Low heat value, or inefficient boiler.

 
VanMarco
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Post by VanMarco » Tue. Dec. 25, 2018 6:52 pm

Hello, I have built an outdoor wood boiler with a barrel stove, it has the heat exchanger (80 feet of 1/2 copper pipe running outside of the drum with 4 inches of glasswoold insulation all around), with WOOD it worked fine, letting the house at 70F rather quickly (it's a small 450sqf house) with around 60lb of wood over the course of 16 hours. Then now I bought lignite, not knowing that even for the higher price nuts were way better and now temperature ranges between 68 and 62, and unlike wood, it needs a lot of tending, as in having to add more coal about every hour or so. (I built grates for coal ofc). It seems like the heat is lower than that of wood coals.

Any idea?


 
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Post by coaledsweat » Tue. Dec. 25, 2018 7:54 pm

Is the combustion blower always running? Does it have a chimney and if so, what is the draft at?

 
VanMarco
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Post by VanMarco » Tue. Dec. 25, 2018 8:18 pm

It has a 6 foot tall chimney, I don't have ablower it's natural draft

 
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Rob R.
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Post by Rob R. » Tue. Dec. 25, 2018 8:25 pm

Low heat value, or inefficient boiler.
Both.

 
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lsayre
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Post by lsayre » Tue. Dec. 25, 2018 8:49 pm

Is this simply a barrel stove with copper tubing wrapped around the barrel?

 
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Post by VanMarco » Tue. Dec. 25, 2018 8:50 pm

Yes. With wood and Coke nut it got super hot :)

 
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Post by VanMarco » Tue. Dec. 25, 2018 8:52 pm

Would the addition of a air blower aid or simply let the heat escape the barrel faster? I run it with the damper 3/4 o the way closed to keep heat in the firebox.


 
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lsayre
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Post by lsayre » Wed. Dec. 26, 2018 6:32 am

VanMarco wrote:
Tue. Dec. 25, 2018 8:50 pm
Yes. With wood and Coke nut it got super hot :)
This is an extremely inefficient way to design a boiler.

 
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coaledsweat
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Post by coaledsweat » Wed. Dec. 26, 2018 6:51 am

It isn't a boiler, it's a stove with a coil. Find a good used stoker boiler and start living the good life.

 
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Post by VanMarco » Wed. Dec. 26, 2018 12:02 pm

lsayre wrote:
Wed. Dec. 26, 2018 6:32 am
This is an extremely inefficient way to design a boiler.
and what's the efficient way to do it?

 
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Post by lsayre » Wed. Dec. 26, 2018 3:21 pm

VanMarco wrote:
Wed. Dec. 26, 2018 12:02 pm
and what's the efficient way to do it?
One method is to purchase a reputable boiler. As to making your own, and also making it operate safely and efficiently, knowledge of that caliber is out of my league.

 
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Post by KLook » Wed. Dec. 26, 2018 8:41 pm

You have attempted to reinvent the wheel, and if it was that easy, everyone would have one like yours.

Kevin

 
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Post by franco b » Wed. Dec. 26, 2018 11:49 pm

It's apparent that the design is better suited to wood than the lignite.

Efficient burning of any fuel is dependent on the even mix of the air with the fuel, along with maintaining adequate heat throughout the fuel bed. This is a generality since I have never burned lignite.

The other problem is heat transfer to the heat absorbing surfaces. Flame and hot gasses must pass in close contact with those surfaces to be effective, and that heat transfer should not in effect, steal heat from the combustion area. The two needs should be separate as far as possible. With a steel barrel wrapped with copper tubing all over, this separation is not possible unless the combustion area is insulated in some way by a fire box spaced away or brick lining.

The short chimney is probably compensated for by the higher stack temperature of wood fuel, while with lignite draft is only adequate with small quantities which accounts for the short burn time. A larger load probably does not burn well or at all.

 
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Post by VanMarco » Thu. Dec. 27, 2018 10:22 pm

hello franco, thanks for your comment.
I do know I should've built something better but it was something to be made quick and cheap.
What you say makes sense. When wood burns, it emits a lot of heat and gasses hence it heats fast.
When coal is in, it burns slower and even with the damper closed almost all the way, all it does is maintaining
water temperature in the system, effectively "locking" temperature to the level wood brought it up to.
I will post a picture of my setup soon. The barrel is lined with firebricks on the bottom, i did add another
2 rows.
also, what you are saying that the heat exchaging surface should not cool down the fire, then why many OWBs
have a big 80 gallons or more water jacket all around? doesn't that cool the fire?
Also the coal itself isn't of great quality, it has carbon, but it has also way too much water for my liking and i am not an expert, but It looks like to me that a lot of the energy contained is used to evaporate water. I stuck one of those wood
moisture meters in it, even the centre of big chunks says 38% moisture.
I am sorry that I posted a silly question, of course a cheap stove I built is not going to be efficient. But I want to figure out what I could build that's better efficiency before next heating season as buying ready-made stuff is out of the question whereas steel and time are cheap here.
Ah and yes a larger load does not burn well. I manage to lay a bed of coals but can't go much thicker than that.

Thanks again!

 
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Post by franco b » Fri. Dec. 28, 2018 12:07 am

Good for you for making the effort. Getting a feel for what works and what doesn't, and why, will be valuable in the next design.

38 percent moisture is awful; that water has to be evaporated at the cost of the heat to do it, along with slow ignition. Worth looking for a better quality supply.

I would try to get a stronger draft by adding sections to the chimney which might allow for a greater burning mass of coal. More burning coal makes for a long burn instead of closing down the air intake or pipe damper to accomplish that long burn. Air must come from below the fuel bed with coal, but you still need some air above to cope with the large amount of gas when the coal is first heated. Best if that secondary hot air is fed by a perforated pipe just above or to the side of the fire box.

Most OWB are bad designs, but a few have secondary burn chambers that are a big improvement. Burning lignite will also need a well planned secondary burn to avoid all the smoke.

To repeat. A good design first has to burn the fuel efficiently, and then transfer that heat to the water. The two needs are separate and have to be thought of that way.

A revolution in boiler design was that of the Scotch Marine boiler which combined compactness with efficiency.
https://en.wikipedia.org/wiki/Scotch_marine_boiler

Where are you located and some pictures of your fire box and grate would be helpful.


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