Here are some pictures of my first forge, it was a process of trial and error - so here is the process I went through, and how I ended up with this design.
Firstly the burner, using some 1/2 inch galve water pipe that I had lying around, I made my first venturi burner. This worked well but there was not enough heat, so I made another and put the two side by side.
The venturi burners were not very efficient in the forge, there was a lot of un-burnt gas coming out of the forge opening - probably because I the burners were not getting enough air.
Also the forge was too big, I had made it two bricks high.
So I cut it down to just one brick high ( as in the photo's) and did away with the venturi system and connected a forced air supply.
WOW this worked GREAT.
The forge works really well, the heat is fantastic, I get almost a white heat, also I can "tune" the burners by controlling the air input and gas supply, I can regulate the heat for a "eco" mode or turn it up for max heat.
Lighting it is a bit tricky, I normally throw in a lighted rag or tissue and then firstly turn on the gas - this lights up the workshop !, then I slowly introduce the air.