New forge V-shape

[Frank S]

When the need to control air flow in a long forge such as yours I might make this suggestion
make 1 tube to fit inside of the other as it appears that your are doing then instead of having just round holes turn them into slots they will then become a variable venturi allowing for maximum flow to be evenly distributed under the entire length of the fire For the times when you find you only require a smaller shorter fire you could have an additional solid or plugged cylinder inserted from the opposite end of the valve which could be slid in or out like that of a child's Kazoo or it could be open ended and be the supply tube this would require capping one end of the inside tube
Here is a quick design I drew up without a third tube