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Down Draft Bench Face Velocity

Down Draft Bench Face Velocity

Down Draft Bench Face Velocity

Several friendly competitors and I have been arguing over the correct methodology for measuring face velocity at the work surface on a down draft workbench. They claim that the true face velocity is calculated by dividing the air volume by the total area of the workbench surface. For example, if the workbench surface dimensions are 2' x 3' and the air volume is 1000 cfm, then the face velocity is 1000cfm/(2x3) = 166.67 fpm.

I contend that the true face velocity is calculated by by dividing the air volume by the total open slot area on the workbench surface. For example, if the workbench surface dimensions are 2' x 3', the surface is 40% open and the air volume is 1000 cfm, then the face velocity is 1000cfm/((2x3)x .40) = 416.67 fpm.

Who's right? If it's them, then please explain. I contend that calculating the face velocity on the work surface of the down draft bench is no different than calculating the face velocity of a slotted ventilation hood (which is calculated using the area of the open slots on the hood).

We've got a steak dinner and drinks riding on this, so I'm looking forward to hearing your thoughts.


RE: Down Draft Bench Face Velocity

I think you have a steak dinner coming. Imagine you are a in a crowd of people in a hallway that is 10' x 10', heading toward exit doors that are 6' x 7'. Will all the people go through the 10' x 10' hall, or through the 6' x7' door?
My money is on a face velocity of 416.67 fpm.

RE: Down Draft Bench Face Velocity

Thanks, Trashcanman. Great analogy. I hope others chime in to give me their thoughts. I can't find any documentation in Industrial Ventilation or online that definitively addresses the calculations for face velocity on a "grinding" bench or down draft bench. The only one I can find is for a cutting torch table which indicates that the face velocity is predicated upon the total area of the table (with no thought for any blockage caused by the grating on the table surface); this is in direct conflict with the methodology for calculating face velocity of a slotted vent... thus the conflict.

RE: Down Draft Bench Face Velocity

Have you checked the ACGIH?

RE: Down Draft Bench Face Velocity

Thanks, urgross. I have checked ACGIH. They really don't have a calculation, per se, for down draft benches. The closest I could find was plasma cutting tables, but that is really not applicable to down draft benches.

RE: Down Draft Bench Face Velocity

From the ACGIH, other down flow arrangements are given, such as with specialized lab hood designs. I used that as a basis for down draft tables for formalin control, and is typical for grossing stations. The ACGIH will provide recommended air flows, generally based on bench area, but the velocity is based on the hole size. I don't see a difference, other than the fact that you have, inherently, the capture distance spelled out. Table size dictates flow, hole size dictates velocity.

RE: Down Draft Bench Face Velocity

Face velocity at the slot opening is one thing, however,I would be more concerned about the capture velocity of the contaminants generated on the work bench.

RE: Down Draft Bench Face Velocity

Face Velocity and Slot Velocity are two different things. Neither one is relevant in this case. The volume of air is what you are looking for over the area of the table, and that is dependent on the necessary capture velocity and the distance the source of the contaminant is from the face of the hood. Face velocity is defined as the average velocity of the air across the face area of the hood. In hood design, slots are usually used for the purpose of achieving uniform distribution across the hood face.

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