One of the latest trends in down draft benches is the addition of return air flow, also called air-makup. The idea is that re-directing the exhaust air back toward the bench will force more particulate down toward the filers, thus increasing the amount of dust that can be collected. There are several reasons why this is a bad idea.
Standard filters used in these systems are capable of filtering up to 99% of dust down to 1/2 micron, which is excellent. However, the 1% of dust that makes it through the filter is typically dispersed throughout the ambient environment to expose workers to safe levels of the dust. By redirecting the exhaust air back at the worker, the contaminated air is concentrated around the worker at the table. Additionally, this is air with the most dangerous dust. At 1/2 micron or less, the dust is easily ingested by the worker and is easily absorbed by the body.
The best scenario for airflow at the work deck is to pull ambient air into the system. The less disruption to this airflow the more efficient the equipment will be in capturing dust. The disruption caused by the return air flow blowing can create “eddys” (just like in a river) around the worker and/or the material being processed on the bench. This again can lead to the worker being exposed to higher dust levels.
It turns out that return air flow is an add on expense that just doesn’t make sense. It sounds like a nice option and it allows manufactures to under power the equipment to achieve a higher FPM (feet per minute) at the deck. Visit www.proventilation.com to see our full line of quality down draft benches.