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May 2020 / 17 Effective dust capture hoods Decide on hood placement. The proper dust capture hood engi- neering and design is based on correctly analyzing the dust-gener- ating source. Care should be taken in determining how close to the dust source the hood can be placed so that it doesn't interfere with production personnel performing their work. If the hood's installed too close to the dust source, aside from getting in the operator's way, valuable material may be captured in addition to the airborne dust. If your dust collection hood captures more than 1 percent of process material, this could prove costly if you can't recycle the material. A well-designed hood should only capture dust that's airborne, which is typically less than 0.5 percent of processed material. Of course, if the hood's too far away, dust capture will be seriously reduced, which could potentially lead to a toxic environment. Measure dust concentration. After determining placement, you'll want to make sure the dust capture hood is up to the job. Dust capture hoods' most import- ant function is to reduce the dust concentration within the process operating personnel's breathing zones. A well-designed dust cap- ture hood can reduce the airborne dust concentration by an average of 90 percent over an 8-hour period. This reduction is compared to the airborne dust concentration in the same area but without a dust capture hood in operation. So, if an industrial hygiene air sample at the dust-generating operation measures 10 micrograms per cubic meter (mcg/m 3 ) without a dust capture hood in operation, a well-designed hood can result in the same sample type of about 1 mcg/m 3 . Put it on paper. And don't forget that detailed design drawings and specifications must be prepared so a new dust collector would solve the problem though, so the plant manager decided to seek outside engineering help. This resulted in the canopy hoods being removed and properly engineered, local dust collection hoods being installed right at the process' dust-generat- ing points. This successful solution didn't require replacing the existing dust collector. Get a second opinion So, be careful if a dust collection system supplier recommends a canopy hood as part of the system to solve your dust issues. Before you commit to purchasing and installing a canopy hood, have an industrial hygienist perform air sampling to determine existing employee respiratory exposure at the dust-gen- erating source. Industrial hygienists are certified, trained experts whose job is to reduce safety risks and haz- ards in industrial settings. Once the hygienist has sam- pled the air, share that information with the potential supplier. Ask the supplier what dust concentra- tion level the suggested canopy hood will provide at the employee's breathing zone in order for you to meet OSHA's permissible exposure limit (PEL) for nuisance dust or toxic dust exposure requirements as appropriate. If you're satisfied with the supplier's answer, put that dust level concentration require- ment in your purchase order. It's imperative to remember why you're purchasing a dust collector, exhaust fan, ductwork, and hoods. You're investing in a system that'll result in decreasing the dust in the pro- duction worker's breathing zone to a specified level for respiratory protec- tion while reducing housekeeping needs as well. If you don't put your requirements in your purchase order, there's a very good chance you won't get your needs met, espe- cially using a canopy hood. canopy hoods are perfect for unen- closed hot processes, I've yet to see a canopy hood that's been effective when placed over a dust-generating source that's at ambient tempera- tures. I've seen many canopy hoods installed over ambient-temperature dust-generating sources that were totally ineffective and, in some cases, resulted in unsafe conditions. Example 1. One canopy hood installation that was ineffective was installed over a manual tote-filling operation in a food processing plant. The dust cloud generated was extremely dense as it drifted out of the tote. Most of the dust could be seen drifting upward, passing through the worker's breathing zone before drifting slowly around the work area. The production worker used a floor propeller fan to try to blow the dust away — to no avail. Testing showed that the resulting dust cloud coming up out of the tote had a density of over 50 g/m 3 . Since the dust was combusti- ble, there was the possibility of an explosion if an ignition source were present. The canopy hood was ren- dered ineffective since most of the dust drifted away from the opera- tion but not into the hood, so it was replaced by a local dust collection hood installed right at the tote. The local dust collection hood's design required more engineering than the canopy hood, but the hood was extremely effective and didn't inter- fere with the tote-filling operation. Example 2. In another situation, a sheet metal contractor fabricated and installed canopy hoods for dust-control purposes at a solids processing operation. After startup, plant management complained about the lack of dust control at the processing plant. In response to these complaints, the dust collector sales representative said that a new, larger dust collector and exhaust fan were needed. The sales repre- sentative couldn't guarantee that