The Basics of Ventilation, Part Three: Overhead hood design

Overhead hoods have a problem: they are non-functional over most of the width of the hood. The only part of the hood that actually functions as designed is an area about 1-2” wide surrounding the suction duct. This is fine for small torches while making small beads, but what happens when you upgrade your torch and start working with larger pieces of glass?

Insertion of large pieces of glass into the flame plume deviates it and the open duct in your hood is no longer functional. And once you start working with powders, enamels and/or fuming, the only draw area is directly in line with the duct opening. How can the standard overhead hood be made to work more efficiently for the lampworker?

Quite simply by changing the way fumes are drawn out of the hood. Instead of using one large duct opening 8” in diameter, change to a slot that runs across the width of the hood. You are now drawing fumes and dust across the entire width of the hood instead of just one fixed spot. The hood presented here has a total draw area of 60 square inches, plenty to handle an 8” duct with 50 square inches of area.

The hood design presented here gives a total coverage area of 5 square feet and requires a minimum of 625 CFM of fan power. The closest fan that Grainger carries is the 1TDT9, which pushes 690 CFM @ 0.30 inches of static pressure.

The hood itself consists of two parts that must be manufactured separately: the hood and the suction plenum. The material should be standard gauge galvanized metal, and can be fabricated by any metal shop or HVAC company.

Before attaching the suction plenum to the hood, insert and attach an 8” duct starter ring to the hole in the suction plenum. It is far easier to do this prior to assembly.

When assembling the two parts together, use a foam gasket between the suction plenum and the hood to close off any possibility of leakage. Cutting the holes in the hood tends to distort the metal, as does making the bends for the mounting flange on the suction plenum. Use plenty of screws, as noted on the design to fasten the two parts together.

The hood can be suspended over the work area in a variety of ways, I recommend using either chains or solid metal brackets. The height of the hood should be such that the flame plume lands at least one inch above the bottom edge of the hood. You can use a yard stick or paper/cardboard tube attached to the top of your torch and parallel to the body of the torch to make this measurement.

Drawing 1: Hood Plenum

Drawing 2: Hood Top

Drawing 3: Side View

NOTE: Drawing 3 (side view) shows an incorrect width measurement of 2 feet 0 inches, this should be 1 foot 8 inches. I apologize for the error!

Drawing 4: Hood Front

NOTE: This document is copyright (C) 2007 by Michael Aurelius. Permission is hereby given to use and reproduce this document for the readers own use only. This document may not be reproduced on any other website or forum without express written permission by the author.


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