An effective LEV system requires a thorough understanding of pollutants, process flows, operation methods and control objectives. If the wrong equipment is purchased, designed or installed, the result will be that the equipment is running but workers are still breathing in harmful gases. Therefore, many employers must clarify a series of basic issues about LEV before adopting an LEV solution.
I. Key Characteristics of Air Pollutants
To control a pollutant, one must first understand what it is and its properties in order to formulate targeted solutions.
Toxicity determines the control target. For highly toxic substances, even extremely low concentrations need to be strictly controlled; for low-toxicity dust, the allowable exposure concentration is relatively higher. The occupational exposure limit (OEL) serves as the judgment criterion: the lower the limit, the stronger the toxicity, and the higher the requirement for the local exhaust ventilation (LEV) system.
Physical form determines its behavior in the air and the method of removal. Dust and smoke are solid particles, mist is liquid particles, and vapor and gas are in molecular form. Solid particles can be removed by filtration, while gas molecules require adsorption or chemical reactions for removal – these are two completely different technical approaches.
Particle size distribution determines the deposition location of particles in the respiratory system. Particles larger than 10 micrometers are usually intercepted by the nasal cavity, causing rhinitis and pharyngitis; particles between 2 and 5 micrometers can enter the trachea and main bronchi, causing tracheitis; while particles smaller than 2 micrometers can reach the alveoli – the core area for gas exchange. The alveoli lack cilia and a mucus layer, and particles deposited here are cleared extremely slowly, leading to long-term accumulation and fibrosis and pneumoconiosis. The smaller the particle size, the greater the harm and the more difficult the control.
Concentration determines the severity of pollution. The higher the concentration, the greater the exhaust air volume required. However, concentration itself is dynamic – it peaks at the moment of arc initiation during welding and then stabilizes; in painting operations, the concentration distribution along the path of the spray gun is highly uneven. What designers need to know are the “peak concentration” and the “average concentration”, rather than a vague “a lot of dust”.
Employers do not need to be toxicologists, but they must be familiar with the basic data of the main pollutants in their workshops. This information is usually available from the safety data sheets (SDS) of raw materials.
II. How are gases, vapors, dusts and mists formed?
Understanding the generation mechanisms of pollutants is essential for thinking about control strategies from the source. Different generation methods determine the form, particle size and movement characteristics of pollutants.
Dust is produced when solid materials are subjected to mechanical forces. Grinding, cutting, drilling, crushing, sanding and polishing – these operations turn large solid pieces into tiny particles. The particle size depends on process parameters and operation methods: rough processing generates large particles, while fine processing and sanding produce fine particles; dry operations generate much more dust than wet operations. The sanding process in wood processing produces the finest wood dust and is also the most health-risky part. Dry cutting of stone produces high concentrations of silica dust, while wet cutting with water can significantly reduce dust.
Fumes are formed when metals or other substances vaporize at high temperatures and then rapidly condense. During welding, cutting and smelting, metals are heated above their boiling points to become metal vapors. After leaving the high-temperature zone, they rapidly cool in the air and condense into extremely fine solid particles. This is a “gas-solid” phase change process, and the particle size is usually between 0.01 and 1 micrometer – 10 to 100 times finer than most mechanically generated dust. Fume particles are extremely small and can remain suspended in the air for a long time, making them the most difficult type of pollutant to control. Different welding processes produce significantly different amounts and compositions of fumes: carbon steel welding fumes contain iron oxide and manganese, while stainless steel welding fumes also contain hexavalent chromium and nickel oxides.
Mists are formed when liquids are atomized by mechanical force or physical action. High-speed rotating cutting tools break metalworking fluids into tiny droplets; spray guns atomize and spray coatings; liquid droplets are produced when bubbles on the surface of electroplating tanks burst; water mists are generated when cooling towers operate. The particle size of mist droplets ranges widely, from sub-micron to tens of microns. Metalworking fluid mists contain emulsifiers, biocides and extreme pressure additives, and inhalation can cause asthma and allergic pneumonia. Oil mist particles formed when engine oil volatilizes at high temperatures and then condenses are extremely fine and require highly efficient coalescing filters to remove.
Vapors and gases are molecular substances produced by evaporation of liquids or chemical reactions. Benzene compounds in paint volatilize from the liquid surface into the air, and the higher the temperature, the faster the volatilization; high temperatures during welding cause nitrogen and oxygen in the air to react and form nitrogen oxides, which are irritating and corrosive; heating or burning of plastics releases toxic gases such as hydrogen chloride and carbon monoxide; volatile organic compounds such as trichloroethylene and n-hexane used in solvent cleaning operations all enter the air. Molecular pollutants have an important characteristic: they cannot be intercepted by any physical filtration material. HEPA filters can trap particles as small as 0.3 micrometers, but not molecules that are 1,000 times smaller than particles. Controlling vapors and gases requires the use of technologies such as activated carbon adsorption, catalytic combustion or chemical scrubbing.
The significance of understanding these generation mechanisms lies in the fact that different pollutants require different treatment methods. A filter cartridge dust collector suitable for capturing wood dust may be insufficiently efficient for welding fumes; while equipment that is very effective for welding fumes is completely ineffective for paint vapors. Using the wrong technical approach means wasted investment.
III. How Pollutant Clouds Move with Surrounding Air
After being released from the source, pollutants do not remain stationary. They move along with the surrounding air, following the basic laws of fluid mechanics. Understanding this movement is the foundation for designing an effective LEV system – the exhaust hood must be placed where the pollutants are “about to pass” rather than where they “have already left”.
The primary driving force is the upward movement of hot air currents. When welding, smelting, hot processing, or cooling of hot workpieces occurs, the generated pollutants have a high temperature and low density, naturally moving upwards. The upward speed of hot smoke can reach several meters per second, forming an upward “thermal plume”. This upward force can be fully utilized–placing the exhaust hood directly above the heat source, the pollutants will “enter” the hood opening on their own, and effective capture can be achieved with a relatively small exhaust volume.
However, the thermal plume has a fatal weakness: it is very fragile. Even a lateral airflow of only 0.3 to 0.5 meters per second in the workshop can deflect the rising hot smoke plume. Lateral airflows have many sources: air conditioning supply outlets, fans, air disturbances caused by forklifts moving, opening and closing of doors, and personnel movement. Once the thermal plume is deflected, it deviates from the capture range of the exhaust hood and begins to spread within the workshop. This is why many exhaust hoods above hot processes appear large, but the actual workstations are still filled with smoke–it is not due to insufficient air volume, but because the lateral airflow has blown the smoke away.
Another driving force is the jet force. High-speed rotating grinding wheels, sandblasting guns, compressed air blowing, and high-pressure spraying–these operations give the pollutants a high initial speed, causing them to move at high speed in a specific direction. In sandblasting operations, sand particles strike the workpiece surface at tens of meters per second, rebounding and carrying dust to continue moving. The receiving hood can be set along this movement trajectory, and the pollutants will “fly” into the hood opening on their own. In grinding operations, the high-speed rotating grinding wheel throws out grinding chips and dust along the tangential direction, and the exhaust hood should be placed in this direction of ejection.
In the absence of obvious upward force and jet force, pollutants mainly spread through diffusion and convection. Dust and vapor diffuse from high concentration areas to low concentration areas –this is the result of molecular movement, which is slow but continuous. At the same time, pollutants are transported along with the air flow in the workshop. Airflows generated by the workshop ventilation system, disturbances caused by personnel movement, and vibrations caused by equipment operation –all these factors gradually disperse the pollutants throughout the space.
The core significance of understanding these movement laws lies in that the exhaust hood should be placed upstream of the pollutant’s movement path, rather than downstream. By upstream, it means the position closer to the source of the pollutant on the path from the generation point to the breathing zone of the workers. You need to intercept the pollutant when it “starts”, instead of waiting until it has spread throughout the workshop to extract it. The fundamental difference between LEV and dilution ventilation that controls the air in the entire room lies here: LEV intercepts at the source, while dilution ventilation deals with the spread after it has occurred. The former requires less air volume, has lower energy consumption and better effect; the latter requires more air volume, has higher energy consumption and poorer effect.