Local exhaust ventilation (LEV) systems provide a positive means of removing airborne contaminants from the working environment by capturing them at their source.
Such systems are required when worker exposure to airborne emissions is not satisfactorily controlled by general dilution ventilation. Factors contributing to the need for LEV include:
- contaminants which are relatively hazardous
- high emission levels which are likely to need excessive volumes of air to control airborne concentrations using dilution ventilation
- location of workers in the immediate vicinity of the emission
- emissions that are intermittent or emission rates which vary with time
- extended duration of potential worker exposure to emissions, making the less desirable alternative of relying on personal protective equipment (PPE) unacceptable due to discomfort and cost of consumables
- emission sources that are relatively fixed, rather than mobile
- legislation requiring exhaust ventilation.
Risk control measures
The effectiveness and efficiency of LEV systems varies depending on design parameters. Engineering expertise with specialised knowledge is usually necessary to achieve the optimal design for effective and economical operation.
To design an LEV system, the supplier will need to know:
- the key properties of the airborne contaminants, to determine capture velocity required
- how the dust, gas or mist is generated (eg the source), which may also affect capture velocity
- the needs of workers performing tasks near the sources (capture away from workers breathing zone)
- how much control is required
- what is the allowable exposure limit, if any?
- are any other controls implemented?
- what is the exposure duration?
- future preventative maintenance requirements and keeping inspection records.
The process should be enclosed as much as possible. Where an enclosed design cannot be used, an efficient flange should be incorporated. The hood should be located as close as practicable to the process and not draw contaminated air through the breathing zone of the worker, which is a frequent problem with canopy hoods.
Ducts should be as short and straight as possible. If bends or changes in diameter are necessary, they should be gradual to minimise resistance to airflow.
Ducts should be self-cleaning and accessible for inspection and maintenance.
The type and specification of filtration and collection equipment will depend on various factors relating to the containment, the process and environmental considerations.
Ventilation systems which recirculate filtered air back into the workplace atmosphere must be designed to be capable of capturing all hazardous contaminants.
Appropriate cleaning and maintenance procedures must be devised and strictly implemented.
Discharge of extracted air
Extracted air must be released to a safe place and not create another hazard. If discharged outside the building, Environment Protection Authority requirements should be considered.