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Figure 8.2 - Example of not recommended dilution ventilation design

 The local exhaust ventilation

Local exhaust system is used to control air contaminants by trapping them at
or near the source, in contrast to dilution ventilation which lets the contaminant
spread throughout the workplace.
Local exhaust is generally a far more effective way of controlling highly toxic
contaminants before they reach the workers' breathing zones. This type of system is
usually the preferred control method if:
1) Air contaminants pose serious health risk.
2) Large amounts of dusts or fumes are generated.
3) Increased heating costs from ventilation (dilution ) in cold weather are a
concern.
4) Emission sources are few in number.
5) Emission sources are near the workers' breathing zones.
In a general way, a local exhaust system operates similar to a household vacuum cleaner
with the hose as close as possible to the place where dirt would be created.

The components of local exhaust ventilation.
A local exhaust system has six basic elements (see Figure 8.3):

1) A "hood" or opening that captures the contaminant at the source.
2) Ducts that transport the airborne chemicals through the system.
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.
6) Make up air that replaces the exhausted air.

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