thermal cutting of metals

dust & fume extraction from laser and plasma cutting applications

Created Friday, 16 June 2023

Laser cutting, plasma cutting and oxy-fuel cutting are different machining processes used in the metal producing and metalworking industries, using energy in various forms to cut virtually any shape from iron and nonferrous materials out of sheets or large slabs. During the processes, the materials heat up, melt and cause harmful emissions. If not controlled properly, this airborne dust and fumes pose health risks for operators and cause damage to machinery.

processes and handled materials

  • Laser cutting: Uses high powered laser beam for the thermal cutting of various metals in different thicknesses and forms such as sheet or tubes so they can be precisely cut leaving an edge with a high-quality surface finish. Over recent years laser cutting technology has advanced to enable the cutting of thicker metals such as structural steel and not only mild steel and stainless steel but also non-ferrous metals including copper, brass, aluminium as well as organic materials. 
  • Plasma cutting: Uses electrically generated plasma and is used primarily for the cutting of thicker metals in various forms, including mild or stainless steel, aluminium, and copper.
  • Oxy-fuel cutting: Oxy-fuel cutting is using gases such as acetylene or propane as energy source and is typically used to cut very thick ferrous metals.  

health risk for operators

A wide range of health risks have been associated with dust and fumes from thermal cutting processes. The nature and severity of the hazard will vary with the type of material and the cutting method.

To understand why a high level of filtration efficiency is so critical, it is necessary to review some of the health risks associated with thermal cutting emissions. Whether you are working with mild steel, stainless steel, aluminium, galvanised or another material, the chemical composition is a good starting point for identifying health risks. Most local health and safety authorities have established Permissible Exposure Limits (PELs) based on 8-hour Time Weighted Average (TWA) for hundreds of dusts.

  • Hexavalent chromium or hex chrome is a carcinogenic substance that results from cutting of stainless steel and other metals that contain chromium. Hex chrome overexposure can result in short-term upper respiratory symptoms, eye or skin irritations. Long-term, the greatest health danger associated with hex chrome exposure is lung cancer.  A common PEL for hex chrome is extremely stringent, at 5.0 μg/m³. When cutting stainless steel, HEPA filtration is required to stay below this threshold limit.

  • Zinc oxide is a pollutant generated by hot work on galvanised steel. Exposure can result in a condition known as “metal fume fever”, a short-term illness in which severe flu-like symptoms occur after a break from work, such as after a weekend or during a vacation. Due to the delayed reaction, it is often confused with regular influenza and many cases go undiagnosed.
  • Manganese, which is present in some steel alloys, can cause workers to feel exhausted, apathetic, weak or headachy. Chronic overexposure to fumes containing manganese leads to a condition known as “manganism”, which is characterised by neurological and neurobehavioral health problems. Manganese discharges are now specifically regulated in most countries by the national health and safety authorities.

  • NOx is generated by all high-temperature processes, through oxidisation of the Nitrogen in the air. There is strong evidence that NOx respiratory exposure can trigger and exacerbate existing asthma symptoms, and may even lead to the development of asthma over longer periods of time.

Fire and explosion hazards

  • Fire prevention is a big issue with lasers due to the nature of the raw material and the use of potentially flammable oil for corrosion protection of this material. Also, un-oxidised metals and their fumes can be combustible and potentially explosive. Prevention measures for a dust and fume collector should, at a minimum, include fire detection with an interlock that shuts down the fan, a sprinkler system to extinguish a fire, a spark arrestor on the ducting to the collector, and flame-retardant filter media.

  • Explosive dusts: Many types of metals you are cutting, may in some situations lead to a combustible dust hazard. The only way to know for sure is to test your dust for explosibility and document a dust hazard analysis. If your dust is found to be an explosion hazard, there are very specific and limiting requirements for your dust and fume collection system.  The type of dust collector, explosion protection and duct isolation required for each application will vary, depending on dust parameters and installation conditions. 

dust & fume extraction solutions for thermal cutting applications

energy recovery & cost savings

Dust collectors consume energy the whole time they are running. The largest portion of the electrical load goes to the fan motor that moves the air through the system. Also a lot of energy is used to heat or cool the fresh air that replaces the extracted process air.

The good news is that there are ways to reduce these costs, using less electricity, using less compressed air and using fewer filter cartridges.

fan motor

The fan motor is the dust collection system component that consumes the most electricity. This consumption is directly proportional to the volume of air the motor is moving through the system. Dust collectors are variable systems. Their resistance to airflow (pressure drop) changes over time, according to the dust loaded on the filter cartridges. So fan efficiency is an important factor and by selecting a high-efficiency fan and using a variable speed drive, energy consumption can be reduced.

compressed air

Producing compressed air is extremely expensive, so pulse cleaning has always been one of the highest operating costs associated with dust collection. Today’s most advanced dust collectors can reduce compressed air consumption by as much as 50% versus less advanced system designs. They use less compressed air because they are able to pulse-clean far less often. When properly designed, the cleaning system will remove the built-up material from the filter cartridges, reduce the pressure drop across them, reduce the fan energy consumption, and in turn, reduce associated energy costs.

heating energy

Energy to heat or cool the extracted air can be a significant part of the total energy consumption that is often overlooked. To reduce this, there are two ways, recirculation or heat recovery. Both of these require the air to be very clean. For recirculation it is crucial to remove harmful substances from being recirculated to the workplace. This is achieved by using a second filter stage with HEPA grade filters.

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