Local Exhaust Ventilation

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Local Exhaust Ventilation
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Local Exhaust Ventilation (LEV)

• Operates by removing a contaminant at the point
  of generation and ducting it away in an air flow
  to a safe place

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LEV Components
Exhaust outlet
Ducting
Fan and motor
Hood or exhaust outlet
Filter or purifying system
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Ducting

• As straight as possible
• Bends or angles should be gentle
• Should be well supported
• Ample access ports should be provided for
  cleaning (think MEMS)
• Provision for testing duct pressure via water
  gauges or access points for air flow meters

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Filter or Purifying System

• Care must be taken to see that correct type of
  filter is used e.g. cyclones are not suitable for
  very fine particles
• Owing to the dynamic nature of cyclones and the
  resultant generation of static electricity the
  potential for dust explosions is very high

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Ventilation Fans and Motors

• Fan must have sufficient power to cope with
  normal working but have sufficient margin to deal
  with overload situations

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Exhaust Outlet

• Exhausted air must be away from any air inlets,
  otherwise a cyclic pollution system is produced
• Variation in wind direction must not reduce
  exhaust efficiency
• Exhaust noise must be controlled

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Fume Cupboards

• Type of LEV widely used in laboratories
• Large ventilated box
• Consists of enclosed chamber accessed through
  vertical sliding sash
• Kept under negative pressure by a fan
• Prevents back release of contaminant

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Multihood Extraction Systems

• Often convenient to extract from different
  locations via hoods linked to a common extraction
  system
• Size of ducts must be selected to distribute
  total airflow amongst the several hoods

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Maintenance

• 14 months if installed as a control measure under
  COSHH
• Transport velocity (anemometer)
• Capture velocity
• Static pressure
• Clean/change filters
• Maintain fans