# Trying to Understand Modulation



## Eric Johnson (Feb 1, 2008)

Some recent excellent discussions about Veissmann's (as yet unavailable in the USA) modulating wood-fired boiler (I think it's the Vitolig 200) have got me pondering the whole concept of modulation. I guess, to start with the fundamentals, "modulation" can be defined as the boiler controller automatically adjusting the boiler's heat output to conditions. So, for example, a 100 KBTU/hour modulating boiler would actually be capable of producing variable heat output, effectively making the boiler many boilers in one.

I know this is what the premium gas-fired boilers being sold today can do, but I guess I don't understand why. Presumably there's some advantage in running a gas boiler for a longer period at a lower output, but I don't what that would be.

When it comes to wood gasification, I can see some real potential advantages to modulation, namely lessening the need for hot water storage and allowing the boiler to run for longer periods with the same amount of fuel during periods of low demand, without idling. The Veissmann literature says that the Vitolig 200 achieves modulation by varying the draft blower speed, which doesn't sound like a big deal, but I assume there's a lot more to it than that. I also notice that the blower on that boiler is mounted on the exhaust side so that it's pulling instead of pushing, like the fans do on the more common gassifiers like many of us have. Somebody explained this the other day and their explanation made sense to me.

Any thoughts or clarifications would be more than welcome.


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## solarguy (Feb 1, 2008)

The theory behind modulation is quite simple.
When there is a large demand on the system the boiler will ramp up to 100%
of it's firing rate. This firing rate is determined by the system water temperature.
As you close the gap towards your high limit setting the micropressor control
will modulate the firing rate down to as low as 30%. This saves energy
by allowing the boiler to sip fuel instead of binge drinking.  When we size
heating systems, we'll spec a larger boiler & fire it at 30% of it's rate. 
What we've found is when a boiler is allowed to modulate, very rarely will it operate
at full fire on a yearly basis.   

Tarm's pellet boilers modulate down to 15% (I think) of their firing rate.

With the right control setup Ekos are prime to modulate due to their VFD combustion air fans.


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## Eric Johnson (Feb 1, 2008)

Thanks for that excellent explanation, solarguy. Now I see why everyone wants to get away from the bang-bang on/off approach. Striving to keep everything on a gentler course would seem to be more efficient and easier on the (increasingly sensitive and delicate) equipment.

I guess the EKO does modulate to some extent since, as you point out, its controller slows the fan speed when you near the high limit. What I think you're saying is that with a more sophisticated controller, the EKO could actually modulate across a much wider temp range with otherwise stock equipment. Pretty cool. I think nofossil and Indiana Bill are already hard at work on that.


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## solarguy (Feb 1, 2008)

That's correct. If you could run an Eko or any other forced draft boiler say at 40% of it's firing rate over a longer period of time it would burn more efficient & you'd get a longer burn.
With the right control you could dial it for 40, 60, 80 or 100% just based upon water temperature.

I haven't had a chance to really wrap my head around the mutiple firing rates yet because I ain't got an Eko to play with yet. But I've considered a 2 stage boiler controller. Stage 1 would run @ 40% of the firing rate, when it could no longer keep up, stage 2 would kick in & ramp the fan up to 100% or whatever percentage you deemed necessary. A 2 stage boiler controller is around 700 bucks & I'm sure there's a cheaper way to do this.


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## wdc1160 (Feb 1, 2008)

Eric, 

I posted a similar thread about this before. I still have questions myself.  And, after having done some research...  I am going to attempt several explanations-  Most unfounded , and likely not believable. 
 I don't think that it would be "easy" to make an EKO mod.  
Furthermore,  I don't think it can be done in the same way as a boiler designed especially for moding(at least the vito200 and HMX.)


Here is why solarguy hop in if you don't think is correct.

Ironically some of my best explanations from this came from engineers at GW.
They face issues with efficiency that I believe are similar, because of their not having the ability to induce draft.  For all intents and purposes it is very similar in nature to purposefully smothering.  They address the problem by advising their clients not to burn wood to dry and not to small.

To paraphrase:
They loose a great deal of efficiency when people out of habit use small splits.  The small splits loose gas to fast.  Its not calibrated for that.(high surface area).  It also has no way to artificially induce draft (speed combust).  So high surface are and low moisture can easily have gases escape from the stack. 

According to NOFO some of the testing that’s done on EKO uses very high surface area.  It can induce a draft.  Clearly an advantage when you want a raging hot flame.  

Slowing the reaction down is tricky.  To slow and your flue gases are turned to creosote on the walls of your stack.  

During most of the discussions here the mechanisms that people talk about when regulating combustion is air control.  Controlling the oxidizer.
A 30% reduction in heat output in an EKO by (smothering) results in similar results as an OWB.  You would infact be approaching this using the same strategy. 


I think you’ll find that the HMX and vito200 have other mechanisms in addition to oxidizer control.

1.    They try to trap flue gas by regulating what type of gas escapes the flue.  
They try to do this by allowing gas to move slowly through SS pipes and allow the gas to “settle” or slow it speed enough to stratify into different densities.

2.    I think they try to expose limited wood to the gasification process at one time; The EKO equivalent of blocking off nozzles on the fly.
This is proprietary/ trade secrets stuff.  They don’t explain this anywhere.  This comes from dealers.  Does it make sense to you??
Or, are they pulling my short Italian leg?


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## solarguy (Feb 1, 2008)

I don't know ABGWD4U. Some of it makes sense but I tend to think you can modulate anything so long as you don't go below the minimum firing rate as set forth by the manufacturer. In the case of Eko, it's 40%. I've read the EKO literature & I'm under the impression you can manually dial in the combustion air fan for whatever percentage you'd like to run it at. If that's not true let me know. If it is true than you should be able to modulate the fan anywhere within the boilers temperature range. 

The cresote issue I think would be directly related to return/boiler shell water temperature. If you modulated too low & allowed the shell temperature to drop off below 130 or 140 that would have an effect on the gasification process & in turn make it a smoker. I also think draft also plays an important part in the equation. If your chimney has a marginal draft than modulating down could have a negative effect so that would be an important thing to check before deciding to proceed. 


I like thinking outside the box, sometimes it works, sometimes it doesn't.....  

Have a good weekend!


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## wdc1160 (Feb 1, 2008)

I have seen that posted by many smart people solar guy.  But, if it were that easy wouldn’t everyone be using a modulating boiler?  If all that is required is a fan that modulates and a demand sensor. I would have the damn thing in by now.  
We have some terms that are confused I think.

Here are some of the axioms I will givem to you point by point if you disagree.  I'll show you who I stole the idea from and we will see if we can figure it out cause it drives me crazy not  knowing such a key feature.

First is the definition of modulate.  I consider modulation to increase and decrease output without "significantly" reducing efficiency.  for this discussion lets say 10 percent.  Gas boilers modulate.  Pellet boilers modulate to a certain point.  At idle I think much of their heat doesn't make it to the system.  

I think viessmann is saying at optimum "lab" we can get 92 percent efficiency.  I will hopefully be able to get 80.

Any lower than 70 percent and it doesn't modulate, now it is sustaining.
My contention is that from 13KW to 40 KW the Viessman corp claims their boiler is at least 70% efficient.  All things being equal (wood, draft, etc…)

I don’t believe you can do that by controlling the air alone. 

Here is why.  Quick disclaimer…  This is not my area of expertise.  


How can the vito go down to 33 percent output w/ 70 percent + efficiency.


Idea 1.
@ 70% efficiency you cannot break out of gasification ie you cannot start having chemical reaction at low temps (like normal wood combustion).
We have all read, and all agree two types of chem. Reactions occur during gasification.  Combustion of low temp molecules and high temp molecules (endothermic and exothermic-- yada yada).  
A ton of people have weighted in on this topic.  I don’t want this to turn into one of those theory discussions.  I take it as gospel truth that certain conditions must be present.

Idea 2.
Air starvation stops chemical reactions from occurring.  Enough starvation and you break out of Gassfication. Also a ton of theory and conjecture about the amounts and finer points have been posted online about this.  However, I have never been able to get the numbers to jive where an EKO running at @ 40 % output and still be gasifying. 
Nofo’s graphs somewhat slam this idea home --if you’re a skeptic.   But he almost never runs @ 40 output unless he messes something up.




Idea3
@ 33 percent output a large portion of the wood just cooks.  During the cooking flammable gases are given off.  If conditions (ambient temp and O2) are not correct it cannot combust-- it goes up the pipe. Often we talk about wood Mo content.
The Mo steals our energy for our chem. reactions and we don’t have the juice to get full temp during some periods of burn time.  Not having air, not having energy/temp.  These are the things as wood burners we control, and so we talk about them the greatest amount.

We control these with our stacks, inducers, fuels type, mo content, and additives.
When we are talking about gasifiers we are indirectly manipulating how and when our wood gas is released.  


Idea three is something I have never seen talked a great deal about.  How to reclaim this gas; Prevent wood cooking in times of little need.  How to preserve heat in the firebox to keep high temps for long periods so rapid gasification can be re-achieved.

I think that these issue are going to be the most elusive when discussing modulating.  Unfortunately.  I only know 3 people have seen these in action.  All say it is different.  But, none can tell in great detail why.


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## Bob Rohr (Feb 2, 2008)

I know the gas boiler modulation folks struggle with ideal operating efficiencies across the modulation range.  i had a good chat with the EBM Papst folks at AHR.  www.ebmpapst.com They make most of the variable speed motors and control boards for mod con boilers.  They recently bought a control company and are planning on the next generation of motors and control.

The trick is the unknown and ever changing calorific content of the fuel we get.  as more and more imported NG is blended the BTU content is all over the place.

Theri new system is called LambdaConstant System, detects combustion quality and adjust accordingly.  barometric pressure, wind effects, length of the flue, all have an effect on combustion.

Sounds like a lot of technology to get the ideal combustion with a fairly stable fuel like gas or LP.  I imagine wood fired combustion would be VERY hard to get a handle on.

Time and money, I suppose.  i can't imagine what the cost or complexity of this techonology would be. Ii wonder that it would be sell-able on a wood fired piece of equipment even if it could be made to perform reliably.

 hr


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## heaterman (Feb 2, 2008)

Simple question/simple answer

Think about your truck and how you drive down the road. Do you get somewhere by flooring the go pedal, getting up to high speed and then coasting with the key off, or do you "modulate" the gas to maintain a consistent speed?  Which is the most fuel efficient way to get from point A to point B?

A standard boiler of furnace operates just like the first scenario. A modulating boiler operates like the second. 

Got it?


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## wdc1160 (Feb 2, 2008)

Eric I appolgize for hijacking your thread.



> Think about your truck and how you drive down the road. Do you get somewhere by flooring the go pedal, getting up to high speed and then coasting with the key off



I understand what your meaning, but since a gas engine stops using gasoline when the key is turned off I think that the anaolgy falls short of some of the realities of gasification smothering.

I think that it would be closer to using the break eventhough the vehicle has an extremely high idle rate.  Say maybe it idels at 50-60% of max rpms. 

Its very hard to have solid fuel compared to liquid or gas, because they so naturally lend themselves to modulation.  They simply do not introduce more fuel to combust.  Its hard to imagine an internal combustion engine that works collectivly with all the gas in the tank at the same time.  




> I imagine wood fired combustion would be VERY hard to get a handle on.
> 
> Time and money, I suppose.  i can’t imagine what the cost or complexity of this techonology would be. Ii wonder that it would be sell-able on a wood fired piece of equipment even if it could be made to perform reliably.
> 
> hr


It certainly is.   I don't think we know all the mechanisms they use.


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## solarguy (Feb 2, 2008)

I agree with many of the points that have been made here &  I second that statement
about many people being much smarter than me but I still think it can be done. Maybe not to the extent of a fossil fueled boiler but any modulation that will extend the burn time in a wood boiler without causing cresote problems is an extra in my book. 
I'm flushing my OWB & next heating season we'll have an Eko in the basement. We'll start pushing the edge of the envelope, see what the old girl will do.


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## Jersey Bill (Feb 2, 2008)

I have a KP Pyro 32, and very little storage (200 gallons). I am on the 3rd season. Its about as simple as a gassifier can be. The fan is on or off and the 2 air vents are either 1/2 or 2/3 closed. The rating is 109 MBH, but my best attempts to measure output show it at about 80 mbh
I have been trying to modulate it down as far as I can because of the need to match output with need. The lowest sustained measured output in in the 40-50 MBH range with the gassification process working. Below that output, I can maintain the fire by keeping very little wood in the upper chamber, but I think that its too cool for gassification. The only way that I can check this is that the lower secondary chamber doesn't glow, so I am assuming that gassification is not "on". Also the chimney shows a light smoke.

When its 24 deg outside, I can burn @ full output with the boiler output matched to the load, air closed 2/3, stack around 300. Below that outdoor temperature, I can push the output up by opening up the dampers to 1/2, and the stack temperature goes to 350.

I am on the verge of buying a combustion gas analyizer. I have a line on a Testo 335 for $2500. It will do O2 and CO. NOx can be added later for another grand. An E- Instruments meter is $1000 less for the same stuff. Does anyone have any experience with these tools?

I am trying to figure out a way to add 1000 gallons of storage to my system. The hard part is where to put it. I thought about the garage, but then my wife would have to park outside (and I would be sleeping there).


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## Eric Johnson (Feb 2, 2008)

Wow, a KP Pryo. It's great to have you on the board, Bill. Where did you get it?


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## wdc1160 (Feb 2, 2008)

> I agree with many of the points that have been made here & I second that statement
> about many people being much smarter than me but I still think it can be done.



Anything that looks fishy in the description?


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## Jersey Bill (Feb 2, 2008)

I got it from Zenon


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## solarguy (Feb 2, 2008)

>Anything that looks fishy in the description? 

duh, I don't know, I guess so


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## wdc1160 (Feb 2, 2008)

solar,  I was asking if you saw anything in my description you thought neeeded looking over?


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## Bob Rohr (Feb 3, 2008)

The EKO already has a modulating fan, and the control has outdoor reset potential built in.

I think you would be better spending time and money on tweaking distribution efficiencies then trying to make them burn more efficiently??  power the entire system with a couple 40 watt circs for starters.  Design around the lowest possible temperatures.  Insulate all piping.  these are inexpensive mods that show immediate and long term payback.

I visited with dozen's of gasification manufacturers at the ISH Frankfurt fair.  They all employee full time engineers to design and squeeze as much energy from solid fuel as possible.  many have labs and equipment we have never even heard of to design with.

I suspect a few more 5 points could be squeezed, but at what cost and complexity.  I suspect the market would be very small if another grand or so of sensors and microprocessors were installed to squeeze more out of them.



 hr


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## wdc1160 (Feb 3, 2008)

sparks,  this is stock from Viessmann.  We haven't even started talking about making mods.  We don't even know how it works????


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## solarguy (Feb 3, 2008)

<solar, I was asking if you saw anything in my description you thought neeeded looking over? 

I'm sorry ABGWD4U, I don't understand the question. Sometimes I just don't see that deer under the stand.....


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## wdc1160 (Feb 3, 2008)

> Here are some of the axioms I will givem to you point by point if you disagree.  I’ll show you who I stole the idea from and we will see if we can figure it out cause it drives me crazy not knowing such a key feature.
> 
> First is the definition of modulate.  I consider modulation to increase and decrease output without “significantly” reducing efficiency.  for this discussion lets say 10 percent.  Gas boilers modulate.  Pellet boilers modulate to a certain point.  At idle I think much of their heat doesn’t make it to the system.
> 
> ...



I was explaining what I am now beginning to believe is done to acheive modulation.


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## solarguy (Feb 3, 2008)

Ok, I see the deer now

For someone that claims this isn't his area of expertise you did a dam good job putting that thought into words.


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## hkobus (Feb 3, 2008)

Very good discussion and ideas. Working with the Eko 40 for the season now, I have to agree with Sparks. I think we may be better of to optimize the systems around the units rather than controlling the burn. There are just too many variables to control at the same time. I find that even through out one burn cycle the parameters change dramatically, not even accounting for issues like bridging and changing drafts and heat demands, and not to mention the fuel it self.
Modulating the fan on the Eko doesn't seem to do enough, it takes to long for it to slow down and way to long to restart. If I were to make a change to this I think I would see if we could control the primary and secondary air independent from each other through out the burn cycle. They serve very different purposes, but I need to think and measure more about how to achieve that.
As far a the Viessman units go, I believe they are build by other manufacturers under their own name with some minor mods to fit their own specs.

Just my thoughts.


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## wdc1160 (Feb 6, 2008)

hogstroker said:
			
		

> Very good discussion and ideas. Working with the Eko 40 for the season now, I have to agree with Sparks. I think we may be better of to optimize the systems around the units rather than controlling the burn. There are just too many variables to control at the same time. I find that even through out one burn cycle the parameters change dramatically, not even accounting for issues like bridging and changing drafts and heat demands, and not to mention the fuel it self.
> Modulating the fan on the Eko doesn't seem to do enough, it takes to long for it to slow down and way to long to restart. If I were to make a change to this I think I would see if we could control the primary and secondary air independent from each other through out the burn cycle. They serve very different purposes, but I need to think and measure more about how to achieve that.
> As far a the Viessman units go, I believe they are build by other manufacturers under their own name with some minor mods to fit their own specs.
> 
> Just my thoughts.



Very interesting.  I am also in the camp of "Its complicated for me to do"  I was hoping the manufacturer would do it.  I was interested in how they could do it.  But its less important that I understand, so long as they understand and can follow through.  Do you think that the manufacturer could be mistating or embellishing that they can modulate??


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## hkobus (Feb 6, 2008)

> Very interesting.  I am also in the camp of "Its complicated for me to do"  I was hoping the manufacturer would do it.  I was interested in how they could do it.  But its less important that I understand, so long as they understand and can follow through.



I believe that most changes in equipment and inventions are innovation by users, that put this board and it contributors right in the heat of it.



> Do you think that the manufacturer could be mistating or embellishing that they can modulate??



I think we are looking at marketing numbers and actual field performance and conditions that, at times, may not always be the same. That doesn't meen that we can't strife for  optimal performance, I just have a hard time believing we can control all parameters or at least at reasonable cost. ( this must be the farmer in me coming out)
The wood boiler market is also much smaller than natural gas or oil, and this will reduce the number of players and the amount spent on R&D;compared to the other fuel types. ( never mind our fuel being less user friendly)

I also get a kick out of some of the pictures in some of the European flyers, where a boiler is placed in a laundry room with spotless white walls and open door to the living space. :lol:

Henk.


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## Gooserider (Feb 11, 2008)

Maybe a dumb question - but has anyone experimented with using an automotive O2 sensor in a gasifier stack to see what the combustion exhaust looks like?  Would this work as a way to function as a control variable that would account for the varying amounts of combustibles in the wood - say by modulating the fans and / or possibly a stack damper to maintain an "optimum" level of O2 in the exhaust stream?  

Another possible place where it might be possible to modulate, though it would probably be difficult to find an appropriate material and control system, would be the size of the gas orifice between the wood chamber and the 2ndary chamber - If it were feasible to adjust that on the fly - possibly by sliding some sort of plate into the opening - then reducing the volume of gas going into the secondary chamber should reduce the heat output, which should eventually reduce the heat back to the wood chamber and slow down the gas output, nice negative feedback loop?  Might want to have some sort of interlock on the door to the wood chamber though, as it wouldn't be good to open the door on a chamber full of backed up wood gas - a potentially explosive combo...

Gooserider


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## Nofossil (Feb 11, 2008)

Gooserider said:
			
		

> Maybe a dumb question - but has anyone experimented with using an automotive O2 sensor in a gasifier stack to see what the combustion exhaust looks like?  Would this work as a way to function as a control variable that would account for the varying amounts of combustibles in the wood - say by modulating the fans and / or possibly a stack damper to maintain an "optimum" level of O2 in the exhaust stream?
> 
> Another possible place where it might be possible to modulate, though it would probably be difficult to find an appropriate material and control system, would be the size of the gas orifice between the wood chamber and the 2ndary chamber - If it were feasible to adjust that on the fly - possibly by sliding some sort of plate into the opening - then reducing the volume of gas going into the secondary chamber should reduce the heat output, which should eventually reduce the heat back to the wood chamber and slow down the gas output, nice negative feedback loop?  Might want to have some sort of interlock on the door to the wood chamber though, as it wouldn't be good to open the door on a chamber full of backed up wood gas - a potentially explosive combo...
> 
> Gooserider



Automotive O2 sensor is on my list of projects. I don't think it will last long, though, and I think that there are some issues with the sensor electronics that may make it difficult to get much data from it. I'm hoping to at least get some comparative data as I play with the adjustments.

The EKO has adjustments that restrict the amount of air going to the secondary combustion nozzle(s). I'm particularly interested in the following:

1) As you open the nozzles, at what point do you start seeing excess O2? At ideal settings, the flue gas should be about 8% oxygen.

2) As you slow down the fan (or restrict the fan inlet), do you need to change the setting for the secondary inlets?

3) Do the secondary inlets need different settings based on the surface area of the fuel? (small sticks/splits vs large rounds)

If there's a body of hard data out there on these questions, I haven't found it yet. I do know that optimum settings for the EKO looks like about 1.6 x ideal (stoichiometric) oxygen. What does that mean in real life? No idea yet....


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## sled_mack (Feb 11, 2008)

In it's simplest form, we are all modulating to some extent - none of us are running the fans wide open.  At least I don't think so?

I think if you could set min/max limits on the fan speed, you could do something simple with modulation.  It would take some experimentation to find out what is the min speed that you can operate and not lose gassification.  The max could be set to limit the stack temp when the water is at max temp.  It would work, I think, and provide more heat when the water is cold and less heat when the water is hot.  Is it enough to make a difference?  I have no idea.

The next level of sophistication would be to monitor water and stack temp.  If the stack gets too hot, cut the fan speed to prevent too much loss up the stack.

But I think that is only part of it.  I think it needs to include independent primary/secondary air control.  And, I think it needs to analyze the gasses to know how to adjust the ratio.  It may also be helpful to know the secondary chamber temp, to ensure that it is kept hot enough to keep secondary burn going.  All of this leading to a true "set it and forget it" system.

As HR stated, this could be a lot of work for very little gain.  If I had the instrumentation available, I'd be tempted to play with it.  But I'm not likely to make the investment, when an equal investment in insulation would get me solid results.


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## Eric Johnson (Feb 11, 2008)

I think you're describing something in a whole different category than a $6,000 or $7,000 boiler, especially considering the vagaries in the fuel quality.

I don't think you'd need much fan pressure to keep gasification going--once you get it going good. I can shut off my blower and load the stove, then close the bypass damper and get flames coming out of the bottom of the nozzles from the draft alone.


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## Gooserider (Feb 11, 2008)

Another thought that occurs to me along the same lines is that does the monitoring miss something if it looks ONLY at flue temps, without looking at the exhaust VOLUME as well?   I'm not sure how to phrase it from a math standpoint, but if you have an exhaust stream at temperature X, the energy contained in that exhaust is a function of both the temperature and the volume - if you doubled the volume of gas, at the same temp, you've doubled the energy, right?

So if you turn up the fans, thus increasing the volume of air going through the boiler, and get no change, or only a slight decrease in exhaust temps, doesn't that mean you are increasing the total amount of energy going up the stack?

I'm just wondering about this because I see a lot of threads where there is a great deal of importance being placed on stack temps, and a presumption being made that lower stack temps = greater efficiency / lower heat loss up the stack, even if the temp is lowered by increasing the fan speeds...  I'm wondering if this is accurate?

Gooserider


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## Nofossil (Feb 11, 2008)

Gooserider said:
			
		

> I'm just wondering about this because I see a lot of threads where there is a great deal of importance being placed on stack temps, and a presumption being made that lower stack temps = greater efficiency / lower heat loss up the stack, even if the temp is lowered by increasing the fan speeds...  I'm wondering if this is accurate?
> 
> Gooserider



You're absolutely right. Blowing a bunch of extra air through lowers both stack temp AND efficiency. That's why I went through the work of measuring the actual heat output from the boiler for a given weight of wood consumed.


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## TCaldwell (Feb 12, 2008)

My interpretation of stack temps is that it  can be a factor for determining efficiency , however it is very specific to each boiler, due to many factors , stack piping, internal boiler heat exchange design and potential, both fixed and make up the "dna" for each boiler installation, then you have the variables, wood types, return water temps and associated flow rates, internal or external storage, and so on that in any day are constantly changing. the complexity is represented by the threshold of the engineering dept or deep pockets thereof. I read that herlt has a controller that is adjusted at installation with a thermocouple at the top of the fluestack set just above condensing temp, modulating the induced draft , they state , that this is not necessarily the optimum effiency for the boiler, but due to on site differences in install , they must at least meet this , claiming only costs a few percentage points in effiency, the herlt is a large downdraft gassifier. With the garn , draft induced, and no way to manipulate the air at all, and you happen to put too much dry wood in the chamber,and its starving for air they tell you to partially block, disrupt the air flow in front of the primary air inlet with a brick, sometimes works. Each manufacturer seems to have their own 90x to a moving target, and it seems most of us want that last 10%. I am not sure what this will actually do , but  i am going to modulate the garn airflow with a 2 hp 3 ph motor controlled by a variable speed drive, from 1 rpm to 4600 rpm, it currently runs a 3450 rpm.On the low side for a batch burner i can reduce rpm at the end of the burn not shed heat up the stack, on the high side i am curious if it will help with the puffing by supplying more combustion air. this is a start, i am afraid it wont end untill the the vsd is controlled by a fixed mount oxygen trim kit!


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## Gooserider (Feb 12, 2008)

nofossil said:
			
		

> Gooserider said:
> 
> 
> 
> ...



Sounds reasonable, so the next question would be how does one simplify that into something potentially useful for modulating?  I would think that getting a handle on the total number of BTU's getting pushed around would help, I see the following as major categories....
1. Heat loss through boiler exterior - presumably this could be figured off an experimentally derived lookup table, or possibly calculated based on a known thermal transmission property of the boiler insulation and the difference between the firebox / water jacket temp and the exterior temps.

2. Heat being sent out the plumbing - should be a very straightforward number - temperature difference times the volume of the water getting moved - presumably you could get this by measurment in some way, or looking at the pressure and calculating against the pumps flow/head output curve.

3. Heat going out the stack - The challenge would be to figure the volume - stack temps are easy to measure, but volume would be much more of a challenge - something on the order of making sure all air enters through the fans, then trying to figure something on the fan's output curve per RPM, along with the static head that the fan is pushing against to get a volume of air going in, which would need to be corrected for the expansion due to heating, to give a volume going out...  Seems like a very ugly equation...

Gooserider


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## sled_mack (Feb 12, 2008)

There are flow meters available that work for air.  The trick would be in the placement.  Either make some ducting like a cold air intake for the fans and put the flow meter there, or mount the fans away from the boiler and push through the ducting and the flow meter.

I know that only gets you air in, not the volume of air out.  But are you concerned with volume of mass of air exiting?  I'm just thinking a small flowmeter on the inlet measuring cold air will be far less expensive than one on the stack that has to stand up to the heat and other goodies in the exhaust.


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## Nofossil (Feb 12, 2008)

A few of the pros here have meters that measure CO, O2, and other flue gas components. I think you'd come close to ideal if you gradually crank up your secondary air until the CO stops dropping (at something pretty close to zero). We know for the EKO 80 at least that an oxygen concentration of 8% is pretty close to ideal, and that's probably true for gasifiers in general. Any more than that means you're blowing extra air through that's carrying away heat and not contributing to improved combustion.


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## Gooserider (Feb 12, 2008)

sled_mack said:
			
		

> There are flow meters available that work for air.  The trick would be in the placement.  Either make some ducting like a cold air intake for the fans and put the flow meter there, or mount the fans away from the boiler and push through the ducting and the flow meter.
> 
> I know that only gets you air in, not the volume of air out.  But are you concerned with volume of mass of air exiting?  I'm just thinking a small flowmeter on the inlet measuring cold air will be far less expensive than one on the stack that has to stand up to the heat and other goodies in the exhaust.



Most of the airflow meters have a certain level of head loss themselves, so measuring with them will change the flow characteristics of what you are trying to measure...  IOW, you'd have to turn up the fans and burn more energy to make up for the flow meter, though it might not be a significant difference...  

As far as air-in vs. air-out, I agree it would be far easier to measure the air-in, just because of the issues you mention on dealing with the exhast.  However the key numbers for looking at heat up the stack ARE temp and air-out...  For that the air-in is only useful to the extent it allows you to calculate, as opposed to measuring, the air-out.  If we just had the air blowing through a heat source, say a water-air heat exchanger, then it would be a very straightforward calculation.  But we don't, the air is going through a fire, where there are a bunch of chemical reactions occurring and we are adding in all the gasses from the wood breakdown, so I'm not sure how, or if it would be, possible to calculate air-out from simply air-in, especially if the amount of wood gasses aren't readily knowable - weighing the wood is a PITA at best, and would only give you info about the entire burn cycle, I don't see an easy way to figure instantaneous wood gas production...

Gooserider


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## TCaldwell (Feb 13, 2008)

I have a anemometer, and know the cfm in and out of the garn at the static 3450 rpm, along with the 2 inches of w/c with the 3/4 hp motor, I have been told that it will take 2 hp to modulate the motor to 4600 rpm, power required to move the added volume of air at that rpm, physically i should be able to move more air, combustion wise i will be anxious to see what happens, with respect to combustion analysis as nofossil suggests, hopefully then it might need some sort of turbulator for longer heat transfer. I have 3 thermocouples to monitor secondary burn, at the back of the appliance ,and at the top of the fluestack. first i want to know whats going on in the combustion chamber, then figure what parameters to mod by, temp, pressure, time, 02 ect. and the associated controller. when installed i will post results, and welcome all opinions


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## brad068 (Feb 13, 2008)

Hey Tom,

How did the leaf blower work? I'm curious to see how you setup works too. The idea of installing turbulators might offset the more cfms that you are trying to get. Did you receive you parts yet?


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## TCaldwell (Feb 13, 2008)

garnification, the leafblower  setup was not one of the more meaningful experiments, over pressureized the burn chamber, backed up at the draft inducer motor, pretty good flame out the door!  all stuff is in, will install friday , i will keep you posted


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## TCaldwell (Feb 20, 2008)

just hooked up the new motor and variable speed drive to the garn, some first observations, experimenting and using the potentometer or hertz control dial, to increase or decrease fan rpm. keep in mind the stock baseline of the original setup was 60 hz representing 3450 rpm from beginning to end of burn, the conversion is 1 hz=57.5rpm of fan speed. as i increased the hz also did the thermocouple reading in the secondary burn chamber, even more so the thermocouple at the rear of the appliance, stack temp increased alot. with the hz at 75 =4312rpm the burn chamber temp dropped off and the stack temp rose, basically i was blowing alot of heat up the stack. when decreased hz to 55 the burn chamber temp climbed to max temp and the stack temp dropped to about 335 , as the burn cycle came to a close and the burn chamber temps dropped , i  lowered hz to 45 and the burn chamber tenp would rise and the stack temp would stay at 300. thus by decreasing the hz as the burn was ending ,i was extracting more heat , than if i kept the hz at 60 from start to finish, i honestly thought i would get more  performance at the high end , so far more at the low end. i wonder if following a target stack temp over 80 percent of the burn might be more efficient than a shorter higher burn temp coupled with a higher stack temp at the first half of the burn followed by lower burn temp coupled with a lower stack temp caused by too much air flow. bring it on


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## Jimxt88 (Feb 25, 2008)

ABGWD4U said:
			
		

> I think you’ll find that the HMX and vito200 have other mechanisms in addition to oxidizer control.
> 
> 1.    They try to trap flue gas by regulating what type of gas escapes the flue.
> They try to do this by allowing gas to move slowly through SS pipes and allow the gas to “settle” or slow its speed enough to stratify into different densities.
> ...



ABG, this reminds me of something I read yesterday in a book called "The Rational Construction of Frunaces" by W.E. Groume-Grimailo, Petrograd, Russia, 1911.  In which he describes the naturally occuring stratification of wood combustion gases.  He states that in a chamber, carbon dioxide which is heavier than air will stay low and be pushed lower by Hydrogen which is lighter than air and floats.  He describes a furnace design in which carbon dioxide is displaced by Hydrogen and forced out a low exaughst opening while the upper chamber stays hot and filled with hydrogen.  In the quote above you describe slowing the gasses (by use of baffles, I assume) and SS pipes that allow the gas to "settle".  It reminds me of the double bell masonry heater design which exploits this characteristic of gases of different densities to stratify.  I am wondering if it is possible to store the hydrogen gasses from a low temperature burn in the boiler and reintroduce them to the gassifying chamber when the charge has been reoxiginated and temperatures in the gassifier restored.  

Can you say more about what you have learned regarding this modulating technology?


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## wdc1160 (Feb 25, 2008)

As I learn more about the strategies used to modulate wood combustion I have found that the most efficient method of storing the gases is leaving them in the wood.  Consequently a great deal of thought in the design of the vito is put into making sure that no excess wood is cooked.  
So although stratification, separation through baffle, and storage are accomplished its not the ideal measure. (the reintroduction of the gases at a more convenient time when demand and conditions are optimum.)  

Some of the best examples of “high tech” wood burning are from WWII.  They had some pretty clever folks using wood gas.  

However, I have stopped doing work on modulation.  I have found that if I take the same amount of extra money and invest that in storage.  I could also accomplish modulation only through the buffering of heat in a tank.  Additionally, I could accomplish a very efficient burn.

It seems that the natural way that wood likes to be burnt is like a forest fire. We can use math and science to stop this natural process, but it takes great expense and complication.  Although, it isn’t technically beyond our capabilities, It currently doesn’t make fiscal sense to work against natures wish to burn fuel in one uninterrupted duration.  

I had made this determination when I decided that solar during the winter could also be helpful-- Again nature not wanting to fit output to demand.  

I have redoubled my efforts into storage e.g. charging, discharging, insulating, burying, constructing.


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## Jimxt88 (Feb 25, 2008)

ABG, Do you have pictures of your energy system?  I am interested in anything that is working for you.  In particular, I would like to see examples of water storage systems.  I also want to recognize TCaldwell's innovation on his GARN described above.


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## dfergx (Feb 25, 2008)

From reading through this thread I think it seams important to size the gasifier properly. to big and it idles wasting fuel. is this correct?


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## Nofossil (Feb 25, 2008)

Thubten Jamepl said:
			
		

> From reading through this thread I think it seams important to size the gasifier properly. to big and it idles wasting fuel. is this correct?



That's always been my hunch, and there are lots of reasonable theories that would support that conclusion. However, I'll have to admit that I have no hard data to prove it. One thing I've learned through all of this is that when you get around to taking real data, you will be surprised by something.


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## wdc1160 (Feb 25, 2008)

Thubten Jamepl said:
			
		

> From reading through this thread I think it seams important to size the gasifier properly. to big and it idles wasting fuel. is this correct?



 I can't see any reason why oversizing couldn't be considered "wasteful"

In the sense that it loses alot of the effciencies you buy a gasifier for.


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