Ventilation Hood Height for torch workers

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.

7 thoughts on “Ventilation Hood Height for torch workers”

1. Roy says:

interesting concept…except your average torch dont all burn the same

2. mikeaurelius says:

How the torch burns doesn’t really matter all that much, Roy – the issue is more the speed of the flame plume as it exits the face of the torch. Typically, on a small torch like the Minor Burner for example, the speed of the gases at the face of the torch are near 6 feet per second. On a larger torch at much higher pressures, the plume is moving much faster.

3. Jenn Casterline says:

Still confused. If I use a short stick, of course the height of the hood would be less. What am I missing?

• Use a stick that is the length of the longest flame plume available on your torch.

4. Jenn Casterline says:

Thank you so much. And that should be the bottom height of my baffle? The whole way around or is just the back of the baffle ok? My hood is about 3-4 feet up from my torch in the front, but much longer in the back. Also, please help me here for one more thing. I have done the proper calculations for my hood/cfms needed and have come up with that my 10 inch ductwork will produce too low of a velocity, only about 1500. If I put 8 inch reducers on the duct, I should be able to get it up to about 2500. Will these reducers have any effect on anything? Am I going about this the right way or should I just suck it up and get en even bigger fan?

Thanks again!

5. Jenn Casterline says:

One more thing Mike. Let me just give you my specs to make sure I figured all this out right. My hood is 29 X 21 1/2 inches, a 790 CFM inline fan, 4 feet of 10 inch duct work, and one 90 degree bend. When I did all the math, this is what I found: Velocity-1437, Velocity Pressure-.13, and Static Pressure-.0468. These number were with 6 feet of ductwork, but now I realized it’s only 4 feet.