Here is my take on the external smoke generator. After multiple other iterations I found that I did not need a soldering iron, did not need a venturi, and did not need direct air injection. I also did not need to cut anything, drill anything, weld, or even bend. I can now build an external generator from off the shelf parts and the tools I need are a hammer and nail, used to poke holes in the bottom of the chamber.
This design drafts fine on its own and produces low temperature smoke. The biggest problem that I have had was with trying to smoke at very low temperatures. If the weather is such that condensation happens in the smoker and you have not done something to increase the airflow, you will get a nasty acrid smoke. At higher smoker temperatures (heat from the smoker's burner) like 200 degrees, it drafts especially well.
The example shown here is a smallish unit. It works with bigger burn chambers and burns for hours.
There are a few tricks. I use only small split wood. I pack the chamber as tightly as possible because if you pack it loosely it is more likely to go out. As you are loading the chamber, shake and tap it on the floor to ensure that it is well packed. Light it with a propane torch and take time to make sure that it is burning well all across the bottom. Using this method and green summer wood, I am getting complete burns, often without even charcoal left. Pull the assembly and shake it mid burn if you happen to pass the smoker mid burn.
In the picture I have a basket holding the burn chamber on but I don't do this anymore. I do make sure that I have a stock pot under the chamber in case it falls.
You can actually fill the chamber and the pipe above it with chips to extend the burn time. The chips only burn where the air drafts in.
Brian
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Materials- Martini Shaker $2 from thrift stores. EMT pipe union from home improvement store $4 (?). Exhaust pipe elbow from auto parts store $5 (?) In this picture I have a martini shaker top jammed into the smoker and the elbow happens to fit perfectly into that. If I were starting from scratch I would start by buying the elbow and pipe union, then go looking for martini shakers. An even simpler design is to skip the pipe union and do what you need to do to attach the shaker to the elbow. If you do it that way you can load the chamber by dropping wood through the pipe and shaking it into place but I do like being able to take this one apart.
There is a computer fan in the picture... I have used this to suck air (smoke) out of the top of the smoker to avoid the low draft condensation problem detailed above. Any computer fan and 12V power supply will do the job and although there is a speed control on the one pictured here, it is not needed. Just set the fan on top of the smoker and slide it over the whole until it pulls to your satisfaction. I have also hung one of these fans inside the smoker to circulate the smoke but I am not sure that there is really any benefit to that. Fan- $2 from a computer recycler Power supply $2 from a computer recycler.