There is a type of side draft forge hood used by blacksmiths who burn coal called a “super sucker” and I’ve never understood the theory of its construction. I gather it uses the Venturi effect to create a natural draft and that it depends on the stack being above the roof.
The design shows that there is a shelf on the inside that narrows the stack before opening again. That said, what are the principles involve in getting one to work? Are there dimension ratios that need to be observed?
Are you asking about the section of the chimney labeled ‘Rumford Throat’ seen here?: https://www.rumford.com/plans/Certified36.gif
Or maybe the ‘Smoke Chamber’?
What’s shown is a pretty basic fireplace design. It’s not really a hood, it’s an open hearth with a side draft chimney. The chimney narrows to accelerate air flow through it and increase the draw through the opening. The entire length of the chimney can taper up to the top but that’s more difficult construction. It’s also easier to install a damper if the chimney gets narrower at a reasonable height. On tall tapered chimneys the damper has to be very large near the bottom, or located at the top of the chimney.
What I am asking is how to design one. It’s already designed, of course, but what’s critical?
One factor that favors that style of side draft hood is that the hood is only literal inches from the flame, so the fumes are still very hot and compact. Any over-hood with a comfortable (easy to work) amount of clearance gives the fumes a chance to cool and expand, meaning they enter the flue at low velocity (and back up in the hood area) and generate a poor draft.
I watched this video from a blacksmith that converted a poor-performing overhead forge hood to a very effective side-draft and he explains the reasoning.
Not much is critical outside of proper materials and construction so it’s not dangerous. You have plans for 3 different sizes so stick to something with proportional dimensions to one of those.
I am surprised no one has mention hot air rises. The suction is caused by the different densities of the cold grown level air and the hot air from the fire. Chimneys have a natural draft, because normally ground air is just a little warmer than the air at the top of the chimney, so air will go up. Light a fire close to the chimney box and it will start pulling in more of the fire’s fumes. The taller the stack the better the draft.
On high ries buildings if you have a ground level opening that travels up to hte top of the building there can be a good up draft.
Saw the same effect on a set of induced draft boilers. When lighting off those boilers on oil it was fun. I would turn the induced draft fans to max speed to create as much draft as possible. When I would open the oil valve there would be a large flare back because the cold fire box and stack were not adding any draft to the induced draft fan. Many of my fellow workers would worry about the flash back and shut the oil off and then make a 2nd or 3rd attempt before they could get the boiler firing on oil. I knew not to be looking down any of the sight tubes or in the air registers. I would watch for the reflection of the flame off the brickwork. But the fire box and stack need to be hot to help the induced draft provide enough air going in to the fire box.
This discussion reminds me of something I learned in Boy Scouts - the Dakota hole.
