor speed control that fits in a typical electrical box for a light switch. It was in my junk box. It worked OK to slow the fan down and I did use it. The issue I had with slowing the fan down was that some burn conditions seemed to require full draft fan speed, while others did well on a slower speed. I typically slowed the fan down after a good load of coals had built up to insure continuous gasification as additional wood was added.

My assessment at this point is that the temp controller on-off produces a more reliable and automatic control. It also is inexpensive. I just ordered a new K-type digital PID controller with SPDT relay for about $30 incl s/h, and I have the high temp K-type probes already.

Experience from others who want to try other methods will be very interesting.

 

[ewdudley]

I had previously installed a fan motor speed control that fits in a typical electrical box for a light switch. It was in my junk box. It worked OK to slow the fan down and I did use it. The issue I had with slowing the fan down was that some burn conditions seemed to require full draft fan speed, while others did well on a slower speed. I typically slowed the fan down after a good load of coals had built up to insure continuous gasification as additional wood was added.

My assessment at this point is that the temp controller on-off produces a more reliable and automatic control. It also is inexpensive. I just ordered a new K-type digital PID controller with SPDT relay for about $30 incl s/h, and I have the high temp K-type probes already.

Experience from others who want to try other methods will be very interesting.

Tonight I tried running a 'JLD-7100' PID controller with on-off control instead of my usual lowspeed-highspeed control, just by disconnecting the lowspeed input, on a 100 second cycle and have been getting nice steady flue temperature, and gasification seems to 'stay lit'.

But I was getting steadier gasification and controllable flue temperatures with the lowspeed-highspeed technique, and I don't like the idea of spinning up from a stop all the time, so I switched back.

--ewd

 

[ewdudley]

A really cheap alternative might be the Nimbus VS controller ...

The manufacturer has a minimum buy, but I have a batch and I've been known to resell them at cost for a good cause.

Dean is offering them retail:

http://smokelessheat.com/productlist.aspx?categoryid=3

 

[kielka]

So I just wanted to say that after reading this thread I went out and adjusted my primary air inlet damper for the first time in three years. Never really realized that such control is optimal for high efficiency burns. I just always let er' rip flat out and it makes so much sense that at those intense burns that an immense amount of hot air is flying right by the hx tubes. So I cranked the damper down a fair amount and noticed the burn went from an intense fire where I didn't even want to open the gassification chamber to a controled blue flame. Amazing. lol

 

[pybyr]

So I just wanted to say that after reading this thread I went out and adjusted my primary air inlet damper for the first time in three years. Never really realized that such control is optimal for high efficiency burns. I just always let er' rip flat out and it makes so much sense that at those intense burns that an immense amount of hot air is flying right by the hx tubes. So I cranked the damper down a fair amount and noticed the burn went from an intense fire where I didn't even want to open the gassification chamber to a controled blue flame. Amazing. lol

One thing to be aware of is that settings that may work best during one part of the burn may work very poorly during another burn stage.

 

[jebatty]

Kiel: I went out and adjusted my primary air inlet damper for the first time in three years.

I followed the advice in the Tarm manual, which is "or set it in the middle and forget it." If your wood is fairly uniform in dryness, you should be able to set the primary/secondary draft control at some middle area and leave it alone. I found that trying to tweak this for every burn, or during mid-burns, was an exercise in having nothing else to do. Set it and forget it. If your wood is not uniform in dryness, then effort is better spent on improving the quality of your drying, not fiddling with the boiler.

 

[TCaldwell]

the one p/s air setting will work better, IF you attempt to maintain a steady differential pressure in the firebox throught the burn. The on/off approach is a compromise to modulation but a step in the right direction.

 

[ewdudley]

Finally got around to building a closed-loop flue temperature controlled inlet-air damper this afternoon.

PID controller with K-type thermocouple senses boiler flue neck temperature and controls Honeywell zone valve heat motor with PWM on a four second cycle.

Seems to control steady in bench testing with a relay, but I'm waiting for an SSR to show up in the mail before I wire it in to the system.

Should work nicely for shutting draft off completely at the end of a burn cycle.

--ewd

 

[jebatty]

Looks impressive and will be watching for more info.

My approach at this point remains focusing on stack temp, but I'm wondering whether focusing on draft w.c. would be a better approach? If the damper was controlled by the draft, keeping it within mfr recommendations, might this better achieve the same result and also result in better burn efficiency? I'm guessing in part that "high" stack temp is related to higher than normal draft. If anyone has a manometer on their stack, do you have any info on the relationship between draft and stack temp?

 

[DaveBP]

If you were to control the damper on the boiler to keep the chimney draft constant, wouldn't it be susceptible to feedback runaway?

If the heat going into the chimney increases, the draft (vacuum) in the chimney increases. Like firing a hot air balloon, higher temperature=more lift.

If the damper control were to respond by opening the damper (or increasing the fan speed) to decrease that vacuum it would increase the burn rate in the firebox.
That would raise the temperature in the flue.
That would increase the draft in the chimney.
That would result in opening the damper or increasing the fan speed.
That would increase the burn rate......

Or is my imagination just running away?

 

[TCaldwell]

dave, the neg pressure that needs to be maintained in the firebox, is directally controlled by the ammt of air consumed by the fire. little or no fire less air flow required through the firebox to maintain a given water column, high fire requires more air to maintain the same wc. this loop is controlled by differential pressure, not fluegas temp. typically the differential pressure sensor is located in the firebox, close to the process. The second loop is the control of a o2 setpoint with primary and secondary dampers. With this approach and a small load of wood, the draft fan speed will be low, and the p/s dampers will be able to maintain o2 setpoint with a lower btu output. typically with a fixed draft fan speed and a small load of wood and the primary damper open [ trying to make more woodgas] and the secondary damper closed [ trying to reduce o2 %] it will still be above your setpoint, because there is too much draft through the firebox. by regulating the differential pressure, it allows the p/s control dampers responses to be predictable or linear throught the burn. there is not a o2 or differential pressure correlation that is reliable to flue temp when you have water intermittantly moving through the boiler influencing flue temp.