# Gassifier Draft Control



## jebatty (Jan 25, 2011)

An idea that may address the variety of questions concerning draft fan and stack temperature. Would not a K-type temperature controller using a stack probe with on/off control of the draft fan work to maintain a set stack temp and control draft at the same time? Example: set the controller at 380F with hysteresis of 20F. Draft fan "on" until stack temperature = 400F, draft fan "off," stack temperature falls to 380F, draft fan "on," etc. K-type temperature controllers and K-type probes are quite inexpensive, and this would be an easy modification to a system.

An extension of this idea revolves around the barbecue temperature controller/ethernet/web control offered by Rock's. This product also will provide Twitter update reports in addition to web control. Other applications of this controller spill into many areas. Plus, a grill fitted into the gasification chamber will sizzle that steak to perfection in seconds!


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## ewdudley (Jan 25, 2011)

jebatty said:
			
		

> An idea that may address the variety of questions concerning draft fan and stack temperature. Would not a K-type temperature controller using a stack probe with on/off control of the draft fan work to maintain a set stack temp and control draft at the same time? Example: set the controller at 380F with hysteresis of 20F. Draft fan "on" until stack temperature = 400F, draft fan "off," stack temperature falls to 380F, draft fan "on," etc. K-type temperature controllers and K-type probes are quite inexpensive, and this would be an easy modification to a system.
> 
> An extension of this idea revolves around the barbecue temperature controller/ethernet/web control offered by Rock's. This product also will provide Twitter update reports in addition to web control. Other applications of this controller spill into many areas. Plus, a grill fitted into the gasification chamber will sizzle that steak to perfection in seconds!



Yes, I've got a '7100' PID controller controlling stack temperature by switching from low-speed-fan to high-speed-fan on a 100 second cycle.  It works quite well and maintains gasification, but I'm still looking for something more elegant.

--ewd


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## jebatty (Jan 25, 2011)

What is your more elegant desire?


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## ewdudley (Jan 25, 2011)

jebatty said:
			
		

> What is your more elegant desire?



With my 'standard' kindling and 'standard' splits and 'standard' draft damper setting, the low-speed-fan is low enough to lower the flue temperature and high-speed-fan brings it up, so normally everything works very nicely.  But at startup I could use a little less draft damper, and also larger/uglier/higher-moisture pieces could use more air.  So I'd like something that leaves the fan set at medium-low and controls an inlet damper.

I'm working with a 7100 controller switching an SSR to control a Taco 555 power head, which looks promising.  The power head only has about a 0.100" throw so I need to develop a linkage that will rotate a butterfly through 70 degrees or so.

--ewd


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## DaveBP (Jan 25, 2011)

But if you change fan speed to control your desired flue temperature won't that throw out your exquisitely tuned air adjustments? 

Which parameter (if you must choose to control only one) is more important to overall efficiency?


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## ewdudley (Jan 25, 2011)

DaveBP said:
			
		

> But if you change fan speed to control your desired flue temperature won't that throw out your exquisitely tuned air adjustments?
> 
> Which parameter (if you must choose to control only one) is more important to overall efficiency?



Dunno, I set mine to the factory specs and on low speed or high speed I see a lean blue flame, probably sub-optimal.

I'm assuming for best system efficiency I want the lowest flue temperature possible while maintaining gasification and generating enough heat to keep up with the load.  

I'm fairly indifferent that it may be burning somewhat leaner than optimal, which is to say I doubt I would ever see a payback on installing and maintaining a lambda control system.


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## DaveBP (Jan 25, 2011)

So can you, at a given constant fan speed, change your air adjustment and notice any change in flue temp?

And same question at high and low fan speed?


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## ewdudley (Jan 25, 2011)

DaveBP said:
			
		

> So can you, at a given constant fan speed, change your air adjustment and notice any change in flue temp?
> 
> And same question at high and low fan speed?



My flue temperature is very sensitive to inlet damper.  

The boiler has a mickey-mouse flap over about a 12 cm inlet port, with a set screw that determines minimum flap opening.  A change of on-the-order-of 0.5 mm in the set screw setting will have an on-the-order-of 20 kelvin effect in flue temperature.  It is of course less sensitive with low-speed-fan and more sensitive with high-speed-fan.


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## Singed Eyebrows (Jan 25, 2011)

I used a cartridge thermostat to control stack temps on a non gasser boiler & it worked well./// My Atmos is similarly sensitive to small changes in intake air. It is much better now that I bent 2 of 6 vanes on the impeller flat as the cast iron manual damper would show it was dropping vacuume, I do not believe it was affecting under nozzle vacuume much at all. The German GSE Atmos uses a Belimo servo valve that the controller uses flue gas temps to regulate. This is a large ball valve that is rotated for air intake. Randy


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## jebatty (Jan 25, 2011)

Put the idea to work. I had a K-type electronic temperature controller, jerry-rigged it into place, and first run this morning I set it to turn the fan off at 430F, back on at 415F (this is adjustable). No change in primary or secondary air settings. Except for the very high burn period, temp stays under 430F. At high burn, fan off at 430, takes about 5-10 seconds for the stack to cool 15F, fan back on, etc. During the off period, still a natural draft and gasification continues. I don't see any benefit or reason to change the air settings for this very limited "off" period. What adverse effects might be anticipated?

First impression is that this works very well to clip the high temperature peaks. I will give it another couple of tests to see if I notice any adverse effects by way of any buildup in the HX tubes or anywhere else. Between roughly 380-430F my Tarm purrs very nicely with high output, and this seems to be the ideal operating temperature.


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## ewdudley (Jan 25, 2011)

jebatty said:
			
		

> Put the idea to work.
> ....
> 
> During the off period, still a natural draft and gasification continues.
> ...



Nice.  

I was assuming I had to have some fan running all the time, which made the inlet damper setting critical, which led to a less-than-robust control in some situations.  I'm going to try it now with bang-bang control and see what happens.

Does anybody know if the fan motor is horribly inefficient when it is spinning up from a dead stop?

--ewd


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## RobC (Jan 26, 2011)

I had just added a Barometric damper to my set up. I mention this because on my second wood loading, to finish getting storage up to temperature. The boiler started to idle. With the fresh load of wood and the extra smoke going by damper there was quite a back-flash at the damper. This may be something to consider in choosing the bang-bang vs, high low speed control.


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## kielka (Jan 26, 2011)

Hello there. I have a Scandtec (formerly tarm) solo plus 40 I installed in 2008. I'm only really getting into this forum lately.  After reading this thread my mind is boggled by the way you guys are tweaking and controlling your boilers. I'm very impressed. Is there a thread that I can read up on to better my burns? are the gains that good?  I've always thrown the wood in and let the baby burn flat out until it reaches water temp. I'm now wondering if there is an aspect of using a gassifier that I'm completely missing.


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## RobC (Jan 26, 2011)

Welcome Kiel. Do you have storage ?


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## kielka (Jan 26, 2011)

No I do not. I just made a post asking about some DIY ideas being as I've been thinking about it since I got my boiler but cost's through the dealer were exceptionally high. Also I have an older home without a means to easily install a large water storage tank. I'm open to ideas and would love to look at options.


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## jebatty (Jan 26, 2011)

> Rob C: The boiler started to idle. With the fresh load of wood and the extra smoke going by damper there was quite a back-flash at the damper. This may be something to consider in choosing the bang-bang vs, high low speed control.



I think the introduction of a barometric damper changes things, and that the flashback is related to the BD in that it allows air (oxygen) into the stack which allows for the possibility of unburned gases in the stack to ignite, and in your case, flashback. This might be a particular issue on a reload, if the boiler has not burned down to low coals and a good fire or substantial coals remain, followed quickly by idle, as the reload results in an immediate high burn and rush of combustible gases, probably more than the boiler can consume, draft fan shuts down, high combustibles still going up the stack, mixing with air from the BD, and ignition/flashback. It probably would be good practice to avoid a reload if an idle situation is imminent. A reload followed by a good burn period without idle would not present the same problem, as the initial rush of gases from a fresh load of wood have then substantially burned out. 

If there is no BD present, bang-bang control of the draft fan does not allow air into the stack. So far as boiler operation is concerned, this is similar to the draft fan shutting down as the boiler goes into idle. Some air may still be pulled through the system by the natural draft, but that air is coming through the firebox, then the gasification chamber, and is consumed by combustion where the combustion is supposed to occur. Even if uncombusted gases go up the stack, there is no air present to cause combustion, and the situation is the same as the initial idle process. But I see an exception, at least in my situation reported above, and the exception being that the draft fan is off only for a few seconds, and then operation continues, although this may repeat several times. Yet, the issue of unburned gases going up the stack remains, and some good thinking and experience may be needed to see how great a problem, if any, this might be. It seems to me, however, that it is little different than a lambda control to maintain a desirable rate of burn. Shutting down the draft fan starves the fire of oxygen, as a lamda control also would do. What do others think?


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## RobC (Jan 26, 2011)

Jim, your description of my experience is accurate. My scenario was adding more wood to the burn to finish getting storage fully charged. When my return temps start to climb the 60 will start to cycle, but well before one would expect.


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## Singed Eyebrows (Jan 26, 2011)

jebatty said:
			
		

> > Rob C: The boiler started to idle. With the fresh load of wood and the extra smoke going by damper there was quite a back-flash at the damper. This may be something to consider in choosing the bang-bang vs, high low speed control.
> 
> 
> 
> Shutting down the draft fan starves the fire of oxygen, as a lamda control also would do. What do others think?


 I had thought that the Lamda regulated primary & secondary air to get a clean burn. Does it also just shut off the draft fan? Why would it need to do this if it can adjust the flaps way down? I believe the Froling has a variable speed fan & this with the control over the 2 flaps & I wouldn't see any need to start stop the fan. Randy PS, Only if boiler & tank were up to temp would it need to shut off fan?


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## TCaldwell (Jan 26, 2011)

What you are discussing really requires 2 closed loops. loop one if you will would need to control draft pressure in the firebox to maintain a slightly negative pressure with respect to ambient air pressure. the result of no control is the example of the garn puffing at startup, it actually vasilates from a negative normal state, to a positive pressure caused by the accumulation of unburned gasses supplied o2 and they ignite. this scenario is caused by a imbalance of primary/secondary control and total air volume with respect to the ammount of fire present. The concept is to maintain a set neg pressure in the firebox no mater how large or small the fire is, this can be done 2 ways. One with a vfd/3ph motor a differential pressure controller. beginning and end of burn would require less air to maintain a neg pressure because there is less fire fo consume o2, as the fire ramps up so would the 3ph motor to supply more air to maintain the same neg pressure. The second way is to use a fan damper in the flue, no vfd and 3 phase, but the same  differential pressure controller. It would modulate the damper to close to restrict airflow at beginning and end of burn. The pitfalls to this method for a wood boiler are periodic cleaning of the damper and flue in a dirty flue stream, the wasted energy of a fan and damper fighting each other and the non linearity of a round damper with respect to airflow. you probably have noticed a round damper does most of its control in 30% of its travel.  the preferred but more costly way is the vfd, but you also get if the need arises the availibility for more than 60hz of fan speed and the vfd is more linear in air volume control.        Once you have that loop set, the second loop is the actual o2 control with a insitu fluegas analyser with  a sensor in the fluestack, monitoring residual o2 after the p/s air control. The o2 range of 0-20.9% is your process variable, the setpoint you want to attain is approx 7%, stiometeric value for wood at 20%mc. The primary air damper burns the wood creating woodgas, the secondary air damper burns the woodgas hopefully to maintain setpoint.  Pid is one method of control by using the o2 signal as your input and output signals to the p/s dampers. pid requires a defined variables, to create a model for controller settings for a given situation, or set of given situations. Fuzzy logic is based on user defined rules, more leniant criteria for varying process. mfa, Model free adaptive control uses a neural network , has memory and only objective is to get the process value as close to setpoint as possible, regardless how non linear the process.  the first 2 are commonly used by oem manufacturers in biomass control, they have extensive r and d into controller tuning.  Also by maintaining a set differential pressure in the firebox it makes the primary/secondary air control more linear and predictable, enabling the use of pid and fuzzy sucessfully, secondally you are able to maintain setpoint longer because the p/s dampers are working longer in there range, with controlled differential pressure. This whole control practice is used to varying degrees by european mfgrs, they simply call it draft pressure and o2 control. There are many mfgrs here that encorporate all of the above  with a burner controller and other peripheral equipt. I am working with one now micromod in rochester ny to control my garn.


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## jebatty (Jan 26, 2011)

There is plenty of technology and science behind draft and O2 control. TCaldwell has a good handle on this.

Perhaps back to one original question. Will bang-bang control of the draft fan (no other air adjustment) like I initially described, and the experience on which I reported, likely to cause issues and, if so, what are they? When I watched the burn yesterday, the on-off draft fan control appeared to work very satisfactorily. It maintained a maximum high stack temp of 430-415, which continued for about 30 minutes, and then stack temp settled under 430 for the rest of the burn. During the "off" period (5-10 sec), hot burning gases continued to swirl in the gasification chamber, and natural draft, albeit much less than the fan forced draft, continued as well, as the fire and stack cooled 15F and the draft fan came back on. So, what are the things to be concerned about?


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## Nofossil (Jan 26, 2011)

I've been following this thread. As long as bang-bang doesn't result in excessive / prolonged idling I don't see a problem.

Where I get curious is the relationship of fan speed to flue gas temp. Since I run many short fires, I'm particularly interested in the beginning and end of the fire.

It seems to me that once secondary combustion is well established at the beginning, there's a strong positive correlation with a lot of gain. A little fan speed increase gives you a lot more flame and heat. 

Once the surface if the wood is charred, it appears to me that the amount of wood gas generation slows considerably, and flue temp is much less affected by fan speed.

At the end, there's not much wood gas at all, and increasing fan speed just pushes more excess air through the system.

I'm a bit surprised and very interested in the idea that you can close the loop around stack temperature - very cool. How well does it work towards the end of the fire, and how do you decide it's time to give up and shut the fan off?


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## ewdudley (Jan 26, 2011)

jebatty said:
			
		

> There is plenty of technology and science behind draft and O2 control. TCaldwell has a good handle on this.
> 
> Perhaps back to one original question. Will bang-bang control of the draft fan work
> ...
> So, what are the things to be concerned about?



You've demonstrated that it works, and if  you're not running too rich or extraordinarily lean you're good.  Extending the burn with lower flue gas temperature equates to better thermodynamic efficiency assuming you're not suffering offsetting losses due to the boiler wasting more heat though its skin by sitting there hot longer or other such secondary effects.

It's possible you'd be promoting more unburnt hydrocarbons and whatnot on one end or more oxides of nitrogen and so forth on the other end, but if you're happy with the amount of wood going in and the amount of heat coming out you can externalize those concerns for now.  At least until it's no longer legal not to have lambda control on wood boilers, even if such concerns have no significance.

--ewd


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## Singed Eyebrows (Jan 26, 2011)

jebatty said:
			
		

> There is plenty of technology and science behind draft and O2 control. TCaldwell has a good handle on this.
> 
> Perhaps back to one original question. Will bang-bang control of the draft fan (no other air adjustment) like I initially described, and the experience on which I reported, likely to cause issues and, if so, what are they? When I watched the burn yesterday, the on-off draft fan control appeared to work very satisfactorily. It maintained a maximum high stack temp of 430-415, which continued for about 30 minutes, and then stack temp settled under 430 for the rest of the burn. During the "off" period (5-10 sec), hot burning gases continued to swirl in the gasification chamber, and natural draft, albeit much less than the fan forced draft, continued as well, as the fire and stack cooled 15F and the draft fan came back on. So, what are the things to be concerned about?


 Jim, I think efficiency & emissions probably suffer under the non running fan. Even though you have good stack temps you have way lower cfm through the stack. You might have a 1000 degree torch flame under natural draft & still have good 400 degree flue gas because of the lower cfm. I think much of the efficiency of a gasser is that hot(2000 degree) flame that is easily lost(transfered). I would guess much more unburned particulates are going up the chimney on natural draft, Randy


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## pybyr (Jan 26, 2011)

jebatty said:
			
		

> Put the idea to work. I had a K-type electronic temperature controller, jerry-rigged it into place, and first run this morning I set it to turn the fan off at 430F, back on at 415F (this is adjustable). No change in primary or secondary air settings. Except for the very high burn period, temp stays under 430F. At high burn, fan off at 430, takes about 5-10 seconds for the stack to cool 15F, fan back on, etc. During the off period, still a natural draft and gasification continues. I don't see any benefit or reason to change the air settings for this very limited "off" period. What adverse effects might be anticipated?
> 
> First impression is that this works very well to clip the high temperature peaks. I will give it another couple of tests to see if I notice any adverse effects by way of any buildup in the HX tubes or anywhere else. Between roughly 380-430F my Tarm purrs very nicely with high output, and this seems to be the ideal operating temperature.



This is a very intriguing set of ideas and experiments, and I am looking forward to following continued results, especially given Jim's knack at instrumentation and methodical documentation-- it'll be interesting to see if this boosts net btu capture per pound of wood.  

I do wonder whether combustion efficiency during the no-fan intervals may drop enough that it counterbalances the benefit of lower average flue temps, but that's just a curiousity, not a criticism.

Re: adverse effects, as far as I know, all of these units use capacitor run motors.  While I don't have expertise in such items, I wonder if frequent cycling may create more strain on the cap/ shorter lifespan (most electronic components seem to experience more stress at turn-on than when steadily running).  Not a reason to not try this, just something to perhaps be aware of.

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)?


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## jebatty (Jan 26, 2011)

Thanks for the continuing input.



> NoFo: Iâ€™m a bit surprised and very interested in the idea that you can close the loop around stack temperature - very cool. How well does it work towards the end of the fire, and how do you decide itâ€™s time to give up and shut the fan off?



What I have described has no impact on the time to shut the fan off. I'm only impacting the very high burn output which occurs, as you described, at the beginning of the burn and before the wood is charred. This seems to be the time that the highest volume of volatile gases are emitted. Other than on-off I am not changing the speed of the draft fan at any time.

The Tarm has an adjustable lo-limit control with manual reset based on smoke-box temperature (area above the HX tubes). I have set this at 100C. After the burn is finished and when the smoke-box temp falls to the lo-limit setting, the lo-limit shuts the draft fan off because at that low temp there is nothing left to be burned. 

I think also I agree generally with your description of the burn process and the diminishing impact of air volume as the burn progresses.



> ewdudley: Extending the burn with lower flue gas temperature equates to better thermodynamic efficiency assuming youâ€™re not suffering offsetting losses due to the boiler wasting more heat though its skin by sitting there hot longer or other such secondary effects.
> 
> Itâ€™s possible youâ€™d be promoting more unburnt hydrocarbons and whatnot on one end or more oxides of nitrogen and so forth on the other end, but if youâ€™re happy with the amount of wood going in and the amount of heat coming out you can externalize those concerns for now.



Getting better thermodynamic efficiency was my goal, as higher than needed or desirable flue gas temp wastes considerable btu's through failure of heat transfer to the water. I would agree that unburnt hydrocarbons likely are being emitted. The net effect on wood burned will be a tough one to measure as there are plenty of other variables at work in any burn scenario, other than the laboratory. Prior measurements of heat transfer to storage water show the Tarm operating at 80%+ efficiency, based on 6050 btu/lb of wood input. I will attempt to see if this changes with the new burn procedure, but that will have to wait a couple of weeks because my old temperature controller quit on me and I just ordered a new one.

Boiler waste through the skin is of no concern. My boiler is in the heated space and all skin "waste" heat just means less need for stored heat later.



> Randy: Jim, I think efficiency & emissions probably suffer under the non running fan.... I think much of the efficiency of a gasser is that hot(2000 degree) flame that is easily lost(transfered). I would guess much more unburned particulates are going up the chimney on natural draft.



On the one hand I agree in loss of *burn* efficiency and increase in emissions during the "off" period and first few seconds of the "on" until gasification is fully reestablished. But on the other hand I don't think I'm losing efficiency in heat transfer to the water; rather I think I may be gaining some efficiency in heat transfer. I'm measuring stack temp, which I have limited at this time to 430F maximum. I would think I'm losing heat transfer efficiency as this temp increases, at the expense of burn efficiency and increase in emissions. The 2000F gases your mention quickly cool as they travel to the HX tubes and up to the stack, exiting in my controlled case at 430F, rather than perhaps a uncontrolled 460-480F, which I have seen on extreme high burn occasions.


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## DaveBP (Jan 26, 2011)

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.


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## ewdudley (Jan 26, 2011)

pybyr said:
			
		

> ...
> 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


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## jebatty (Jan 26, 2011)

> 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?


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## TCaldwell (Jan 26, 2011)

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.


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## jebatty (Jan 26, 2011)

> Kiel: After reading this thread my mind is boggled by the way you guys are tweaking and controlling your boilers. Iâ€™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!


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## TCaldwell (Jan 26, 2011)

some available control technology in europe,  fumis controller-- sales@atech.si     and  micatron mog 1000-02K  --  www.micatrone.se


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## Singed Eyebrows (Jan 26, 2011)

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


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## jebatty (Jan 26, 2011)

for the *atech.si* site, see Fumis.


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## TCaldwell (Jan 26, 2011)

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.


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## jebatty (Jan 26, 2011)

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.


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## pybyr (Jan 26, 2011)

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.


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## Singed Eyebrows (Jan 26, 2011)

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


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## jebatty (Jan 26, 2011)

> Along with data (once youâ€™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.


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## Nofossil (Jan 26, 2011)

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.


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## jebatty (Jan 27, 2011)

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.


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## ewdudley (Jan 27, 2011)

jebatty said:
			
		

> 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


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## ewdudley (Jan 27, 2011)

nofossil said:
			
		

> 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


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## kielka (Jan 27, 2011)

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


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## pybyr (Jan 27, 2011)

kiel said:
			
		

> 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.


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## jebatty (Jan 27, 2011)

> 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.


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## TCaldwell (Jan 28, 2011)

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.


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## ewdudley (Feb 5, 2011)

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


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## jebatty (Feb 6, 2011)

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?


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## DaveBP (Feb 6, 2011)

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?


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## TCaldwell (Feb 6, 2011)

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.


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## pybyr (Feb 6, 2011)

jebatty said:
			
		

> 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?



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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## TCaldwell (Feb 6, 2011)

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.


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## TCaldwell (Feb 9, 2011)

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


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## ewdudley (Feb 10, 2011)

TCaldwell said:
			
		

> did ssr arive, curious how it works?



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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## ewdudley (Feb 12, 2011)

TCaldwell said:
			
		

> curious how it works



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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## steam man (Feb 13, 2011)

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.


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## ewdudley (Feb 13, 2011)

steam man said:
			
		

> If the valve flow characteristics are not linear you may have a problem trying to tune the controller for stability.
> 
> It seems to me you shouldn't have to open the valve all the way to get full flow.


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.


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## jebatty (Feb 13, 2011)

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!


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## ewdudley (Feb 13, 2011)

jebatty said:
			
		

> 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?


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. 



> 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.


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.


> 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!



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.


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## jebatty (Feb 13, 2011)

> Iâ€™m running out of excuses for the hot tub not being online yet, so my priorities might get shuffled.



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



> From what Iâ€™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.



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.


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## DaveBP (Feb 13, 2011)

> Iâ€™m wondering though, in addition to dropping flue temp, I think Iâ€™m slowing the burn rate and getting longer burns at lower temp for the same load of wood. Any thought on that?



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.


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## jebatty (Feb 14, 2011)

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.


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## ewdudley (Feb 15, 2011)

jebatty said:
			
		

> 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.



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


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## ewdudley (Feb 15, 2011)

DaveBP said:
			
		

> > Iâ€™m wondering though, in addition to dropping flue temp, I think Iâ€™m slowing the burn rate and getting longer burns at lower temp for the same load of wood. Any thought on that?
> 
> 
> 
> ...



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.


> 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



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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