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Post by jkirk3279 on Nov 13, 2016 10:14:21 GMT -8
I would like to ask if anyone has done a modification to a standard wood furnace using a Bell to harvest the heat from the smoke.
The flue is a six inch, and comes off the back of our Hotblast wood stove.
We usually burn it at full blast, and I know a lot of heat is being lost.
The chimney goes up over twenty feet, so there’s plenty of draw.
My image of this would be making a bypass damper and attaching it at the back of the Hotblast, then a Bell made of 55 gallon drums with the air return being higher than the damper.
When started, the damper would be in the down position and the chimney would function normally.
When the damper is swung up, the draft pulling on the exit from the Bell would pull the hot gasses through the Bell.
If anyone has done this, I’d appreciate a link.
As for the Bell, there’s 11 feet of headroom so a stack of drums with the smaller 30 gallon drums inside would work.
The hot flue gas would rise inside the smaller drums and drop down to the base of the larger drums, then the exit pipe up to the chimney at the firestop.
I know that I need to account for “water column” to tell how big the Bell can be... this is the part I most need help with.
I figure by now that Peter must have tables for all of this.
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Post by peterberg on Nov 13, 2016 11:41:07 GMT -8
Hi Jkirk, No, I don't have any tables for this application. But, let's start with a proper bell description as I suspect you've got the wrong picture.
A bell is in essence a hollow vessel, closed at the top, with two holes close to the bottom. One is the inlet and the other is the exhaust. When those openings are reasonably close to each other the inlet is situated a bit higher than the other to prevent the in-streaming gases to shortcut to the exhaust opening. That's about all, when hot air is fed into the inlet colder air is streaming out of the exhaust. The difference in temperature is extracted by the bell because hot air tend to rise as compared to cold air, so feeding in hot air will displace the colder air already in there.
In order to slow down the forward velocity of the gases the bell need to be a fair deal wider than the pipe which is feeding in, about five times as a minimum. The slower the gases are streaming, the more it is affected by gravity which is driving the segregation of hot and cold gases.
Now to your heater. You could build a bell out of one or two barrels on top of each other or on top of the stove itself. Let's see to the bell behind of beside the stove first. The stove need to feed into this barrel/bell, preferably horizontally with a simple stove pipe. That'll mean the bell isn't fed close to the floor but that isn't really important. What is important though is the position of the exhaust opening. This need to be close to the floor, about 4" above it. A 90 degree elbow out of that hole and then straight up would be ideal.
A bell on top of your heater could be another option when the exhaust is on top. This will call for a hole in the bottom of the barrel and sticking a short piece of stove pipe through it just to elevate the inlet slightly above the exhaust opening of the bell. Exhaust the same placement as in the other example. Height permitting, you could even place another barrel on top if the first in such a way that both are combined to form a single hollow cylinder. Should be the same for the other example I must add.
This will work beautifully, skip the idea of having smaller barrels inside, this will kill the bell effect. Bells are the heat extractors which sports the lowest friction of all because it is operated by gravital force, nothing else. And since gravital force is in abundance everywhere on the planet there's hardly a possibility to espace from it. Leave out the bypass valve, since you are running the heater at full tilt all the time and there's a good chimney stack it will almost certainly work right away. Of course I don't know what the level of heat is that is going into the chimney. Because of this, I recommend you'd best to try it out with one barrel. Don't forget to burn it out so all the paint is off, otherwise this paint will inevitably burn off when the barrel is heated up inside the house.
To conclude: I have seen it working, started up without a hitch. Lots of people wondered why this technique isn't widely used to ramp up the efficiency of ordinairy box stoves. There's a reason though: you can't "tune" the stove down to a smoulder, by doing that the exhaust gases will get into the house and the CO in that will sniff the inhabitants out. When you want heat overnight you'd need to look at the possibility of a mass heater of some sort.
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Post by jkirk3279 on Nov 13, 2016 14:25:39 GMT -8
I appreciate the rapid response.
Your description simplifies the construction.
Start with one barrel, and go to two after that works. The Exhaust needs to be 4” from the floor and the input needs to be higher (about three feet from the floor in reality).
*******
One thing I had wondered about was using a booster fan I have from a pellet stove I upgraded.
That would require a bit of welding to fabricate a box, and the fan would force the draft to keep going even if the fire died down.
Given my Mechatronics training, I immediately jump to a three-phase motor for the fan, with a motor controller regulated by a K-type thermostat.
The motor would only kick on at start up and again at flame out, stopping when the temperature got too high.
Of course, I realize you use analog techniques for simplicity, but with a CO detector it could be quite safe.
It could even be run by 12 volt backup.
For heat storage, I was thinking of running a copper loop around the bell, using a drainback system. Water would be pumped into the copper only when the bell is hot.
Sucking that much heat out of the flue gas would definitely require fan backup lest the gas cool too much.
The unknown in all of this is the need for cleanout hatches. Logically, there will be some fine ash that will accumulate.
That’s the same concern I have with using a flue-based “bench”.
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Post by peterberg on Nov 14, 2016 1:41:46 GMT -8
In order to have the barrel function as a bell the inlet should be a little bit higher than the exhaust opening. That barrel isn't even 3' high so it is fed at the top and even then you have to place it on a pedestal. The exhaust of your existing stove is at that height I presume, so in order to achieve some bell action you have to start with two barrels.
Please, now the try-out is so simple, don't start to make it complicated again. If the chimney is as good as you say it is, you don't need an extractor fan or booster. As for a clean-out hatch, there is one hole close to the bottom already, the exhaust opening. Just pull out the stove pipe elbow and the bell's floor can be reached easily. I would suggest you start with the most simple setup and go from there as needed.
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Post by jkirk3279 on Nov 16, 2016 14:54:26 GMT -8
Right, I was thinking the inlet was rather high.
Start with two.
There’s headroom for a third barrel later if it all works out.
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