Reducing workplace contaminant with Local exhaust ventilation

According to Health and Safety Authority, Local Exhaust ventilation (LEV) is an engineering system designed to reduce employee exposure to airborne contaminants (dust, mist, fume, vapour, gas) in the workplace by capturing the emission at source and transporting it to a safe emission point or to a filter/scrubber.

It is vital that the correct LEV is chosen for a particular task and kept correctly maintained. If a process or activity with which the LEV is associated is changed then the suitability and specification of the LEV system must be re-assessed.

Local exhaust ventilation

 

Components of the local exhaust ventilation

A local exhaust system has five basic elements;

  1. The “hood” or opening that captures the contaminant at the source.

This captures, contains, or receives contaminants generated at an emission source. It converts duct static pressure to velocity pressure and hood entry losses (e.g., slot and duct entry losses).

Hood entry loss (He) is calculated according to the following equation:

He = (K)(VP) = |SPh| = VP

where:

K = loss factor

VP = velocity pressure in duct

|SPh| = absolute static pressure about 5 duct diameters down the duct from the hood.

2. Ducts that transport the airborne chemicals through the system (exhaust air) and the air that is re-circulated.

3. An air cleaning device that removes the contaminant from the moving air in the system (not always required).

4. Fans that move the air through the system and discharges the exhaust air outdoors.

5. An exhaust stack through which the contaminated air is discharged

Advantages of Local exhaust system (LEV)

  • Captures the contaminant at the source and removes it from the workplace.
  • Handles many types of contaminant including dust and metal fumes.
  • Requires smaller amount of makeup space, since smaller amount of air is being exhausted.
  • Less energy costs since there is less makeup air to heat or cool.

Disadvantages of Local Exhaust ventilation

  • Higher cost for design, installation and equipment.
  • Requires regular cleaning, inspection and maintenance.

There are some documents that will help you to understand more about the local exhaust ventilation and also help you maximize its use. These documents includes:

  • Guideline on Occupational Safety and Health for Design, Inspection, Testing and Examination of Local Exhaust Ventilation System. Check document
  • Local exhaust ventilation check list. See document

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