This is a topic that is very rarely addressed, and is one of the most important issues in setting up your workstation.
The hood needs to be high enough that you have plenty of clear area underneath so you can work safely without interference, but low enough that the hood captures the flame plume from the torch.
So, exactly how high is that? The answer is that it will vary from hood to hood, depending on the angle that the torch has to the table.
Let’s use a little basic trigonometry (don’t worry, very easy ). If your torch is at a 45 degree angle to the work bench, this angle is the basis of an equilateral triangle, which means that the opposing angles and opposite sides are equal. Therefore, if your bench is 30″ deep, the hood height (as measured to the bottom of the hood baffle) needs to be 29″ (30″ less 1″ so the plume is captured entirely by the hood) from the benchtop.
But what if your torch isn’t at a 45 degree angle? Do you have a yardstick or long paper/cardboard giftwrap tube? Making sure your torch is off and cool, place the yardstick or tube on top of the torch so that the tube runs parallel to the torch body. This will simulate the run of the flame plume.
Now, the plume does curve slightly upwards (heat rises), but by using the parallel stick or tube, we are setting the minimum height. Lower the hood until the end of the stick or tube is at least 1″ inside the hood (as measured to the bottom of the hood baffle) at the back end.