Local Exhaust Ventilation (LEV) is extremely important in workplaces with airborne substances which can cause damage to the respiratory system of those breathing them in or can cause damage to machinery in the area.
What is a Local Exhaust Ventilation System?
An LEV system is any system that uses extraction to control airborne contaminants to stop harmful substances from being inhaled by people in the area. An LEV system will take this polluted air from where it will cause risk and, with a series of ducting and filtration, the air will be cleaned and discharged either back into the workplace or to a safe place outside..
An LEV system consists of an inlet, which can be a hood, booth, slot, canopy, cabinet or enclosure – this will be placed close to the source of pollutant and where the hazardous air enters the system.
From the inlet the extracted air will then enter ducting, which can be round or rectangular and can contain a series of junctions, bends, dampeners and changes in section. This ducting transports the extracted air to the air cleaner, or filtration by utilising a fan or other type of air mover.
There are several filters or air cleaners which can be used in LEV, depending on the nature of the pollutant. A competent LEV engineer can help you to design a system which efficiently fits your needs not only in terms of placement and sizing but also in filter type. The type of filter that would meet your system requirements best will depend on the contaminant in the air.
Lastly, the cleaned air will be released back into the atmosphere via a diffuser, grille or stack.
What Filter Is the Best Choice for My LEV System?
As previously mentioned, there are several different filtration types which have their pros and cons within different industries. Particle collectors are a commonly found type of filter and even within this, there are a lot of options and considerations on what is best for the circumstance.
Fabric filters are suitable for removing dry dust from the atmosphere. The air will enter the LEV system via the inlet and a fan will draw it into the filter where there is a porous fabric layer (this layer may be treated to carry an electrostatic charge to increase attraction and retention of dust). There are three ways in which the dust particles are then removed from the dirty air, which are as follows –
- Impaction – Larger dust particles meet the surface of the filter and remain there
- Impingement – Medium sized dust particles reach and are retained by fibres within the fabric weave
- Diffusion – Small dust particles are attracted toward the fibres
Fabric filters are extremely efficient when correctly maintained and used in the right types of industry. Collection efficiency can rise to over 99.9%.
Disadvantages to fabric filters are that the filter material itself can be quite costly and needs changing periodically to ensure it is working at optimal levels. When you have a fabric filter fitted the LEV engineer will let you know how often this material will need changing. Another disadvantage is that any waxy or greasy material can clog the filter. Abrasive materials can also cause wear and may lead to more frequent replacement.
It is important to keep on top of cleaning for fabric filters, which can be done with mechanical shaking, reverse airflow, or a pulse jet; again, your LEV engineer will explain the best methods and the time intervals that this should be carried out at.
A cyclone filter has a circular chamber which tapers off at the bottom. Dusty air enters at the top of the chamber and the air swirls around the chamber. This throws particles out at the chamber wall using centrifugal force. As the velocity of the particles decreases, they will fall into the hopper/collector at the bottom of the chamber and clean air will exit through the bottom of the chamber.
Cyclones are a good method of filtering for larger particles. Any particles under 2μm will not be able to fall into the collector at the bottom and will just exit the chamber. Particles around 5μm have a 50% collection efficiency. Cyclones work best for particles larger than 8μm where efficiency is 100%.
Not only are cyclone filters good for large particles, but there is also a lower pressure drop in comparison to other dust collectors. Where the air is being passed back into the working environment cyclones will need a secondary level of filtration to remove any respirable dust particles carried over.
Electrical precipitators give dust and fume particles an electrical charge which makes them attracted to collection surfaces with the opposite charge. Clean air can then flow out. Electrostatic precipitators are better at collecting larger particles (5 to 10μm has a 99%+ efficiency vs 1 to 5μm which has an 80-99% efficiency.)
Electrostatic precipitators are especially useful for corrosive conditions and high temperatures.
This type of filter benefits from low running costs; however, the initial investment cost is high, and the filter does require specialist cleaning. It is important to keep on top of cleaning for electrostatic precipitators as when they get too dirty it can lead to the system shorting or sparking, which can be a hazard in itself.
Electrostatic precipitators work best for particles that are moderately electrically conductive; efficiency of filtration is reduced for particles with especially high or low electrical conductivity.
If space is an issue, then an electrostatic precipitator may not be the best option as they can be quite large.
In dust and fume extraction the act of “scrubbing” means to wet the particles and wash them out of the contaminant cloud. When designing a scrubber system, it must be able to wet the particles, allow the particles to settle in the water, dispose of the particles suitably, prevent dust build-up at the inlet and prevent water carry-over in clean air. There are many different designs for scrubbers, which are as follows –
- Venturi Scrubbers – Dusty air passes through the narrow throat of the system and is injected with water. There are turbulent conditions inside the system which cause the water to separate into droplets which will then collide with the particles. The droplets are then separated to produce a sludge which contains the dust, and clean air then passes out of the top of the system.
- Self-induced Spray Collectors – The dusty air enters under a baffle in a water trough. The dust will impact on droplets as well as on water in the trough. A spray/dust eliminator then separates the clean air and water droplets, and the dust will settle as sludge at the bottom of the collector. It is important to regularly clean self-induced spray collectors as it can cause bad odours and bacterial growth if not; it can also present a Legionella risk, which is a type of pneumonia.
- Wet Cyclone Scrubbers – Contaminated air enters a cyclone which has a water spray in the centre which is directed outwards. The cyclone separates the droplets which combine with the dust particles and produce sludge in the collector and the clean air can leave through the top of the cyclone.
Wet scrubbers have a 96% efficiency for particles over 5μm and 20 to 80% efficiency for particles under 5μm. Wet scrubbers are good for cleaning the air of hot gases and due to the use of water can reduce the risk of fire and explosions, they can also easily deal with sticky or waxy particles without clogging.
The disadvantages of wet scrubbers are that they produce a lot of noise, and they can also not work in colder conditions due to the water freezing; also, if the sludge is not regularly cleaned out, this can cause bad odours.
It can be daunting to understand what the best filter is for your LEV, and it is extremely important to get it right to prevent illness for your employees. At Vent-Tech we are leaders in all things ventilation and can assist with consultations, LEV design, installation and even servicing and testing. Give our knowledgeable team a call today for help with your LEV system.