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Source: HSE
Figure 10.5 - Capture Zone or Capture Bubble on a welding hood
Important considerations about the source of contamination that may need to be considered during the design and construction of captor inlets include:
The size, shape and position of the source. The physical nature of the contaminant.
The speed and direction of the source. The rate of generation of the contaminant.
The nature of the operation. The positions and movements of plant and personnel.
Any local air movements.
10.2.3 Ductwork
Ductwork carries the extracted air and the contaminant from the inlet to the air cleaning device. For particulate matter, the air velocity within the duct must
be sufficiently high to ensure that the particles remain suspended in the air stream. Recommended transport duct velocities for various contaminants
are given below.
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Type of contaminant Duct velocity
m sec-1
Gases non-condensing No minimum limit
Vapours, smoke, fume 10
Lightmedium density dust e.g. sawdust, plastic dust
15 Average industrial dusts e.g. grinding
dust, wood shavings, asbestos, silica 20
Heavy dusts, e.g. lead, metal turnings and dusts which are damp or that tend
to agglomerate 25
Ductwork should be sufficiently strong, well supported and capable of withstanding normal wear and tear. The number of changes of directions
should be kept to a minimum and, where required, they should be made smoothly. Access to ducting may be required to facilitate cleaning, inspection
and maintenance.
10.2.4 Air Cleaners
There are three basic types of air cleaning devices.
Air filters
These are mainly used for cleaning air in ventilation and air-conditioning systems and are designed to handle large air volumes with low resistance to
air flow. High resistance high-efficiency particle arresters HEPA filters are used for ultra-clean applications and where particularly hazardous dusts are
found e.g. asbestos.
Particulate dust and fume collectors
These are designed to extract large quantities of particulate from the airstream at much higher inlet concentrations than can be handled by air
filters. These collectors include cyclones, fabric filters, wet collectors and electrostatic precipitators. These are the most common air cleaning devices
associated with LEV systems.
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Devices to remove mists, gases and vapours
Mists, gases and vapours can be removed from an air stream by a variety of means involving chemical absorption, combustion and condensation.
Others points to be considered: Greasy or waxy materials may clog filters.
Abrasiveness of particulate. Flammability and explosion potential.
Corrosiveness and oxidising capacity. Gases and vapours will not be removed by particulate filters.
High temperature materials.
10.2.5 Air Movers
