FUME EXTRACTION SURVEY
The first thing to consider is the process that you need a fume extraction system for, for instance:
• plating tanks
• paint spraying
• fibre glass laying out
• screen printing
Once this is established, and the type of fumes that are to be removed, thought can be directed into what method of capture will be used, the ducting route needed, and a specific method of fume removal from the air before discharge into the atmosphere. All LEV systems should comply with the government guide “Controlling airborne contamination at work” reference HSG258.
The nature of the fumes produced should be considered, are they flammable or explosive under certain circumstances (for example Toluene or other solvent based fume)? These need special treatment to prevent any workers being injured in the event of an explosion occurring.
Reference to the Dangerous Substances and Explosive Atmospheres regulations 2002 is recommended to prevent unnecessary problems down the line.
How many machines are working at one time and for how long?
The number of machines working at one time will determine the amount of air required for the system, and the time working will determine whether a continuous operation filter is needed.
Some of the filter types available are as follows:-
No filters fitted
Some operations produce fumes which can in certain circumstances be discharged to the atmosphere without any filtration. Welding fumes where the operation does not happen for long periods and not very often could possibly be vented straight out to atmosphere without filtration, but this would need confirmation by the local environmental authority. Care would need to be taken to ensure the vent did not impact on neighbouring properties or create short circuiting of the fumes back into the workshop via an open window or door.
Mechanical filter housing
These units usually have the filter media inside a fabricated housing.
The most basic and commonly used are grease filters fitted above a kitchen cooker hood. The filter has a frame usually made from stainless steel, into which a mesh or baffle plates have been fitted. As the air and any airborne grease is passing through the mesh, the droplets of grease gradually touch the mesh and drain downwards towards the built-in drip tray where it will accumulate for disposal.
The baffle filter works in a different way. The baffle plates are designed so the air has to change direction as it passes through the filter. The grease content cannot change direction as quickly as the air and is sent by centrifugal force onto the baffle plates before running down to the drip tray.
To prevent the loss of suction the collected material needs to be removed from the media. This must be carried out regularly to prevent fires and keep up to hygiene levels required.
Other mechanical filters are available for dry fumes, mist and smoke.
These filters use a number of different methods to remove the particulate from the airstream:
Impingement is the mechanism by which large, high density particles are captured. As the particulate laden air passes through the filter media, the air tends to pass around the fibres. Inertia in the particulate causes it to separate from the airstream and to collide with the fibres to which they then become attached.
Interception occurs when a particle follows the airstream but still comes into contact with the fibre as it passes around it. If the forces of attraction (electrostatic in nature) are stronger than those provided by the airstream to dislodge it, the particle will be retained.
Diffusion occurs specifically with very small particles which follow irregular patterns in a manner similar to gases, not necessarily following the airstream. This irregular pattern is known as “Brownian Motion” and increases the particles chance of being captured through contact with the fibres.
Straining is the most basic form of filtration, where the smallest dimension of the particle is larger than the space between the adjoining fibres, and is caught. These then create smaller holes by blocking off the bigger holes and improving the efficiency even more.
The efficiency of each grade of filter increases with improved effectiveness of removal moving from G1to G4 (general grades), moving to F5 to F9 (fine dust filters) up to H10 to H14 (HEPA filters) and U15 to U17 (ULPA filters). The efficiency is achieved by using different fibre diameters and increasing finer material to capture the finest particles. H10 to U17 are used in hospitals, pharmaceutical production and nuclear industries.
This is the mechanism used by carbon filters to remove gases and vapours from the air. The vary porous characteristics of the surface of each carbon molecule allows it to adsorb vast amounts of contaminants as they come into contact with it much like the ability of a sponge to retain water.
Each 450grams of carbon has the equivalent surface area of approximately 100 acres (404,830 sq Metres)
These are necessary when handling certain high concentration laden air which would fill up carbon filters very quickly or where the contamination needs to be kept wet.
There are a few types available:
This type has a tall vertical cylinder, usually made from plastic, inside which there is a section of specifically shaped ceramic or plastic mouldings.
The contaminated air flows upwards from a bottom inlet, up through the tower of shapes as water from jets at the top flows downwards, making for a lot of contact between the mouldings, water, fumes and contaminant. The contamination is scrubbed from the air, draining with the water to a sump at the base which will need emptying periodically.
This type also has a vertical tower, but this has a number of horizontal “plates” located across the whole diameter of the tower. Within the plates are a high number of holes through which the rising air passes, but also the falling water is trying to flow downwards. At the holes there is a lot of process reaction which cleans the contamination out of the air and down into the sump at the base.
This type uses a narrowing within the scrubber body to speed up the flow of air, while at the same time injecting the cleaning water as a spray. The speeding up of the air mixes the water with the contamination causing it to fall out of the air and into the sump. This uses the most power consumption but is also the most effective.
All of the wet scrubbers may need additives to be used to counteract any process issues with alkalizes or acids. Circulation pumps would be needed to provide the correct spray and dosing of chemicals if needed.
After consideration of the type of contaminants that will be present and the type of filters required, a layout of the machines and the ductwork is needed. Establishing the number of machines that would be in use at one time is also required. If there are many more machines than operators, dampers will be needed to keep the air extract volume down, but still give a degree of flexibility of use. The design of the ductwork should be sized to keep any captured fume airborne throughout the entire ductwork route, and have no settling out sections. Drain points are a must in systems handling potential liquids.
To reduce the motor power needed, always allow for any branch ducts entering into the main to be branched in at 30 degrees. This has one third of the pressure drop of that by connecting a duct at right angles like so many other suppliers do.
It is imperative that the hoods, ductwork and filter are all designed to do the same job. We have come across some systems which have reused a fan and filter from another project, and it has lead to major fall out problems, and un-safe conditions for staff using the equipment.
When installed, the dampers should be set (and marked if less than all the dampers are not used at the same time), so that the airflows required can be matched by any operator simply setting to the marks.
Heat recovery from any extracted air seems to be in the news at the moment. The amount of heat that can be recovered depends on the temperature of the air leaving the building, and if low will not achieve any savings at all. If the air is hot (from an over) or very cold (from a chilling tunnel) the amount of heat transfer and recovery can be worthwhile. Depending on the fume laden air involved it may not be possible to reuse the extracted air
Issued in 2008, HSG 258 (Controlling airborne contaminants at work) is a government publication guide, concentrating on the design and maintenance of LEV systems. This gives a structured procedure for evaluating the best design for hoods, ducting, and safe removal of fume and dust in LEV systems.
One of the many points discussed is the installation of a visual indicator to show that there is extraction to a particular machine and that it is working as designed.
These can be a simple manometer connected into the hood duct, or a complex device using pressure switches with pre set trigger levels.
Although not yet compulsory, more and more companies are starting to retro fit gauges to the working hoods.
Click on the link for more information regarding the Dangerous Substances and Explosive Atmospheres regulations.
Post time: 03-21-2017