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LOCAL EXHAUST VENTILATION

Two useful standards for Local Exhaust Ventilation (LEV) are the HSE publications An Introduction to Local Exhaust Ventilation (HSG37) price £4.50 and Maintenance, Examination and Testing of Local Exhaust Ventilation (HSG54) price £8.50.

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.

LEV has an important role to play within the hierarchy of control measures required by the Control of Substances Hazardous to Health Regulations 1999 (COSHH). Although it should always be remembered that COSHH strictly requires exposures to hazardous substances to be prevented and control measures only to be introduced where prevention is not reasonably practicable to achieve.

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It is important to consider how the air withdrawn from a workplace by a large LEV system is to be replaced and also, if necessary, re-heated.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 flowrate 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 vapours.

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.

All LEV systems need to be subject to commissioning to ensure that they are capable of meeting their design specifications. Under the Control of Substances Hazardous to Health Regulations 1999 all control measures need to be maintained in an efficient state, in efficient working order and in good repair. Maintenance procedures need to include information on: how frequently maintenance needs to be carried out for each component of the system, what maintenance tasks are necessary and how defects are to be detected and remedied, and who is to be responsible for the maintenance. The maintenance procedures should cover the full range of maintenance activities from simple visual checks to detect obvious defects, to major overhauls for preventative and remedial purposes. In addition to effective preventative maintenance, the Control of Substances Hazardous to Health Regulations 1999 and other regulations contain statutory requirements for the undertaking of formal examination and testing of LEV systems. It may be prudent for these examinations to be carried out by persons not normally responsible for the system maintenance in order that an independent second opinion can be obtained. For effective examination and testing comprehensive information on the system and its design specification needs to be provided. The Control of Substances Hazardous to Health Regulations 1999 require the thorough examination and testing of LEV systems at least once every 14 months.

However, more frequent thorough examination and testing is required in the following processes:

When deciding the frequency of thorough examination and testing: treat parts of equipment such as the machine casing and guards as LEV if they are directly ventilated and if one of their functions is to control emissions, regard make-up air systems that replace exhausted air as LEV if they are an integral part of an exhaust system, treat flues from furnaces, ovens etc as LEV where the draght created by the flue is necessary to control the release of hazardous substances, and only treat vacuum cleaners as LEV if they are connected to a portable machine or tool.

The Control of Substances Hazardous to Health Regulations 1999 specify that records be kept of the results of the tests including details of any repairs carried out as a result of the examination and tests, these records being kept for a minimum of 5 years.

Maintenance and thorough examination and testing need to be planned together in 3 stages:

1- Initial appraisal.

2- Regular maintenance including frequent visual inspections, perhaps daily, weekly or monthly.

3- Thorough examination and testing.

The information required while carrying out the above includes:

Regular inspection and checking of LEV is not the same as the thorough examination and testing.

The aim of inspection and testing being to identify potential problems so that they can be rectified before performance deteriorates. Weekly visual checks should be carried out to identify any obvious defects, although these may need to be more frequent where certain hazardous substances are involved. The inspection and checking should cover: ensuring that the LEV is always running when hazardous substances are either being emitted or are likely to be emitted, observing the condition of the suction inlet such as the hood or booth to see if it has moved or been damaged, observing the condition of any visible ductwork etc, observing any evidence of control failure such as unusual dust deposits or stronger odours than usual, observing any local instrument fitted to the LEV to indicate its performance and undertaking any minor servicing such as the emptying of filter bins etc. A formal system for dealing with verval reports from employees should be in place in order that details can be recorded into maintenance reports.

Thorough examinination and resting of a LEV system represents a regular audit of the performance of the system and should reveal whether or not the plant is performing correctly and effectively, although it may not reveal the precise cause of the unsatisfactory performance that has been identified. The thorough examination and testing will comprise of: a visual check, a measurement of plant performance and an assessment of control, and an assessment of the performance of the air cleaner or filter where the air is recirculated.

The most common categories of instruments and techniques used for the examination and testing of LEV systems are: direct measurement of emissions through air monitoring (in the breathing zone close to the source), measurement of plant performance (static pressure and air velocity) and visualisation techniques (smoke generators and dust lamps).

The type of information kept in the record for a thorough examination and test should include: the conditions of the LEV system at the time, the intended performance of the LEV system and the way it should be used, methods used to judge the performance of the LEV system and whether it achieves the intended performance, results of routine ventilation measurements, results of tests of the concentration of airborne material and request for remedial action with details of repair or modifications needed. The record should be kept for 5 years with a copy being available at the workplace in which the LEV is located.

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