93
Hood Air Cleaner
Fan Duct
Discharge
Source: Adrian Hirst
Figure 10.1 – General Features of an LEV System
10.2.1 General Considerations
LEV can be simple systems that serve a single machine or be complex and serve an entire factory. For a LEV system to be effective, all of the
component parts must function correctly; a well designed and correctly positioned captor hood will be useless if the fan cannot provide the correct
airflow. A LEV system removes air from the workplace and therefore there must be a means of ensuring a sufficient supply of make-up air to compensate
for this. In large LEV systems this may mean that grilles or air vents have to be fitted to doors or walls and a supply fan may need to be installed. It should
be remembered that the cost of heating air can be substantial, therefore, poor design may lead to unnecessary energy costs and it may be worthwhile
installing a heat recovery system. It‟s essential that any LEV system is designed for the process which it is
intended to control. The diagram below shows the interdependent factors
which lead to effective control. It is important that the nature of the contaminant being controlled is fully understood. Gases released under
ambient conditions will behave very differently to dust particles which are released with a high velocity. This affects the design of the capture system as
well as any cleaning system which is incorporated.
94 It is essential to consider work process requirements as well as operator
requirements. Inevitably some compromise occurs for both the operator and the process, however if this compromise is too great then the LEV is unlikely
to be employed after it is installed.
Source: Adrian Hirst adapted from HSE Publication – HSG 258
10.2.2 InletsHoods
The design of the LEV inlet is one of the most important factors in achieving effective control. Hoods can be broadly classified into three types:
An enclosing hood is usually the most effective at capturing a contaminant
as it contains and separates the contaminant from the worker. This is the type you might find in a laboratory fume cupboard partial enclosure or a shot
blasting unit full enclosure.
Source: HSE
Figure 10.2 - Enclosing hood
95
A receptor hood takes advantage of the any natural buoyancy or velocity
which contaminants have causing them to move towards the hood. Whilst this type of hood gives minimum interference with the operator and process, it can
be prone to effects from other airflows in the area.
Source: HSE
Figure 10.3 - Receptor hoods Capture hood is the most common type encountered and is one in which the
contaminant is generated outside of the hood. The hood therefore has to generate sufficient airflow to “capture” and draw in the contaminant. This
means that the velocity of air and the proximity of the hood to the source of
contaminant are crucial. e.g. welding extraction.
Source: HSE
Figure 10.4 - Capture hood
The precise design of each of these systems has to be tailored to the process it controls. The table below lists some examples of common industrial
processes together with the types of LEV which can be installed to control exposures taking into account the type of hazardous substance that is present
and the way in which it is released.
96
Types of LEV used for various processes Industrial
process Nature of hazardous
substance Types of LEV
Welding Welding fume: fine
particulate with some natural buoyancy
Capture hood positioned close to the welding activity; or
Tip extraction fitted to the end of the welding gun
Paint spraying Mist and solvent vapours
released in controlled direction with velocity
Walk in paint spray booth Down flow booth
Polishing Metal and polishing dust
released in controlled direction with high
velocity Receptor hood and enclosure
around the polishing wheel
Shot blasting Steel shot and metal dust
from components released at high velocity
in variable direction Fully enclosed glove box type
cabinet with airflow managed to compensate for compressed air
input and shot recycling system Hand held orbital
Sander Wood dust released in
variable directions Extraction integrated into the
sander disc Paint curing Ovens
Hot air and curing vapours with strong
thermal buoyancy Extractvent from top of oven
combined with a receptor hood over the doorway
Laboratory analysis
Acid and solvent vapours released with low velocity
and little direction Partial enclosure and extraction
within a fume cupboard
Source: HSE
The inlets to LEV systems can only exert effective control close to the inlet itself. For example, for a hood with a 0.3 metre diameter circular opening with
a face velocity of 5 metres per second will only capture contaminant released within 0.3 metres of the opening. The velocity at one diameter distance i.e.
0.3 m from the opening drops to approximately 10 of the velocity at the opening 0.5 ms. Outside this capture zone, external influences such
moving machinery or personnel can overcome the capture effect of the inlet. This is illustrated overleaf.
97
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
