# 48% gasifier efficiency - Ooops, 56%



## Nofossil (Jan 13, 2008)

EDIT: I mis-clicked and left out part of the burn data. Corrected efficiency is 56%. I changed numbers to match corrected data. Balance of post stands, but improved efficiency from dry wood in later posts is blown out of the water.

Pretty sobering data. I ran a detailed analysis of the live burn that I did two nights ago. Worked out to 48% efficiency. Either I'm missing something big, or I have a lot of room for improvement. Here's how it came out:

94 lbs wood at average 30% moisture = 65.8 pounds 'bone' dry at 8600 BTU/lb = 565880 potential BTU

Top floor: 94 minutes at 180 BTU/min = 11,700 BTU      EDIT: 17010
Main floor: 93 minutes at 300 BTU/min = 15,900 BTU     EDIT: 27750 
Bottom floor: 71 minutes at 380 BTU/min = 7410 BTU EDIT: 26980

Hot Tub: 36 minutes at 586 BTU/min = 21,120 BTU
Hot water: 47134 BTU -> 50979 BTU = 3845 BTU
Storage: 81717 BTU -> 301817 BTU = 220,100 BTU

Total delivered heat energy = 316,805 BTU

Storage BTU is calculated as usable BTU, assuming that 120 degrees is the lowest usable temperature. In the storage tank, the top and bottom temperature probe covers the top and bottom quarter of the tank, while the middle covers the middle half.

The hot water tank is 40 gallons.

Hot tub heat load is based on the previously measured ability to raise the temperature of the 550 gallon hot tub by 8 degrees per hour.

I've clearly lost some efficiency in that the EKO, refractory, steel, and water, is at 140 degrees when the circ shuts down. That heat energy is lost to these calculations, although some percentage of it ends up heating the basement as opposed to going up the stack. This was also a really short burn, with the house needing very little heat and the storage and hot water tanks not depleted at all.

Am I missing something, or am I really only getting 56%?

My wood consumption has been right exactly on track from previous seasons - a bit under two cords from October 15th until now.


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## wdc1160 (Jan 13, 2008)

I'll call you crazy if you use the number 8600
I never use the number 8600 usable btu's-- because the water gets boiled off and that isn't ineffcient on your part -- is it?
Also on the best day your wood wound't have 8600 with 0 moisture.  I think the most I have ever seen manufactuerer use is 7K.




@ 20% moisture your looking at 5800 btu
@ 60% moisture 4000 btu

but,


Another way to look at it



Heating Value per Air-Dried Cord in BTUs 
White Oak 30,600,000 Poplar 17,260,000 
White Elm 24,500,000 White Pine 17,100,000 
Tamarack 24,000,000 Basswood 17,000,000 
White Birch 23,400,000 White Cedar 16,300,00 
Black Ash 22,600,000 White Spruce 16,200,000 
Maple 19,300,000 Balsam Fir 15,500,000 


I don't have a source for this info.....

but my estimate earlier was gross btu's in the 480k range, not 560...


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## SciGuy (Jan 13, 2008)

NOFOSSIL,

As the latent heat of vapoization is 970BTUs per lb of water, the 28.2 lb of water your unit evapoated required an additional 27,354 BTUs to do that task. 
Hmmmmm that bumps you up to 52.5% energy recovered.

Hugh


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## wdc1160 (Jan 13, 2008)

I am also curious how you came to the btu/min absorbtion rates you calculated.  I know your probably being cautious/conservative in your calcs, but

I don't think the absorbtion in your house is going to be a very agressive version of "what could be"

Recalc with 10-8 btu per min per foot  -- Unless your telling me upstairs in your house you only have 18ft of baseboard....


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## Nofossil (Jan 13, 2008)

I used 8600 BTU/lb for wood at 0% moisture. I think that number is good. I had 94 pounds of wood at about 30% moisture. If the right number for potential BTUs is not somewhere around 566,000 BTU, then I'm wrong. I started trying to convert it to the equivalent weight if it had been 20% moisture and then using the 6880 number, but I skipped the intermediate step. I think I had about 66 pounds of actual wood in my load.

I figured that I'm losing a few percent from vaporizing the extra water, but it doesn't come close to explaining the numbers.

I do have exactly 18 feet of baseboard in my top floor. 30 on the main floor and 38 in the basement. I assumed 600 BTU/ft/hr for baseboards.

This is the first time I've weighed the wood. What I'm burning this year is no worse than what I've burned in previous seasons. I did a back-of-the-envelope calculation based on degree days, cords of wood, and historical oil consumption and came up with a delivered efficiency of around 65% or 18,000,000 BTU/cord in previous seasons. This year is exactly on track, so this efficiency calculation really surprised me.


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## wdc1160 (Jan 13, 2008)

> I do have exactly 18 feet of baseboard in my top floor. 30 on the main floor and 38 in the basement. I assumed 600 BTU/ft/hr for baseboards.



Sorry,  I didn't realize.  I thought your upstairs was bigger



According to the cords of woods I listed up above your aproaching 80% of many of the woods listed per cord.  Are those numbers too pessimistic in their calcs??



> I did a back-of-the-envelope calculation based on degree days, cords of wood, and historical oil consumption and came up with a delivered efficiency of around 65% or 18,000,000 BTU/cord in previous seasons.



In order to calculate getting 65% efficiency from a cord @ 18M BTU.  You would have to assume 27.7M BTU per Cord?

What I have
is 56% effcient. from your 94 lbs of wood.  I figured 5100 btu with your wood types.  

Where do you think your losing it??? or are we figuring usage incorrectly?


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## webbie (Jan 13, 2008)

I think you have to adjust for the moisture before you even start figuring on the burn......at least to be fair to the wood!

20% moisture is considered to have 7000 BTU per pound
So, if the scale remains the same, 30% moisture would have about 6200 BTU per pound input.

Additionally, since most units are not built to handle high moisture wood, the burn could be worse than just that. I found that with Tarm boilers, a moisture content over 25% lowered the firebox temps so much that the whole thing was tough to operate. In other words, then the right calcs are done, 48% might seem GOOD.

Your moisture adjustment seems to consider that the moisture burns off "for free".

Check the work.


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## wdc1160 (Jan 13, 2008)

To be clear.  Craig  Nofo -- You didn't burn white oak -- If your going to calc 6200 btu per pound we have a very large dispersion in what we think would btu per pound should be.


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## Buck1200 (Jan 13, 2008)

New guy here... Lurker... love this type of stuff.  I also wonder about the measurement of heat delivery.  Would you be better off monitoring flow rate and delta T straight from the boiler, rather than attempting to calculate delivery at 5 or 6 downstream destinations?  I realize that requires yet another analog in, but it might relieve you from what I see to be quite a few sources of measurement inaccuracy.


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## webbie (Jan 13, 2008)

ABGWD4U said:
			
		

> To be clear.  Craig  Nofo -- You didn't burn white oak -- If your going to calc 6200 btu per pound we have a very large dispersion in what we think would btu per pound should be.



Explain.

From what I read, air seasoned wood has a normal BTU input of 6200-7000 BTU per pound after adjustment for moisture. It is also claimed that the higher BTU woods (if any) are resinous (softwoods...that is why folks like softwood pellets).

So if 6200-7000 at 20-24% moisture, I would assume the lower end at 30%.

In the case of these fuels, I think it is best to always be very conservative in terms of numbers.


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## Nofossil (Jan 13, 2008)

Webmaster said:
			
		

> ABGWD4U said:
> 
> 
> 
> ...



Works out perfectly. I calculated that 94 pounds of wood at 30% moisture would yield 566,000 BTU. That works out to 6021 BTU/lb, so my 48% efficiency figure stands. I use really dry wood (<20%) to start the fire, and burn the greener wood once it's going. 

Gasification is sustained and vigorous throughout, with secondary combustion chamber temps running 900 - 1200 outside the labyrinth. No creosote, no visible smoke, and no odor at all.

I'm quite sure I'd do a bit better with drier wood, but it's not damp enough to interfere with gasification as far as I can see.

My flue temps are around 500 degrees - much higher than I expected. Have to get moving on those turbulators.

I'm curious about the real world difference between combustion efficiency and system efficiency. Could I be running 80% combustion efficiency and losing a bunch to heating of the boiler itself, piping losses, and so on? Anyone else have any data on system level efficiency?

I'd love to add a flowmeter, but I don't have one at the moment. Donations cheerfully accepted ;-)


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## webbie (Jan 13, 2008)

It's great to fiddle with this data, because it shows the folly (which I am constantly trying to point out) of people talking about their Pellet Stoves being over 80% efficient their wood stoves being the same. There is a vast different between combustion, steady-state, AFUE and then the REAL AFUE (such as Energuide from Canada)....and, then, the actual delivered efficiency to a single point (indoors of the home).....

In theory, if a wood stove is making one room 78 degrees, that could be a big loss if the room does not need to be that hot...if you only wanted it to 70. The extra 8 degrees becomes a waste. 

As an example of this, look at the canadian Energuide ratings of freestanding gas stoves. There we are dealing with a very stable flame and heat exchange system. Some unit which are rated at over 80% efficiency by their makers, come in at 60% in Energuide.....that is a 25% difference on the way down, or a 33% difference on the way up - substantial.

I get the feeling that folks think I am being pessimistic when I always figure LOW for the claimed efficiencies of much of this stuff. Yes, the turbulators will help and I think a couple runs with 20-22% wood might make a big difference also....but if your existing numbers are accurate, it might be hard to get to over 60% system efficiency. 

I think the boiler itself may exceed 80% in steady state when it is running. After that, everything goes downhill.


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## wdc1160 (Jan 13, 2008)

> Explain.
> 
> From what I read, air seasoned wood has a normal BTU input of 6200-7000 BTU per pound after adjustment for moisture. It is also claimed that the higher BTU woods (if any) are resinous (softwoods...that is why folks like softwood pellets).
> 
> ...




According to the oregon dept of ag.-- department of measurments standards division
dry hickory has 6.4k btu @ 20 percent
dry black locust "I don't know what locust was used" @ 20 percent =  6.8Kbtu <- didn't realize this was so high

@ 30 percent you could do 6k.  thats believable.


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## Nofossil (Jan 13, 2008)

I teach my clients that they should look at problems as 'gold', because the only way to get better is by finding problems and solving them. I'm looking at this as a problem, and trying to convince myself that it's a good thing in the spirit of practicing what I preach. So here goes:

If my 48% number is correct or even close, that's a good thing because there's a lot of room for improvement. I'm currently burning about 4.5 cords/year to completely replace my oil from mid-October to mid-April for heat, hot water, and hot tub.

If that's at a system efficiency of 48%, then it might be reasonable to think that if I can get to 65% or so, I could get by on 3.5 cords per year. That would be amazing.

So.... where am I losing 50% of my potential BTUs?

 - 15% heat up the flue and incomplete combustion based on EKO claim of 85%
 - 5% additional loss due to latent heat of vaporization for additional water

Where's the other 30%? That's about 180,000 BTU during the course of this fire - not peanuts.


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## leaddog (Jan 13, 2008)

maybe I missed it but I think there is a very sizable loss of heat from the boiler and piping. I know that if you stand very close to the door or put your finger on the edge you will notice. They are insulated but there is parts that aren't and they radiate alot of heat. I have mine is an uninsulated shed and there is a noticeable difference inside and outside.
leaddog


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## cbrodsky (Jan 13, 2008)

nofossil said:
			
		

> Storage: 81717 BTU -> 300619 BTU = 218902 BTU
> 
> Total delivered heat energy = 269858 BTU
> 
> ...



Interesting thread - I haven't been following all the details, but I'm curious to understand more on how the storage value is measured since it seems to dominate the production side of the calculation.

As for delivery loss, assuming you have a standard baseboard system that is turning on/off regularly, you might be able to estimate some value for how much piping you have going to the zones through unconditioned space, then assume it goes to ambient after each heating cycle, and see how many BTUs are being lost to that water/copper cooling down on each cycle.  I've never calculated it - but it's probably simpler than calculating forced air delivery losses.

-Colin


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## kuribo (Jan 13, 2008)

[/quote]


"My flue temps are around 500 degrees - much higher than I expected. Have to get moving on those turbulators.

I'm curious about the real world difference between combustion efficiency and system efficiency. Could I be running 80% combustion efficiency and losing a bunch to heating of the boiler itself, piping losses, and so on? Anyone else have any data on system level efficiency?"

[/quote]


I would think that it would be easy to have 80%+ combustion efficiency and only half of that on a system basis in light of pumping losses. frictional losses, less than perfect heat exchange, heat losses through piping, storage tank, etc., AND 500F flue temps. A lot of heat is being lost up your flue; if your temps outside the combustion chamber are 1200F, you are losing maybe 20-30% up the flue... . The 90%+ efficiency gas condensing boilers have extremely low flue exhaust temps....


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## leaddog (Jan 13, 2008)

If you take your ir thermometer and take some readings all over your boiler you will get some very sobering readings. Be sure and check the bottem legs and top surface. I put some foil/bubble/foil on mine and dropped the readings down but I haven't followed up on it more because of lack of time. maybe I should. There is alot of surface area there. You have it inside so it isn't "heat loss" but I think you will see that it is something that you need to figure into your figures.
leaddog


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## Buck1200 (Jan 13, 2008)

Since nofossil's boiler is within the conditioned space, these losses add to the heating of his house, and so should not necessarily be considered 'wasted'.  Counting them up and controlling them (this heat energy) is another matter, however.   This is where knowing the mass flow rate and composition of your flue gas, as well as accurate temperature, of course, over the entire burn cycle from dead cold to blazing hot to dead cold, and subtracting that from the theoretical energy capacity of the wood being burned would be the only way to accurately determine the absolute efficiency of the boiler, at least as far as liberating heat energy from the wood is concerned.  Again, you might not know where it's all going, but if it's not going up the stack then at least it's heating the house.


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## Eric Johnson (Jan 13, 2008)

That's an excellent point, leaddog. My little boiler room probably stays around 120 from boiler/chimney connector radiant, and the vermiculite-filled cinderblock wall is noticeably warm to the touch. The wood stacked against it is extra dry.

Did you factor the heat produced by your boiler/stovepipe/chimney into your calculation, nofossil? You're recovering a lot of radiant heat that I'm not.

By the way, I agree that a lower-than-expected number is nothing more than an opportunity for improvement. Sounds almost Buddhist. That's my old song about the U.S. energy situation: We waste so much energy that just tightening up our wasteful habits would generate a huge drop in demand, with no perceptible effect on our (relatively lavish) lifestyles.


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## Nofossil (Jan 13, 2008)

NY Soapstone said:
			
		

> As for delivery loss, assuming you have a standard baseboard system that is turning on/off regularly, you might be able to estimate some value for how much piping you have going to the zones through unconditioned space, then assume it goes to ambient after each heating cycle, and see how many BTUs are being lost to that water/copper cooling down on each cycle.  I've never calculated it - but it's probably simpler than calculating forced air delivery losses.
> 
> -Colin



I'll take a crack at it, but the heating zones didn't cycle much during the burn. 




> That’s an excellent point, leaddog. My little boiler room probably stays around 120 from boiler/chimney connector radiant, and the vermiculite-filled cinderblock wall is noticeably warm to the touch. The wood stacked against it is extra dry.
> 
> Did you factor the heat produced by your boiler/stovepipe/chimney into your calculation, nofossil? You’re recovering a lot of radiant heat that I’m not.



I don't seem to see so much heat from the boiler and stovepipe. My boiler room is in the basement, and it's pretty much the same temp as the rest of the basement.





> I would think that it would be easy to have 80%+ combustion efficiency and only half of that on a system basis in light of pumping losses. frictional losses, less than perfect heat exchange, heat losses through piping, storage tank, etc., AND 500F flue temps. A lot of heat is being lost up your flue; if your temps outside the combustion chamber are 1200F, you are losing maybe 20-30% up the flue… . The 90%+ efficiency gas condensing boilers have extremely low flue exhaust temps....



I don't know what flue temp EKO used when calculating their 85% or 87% efficiency. I'm sure that you would need really low flue temps to get above 90%, but I don't think the ratio of combustion and flue temps tells the whole story - a lot of heat comes from the primary chamber as well. If I had a way to estimate the flue gas flow rate, I could calculate the losses there.

Maybe the 180,000 missing BTUs are scattered around, a little here and a little there...



I'm doing another burn tonight. As it happens, it's MUCH drier wood - about 20% average this time. Also smaller pieces.


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## Bob Rohr (Jan 13, 2008)

A btu meter on the output of your tank to the distribution system would be a great place to measure DELIVERED energy.  I think Caleffi has one about to be introduced.  It's designed for solar, not sure the capacity.

BTU metering is very common in Europe, both for district heat systems and apartments.

Most are located in a cabinet in the hallways.  They measure cold water, heat, and sometimes DHW.

In some rental properties you swipe a credit card to prepay an amount of water.  Even the drinking fountains in Germany, when you can find one, have water meters.

You think oil is expensive, just wait.  Water will soon be the most precious resource.

 hr


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## kuribo (Jan 13, 2008)

"I don’t know what flue temp EKO used when calculating their 85% or 87% efficiency. I’m sure that you would need really low flue temps to get above 90%, but I don’t think the ratio of combustion and flue temps tells the whole story - a lot of heat comes from the primary chamber as well. If I had a way to estimate the flue gas flow rate, I could calculate the losses there."


There are several different ways used to calculate "efficiency". You might talk to your manufacturer to find out which method they are using...steady state efficiency. cycle efficiency, or combustion efficiency....

By the way according one text, using wood with a high moisture content can cut the efficiency figures in half...


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## Nofossil (Jan 14, 2008)

New fire burning now with dry wood - check it out using 'voyeur' link in my signature. Secondary combustion and flue temps are both higher. Way too soon to calculate efficiency, but it will be interesting.

One thing that's easy to see in the graph is the seven minute match-to-gasification time. When I close the lower door and turn on the fan, secondary combustion jumps to 900 right away, and is over 1200 within 15 minutes.


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## brad068 (Jan 14, 2008)

nofossil said:
			
		

> New fire burning now with dry wood - check it out using 'voyeur' link in my signature. Secondary combustion and flue temps are both higher. Way too soon to calculate efficiency, but it will be interesting.
> 
> One thing that's easy to see in the graph is the seven minute match-to-gasification time. When I close the lower door and turn on the fan, secondary combustion jumps to 900 right away, and is over 1200 within 15 minutes.



You know NoFo, your graphing reminds me of my job. I use a SCADA system to monitor DO, temps, RAS rates, cycles, etc. If only I could employ this system to my boiler ops.


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## heaterman (Jan 14, 2008)

Raw Combustion efficiency vs system efficiency

Combustion efficiency is relatively easy to measure if you have the tool I.E., a combustion analyzer.

System efficiency, which is what nofossil is trying to measure is a completely different animal and orders of magnitude harder to calculate. There are a host of areas that present parasitic losses on a system, some of which are mentioned in the posts above. 

Trying to calculate system efficiency would involve starting with your combustion numbers and then measuring heat transfer throughout the entire system. This is virtually impossible to do because the whole time you are measuring heat output into your structure in order to compute temp rise, the structure is at the same time losing heat into the great outdoors. So how do you actually know the amount of heat you put into the structure? (This factor alone may be what is skewing your numbers so low.)
 Tough tough tough to do. You're talking lab grade equipment to do the measurements with and a Cray to crunch the numbers.  

One could roughly estimate what's going on if you have a good handle on your flow rate and temp drop from supply to return. This will give you an approximation of the total heat transfered outside the boiler. Then you'd have to factor in the jacket losses of the boiler, the cubic feet and net temp rise of your flue gas going up the stack. Actual btu content of the fuel being burned is obviously a matter of differing opinion...............but you'd have to start there to determine actual input. Without that as your starting number you're just "blowing smoke". (feeble attempt at joke there)  AHHHHHHHHHHHHHHHHHHH!! my head is starting to spin just thinking about it. 

I went to a training class on Riello burners last year and while there talked to a couple of their lab guys doing an AFUE rating test on a boiler with their burner installed on it. They spent the better part of 4 hours just dialing in the flow rate to get the 20* drop called for in AFUE testing protocol. AFUE specs call for testing at 140 supply and 120 return. (Not reflective of 99% of the boiler systems actually out there but hey.....it's a government spec so what did you expect.....) At that point you can begin to measure what the actual output of the boiler is. Next they hooked up their gear to precisely measure fuel input (this one was oil) which was measured to the milligram.........you get my drift........  Then there were flue gas measurements which were factored in to the equation. And on and on and on and this is just measuring boiler/burner efficiency.  All I can say is in less than lab conditions, I don't know how you would get anything close to valid.  

Gotta say it is fun exercising the ol noggin thinking about it.


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## Nofossil (Jan 14, 2008)

> You know NoFo, your graphing reminds me of my job. I use a SCADA system to monitor DO, temps, RAS rates, cycles, etc. If only I could employ this system to my boiler ops.



So... is that a good thing or a bad thing?



			
				heaterman said:
			
		

> Raw Combustion efficiency vs system efficiency
> 
> Combustion efficiency is relatively easy to measure if you have the tool I.E., a combustion analyzer.
> 
> ...



I'm starting out trying to look at it as a 'black box' - I know what goes in in terms of fuel, and I can measure to some reasonable degree of accuracy what comes out as usable heat energy.

I've long since given up trying to analyze every detail, but I'm hoping I can get my hands around any chunks of missing energy that are big enough to be interesting. I'm hoping to learn, and I'm hioping to share whatever I get out of this with the community.


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## Buck1200 (Jan 14, 2008)

One thing you could do to measure the losses from your piping and boiler would be to calculate the steady state heat loss of your basement and, assuming it's the same temperature as the rest of the house, you'll likely find that the number of btu's heading that way through the distribution system is significantly less than what is actually required to maintain that temperature.  The difference is your losses from the boiler jacket, piping, and smoke pipe (as well as light bulbs, transformers, pets, etc).

BTW- I've been through your site from top to bottom: great job.


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## barnartist (Jan 14, 2008)

Nofo,
Im really glad your working on this. At the same time, you heat more with your tiny stove than me with my monster, and I used freakin 5 cords of wood already, AND I did not start burning until Halloween, AND I have turbulators Id give my left !@%&^ to only burn what you burn.
Keep working on the "problem" so I can learn from it man. Ypu house must be R-1000.


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## bbb123 (Jan 14, 2008)

Thats close to the % I got NoFo when I weighed the wood.  I came up with around 50% when I did it.


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## Nofossil (Jan 14, 2008)

bbb123 said:
			
		

> Thats close to the % I got NoFo when I weighed the wood.  I came up with around 50% when I did it.



That's encouraging. Now if we all can figure out how to close the gap between combustion efficiency and system efficiency, life will be better.

Here's the data from last night's burn. First hard data that I have that might start to quantify the difference between dry and really dry wood. 

119.5 lbs wood at average 20% moisture = 95.6 pounds ‘bone’dry at 8600 BTU/lb = 822,160 potential BTU

Top floor: 182 minutes at 180 BTU/min = 32,760 BTU
Main floor: 214 minutes at 300 BTU/min = 64,200 BTU
Bottom floor: 188 minutes at 380 BTU/min = 71,440 BTU

Hot Tub: 85 minutes at 587 BTU/min = 49,867 BTU
Hot water: 47134 BTU -> 50979 BTU = 12,507 BTU
Storage: 81717 BTU -> 300619 BTU = 233,608 BTU

Total delivered heat energy = 464,382 BTU

System efficiency works out to 56%. I would have thought that wasn't too good if I hadn't gotten 48% on the previous burn. This works out to about 16% more delivered energy per pound of bone dry wood, and a 32% increase in BTUs per 'as measured' pound.

Dry wood was not the only difference. For this burn, the house, outside temp, hat water tank, and storage tank were all cooler. The burn was longer as well. with about 50% more actual fuel.

This was the driest wood I've burned this year. It created another problem: The delta T through the boiler was so high that when the controller tried to keep the inlet above 140, it caused the outlet to exceed 180, which shuts down the EKO fan.

I think this means that I need to either install my three speed Grundfos which at high speed pumps more than my Taco 007, or I need to throttle back the EKO when burning wood that's this dry. Am I on the right track here?


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## atlarge54 (Jan 14, 2008)

barnartist said:
			
		

> Nofo,
> Im really glad your working on this. At the same time, you heat more with your tiny stove than me with my monster, and I used freakin 5 cords of wood already, AND I did not start burning until Halloween, AND I have turbulators Id give my left !@%&^ to only burn what you burn.
> Keep working on the "problem" so I can learn from it man. Ypu house must be R-1000.


                                                                                                                                   I've wondered about nofossil's home insulation, windows etc. All the glass people I talk to say don't buy the argon filled windows, they claim seals will go bad. Are those flat roofs? I can't recall, wasn't living space over 3000 sq ft.?


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## bbb123 (Jan 14, 2008)

Nofo I was lookin at your graphs your starting your fire when tank is around 120 and it starts idleing a couple hours after that.  I start mine at around the same temp and mine will not idle until the second load of wood and tank gets up around 170 top 160 bottom (4-6 hours).  I believe I'm running the basic taco cirulator I'm thinkin you HX in your tank isn't workin fast enough.  Or maybe your circulator isn't running 100% it would def. be easier to change that out.  What's the gpm rating on these circulators?


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## slowzuki (Jan 14, 2008)

The 85% would be a steady state efficiency of the boiler, a short burn cycle would be considerably less.  This efficiency would not be seen without turbulators, 500F flue gas is tossing lots of btu's up the chimney.


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## Eric Johnson (Jan 14, 2008)

Would it be possible to attain and sustain a steady state of operation with one of these boilers, or does the need to refuel periodically practically blow that possibility out of the water?

And presumably, your efficiency will vary during the course of a burn cycle. I'm guessing it's a lot lower at the start than in the middle or the end, just because of the moisture content of the wood.


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## Donl (Jan 14, 2008)

nofossil said:
			
		

> 119.5 lbs wood at average 20% moisture = 95.6 pounds ‘bone’dry at 8600 BTU/lb = 822,160 potential BTU



I may have this all wrong, but the way I'm seeing it is that yes the potential for 95.6 pounds of bone dry is 822,160 BTU. However, I don't see where there is a loss included to account for the 23.9 ponds of water that needs to be taken care of.


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## Nofossil (Jan 14, 2008)

bbb123 said:
			
		

> Nofo I was lookin at your graphs your starting your fire when tank is around 120 and it starts idleing a couple hours after that.  I start mine at around the same temp and mine will not idle until the second load of wood and tank gets up around 170 top 160 bottom (4-6 hours).  I believe I'm running the basic taco cirulator I'm thinkin you HX in your tank isn't workin fast enough.  Or maybe your circulator isn't running 100% it would def. be easier to change that out.  What's the gpm rating on these circulators?



I'm using a Taco 007. These circs are very sensitive to pressure drop. I'm planning on replacing it with a Grundfos 3 speed, which is less sensitive and should pump a bit more at its highest speed. I'd agree that my in-tank HX could be bigger. I'm thinking of a retrofit where I form a bunch of copper fins that clip over the HX tubing.

There's been some discussion of HX losses. In a system like this, those aren't really losses - they're just failure to move energy from one place to another. The energy in this case doesn't exit the system except to the extent that a higher boiler temp results in higher flue temps.


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## Nofossil (Jan 15, 2008)

*Another data point - last night's fire (1/14/08).*

Ran a much shorter fire last night - 70 lbs of wood at 20% moisture. Bottom line - 56% again. I did make a small software change to try and reduce the intermittent idling so it's not a perfect comparison, but it looks like moisture makes more of a difference than short vs. longer burns.

Got some cold weather coming, so maybe I can get some data on a longer burn. Might have to give back the bathroom scale, though.

As of last night, I've burned exactly 2 cords since October 15th.


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## barnartist (Jan 15, 2008)

im going to weigh my wood I load in today and try to borrow your numbers. If your 56%, I must be 10%. I dont have a meter though.
Your right some cold on the way. I wonder what effect insulating the bottom of the boiler would have on efficiancy?


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## Nofossil (Jan 15, 2008)

My bad - I messed up on the initial calculation and left out some data. Actual efficiency was 56%, not 48%.

Still lots of room for improvement, but it raises an interesting question. I thought that the dry wood was giving me better numbers, but apparently not.

Wood at 30% moisture - 56.0%
Wood at 20% moisture - 56.5% burn 1, and 56.6% burn 2.

Nothing if not consistent......

Hard to get excited over 1/2%.

I'm doing a longer burn tonight, and I closed the fan inlet shutter from 1.35" to 1.25" hoping to reduce overtemp idling.

Turbulators next!


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## barnartist (Jan 15, 2008)

what did you notice Nofossil as far as ash and such with the 30% wood? Much difference compared to 20%?


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## slowzuki (Jan 16, 2008)

I think you should find you're heat output is much lower on wetter wood but the calc, once accounting for the water, should show about similar efficiency.  With your boiler idling all the time the dry wood may even penalize you a bit more.
Ken


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## Nofossil (Jan 16, 2008)

slowzuki said:
			
		

> I think you should find you're heat output is much lower on wetter wood but the calc, once accounting for the water, should show about similar efficiency.  With your boiler idling all the time the dry wood may even penalize you a bit more.
> Ken



You're exactly right. I'm deducting the weight of the water before calculating the fuel value. 

I've been preaching the gospel of really dry wood, but based on this one data point, perhaps there's very little actual advantage to having wood that's dryer than 30%.

I ran a longer burn last night, still with dry wood. I closed down the fan shutter a bit (from 1.35" to 1.25") and made a software tweak to try and reduce idling. Still got idling, but system efficiency was 58.5%. Flue temps were down slightly.

I'm having a hard time resisting the temptation to change several things at once, so this isn't as scientific a series of experiments as it might be.


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## slowzuki (Jan 16, 2008)

I don't think you're reading into it right, the efficiency is based on the max theroetical energy you can get out of the wood and what you actually got out.  So wet wood the max you can get out of a pound of it is lower.  So burning wet wood wastes wood, even if your boiler can burn it efficiently!

Trying to think of how to state this right, there is probably a wetness of wood say 150% where all the energy in wood fiber would be used to evaporate the water in the wood, so there is 0 net energy available in the wood.  If you could theoreticly get this wood to burn, and get no heat off it, you would have a 100% efficient boiler with no heat output, you follow?


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## Eric Johnson (Jan 16, 2008)

You would have 100 percent combustion efficiency and 0 percent heat transfer efficiency, right?

What would the "overall efficiency" of the boiler be in that instance? One way to look at it would be 50%, but we know that can't be. I'd have to say zero overall.

I agree that more moisture = less efficiency. I don't see any way around it.


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## Nofossil (Jan 16, 2008)

slowzuki said:
			
		

> I don't think you're reading into it right, the efficiency is based on the max theroetical energy you can get out of the wood and what you actually got out.  So wet wood the max you can get out of a pound of it is lower.  So burning wet wood wastes wood, even if your boiler can burn it efficiently!
> 
> Trying to think of how to state this right, there is probably a wetness of wood say 150% where all the energy in wood fiber would be used to evaporate the water in the wood, so there is 0 net energy available in the wood.  If you could theoreticly get this wood to burn, and get no heat off it, you would have a 100% efficient boiler with no heat output, you follow?



You're right about getting less out of a pound of wood. However, my question is how much benefit there is in getting an individual piece of wood dry before burning it. Of course, it weighs less when dried. Do I get mor eusable heat out of it?

I assumed that a log burned wet would generate less usable heat than the same log burned after drying. Note that the dry weight of the log is the same either way, and the dry weight is what I use to calculate potential heat energy.

It's not a huge difference, by the way, but I expected noticeably better efficiency with dry wood. I've also assumed that the excess moisture would dampen the secondary combustion - it certainly makes it hard to get secondary combustion started. As you point out, there has to be a moisture level high enough to cause problems. All I can say is that 30% doesn't seem to hurt.

However, the little data that I have suggests essentially no difference in usable heat from the same log at 20% or 30%. I really need to run another few burns with 30% wood. I just don't have any right now.

I've actually put a lot of time and effort into planning to cut and store wood covered two years in advance. Maybe there's no reason to do so.


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## wdc1160 (Jan 16, 2008)

You may be onto something big here nofo

I have read a great number of threads by people who swear their 2 year old wood is the efficiency king.  Maybe not in your boiler -- maybe its an EKO thing?


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## Nofossil (Jan 16, 2008)

ABGWD4U said:
			
		

> You may be onto something big here nofo
> 
> I have read a great number of threads by people who swear their 2 year old wood is the efficiency king.  Maybe not in your boiler -- maybe its an EKO thing?



I'd be cautious about overstating conclusions at this point - we have ONE data point for wetter wood, and that's 30% - hardly dripping. Actually, most of the wood was 32-34%, but I had some 20% mixed in. It's seasoned for at least a year. In the case of the locust, it's seasoned 8 years (uncovered) before being split, and half a year (covered) afterwards. Don't want to get carried away and assume that we can burn it the same day we cut it.

I guess it's fair to say that I don't have all the answers yet.


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## wdc1160 (Jan 16, 2008)

Yah,  I hope no one threw away their old wood or even traded it in for newer wood.


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## slowzuki (Jan 16, 2008)

On a lb basis completely dry, you get more energy out of dry wood.  Evaporating water uses a lot of your energy.

Eric, combustion efficiency is basically how much fuel gets xfered completely to H20 and CO2 and how much fuel is lost out the stack as CO and soot and unburned tars and HC's.

Heat transfer efficiency is how much of the energy inside the boiler combustion area makes it out into the water (outlet of the boiler I think is fairly std to account for heat loss of the boiler).

So to keep with the same example, my extra wet wood that only had enough energy to evaporate the water in it, we can't evaluate the xfer efficiency because we had no spare heat to xfer.  But I said all the wood was converted to heat so we had 100% combustion efficiency.

I know this is a made up situation but it is easier to understand that way.  I'll try to come up with a good example here.


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## slowzuki (Jan 16, 2008)

Ok a quick example/problem, skipping the data conversions:
100 kBTU of dry basis wood is burned at 95% combustion efficiency.  20 kBTU is required to free the water contained due to the latent heat of evaporation and the specific heat of the same water.  With a 95% efficient heat transfer to the boiler jacket (assume no heat loss in the boiler), how many kBTU are delivered to the water jacket? What is the total efficiency of the conversion of the wood to usable heat?

100 kBTU x 0.95 = 95 kBTU
95 kBTU - 20 kBTU = 75 kBTU
0.95 x 75 kBTU = 71.25 kBTU Delivered to water jacket

71.25 / 100 = 71.25% Of the energy in the wood is useable.

Now as I understand, most of the methods account for the water due to latent heat so you can compare two devices using different fuel.

Now remember, there is also the water from the combustion to deal with as well.  Natural gas condensing boilers condense out the water from combustion to gain most of the few % efficiency they gain.  We have a wet fuel to begin with.  I've got to read up on my thermo studies again, been awhile.  We even had to compensate for if it was 0% to 100% humidity in the combustion air being supplied.


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## Nofossil (Jan 16, 2008)

*Moved this to a new thread.*

From the initial post, we have 94 lbs wood at average 30% moisture = 65.8 pounds ‘bone’ dry at 8600 BTU/lb = 565880 potential BTU

Calculating the energy lost to the water in the wood:

It takes 1 btu per pound to raise water 1 degree. It takes 970 BTU to boil one pound of water that's at 212 degrees. It takes .5 BTU per pound to raise steam 1 degree.

We have about 30 pounds of water in our wood. If the wood starts out at 70 degrees and the steam goes up the flue at 600 degrees, it will require the following energy which will be lost to the system:

30 lbs water, 60 to 212 degrees = 4560 BTU
30 lbs water to 30 lbs steam = 29,100 BTU
30 lbs steam 212 to 600 degrees = 5820 BTU

Total loss = 39,380 BTU at 30% moisture. This represents 7% of the energy available in the wood itself. Thus, I'd expect a 7% loss compared to bone dry wood. Wood at 20% should represent a 4.6% loss.

The theoretical difference between the energy loss due to water at 30% vs. water at 20% is less than 2.5% assuming the same piece of wood in both cases.

Not a big number. I should have done the math earlier.

Of course, if you flip that around, this 7% number could account for a sizable majority of your combustion efficiency losses if you're somewhere near 90%. From a system level, it's not such a big deal. According to these numbers, I need to burn another tenth of a cord if it's 30% instead of 20%.


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## slowzuki (Jan 16, 2008)

Thats 39,380 BTU of the theoretical max in the log, which is higher than we could hope to extract.  Say 50% efficiency getting from log to house, that 39380 becomes a direct 14% difference.


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## Nofossil (Jan 16, 2008)

slowzuki said:
			
		

> Thats 39,380 BTU of the theoretical max in the log, which is higher than we could hope to extract.  Say 50% efficiency getting from log to house, that 39380 becomes a direct 14% difference.



I suspect that the other losses are proportional to the total amount of energy that you get from your combustion. I think that there are three main losses that occur during combustion: Incomplete combustion, flue gas heat, and water related losses. For every 100 potential BTU that you have, you'll lose 7 to water if your wood is at 30%. Another few percent is lost due to incomplete combustion.

Air has a specific heat of .24BTU/lb. Making a wild assumption that the specific heat of all the non-water flue gas is about that value, we can calculate heat loss up the flue.

For the original example, 6 pounds of air per pound of wood = 455 pounds of flue gas at 600 degrees = 59,000 BTU of non-water flue gas loss for another 10%. At this point, we're down to around 80% depending on the unburned fuel losses. I actually got 56% measured, so I'm losing energy elsewhere.

I'm copying this to my new efficiency thread - sorry for the double posting.


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## kuribo (Jan 16, 2008)

at standard pressure and temp, a cubic foot of air weighs about 0.0807 lb/cf. If your blower is providing 100 cfm, that is 484.2 lbs of air per hour the blower is running. How many cfm and for how long does the blower run???


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## Nofossil (Jan 16, 2008)

kuribo said:
			
		

> at standard pressure and temp, a cubic foot of air weighs about 0.0807 lb/cf. If your blower is providing 100 cfm, that is 484.2 lbs of air per hour the blower is running. How many cfm and for how long does the blower run???



Got no data. It's shut down to a smallish opening (a pair id pie-shaped wedges about 1.25" wide at the ends). I don't have a good way to measure flow, and I haven't been able to find specs on the blower.


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## kuribo (Jan 16, 2008)

100 cfm is pretty small, I think a hair dryer probably outputs more, so I would guess your blower is putting out at least 150-200 cfm....How long does the blower run during a burn?


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## Nofossil (Jan 16, 2008)

kuribo said:
			
		

> 100 cfm is pretty small, I think a hair dryer probably outputs more, so I would guess your blower is putting out at least 150-200 cfm....How long does the blower run during a burn?



It runs pretty continuously, but at varying speeds. I also have no idea what the effective pressure drop is. Again, best estimate that I can justify is that EKO might have set it up to be somewhere near the 1.6 x stoichiometric that's supposed to be optimum for gasifiers. Of course, the setting would have to assume something about the surface area of wood that's pyrolizing.


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## Eric Johnson (Jan 16, 2008)

Here's the label on one of my blowers (mine has two of these). I have no idea what these numbers mean, but I bet you guys do. I was pleased to note that they're made in Germany, so they should be relatively easy to replace if it ever came to that.


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## kuribo (Jan 16, 2008)

The fellow at New Horizons tells me that he calculated an 85% efficiency at 300F flue gas temp and a 45% efficiency at a 650F flue gas temp when testing the EKO (EKO 25 I think he said...)

That doesn't answer the question about efficiency and moisture content, but it does show how high flue temps can drastically reduce efficiency....


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## kuribo (Jan 16, 2008)

I have here a factory test report on the EKO 80. It seems it used about 140cfm during their test cycle.....

Another thing that needs to be figured in is the water in the combustion air....This is also heated to steam and goes out the flue....


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## Nofossil (Jan 17, 2008)

kuribo said:
			
		

> The fellow at New Horizons tells me that he calculated an 85% efficiency at 300F flue gas temp and a 45% efficiency at a 650F flue gas temp when testing the EKO (EKO 25 I think he said...)
> 
> That doesn't answer the question about efficiency and moisture content, but it does show how high flue temps can drastically reduce efficiency....



That's curious. Given that it's the same boiler in both tests. I wonder what you can do to it that would raise the flue temp and affect efficiency that drastically. I calculate less than 20% total heat loss at 600 degrees flue temp, even assuming that the fan flow rate is twice stoichiometric. Worse still, a too-high fan rate would lower the flue temp, not raise it. Poor combustion would lower it as well. The only thing I can think of that would raise the flue temp and lower efficiency is a thick ash buildup on the HX surfaces.


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## kuribo (Jan 17, 2008)

He said he varied the airflows and "played" with it.....Maybe give him a call and ask him....Zenon at New Horizons....


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## sled_mack (Jan 17, 2008)

nofossil said:
			
		

> kuribo said:
> 
> 
> 
> ...



Make sure you're all talking about the same efficiency.  By removing the turbulators and increasing the fan openings, you could get a much higher stack temp - even with the same COMBUSTION efficiency.  But, efficiency measured from raw wood to storage tank could be much different.  Not knowing what Zenon was referring to, means you have no idea how to compare it to your discussion.


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## Eric Johnson (Jan 19, 2008)

I don't know how inclined you are to hustle for a buck, nofossil, but if I were you, I'd consider putting that thermocouple temp monitoring kit into a marketable package. It's something I'd like to have, but I lack the knowledge and motivation to do it myself. I bet others are in the same boat.

That's the kind of thing dealers should offer as an option with a new boiler, along with clear instructions on how to dial these things in. I think the more you understand about how the boiler works from the get-go, the happier you're going to be.


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## wdc1160 (Jan 19, 2008)

Eric Johnson said:
			
		

> I don't know how inclined you are to hustle for a buck, nofossil, but if I were you, I'd consider putting that thermocouple temp monitoring kit into a marketable package. It's something I'd like to have, but I lack the knowledge and motivation to do it myself. I bet others are in the same boat.
> 
> That's the kind of thing dealers should offer as an option with a new boiler, along with clear instructions on how to dial these things in. I think the more you understand about how the boiler works from the get-go, the happier you're going to be.




Eric, I am convinced people nearly kill themselves before they figure out they need to make an adjustment or a tweak.  Especially if the tweak may be more technical than they are conforatable with.  I think that its realistic to believe you could cut your learning curve in 1 year proportional to what it would take someone tweaking 3 years to find.  And, even then your more likely to find things the tweaker won't.
There are 100 sayings like:  If you can't measure it you can't control it, if you can't quantify it you really don't know much about it, and you can't measure improvement if you don't measure.
Flashbacks from every math class I have ever taken.
I think if you could quantify a boiler system easily its probably worth doing.  

Nofo so what kind of monitoring plans you selling?


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## Nofossil (Jan 19, 2008)

ABGWD4U said:
			
		

> Eric Johnson said:
> 
> 
> 
> ...



Got no plans to sell, but I'll give away my design. I have a thermocouple signal conditioner that I found - the Analog Devices AD595 - that makes it really simple and cheap to get a signal from a thermocouple that reads degrees on a standard voltmeter. Chip is $12. I'll post suppliers and part numbers as soon as I get a few minutes. If there's enough interest, I'll see if I can draft a college student to wire up a few and figure out a package price. Data logging is a little more complicated, but there's a pretty reasonable device called the LabJack that would do the job, I think.


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## wdc1160 (Jan 19, 2008)

nofossil said:
			
		

> ABGWD4U said:
> 
> 
> 
> ...




What will your business slogan be...

Here at Nofo's Controller's Limited-- if we can't measure it it don't exist.


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## barnartist (Jan 19, 2008)

I second what Eric said. I'll buy one right now.


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## barnartist (Jan 19, 2008)

Also once again, can a higher flow rate lower the stack temp. Nofossil, im not sure how a higher fan rate lowers stack, do you mis type that? I think Zenons right on this one. Sled_Mack has great results in keeping his stack a hair above the 200 mark. I cant seem to run below 300 and get enough heat.

Are those for sale yet Fossil?


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## Nofossil (Jan 19, 2008)

I KNEW this thread would get some discussion going!

If the amount of material that's burning is constant, the more air you push through, the lower the stack temp. I think this could happen two ways. First, any excess air added to the secondary combustion can't increase burning, so all it can do is dilute the hot combustion gases with cooler air. Second, if you have a bunch of large chunks of wood in your primary chamber, there's not much surface area for gas generation to happen. I can imagine in that case you could be blowing a bunch of extra air though that has nothing to react with and just passes through the nozzle without adding to gas generation. That would also have the effect of lowering the flue temperature. That happens as the fire dies down - fan running on high, not enough fuel surface area to react with it. Slowing the fan down would increase the flue temperature in that case for sure.

When you've got a good bed of coals and plenty of fuel, adding more primary air increases gas generation and thereby generates higher output, presumably raising flue temperature, all other things being equal.

Heat loss up the chimney is a big percentage of total loss for these units. I'm still wrapping my head around the factors that affect flue temp.

An observation: The HX pipes on the EKO 40, 60, and 80 are much longer than mine. I wonder if that makes them inherently more efficient. The test results that I've seen are for the 80.

I'm having a hard time imagining what I can do besides turbulators to drop my stack temp without throttling the boiler way back.


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## kuribo (Jan 19, 2008)

One sould also be able to lower the flue temps by removing more heat from the combustion gases via the water jacket....


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## wdc1160 (Jan 19, 2008)

nofossil said:
			
		

> I KNEW this thread would get some discussion going!
> 
> If the amount of material that's burning is constant, the more air you push through, the lower the stack temp. I think this could happen two ways. First, any excess air added to the secondary combustion can't increase burning, so all it can do is dilute the hot combustion gases with cooler air. Second, if you have a bunch of large chunks of wood in your primary chamber, there's not much surface area for gas generation to happen. I can imagine in that case you could be blowing a bunch of extra air though that has nothing to react with and just passes through the nozzle without adding to gas generation. That would also have the effect of lowering the flue temperature. That happens as the fire dies down - fan running on high, not enough fuel surface area to react with it. Slowing the fan down would increase the flue temperature in that case for sure.
> 
> ...





I thought from my watching the thread, you guys could go over some of the assumptions you have made.  Some of the ones that seem obvious may not be.


These are three that I thought could use more thought
One example would be the outlet temp being lower = more efficient / lower stack temp.
An influx of air can be to much of a good thing if it pushes uncombusted gases out the stack.  
This may also be said about the surface area of the fuel.


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## Nofossil (Jan 19, 2008)

For those inclined to roll their own, here's a picture of my two-channel thermocouple signal conditioner. Next to it is an Omega SMCJ-K which does the same thing, one channel only, for $100. I use it as a reference for calibrating mine, but it's a simple solution.

List of parts required: Analog Devices AD-595, 6.8K resistor, prototyping board, type K thermocouple, DC power source. A 9V transistor battery would work, but you'd want something a bit higher for high temps. The output is 10mv/ degree C, so 9 volts would limit you to 900 degrees C, or about 1600 F. The chip will tolerate up to 30V supply.

Digi-key provides data sheets, which are a bit thick for non-electrical-engineer types. I managed to slog through. The resistor is to scale the output to fit the range of my controller's inputs.


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## TCaldwell (Jan 25, 2008)

nofo, maybee something like this would be a little more potent than the turbulators in the eko, www.hpac.com.  compact efficient heat transfer.


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## Nofossil (Jan 25, 2008)

TCaldwell said:
			
		

> nofo, maybee something like this would be a little more potent than the turbulators in the eko, www.hpac.com.  compact efficient heat transfer.



I'm getting turbulators made, and trying to get a quote from Zenon as well.

This design is great - I considered inserts that would have a similar effect, and be thermally coupled to the tubing walls. I think cleaning the fly ash would be a challenge.

Here's a photo - the page seems to change and didn't have the article when I looked at it.


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## barnartist (Jan 25, 2008)

That looks great! but maybe a few less fins, and a spiral motion, but yeah, to clean the things. Man, that is VERY interesting.


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## TCaldwell (Jan 26, 2008)

barnartist, I know a boiler maker in ny. that has experimented with 6'' dia auger bits slid into horizontal  heat exchanger flue tube, claims it reduces the need for about 3 times its length of conventional  heat exchange pipe. to clean, just pull out of the pipe!


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## Eric Johnson (Jan 26, 2008)

A couple more points on turbulators:

Joe Brown made the point awhile back that turbulators should be made of some kind of tempered (or maybe it was annealed) steel so that they don't warp in the tubes and get jammed up so that you can't slide them out.

Slowzuki's JetStream gasifier has little stainless steel turbulators that, as I recall, are about four inches long and slide into the ends of the tubes. Ken said that they're not hard to make and that he has the pattern (or design). I'm not sure if they could be adapted to the EKO, however, since I suspect they go into the bottoms of the tubes, not the tops, which are easily accessible on the EKO. But I'm sure you could modify the basic design to work.

The turbulators-as-hx-cleaners that come on the current EKO models is such a cool design that I'm surprised that it's not an option on other high-end gasifiers like the Tarm. As far as I know, only the EKO, Econoburn and BioMax do it that way. All I can say is that it sure beats cleaning them by hand.


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## slowzuki (Feb 7, 2008)

Although we only pulled them part way out, they are actually about 2 ft long.  They are just twisted strips of sheet metal with the right clearance to pull out of the boiler tubes.  They do go in from the top and have a little hook to keep them from dropping too far.



			
				Eric Johnson said:
			
		

> A couple more points on turbulators:
> 
> Joe Brown made the point awhile back that turbulators should be made of some kind of tempered (or maybe it was annealed) steel so that they don't warp in the tubes and get jammed up so that you can't slide them out.
> 
> ...


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## Nofossil (Feb 7, 2008)

I got my turbulators. Haven't had a chance to take pictures or install them yet - probably Friday. That will give me a chance to do a comparison run lloking at flue temps. Detailed efficiency will have to wait a bit longer.


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## jebatty (Feb 7, 2008)

Reviewed the thread and a few comments and considerations.

Wood moisture content as related to efficiency
There is a factor which I did not see discussed which is the extra energy required to reduce wood moisture from around 25-35% to a lower level. Moisture in wood is not the same as a wet sponge. Water in green wood exists between the cells and within the cells. The water between the cells is pretty much free water and is lost relatively quickly, resulting in an initial moisture content of around 25-35% depending on species. This is similar to a wet sponge. To reduce water content further, the water bound up within the cellular structure needs to be released. This takes extra energy. It is somewhat similar to the extra energy needed or released in a phase change. There are resources on kiln drying of woods that explain this in detail and likely provide energy calculations. My understanding is that this extra energy is material and if not considered will result in an otherwise unexplained error factor.

Stack temp vs efficiency – heat loss up the chimney
This source cites a 1% efficiency gain for each 40 degree reduction in stack temp, as might be achieved through use of tubrulators. 
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Turbulators.asp
Another source, which now I cannot find, claimed each 100 degree drop in stack temp related to a 5% efficiency gain. I assume there is a "sweet spot" in these calculations and the number changes above or below a specified stack temp.

Efficiency related to water temp
This source states that system efficiency rises as water temp falls, and to achieve maximum system efficiency to achieve the purpose of the boiler: For hot water boilers used in a space heating application, reduce water temperature to the lowest temperature that will meet the demand
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Efficiency_Tips.asp

Interesting sources on boiler efficiency and turbulator science
http://mistug.tubitak.gov.tr/bdyim/abs.php?dergi=muh&rak=97077
http://www.wargaboiler.com/services.html
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5782217

Maybe some hyperbole or an agenda here, but some interesting points on efficiency in general
http://www.boilerspec.com/EmmisEffic/boiler_efficiency_facts.pdf


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## Eric Johnson (Feb 7, 2008)

Great information and observations, Jim. Gracias!

I could be wrong, but while your observations about bound vs. free water in wood is correct when it comes to air- or kiln-drying wood, I'm not sure that the same logic applies to wood being burned. It seems to me (just thinking about it), that once you toss that chunk into the firebox, water is water and wood is wood......


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## jebatty (Feb 7, 2008)

Eric, what you say seems to make sense on a first look. But the cell wall needs to breakdown to release all of the water, and that takes energy. 

Wood moisture content as related to efficiency 
I think I was in error though in drawing the distinction as water between the cells and water in the cells. The proper distinction is water in the cells (free water) and water in the cell walls (bound water). The bound water is chemically attached to the cell wall. It is the release of the bound water which requires extra energy. Drying Lumber Hardwood, USDA, General Technical Report FPL-GTR-118, pg 2.

Because MC of wood cells varies throughout the log, on average approximately 1,100 BTU/lb (2.6MJ/kg) of water are required to evaporate water from green wood. Same source, pg. 2.

Using this number and first calculating the MC in the wood being burned, and converting that to lbs, one can calculate the approximate BTU's needed to exhaust all water from the wood during the burn process.

Relative humidity as related to efficiency
Another factor which I did not see discussed is the effect of relative humidity on efficiency. The higher the relative humidity, the more energy will be required in the burn process. This is one more variable, or error factor, to be considered in determining various levels of efficiency.


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## slowzuki (Feb 20, 2008)

Just found Dr. Hill's tables of energy content in wood, I'll type out a few:

Available Heat Per Cord in Millions of BTU

Ash, Green-16.5, Air dry-20.0
Aspen/Poplar - Green-10.3, Air Dry-12.5
Red oak - Green 17.9, Air dry - 21.3
White Pine - Green 13.1, Air dry - 13.3
Douglas Fir - Green 13.0, Air Dry 18.0

There is no desription of the calc to arrive at these values though.


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## wdc1160 (Feb 20, 2008)

slow where were you 3 1/2 weeks ago.  Is that how you got the name slowzuki?


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## slowzuki (Feb 21, 2008)

3 1/2 weeks ago I was working 80 hours a week at work and plowing 10-15 hours a week at home and driving 10 hours a week to get to work!


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## FishHawk 210CC (Feb 28, 2008)

I'm new to this site and have been reading up on the discussion of overall efficiency of the boiler. Another way to look at this is based strictly on flue gas temps and how it relates to the process of heat extraction.

You begin with wood and air at 70 deg F, utilize the energy of the wood (minus the heat for the mositure, wood quality, etc) and heat this mass up to a final temperature of 1800 deg F and then begin the process of recovering the heat for use in the multiple uses around the house. Your exit temp is 600 deg F

1800 - 600 is a delta T of 1200 deg of temperature removed. This would yield be a maximum efficiency of 66% without taking into account boiler heat losses, moisture in the wood, combustion efficiency, etc. If you are getting 56% out a possible 66% you are doing well @ 85% capture of heat not going up the chimney.

If you decrease your flue temp to 200 deg F your potential efficiency would go up to 89% from 66%.


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## buickpwr (Feb 28, 2008)

Wow what a read, some guys here have it figured.

I need the magical btu/lb number and finally found it, here goes;

- 8660 btu/lb may be possible in a laboratory. problem is the number is often used in factoring outputs and
efficiencies of appliances/boiler

This "high heat value" is only obtained in perfectly dry wood 0% moisture and only in an environment of pure oxygen.
so our practical world 8660 is completely unrealistic.

After being cut split and stacked to sit for a year or two you reach about or around 20%.
- 1.25 lbs of well seasoned wood(20%) is made up of;
1 lb of fiber and 0.25 lb of water resulting in 6928 btu/lb (8660 x 0.80) of actual total weight.

problem of 0.25 lb of water in the piece?
- evaporate the water and raise it to the flue gas temp.

- additional water in the air used to combust our piece.
- additional water/moisture produced as a by-product of the combustion cycle 
  (hydrogen atoms from the wood combine with the oxygen atoms in the induced air to form water vapour).

By using the High Heat Value (8660 btu/lb or 6928 btu/lb 20%) by way of thought that all latent heat can be recovered to produce more
useable output... so in reality to use those figures you would have to have flue gas temp exhaust at the temp of the initial incoming 
induced air, which this would be near 0 degrees F. 
The latent heat put into the 3 water vapours will be ignored due to not practical in recovery and removed from calculations.

So account for the Latent heat effect. because its much closer to the actual conditions of actual use that we go thru daily.

1050 btu to boil/evaporate 1 lb of water
1 btu additional is needed to raise the pound's temp 1 degree

Total weight of moisture given off by fire:
.25 lb of moisture in piece
add about .54 lb of water vapour as product of combustion
@ 60 degrees have 0.79 lbs of water vapour
Heat to flue temp of say 400 degree
0.79 x [1050 btu x (400 - 60)] = 1098 btu per 1.25 pound piece; and 879 btu/lb (1098 x 0.80)

6050 btu/lb (6930 - 879) is show to be realistic and that is what the average Joe should use if he want to weight his wood and toss
it in the firebox, seeking some boiler efficiency numbers that are accurate and real.

Be careful of charts described as output per cord or pound of wood. There lies a assumption that is based upon some efficiency of
the boiler used, when assuming and stack temp changes etc then the chart figures become inaccurate. or your boiler has to match test unit
in-order to use chart.

Comparison using High heat value or Low heat value; LHV gives 8% higher than the same results using HHV
example 80% boiler efficiency (LHV) using 6050 btu/lb is 74% efficient using (HHV) 8660 btu/lb

Lastly 
Green piece say 50% that weights 2 lbs
2 lb piece = 1 lb wood fiber = 8660 btu
1.54 lb of water to vapourize and heat ( taking away 2200 btu)
2 lb piece has a net available energy of 6460 btu or 3230 btu/lb

reference: Technical Series; The amount of energy in wood

 thx
just some thoughts on everyone using the same factor would be nice


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## Nofossil (Feb 28, 2008)

Nice summary.

I include the latent heat of vaporization since it's possible to reduce the amount of water in the wood, and it's possible to build condensing boilers.

I take the actual weight of the wood, subtract the weight of the water, and multiply the result by 8660 to give me the theoretical heat value. To actually attain that, you'd have to have 100% perfect combustion and the flue temperature at ambient - obviously not possible, but a worthy goal nonetheless.

By this criteria, the EKO 80 can achieve 90% efficiency under ideal conditions.


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## jebatty (Mar 15, 2008)

nofo -- when you have an extra minute or so, look at this site on boiler efficiency. It deals directly with coal, but you might find it useful anyway.
http://www.firecad.net/boilerblog/


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## Nofossil (Mar 15, 2008)

Thanks - nice link. Tmonter sent me a spreadsheet that he uses to do the same calcs for other solid fuel boilers.

Eric was threatening to create a sticky thread with links to other high-value threads. Might be nice to collect efficiency related resources like the one you referenced.


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