BOFA's range of ADVANTAGE laser fume and filtration extraction systems are designed for applications that generate particulate and gaseous organic compounds within the Laser Marking, Coding and Engraving Industries.
Most laser fume purification systems have 3 stages of filtration. A pre filter to protect the higher cost HEPA (High Efficiency Particulate Air) for the finer particulates and finally a gas/chemical filter.
The efficiency of the pre filter is directly related to the life of the HEPA in that the more particulates captured in it, the less can pass through and prematurely block the HEPA. Most pre filters have an efficiency grading of F5 - F7. When plastics are being lasered, the particulate size generated is typically 0.2microns. At this level of filtration the media has an efficiency of 35% in F5 to 65% in F7. When metals are being lasered the particulate size being generated is typically below 1micron. This equates to an efficiency of 65% in F5 to 80% in F7.
The Bofa Advantage range of laser fume purification systems has an F8 grade media which gives an efficiency of 80% at 0.2 microns and 95% at 1micron.
Most panel filters have a nominal depth of 50mm. They have few pleats to obtain some increase in surface area. In short we would not recommend the use of this type of filter in any laser marking or cutting exercise as the surface areas are far too small for the amounts and the size of particulate generated.
Bag filter are commonly used in laser fume elimination for their volumetric containment properties. These bags have a variety of shapes including multi pocket styles to increase surface area and reduce pressure drops.
As the contaminated air enters the inside of the cartridge chamber, the velocity drops slowing down the airflow causing in the larger and heavier particulates to fall within the cartridge sump. The sump also allows larger particulates that build up during operation to drop away from the filter through the true nature of reverse airflow design. See fig 1.
Filter life is directly related to surface area. As previously mentioned the larger dry particulates build up and fall away during operation. Sticky particulates on the other hand will hit the media surface and dry on contact irrespective of the progressive depth of the media. The end result of this is skinning or masking off to the surface media. Therefore the greater the surface area the longer the filter will last.
It should be worth mentioning at this point that electrostatic charged fibres have in recent years been applied to a range of panel and bag filtration products. Extensive field trials have shown that the charged fibres are adversely affected and the charge can to a large extent be negated by humidity or by coming into contact with carbonaceous contaminants. This can result in a severe decrease in efficiency after a few weeks operation. The particles are held within a much more open matrix, in the event of vibration caused by air movement through the filter, particles can become dislodged and carried into the 'clean' airstream.
All pre filters used in the BOFA Advantage range have a very high degree of particulate separation (F8, which is 95% @ 1 micron). The Advantage 180 and 400 having 9m2 of surface area and the 1500 and 4000 being as high as 20m2. The whole filter is designed as a cartridge with a sump capacity of 10Kgs for the 180 and 400 series and 25Kgs for the 1500 and 4000 series. Where bag filters are used in conjunction with a reverse airflow system the whole bag acts as the filter. This results in larger particulates falling back within the filter media itself. If that media is pleated, the surface area for fall back contaminants can lay in up to 50% of the construction of that filter. See fig 2.
Multi pleat bag filters are used to obtain greater surface area within the media. In cases where these filters are positioned horizontally being supported by a vacuum inlet pipe secured through it centre, the pleats tend to collapse masking off large sections thus nullifying the effectiveness of the design. This problem is magnified where more pleats are added.
Most micro or HEPA filters used today are of the mini pleat design. They are generally manufactured by specialist filter companies to ensure that the latest manufacturing and test methods are adopted. Each filter incorporates a series of webbing between the pleats. This feature not only guarantees even spacing for full filter coverage between each pleat, but also alleviates the possibility of airflow vibration or collapse.
Filters should be individually tested and certified to a minimum efficiency of 99.997% down to 0.3 microns and 95% as small as 0.1 micron.
The Bofa Advantage micro filters are positioned within the system in such a way that all the particulates are not only captured but held within the media. This is not the case with conventional reverse airflow systems were particulate can fall away and rest on the outside of the pre filter creating a health risk on filter replacement.
Mini pleats can be produced with depths of up to 200mm within each pleat, depending on the application.
Most standard chemical filters are made up of a 50/50 mix of Activated Aluminium Potassium Permanganate and Active carbon. The media neutralises a broad base of chemicals and VOC (Volatile Organic Compounds) pollutants. Where PVC is lasered a third gas media should be introduced that is specifically designed to absorb and neutralise harmful and extremely corrosive HCL (Hydrogen Chloride) gas.