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What is the recommended face velocity range for a Benchtop Downdraft Hood

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Laboratory fume hoods are one of the most essential and abused hazard control device. The integrated use of safety glass, safety gloves, brilliant safety practises and laboratory fume hoods ensure the safety of laboratory personnel from any dangerous chemical exposure. Literally, laboratory fume hoods are a sort of ventilation system with the main function of removing chemical fumes, vapours gaseous, dust, mist and aerosol. Additionally, fume hoods also serve the function of physical barrier between variant reactions and the laboratory thereby offering a solid security against inhalation exposure, chemical pills, scattered reactions and fires. Fume hoods accomplish this safety purpose by mitigating the hood sash.

The hood operate by maintaining relatively negative pressure in the interior of the hood to inhibit any impurityfrom escaping while drawing air in through the hood opening at a steady rate. In laboratory ambience appropriate hood face velocity plays immense role in order to ensure safety and effective operation of fume hood. Although extreme face velocities can lead to turbulence and can mitigate containment. But at the same time insufficient velocity can also compromise overall hood performance.

Generally hood’s velocity is recommended to be between the range of 0.3 m/s(60 fpm) to 0.5 m/s (100 fpm). Althoug it is crucial to maintain a face velocity between the range of 0.3 m/s (60 fpm) to 0.5 m/s (fpm), but velocities higher than this prove to be hazardous. Actually when face velocity exceeds 0.6 m/s (125 fpm) then eddy currents are automatically created which permit contaminants to be drawn out of the hood thereby accelerating worker exposure. So hoods must be thoroughly evaluated by the user before each use in order to ensure adequate face velocities and the absence of excessive turbulence.

However as far as face velocity is concerned, it is crucial to check with the local safety norms and regulations on the face velocity recommendation prior using the Benchtop Downdraft Hood. Majority of the hoods are sized for a minimum face velocity at full sash opening, but for the purpose of conserving energy some hoods size the minimum face velocity of the hood at half-sash opening creating new low flow fume hoods, which are now available in the market.

Numerous reputed organisations are complying with the norms of laboratory hood safety standards, in order to determine the effectiveness of a hood in limiting occupational exposure. One such technique of deciding the effectiveness of the hood is by measuring the speed of air entering the hood which is termed as face velocity.

In fact fume hoods should be strictly certified at least on annual basis, in order to ensure that they are operating with immense safety. Typical tests comprises of face velocity measurement, smoke tests and tracer gas restriction.

Although face velocity is not considered the only test method for determining whether a hood has the efficiency to hold the contaminants but at the same time face velocity is the only performance standard referenced by numerous reputed organisations, for instance OSHA,NIOSH, ANSI, ACGIH.

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