LEV SYSTEM

LEV is a system that uses extract ventilation to prevent or reduce the level of airborne hazardous substances from being breathed by people in the workplace, which draws pollutants away from a process or operation that is likely to release a hazardous substance into the air and which consists of an inlet, such as a hood, slot, booth or cabinet placed around or close to the point of release of the substance. This device is connected via ducting to the inlet of a fan or air mover. The extracted air is usually discharged to the atmosphere or returned elsewhere in the workplace, having first been cleaned to make it safe for release.

There are two main methods of ventilation which can be used to control airborne contamination: dilution ventilation which provides a flow of air into and out of the working area and does not give any control at the source of the contamination, and LEV which intercepts the contamination as soon as it is generated and removes it from the workplace before it can be inhaled.

It is important to consider how the air withdrawn from a workplace by a large LEV system is to be replaced and also, if necessary, re-heated. Where re-circulation is involved, it is important to ensure that effective filtering is in place in order that all hazardous contamination is removed from the re-circulated air.

When designing an effective LEV system, account must be taken of the nature and size of the airborne contaminates that need to be removed. For example, are they dusts, fumes, smoke, mists, vapours or gases.

With regard to the inlets, consideration needs to be given to the size, shape and position of the source, the physical nature of the contaminant, the speed and direction of the contaminant as it moves away from the source, the rate of generation of the contaminant, the nature of the operation being carried out, the position and movement of the plant or person involved and local air movements due to general ventilation and the operation of nearby machinery. It needs to be remembered that inlets can only exert effective control at points fairly close to the inlet itself. Therefore the inlet needs to enclose the source as far as possible if good control is to be achieved. Although, in practice, a compromise has to be reached when constant access to the work is required.

The two main types of inlet are partial enclosures and LEV hoods. With partial enclosures the source of contamination is located inside the enclosure. Air flows from the open face of the enclosure and across the source, to extract openings located in the rear, top or bottom of the enclosure. LEV hoods should be located as close as possible to the source of contamination and are designed to capture or collect the contaminant and to direct it into the connecting ductwork.

Partial enclosures must be large enough to contain the work and the air flow must be capable of guiding the contaminant towards an exact point once the contaminant is released into the atmosphere. Along with adequate air velocity the booth must be designed to prevent the contaminant spilling out at the front of the enclosure. For applications such as spray painting the minimum recommended velocity is 0.7 m/s. As a general principle operators should never be positioned in the airflow path between the source and the opening of the extractor. The airflow in partial enclosures should be smooth and sudden changes in cross-section and protrusions into the enclosure should be avoided as they may lead to local air turbulence. Large-scale turbulence is less likely in deep enclosures than in shallow enclosures.

LEV hoods vary in size from small nozzles to large canopies and can be positioned above, below or to the side of the source. They should be located close to the source, enclosing it if possible. There are two main types of LEV hoods: receptor hoods and capture hoods.

A receptor hood is used where the contaminant is generated with considerable momentum and the hood is placed in the path of the moving airstream to collect and remove the contaminant.

A captor hood is used where there is no initial tendency for the contaminant to enter the LEV system and the energy required to provide movement in the right direction is supplied by suction at the hood. The minimum air velocity required being termed the capture velocity.

Capture velocity and the degree of enclosure are the two most important features of capture hoods. They determine the volume of air that needs to be extracted to give effective control. The lowest volume flow rate will be achieved with a hood design which encloses the source as much as is possible. Due to not being complete enclosures, operator movement and random air currents can be disturbing influences. These effects can be reduced through use of hoods which enclose the source to a high degree.

Ductwork needs to be designed so that the air velocity in the duct is high enough to keep the particles suspended in the airstream, particularly with regard to long horizontal runs of ductwork. Runs of ducting should be provided with access holes for internal cleaning and flexible ducting should be frequently inspected for leaks, partial connection and damage.

LEV systems with more than one inlet need to be designed and constructed so that each branch extracts the right amount of air through the inlet it serves. This involves consideration of airflow distribution and balancing. The airflow in each branch being determined by the resistance of the inlet, the length, diameter and flow resistance of the branch duct and the flow conditions at the junction with the main duct. Standard procedures exist for balancing ductwork.

Air cleaners can be classified as: air filters, particulate dust and fume collectors and devices to remove mists, gases and vapors.

Filters are mainly used for cleaning air and are designed to handle large air volumes with low resistance to airflow, although high-resistance high-efficiency filters are used for ultra-clean applications and for the control of hazardous dusts such as asbestos.

Particulate collectors extract large quantities of dust and fume from an airstream at higher inlet dust concentrations than filters. These collectors include: cyclones, fabric filters, wet collectors and electrostatic precipitators.

Mists gases and vapours being removed by chemical absorption, combustion or condensation.

When selecting a suitable air cleaner, obviously the features and properties of the contaminant need to be considered. The following points also need to be taken into account: greasy or waxy materials may permanently clog fabric filters, abrasive material may cause problems with fabric filters, flammable and explosive materials require special precautions, corrosive and highly oxidising substances will require special materials of construction and neutralising agents may be needed in wet collectors, some dusts may be difficult to wet, any gas or vapour components will not be removed by particulate air cleaning and filtration systems for hot processes will need to be suitably temperature resistant.

Fans in LEV systems fall into two main categories- centrifugal and axial flow. Although, for special purposes, turbo-exhausters and compressed air driven movers can be used.

In a centrifugal fan, air is drawn into the centre of the impeller, picked up by the rotating blades and thrown off at high velocity into the fan casing which collects and guides it towards the discharge opening. In this way, airflows can be delivered against considerable resistance. Fans can utilise radial, forward curved and backward curved blades.

With axial fans, air passes along the duct and is accelerated by the rotating blades. As a result, only low resistance can be overcome and they are mainly used as roof mounted extractor units.

In selecting the correct type of fan, consideration should be given to: required airflow, the total flow resistance of the system, the type of contaminant, the flammability of the contaminant, space limitations, the method of fan mounting and the type of drive to be used, operating temperature and the level of noise and the need for silencers.

Downstream of the fan, ductwork is required to carry clean air to a suitable point for discharge to the outside atmosphere in a way that avoids re-entry to the building. This may involve the discharge stack extending to a height of at least 3 m above the roof level. Integrated pollution control may require further measures where harmful or odorous contaminants are involved. "Chinese hat" type discharge terminals should not be used as they directed contaminants downwards and may cause re-entry into buildings. Additionally, they have a high flow resistance.

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