Cutting through the smoke

Considerations when choosing the right dust and fume collector

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The airborne “smoke” generated during the laser cutting process is actually a fume cloud comprised of very fine dust particles that can harm workers and equipment. These particulate contaminants, however, can be properly controlled through high-efficiency dust and fume collection.

Fortunately, today’s laser cutting machinery typically is designed to allow plug-and-play installation of fume collection equipment. A good fume collector should be as reliable as the lighting in a facility: You should be able to turn it on and not have to think about it, allowing you to concentrate instead on people, processes and production schedules.

Selecting the right fume collection equipment is an important and multi-faceted decision. This article examines four key areas of concern: health and safety issues, design considerations, maintenance and operational factors, and air recirculation.

Health and safety issues

A range of health and safety hazards are associated with laser cutting fumes, and the nature and severity of the hazard varies with the type of material being cut. Whether working with mild steel, aluminum, galvanized or another material, a material safety data sheet is a good starting point for identifying health risks.

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A Camfil APC promotional part is produced on a 4-kW laser table.

Air quality concerns: OSHA has established permissible exposure limits based on an 8‐hour time-weighted average (TWA) for hundreds of dusts, including the numerous metal dusts generated in laser cutting. One of the most dangerous dusts is hexavalent chromium or “hex chrome,” which results from the cutting of stainless steel and other metals that contain chromium. Overexposure to hex chrome is proven to cause lung cancer and a long list of other major health problems.

As a result, in 2006 OSHA published a new rule for hex chrome exposure, which drastically reduced the permissible level of exposure in the workplace, from 52 to 5 micrograms per cubic meter (µg/m3) – a tenfold reduction. OSHA stipulates that every company with a process that generates hex chrome must use air-quality monitoring to determine the 8-hour TWA exposure for each worker who is affected.

A well-designed cartridge fume collector properly filters laser cutting fumes and other hazardous contaminants. This collector uses self-cleaning mechanisms that pulse dirt off the filters, allowing units to run for extended periods between filter changes. The best systems use source capture to control the dust at the point of generation, ensuring that it will not spread and be inhaled by workers in other areas of the plant.

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Sheet metal part fabrication on a laser table.

Fire and explosion risks: Fire prevention is a big issue with lasers due to the nature of the raw material and the abundant use of potentially flammable oil in the process. Prevention measures should, at a minimum, include a smoke detector with fan shutoff, a sprinkler system, a spark arrestor at the inlet of the fume collector and a fire-retardant filter media. More sophisticated fire protection systems may be required for some installations.

Much of the dust generated in laser cutting is inert and, therefore, does not pose an explosion risk. However, there are situations in which a combustible dust hazard may be present. The only way to know for sure is to test the dust for explosiveness.

Approaching health and safety risks as a whole, the only way to get a proper assessment is through a hazard analysis, conducted by an engineer knowledgeable about the process. This is needed to safeguard employees from hazardous exposure of any kind and to comply with applicable requirements from the National Fire Protection Association and OSHA.

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Fire-retardant filter media should always be used in fume collection equipment for laser cutting applications.

Design considerations

If a fume collector is undersized – which sometimes happens in an attempt to minimize capital investment – it can’t perform its job properly; if oversized, it may be prohibitively costly to purchase and operate while hogging valuable floor space.

Correct sizing: Sizing is critical and is a function of the open area on the table zones, the thickness and type of material being cut, the laser wattage (power) being used, the feet per minute of cut and the hours of operation – including whether it is a lights-out programmable system designed to run at night. An air filtration specialist can help calculate the right size fume collector for an application including the proper air-to-cloth ratio, which refers to the cubic feet per minute of air entering the collector divided by the square feet of filter media.

Ductwork must also be sized properly to maintain the proper conveying velocity through the system, a step that is sometimes overlooked. If the duct diameter is too small, it may create a high conveying velocity, possibly drawing small metal pieces out of the table and causing too much static pressure on the fan. If the duct diameter is too large or any shape other than round, dust can fall out of the airstream and create a fire hazard. The Sheet Metal Air Conditioning Contractors’ National Association has published standards establishing proper configuration and sealing of ducting, joints and flanges, and these should always be followed.

Proper noise control: This must be designed into the system, as well. Fan exhaust from fume collection systems can be very noisy, creating a fatiguing and hazardous environment. An exhaust silencer reduces the noise level and can often be customized to meet varying needs.

Filter mounting: A fume collector designed with vertically mounted filter cartridges helps minimize fire and explosion risks. With horizontally mounted systems, dust becomes entrained at the top of the filters, and there is no pre-separation of heavy or abrasive particles from the airstream. This condition can shorten filter life and provide a dusty surface for sparks to ignite. Vertical mounting reduces the load on the filters and helps eliminate these problems.

Also, make sure the fume collector is being used for its designed function. Sometimes, a shop purchases a laser table to cut mild steel and then uses it to cut other metals that might have different explosive or biological hazards – or even to cut wood or acrylic, which could create a fire risk or plug the filters because the fume collection system is not designed to handle those materials.

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An eight-cartridge dust collector captures laser cutting fumes from an automated tube cutting application.

Maintenance and operational factors

A well-designed and properly sized fume collector is engineered to keep maintenance to a minimum. The only operator inputs should be occasional changing of the pulse-cleaning/pressure settings as the filters wear and the eventual replacement of filters when differential pressure through the system reaches the maximum level specified by the filter manufacturer. This is very important to ensure that filters are effectively controlling dust and fumes. If filters start plugging due to overdue change-out, there will be a noticeable increase in fumes.

Collector location: When a fume collector is designed to be operated and maintained “from the face,” i.e., from one surface, it can be more readily tucked into a tight corner or between columns to solve space constraints. Another solution is a structural mezzanine that bridges over the laser table. The mezzanine can hold the fume collector, laser resonator and chiller, opening up floor space and consolidating all the laser support on one clean, safe, serviceable platform. In addition to easy access, the operator should also be able to change filters quickly with no need for tools.

Filter “seasoning”: A fume collector fan is typically designed to pull the manufacturer’s recommended airflow when a filter is dirty or at the end of its life. When filters are clean, the initial startup pressure is very low. If the damper is left wide open, the fan pulls more than the design airflow. This wastes energy; and in addition, particles moving at high velocity may cause filters to plug prematurely. This problem can be avoided by damping the fan down when filters are new.

Dust removal: Cleaning out collected dust is a simple but sometimes overlooked maintenance task. If dust accumulates over the top of the storage drum, it can back up into the hopper and cause a fume collector malfunction. Alternately, the overflowing dust can fall out on the floor when the drum is moved, creating an unsafe mess and a possible fire hazard.

Monitoring: Remote monitoring of critical information is recommended. Many laser suppliers have central diagnostic centers where they can pull up and diagnose problems for customers. Monitoring of the fume collection equipment can be tied into this function or handled independently through new web-based diagnostic systems. These systems can electronically monitor an entire network of fume collectors and provide automatic alarming of fault conditions as soon as they occur – enabling you to stay connected to vital information whether you are in or out of the shop.

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A collector with 20 filter cartridges, shown in back right, captures dust and fumes from three laser tables at an engine manufacturing facility.

Air recirculation

Air recirculation is the single best way to save energy and maximize return on investment with a fume collector. By recirculating heated or cooled air back through the plant instead of venting it outdoors, the cost to replace that conditioned air is eliminated.

Facilities in all regions report five- to six-figure annual energy savings, with the greatest savings seen in northern climates, which experience longer, colder winters. In addition, the U.S. Department of Energy offers public utility-sponsored rebates and incentives for facilities that use recycled heated or air conditioned air. Most equipment suppliers have cost-calculation software to help project the savings based on system airflow, climate, local utility costs and other factors.

When recirculating air downstream of the fume collector, a HEPA after-filter (sometimes called a safety monitoring filter) is recommended and, when filtering hazardous dusts, it may be required. These high-efficiency filters provide backup protection and a final scrub of the air before it returns to the facility.

In summary, a high-efficiency dust and fume collector can greatly reduce or nearly eliminate employee exposure to airborne contaminants generated during laser cutting, resulting in a cleaner and greener work environment that improves comfort and morale, boosts productivity and enhances manufacturing reliability. When you add air recirculation to the equation, you achieve the trifecta of compliance, health and wellness, and energy savings.

Camfil Air Pollution Control

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