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I Forge Iron

chimney problems


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my stack is tall and black but still just doesn't draw well enough. I used a barrel standing on end with a 10x10 hole cut in the bottom and placed right at the fire pot. it makes a 90 straight out of the barrel to get outside and then 90 up but its not perfect 90 or flat it angles upward the whole way. i wonder if the barrel is the problem. Oh yeah i took the rain cap off and it made no difference. I don't really understand the injector .

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an injectorejector is a means to suck out a fluid, gas or even small particles entrained in a gas with a second fluid (keep in mind a fluid can be liquid or gas, ie air) the blower would be aimed at a venturi, a narrowing of a chimney duct, there is a slight convergent nozzle at the end of the blower which goes through a chamber connected to the forge, your blasting cool air (or whatever) through the narrowed chimney constriction, the air speeds up, which also means the pressure drops, sucking in the flue gas with the blower air.

Its how a carburetor works, or an airplane wing or a number of other useful inventions
Bernoulli's Principle Animation
Venturi effect
Choked flow tutorial pdf
De Laval nozzle(not your recommended venturi profile just interesting)

there are three possible disadvantages
1. the power to run the blower, a properly designed chimney employs a passive stack effect to work which uses the energy from the exhaust (and the lack of energy at the end of the stack). If you can make the blower do other useful work, this might not be a problem.

2. if what your feeding the blower is inside air, you could have a heat loss in winter, on the other hand, you might look at it as a ventilation advantage at other times of the year, you could even exhaust other equipment, provided its not too hot. The main point is the blower isnt getting exposed to the flue gases directly.

3. because the flue gases are being diluted with air before they go up the stack, its cooling faster, with the distinct possibility of condensing out chemicals inside the stack, leading to more corrosion, aside from the energy employed running a forced air system thats the main possible problem. Ideally a youd either use a cheap chimney pipe easy to replace when needed or a corrosion resistant one like stainless steel.

the forth possible problem would be forgetting to turn the blower on, which would be bad, as in carbon monoxide dead. If the blower is drawing air in from the shop, with it off the exhaust could contaminate the shop directly


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an additional thought, in the diagram I drew the blower is directly below the chimney, it doesnt need to be, infact it would be better if soot from the chimney couldn't fall back into the blower, so adapting the basics to your current angled ninety would be an advantage. Not placxing the motor over the forge would also be an advantage...so

a few more thoughts :rolleyes::P
the cooler the blower motor can be kept the longer it will last, there is a trade off there, in that if its further from the forge the path of the gas is longer and the blower needs to overcome more friction as the length of the pipe increases (meaning you need a blower big enough to deal with both the bit we are adding and the total length of the chimney plus any friction loss for corners, the likely reason your passive chimney isnt currently working), adding a simple shield to block radiative energy (not shown) would be helpful, I drew square duct because its easier to fabricate, you could elect to do the whole thing in round if your up for it. the venturi could be longer in length this is seat of the pants DIY engineering and has nothing whatsoever to do with an optimal solution like youd get from a real engineer, we havent even established what blower you might have available :P

flaring the end of the venturi to meet the round chimney tube shouldn't prove too hard for a blacksmith, but a good seal there is advisable.
again this is just a concept drawing not a blueprint no dimensions for the injector or the side draft are anything but a wild guess ;)

doing this might make some sense if for some reason you cant fix your passive chimney, or if you can come up with some other useful work the blower can do. What it does avoid is the need to get a thermally robust blower able to deal with flue gases, those are atrociously expensive



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Many of the old forges had from the blower, a secondary air feed that went into the side of the main chimney at an angle so the air would suck up the smoke to the chimney from the hearth.

Some had a slide valve or other means to shut this secondary pipe off if required

Usually the forge had a slide valve for controlling air flow through the tue iron, not an electronic controller, fans are usually over capacity to the tue, so surplus air may as well go up the flue. No extra costs or motors to burn out

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  • 2 months later...

If your chimney pipe is too small, you could try a wind turbine attic ventilator. They work good on even mildly windy days. The best solution is to have a side draft chimney with an opening within 6 inches of the firepot, no larger than 10 inch by 15 inch (150 sq. inches), at least a 12 inch diameter flu pipe (14 inch is better and 16 inch is even better than that) that doesn't have any elbows or bends, is at least 16 ft. tall, and at least 2 ft. taller than anything within 10 ft. of the chimney. Also, trees and buildings that are close can affect the chimney depending on which direction the wind is blowing. Also keep the bottom of any chimney caps above the chimney at least as high as the diameter of the pipe.

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