Health and Safety
Executive / Commission
Local exhaust ventilation systems (LEV)
The effective application of LEV requires a good understanding of processes and the sources they create.
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Unguarded circular saw [MPEG 1.7MB]Air movement is made visible with smoke. The rotating circular saw blade acts as a crude fan. It draws air in along the axis of blade rotation and throws it out as a jet in line with the blade. The airflow is directed and shaped by the saw table. A narrow and a fan-shaped air jet are created.
Circular saw with LEV hood (guard) [MPEG 653KB]Air movement is made visible with smoke. The saw-blade LEV hood (guard) copes with and contains the vertical air-jet is contained. The wide, fan shaped, air jet is partially extracted but part of it escapes and flows towards where the saw operator would stand. The LEV hood isn't big enough and the extracted airflow is too low i.e. the hood isn't emptied as fast as it is filled.
Free falling powder 32mm pipe 1m from a flat surface [MPEG 1.1MB]Falling powder into a bin and onto a flat surface (from two heights). As the stream of powder falls, it induces a downward flow of air. If the powder lands on a flat surface the powder, and induced airflow 'splashes' outwards. The greater the height of fall, the higher the terminal velocity of the falling column of powder, and the amount of induced airflow. If the fall is into a container, the airflow will induce upward air displacement.
Also in this set:
Spray gun [MPEG 1.7MB]Air movement is made visible with smoke. The narrow compressed air-jet leaves the spray gun "air-cap" at over 100 metres per second. It flows rapidly away from the spray gun growing in size by induction of spray room air. The total 'throw' may be over 12 metres. The air jet is an energetic and a difficult-to-control.
Cleaning badly contaminated clothes using compressed air [MPEG 800KB]The compressed air jet travels at high velocity and moves a large volume of air. When used to clean clothing it creates a large dust cloud surrounding the worker.
Cleaning badly contaminated clothing using hands [MPEG 733KB]Dust contaminates work clothing by direct contact with powder and settled dust. Patting the clothing releases the retained dust and makes it into an airborne cloud surrounding the worker. Some fabrics, such as cotton, retain and release far more dust than more modern fabrics.
Compressed air used to clean a surface) [MPEG 437KB]The compressed air jet travels at high velocity and moves a large volume of air. When used to clean surfaces or components it makes solid debris airborne as dust, or liquids airborne as mist. Because it is so energetic the airborne contaminant cloud created is large, fast moving and very difficult to control.
Shock - hammer strike to the underside of a dusty surface [MPEG 451KB]Settled dust or spilt powder can be released from a contaminated surface and propelled into the air by impact. The amount of airborne dust release released will depend on the degree of surface contamination, the nature of the dust, and the energy (frequency and amplitude) of the shock applied to the surface.
Sack stacking [MPEG 735KB]The workers clothing becomes directly contaminated each time he picks up a powder covered sack. Each time he drops a sack onto the pile dust is made airborne by the direct impact and by air displacement between the falling sack and the one below.
Wood stacking - bellows effect [MPEG 605KB]The top sheet of wood suddenly falling onto the one below causes a “bellows effect” causing rapid displacement of dust-laden air.
Hand sanding [MPEG 2.4MB]The fine dust created by hand sanding of and made airborne is shown up using a dust-lamp. In normal lighting the fine dust cloud is not visible.
