Integrated dust collectors in cutting systems help to reduce worker exposure to fume and help protect expensive machinery. When selecting and designing a dust collector, consideration of thermally generated dusts should include the filtration challenges such as particle size, loading and combustible dust risks.
Dry vs. Wet Tables
Dry tables, often utilise cartridge dust collectors with dust conveyed to the collector via ducts built within the downdraft table. An alternative collection system is a wet table, which uses a water solution in which the work piece is submerged. Cutting done at or below the water surface allows fume or particulate generated during cutting to then deposit within the water solution.
Benefits of Surface-Loading Filter Media
The fine particulate generated by the thermal cutting applications requires high efficiency cartridge collectors to perform well. Surface-loading, nanofibre media and downward airflow patterns increase collector performance dramatically.
Nanofibres aid in the removal of the very fine particulate from the airstream, through several filtering phenomena including, interception, diffusion and impaction. The nanofibres increase the overall efficiency of the filter media, and they force the particulate to accumulate at the surface of the media where pulse cleaning can be effective. The downward airflow patter is also critical for stabilising differential pressure on thermal cutting applications.
As the cleaning system pulses, the downward airflow assists with evacuating fine particles and fume from the surface of the filter. Cartridge filter media without nanofibres often allows particulate to embed within the media fibres, resulting in a reduced airflow to the cutting table and shorter filter life.
Determining Dust Collector Size
The size of a dust collector is dependent on required airflow to contain the fume and particulate. Generally speaking, the wider the cutting table, the greater the airflow required to contain the dust and, consequently, the larger the dust collector required. Other variables also influence airflow, including the size of the table, the fraction of the table covered by the work piece, and the number of open zones within a table during the cutting process.
Downdraft tables are often partitioned or zoned to reduce the total airflow required to capture dust effectively, thus reducing the size of the collector. The collector size is then tailored based on factors including the number of cutting heads, the cutting technology (laser, plasma, oxy-fuel etc.), the material being cut, and the cutting speed and torch on-time.
The airflow through the cutting table must be sufficient to generate a downdraft velocity at the surface of the table to overcome rising fume.
Downdraft velocity is the minimum velocity of air required to prevent fume and particulate from escaping the cutting table (typically) 0.76 - 1.27 m/s for thermal cutting applications) but can vary with table size and design.
The Need for Hazard Analysis
As with any process, customers should perform hazard analysis before selecting a dust collection strategy. Sparks are typically present in any thermal cutting process, and collected dusts may be combustible - presenting explosion and/or fire risks. Ignition source mitigation strategies should be a consideration for any dry dust collector strategy. In addition, consideration of risks capturing particulate from different metals should be included.
Bring in the Experts
Considering the filtration challenges posed by thermal cutting applications, manufacturers should consult filtration experts for dust collection solutions that meet the requirements of the EPA (if applicable), protect cutting machinery, and minimise worker exposure to harmful fumes and particulate.