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Backdraft Bench For Welding Fumes

We noted previously that the American Conference of Governmental Industrial Hygienists (ACGIH) recommended a number of possible engineering control solutions for welding fume. In the ACGIH volume, “Industrial Ventilation: A Manual of Recommended Practice for Design,” the ACGIH recommends ventilation controls for welding fume in the following order of preference:

  1. Enclosing hoods
  2. Vacuum nozzles (for MIG and FCAW)
  3. Fixed slot or plenum hood on a worktable
  4. Movable hood hanging freely
  5. Overhead canopy
  6. Dilution ventilation

So what about the 3rdh option, “Fixed slot or plenum hood on a worktable?” Take a look at a schematic illustration re-drawn from “Industrial Ventilation…”:

backdraft bench

Commonly called a “Backdraft Welding Bench,” this solution has the advantage of not requiring constant re-positioning of the movable hood previously described. Baffles, or “wings”, at the ends of the bench provide better fume capture by reducing the effect of cross drafts.

Like any other engineering control utilizing air flow for contaminant control, there are certain airflow criteria that must be met for effective welding fume capture. These airflow criteria are set to create an adequate low pressure area at the back side (where the slots are) of the bench to ensure that welding fumes are largely captured before they can reach the welder’s breathing zone. The criteria are:

  • The bench must be large enough to hold the work being welded. This criterion obviously eliminates trailer frames from the list of likely candidates for this engineering control, but it is effective for many small parts and sub-assemblies.
  • The airflow requirement for the bench is equal to 350 cubic feet per minute (350 cfm) for each foot of bench length. Therefore a 6’ long bench would require 350 cfm x 6’ = 2100 cfm.
  • The slot width should be sized to create a flow velocity at the flow velocity of 2000 feet per minute (2000 fpm). Since there are two slots, the total slot length in our example is 12 feet. 2100 cfm¸ 12 feet = 175 cfm per foot of slot length.
  • Slot width equals 175 cfm per foot of slot length divided by 2000 fpm = 0.088 ft, or a little over 1 inch for the slot width.

While a backdraft welding bench is a useful tool for controlling fume exposures, it should be borne in mind that exhausting 2100 cfm of heated or conditioned air outside can be a hidden, or unanticipated expense. Air cleaning is a viable solution, allowing recirculation of the filtered air. The same filtration system described in the previous blog (without the LEV arms) would be a good fit for this application.

When investigating application of this engineering control for your welding facility, remember that respiratory protection may still be required. A back draft bench, however, may reduce fume concentrations to a level where a half-face air-purifying respirator (HFAPR) may be sufficient rather than a powered air-purifying respirator (PAPR). In either event, use of respiratory protection should be carried out in accordance with the OSHA Respiratory Protection Standard at 29 CFR 1910.134. Respiratory protection will be addressed in a future blog.

Air Quality Engineering

Air Quality Engineering