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NOTICE: This is an OSHA Archive Document, and no longer represents OSHA Policy. It is presented here as historical content, for research and review purposes only.

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This image shows the back side of a grinder. This image shows the back  side of a grinder. The grinder is enclosed only on three sides by an incomplete, ventilated doghouse. The back side of the enclosure is missing and allows silica containing dust to escape the grinding operation.

The back side of this enclosure should be closed in to knock down dust and provide solid surfaces to improve the efficacy of the available exhaust ventilation capacity.
   
This image shows an exhaust intake inside a doghouse enclosure. This image shows an exhaust intake inside a doghouse enclosure. Exhausting air from the enclosure in this way is very inefficient in terms of dust capture.

A well designed doghouse will draw the dust into a slotted plenum at one end of the enclosure and then into a smooth fishtail duct transition.
   
This image shows a bag dump hood. This image shows a bag dump hood. The raw duct take-off from the back of the hood (see arrow) does not support the optimum aerodynamics in the hood and thus compromises the capture efficiency of the hood.

A baffled or slotted plenum at the back of the hood would be more efficient.
   
This image shows a local exhaust hood for a transfer point. This image shows a local exhaust hood for a transfer point.

The opening to this hood could be smaller thus increasing the capture velocity at the face of hood. It could, also, be equipped with flexible rubber strips to minimize the face opening and maximize capture efficiency.
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