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LEV Central

The Knowledge Base for LEV

LEV Glossary of terms

This is a glossary of basic LEV terms:

  • ACoP –  Approved code of practice. This carries greater legal weight in Great Britain courts than guidance.
  • Aerodynamic diameter – The diameter of a unit-density sphere that has the same settling velocity in air as the particle in question.
  • Air cleaner – A device also known as filtration unit which removes (filters out) contaminants from air, eg filter, cyclone, sock, wet scrubber, electrostatic precipitator (EP).
  • Air mover – Device also known as a fan that move air.  Types of air movers include centrifugal, axial, propeller and Turbo Exhauster.
  • Boundary layer – The stationary or turbulent layers of air near a surface which can hold a contaminant cloud.
  • Breathing zone – The region around operators from which they draw air for breathing. Commonly defined as being within 300 mm of nose/mouth.
  • Canopy hood – A receiving hood used over a hot process.
  • Capturing hood – The source and the contaminant cloud are outside the hood. A capturing hood has to generate sufficient airflow at and around the source to ‘capture’ and draw the contaminant-laden air into it.
  • Capture velocity – The air velocity, (m/s) required around a source to capture the contaminant cloud and draw it into the hood.
  • Clearance time – The time taken for a contaminant to clear from a room or enclosure, once generation has stopped.
  • Commissioning – This is a Thorough Examination and Test of a system only you are benchmarking the results recorded against the design criteria. Normally undertaken when a system is first installed or when there have been any alterations to the system.
  • Contaminant cloud – This is the cloud of harmful contaminates which have been generated at the source.
  • CoSHH – Control of Substances Hazardous to Health.  CoSHH is the law that requires employers to control substances that are hazardous to health.
  • Cyclone – An air-cleaning device to remove particles from air by centrifugal force.
  • Dilution ventilation – A supply of clean air into the workplace, mixing with and diluting the contaminated air.
  • Duct velocity – The average air velocity measured across a duct cross section (m/s).
  • Dust lamp – Also known as Tyndall lamp is a parallel light beam illuminates the dust cloud to produce forwards light-scattering.  Similar effect to the sun making dusts visible in a room.
  • Eddy – A region in airflow with a circular motion, contrary to the main flow.
  • Electrostatic precipitator – A type of air cleaner/particle filter. Charged particles are attracted to a plate of opposite polarity, to which they attach.
  • Enclosure hood – A hood contains the process by enclosing it. Separating the contaminate from the operator.
  • Exposure limit – Workplace Exposure Limit.  Normally documented in EH40 and MSDS sheets.  They are maximum limits defined for when exposure to harmful substances cause symptoms.
  • Face velocity – The average velocity of air at the open front face of a hood (m/s).
  • Flow rates – the amount of air that flows in a given time.
  • General ventilation – Air extracted from a space as a whole and replaced.
  • Hierarchy of control – is a system set out in CoSHH to minimize or eliminate exposure to hazards.
  • Hood – A device to enclose, receive or capture a contaminant cloud.
  • Hood face – The area at the entrance of a hood; the plane (or planes) between the workplace and the hood interior. For an enclosing hood, the face is all openings that are entry points for workplace air.
  • Inhalable particles – Its a definition of the size particle (i.e. dust) which enters the body, this size gets trapped in the nose, throat, and upper respiratory tract. The median aerodynamic diameter of this dust is about 10 µm.
  • Lip extraction – Extraction slot along one or more sides of an area source such as an open surface tank (note not appropriate for tanks larger than 1.2 m wide)
  • Local air displacement (LAD) – A wide, directional, relatively slow moving jet of air blown into the operator’s breathing zone, to displace contaminated air and prevent it entering the operators breathing zone.
  • Local exhaust ventilation (LEV) – LEV is an engineering control system to reduce exposures to airborne contaminates such as dust, mist, fume, vapour or gas in a workplace.  Normally by removing at or as close to the source of contaminate.
  • Low volume high velocity (LVHV) – A method of LEV which uses very small hoods to capture contaminants very close to a source using high-velocity air extraction.
  • Make-up air – This is air required to replace extracted air.
  • Manometer –  A simple pressure-indicating device, eg on hoods.
  • Negative pressure – Air pressure lower than that in the workplace.
  • Pitot tube – A device used with a manometer to measure static and total pressure.
  • Plenum – A device to smooth airflows, eg behind the filter in a walk-in booth or in an LAD system.
  • Positive pressure – Air pressure higher than that in the workplace.
  • Pressure – It is a unit of measurement which measures the force exerted by the weight of the air.  Units of measurement: Pascals (Pa) Torr = millimetre of mercury (mm Hg) Millibar (mbar) Pound per sq inch (psi) Inch water gauge (WG)
  • Process – The way that airborne contaminants are generated.
  • Push-pull – An air supply on one side of a contaminant source blows the contaminant cloud towards an extraction hood on the other side.
  • Qualitative assessment – a visual observation.
  • Quantitative assessment – Relating to, measuring, or measured by the quantity of something (readings).
  • Receiving hood – These hoods are designed to “receive” or catch the emissions from a source that has some initial velocity or movement.
  • Respirable particles –  Its a definition of the size particle (i.e. dust/fume/gas/vapour) respirable particles are small enough to pass the nose and upper respiratory system and be inhaled deep into the lungs.  Particles that penetrate deep into the respiratory system are generally beyond the body’s natural clearance mechanisms of cilia and mucous and are more likely to be retained.
  • Slot – A long, thin hood with an aspect ratio of 5:1 or greater.
  • Source – A process creates a source; the creation of the contaminant cloud.
  • Source strength – A combination of the volume rate of release of the contaminant cloud, the cloud volume, shape and speed and the contaminant concentration.
  • Static pressure – Air pressure, measured normal to the flow direction, ie the difference between inside and outside air pressure measured by, eg, a manometer.
  • Total pressure – The algebraic sum of the static and velocity pressures.
  • Transport velocity – The velocity required to transport particles within duct to prevent settlement and worst case blockages.
  • Turbulence – violent or unsteady movement of air.
  • Tyndall Lamp – Also known as Dust lamp is a parallel light beam illuminates the dust cloud to produce forwards light-scattering.  Similar effect to the sun making dusts visible in a room.
  • Velocity pressure – Pressure exerted by air due to its motion.
  • Vena contracta – The section within an opening at which the streamlines first become parallel after entering that opening.
  • Wake – A low-pressure region that forms downstream of a body in an airflow.

This glossary is not exhaustive.  If you have not found the term you are looking for here then submit a suggestion to us here at LEV Central.

*Referenced from OSHA, ACGIH IV design manual & HSG258.