If a local exhaust ventilation system is not equipped with a purification device, the air carrying dust, smoke, oil mist or chemical vapors will be directly discharged outdoors. This merely shifts the pollution – from the workshop to the outside, and from the breathing zone of the workers to the windows of their neighbors.
I. Position of the Purification Device in the LEV System
Is the purification device usually placed before or after the fan? The answer is that it is usually placed before the fan.
This means that the purification device is in the negative pressure section. The air is first treated and cleaned by the purification device and then enters the fan impeller. The benefits of this design are obvious: the fan impeller comes into contact with clean air and is not subject to dust wear, oil mist adhesion, or chemical corrosion. The service life of the fan is greatly extended, and the maintenance frequency is also significantly reduced.
II. Four Main Purification Principles
There are numerous types of air purification devices used in industry, but their basic principles can be classified into four categories: filtration, washing, electrostatic precipitation, and adsorption.
Filtration – Trapping Particles
The principle is straightforward: dusty air passes through a layer of fibrous material. Particles larger than the gaps between the fibers get trapped, while clean air passes through and enters the next section of the pipeline.
Filter bags are long, tubular bags. Air flows from the outside to the inside, and dust remains on the outer surface. Periodic pulse jet cleaning shakes the dust off into the ash hopper. Filter bags are suitable for handling high-dust concentration conditions, such as in cement plants and grinding workshops.
Filter cartridges are folded into a star shape, providing a much larger filtration area than the same volume of filter bags and a more compact structure. They are suitable for handling medium concentrations of fine dust.
High-efficiency filters use ultra-fine glass fibers and can capture over 99.97% of particles around 0.3 microns. This level is typically used in nuclear industries, pharmaceuticals, and biosafety laboratories where extremely high cleanliness is required.
The advantages of filtration include high efficiency, mature equipment, and wide application. The disadvantages are that resistance increases as dust accumulates on the filter material, requiring regular cleaning or filter element replacement. Wet dust can cause bag clogging, and high-temperature dust requires attention to the temperature limit of the filter material.
Washing type – washed down with water
Washing type purification uses water or other liquids to capture pollutants. When the dusty air enters the scrubber, it comes into full contact with the water mist sprayed down. Dust particles will be moistened and adhered to water droplets, and then settle to the bottom of the tower under the force of gravity. Clean air is discharged from the top of the tower.
The washing type is particularly suitable for treating flammable and explosive dust and high-temperature and high-humidity gases. Water is non-flammable, and the washing process also has a cooling effect. It is also suitable for treating sticky dust – sticky dust will clog the filter material in the filtering device, but for the scrubber, it just means there is more sludge to be treated.
The disadvantage of the washing type is that it generates wastewater. The collected dust turns into sludge or mud, which needs further dewatering, drying and disposal. This is a considerable burden in some regions with strict environmental protection requirements. In addition, the humidity of the air after washing is very high. If the subsequent pipelines and fans are not corrosion-resistant, anti-corrosion measures need to be considered.
Electrostatic type – Attracted by electricity
The principle of electrostatic purification devices is similar to that of the air purifiers or the electrostatic modules in kitchen range hoods used at home. Dust-laden air passes through a high-voltage electric field, where dust particles are ionized and charged. Then, they pass through a set of dust collection plates with opposite charges, and the charged particles are adsorbed onto the plates. Clean air passes through.
The greatest advantage of the electrostatic type is its extremely low resistance. Filter-type devices need to overcome the resistance of the filter material, while the airflow path of the electrostatic type is almost straight, with a resistance of only one-tenth or even less than that of the filter type. This means that the accompanying fan can be smaller and consume less energy.
The disadvantage of the electrostatic type is that it has requirements for the specific resistance of dust. If the specific resistance is too high, the dust cannot discharge electricity when it reaches the dust collection plate and will accumulate thicker and thicker, causing a “reverse corona” phenomenon and a sharp drop in efficiency; if the specific resistance is too low, the dust will lose its charge as soon as it reaches the dust collection plate and will bounce back into the airflow. In addition, the electrostatic type is not suitable for handling high-concentration dust and is usually used as a secondary fine purification device.
Adsorption – Capturing Molecules with Pores
For gas molecules, filtration is largely ineffective – the molecules are too small to be blocked by fibers. Washing has some effect, but it is limited for organic solvents. In such cases, adsorption purification is needed.
The most commonly used adsorption material is activated carbon. Activated carbon has an extremely rich network of micropores, with a specific surface area of one gram of activated carbon reaching 1,000 to 1,500 square meters. When gas molecules enter these micropores, they are adsorbed onto the pore walls due to intermolecular forces and are removed from the gas flow.
Another commonly used adsorption material is molecular sieves, which have uniform pore sizes and can screen molecules based on size, making them suitable for treating specific gases. Molecular sieves are heat-resistant and non-flammable, and are widely used in the chemical industry.
The drawback of adsorption is its limited adsorption capacity. Activated carbon will eventually become saturated and will no longer adsorb after saturation, requiring replacement or regeneration. Saturated activated carbon that has been replaced is considered hazardous waste and its disposal cost is not low. Regeneration requires heating or reducing pressure to release the adsorbed gas – the released gas can either be condensed and recovered or burned.
III. Several Questions to Be Answered When Selecting a Model
Frankfurt’s professional air purification equipment supplier, TrennTech, suggests that before selecting a model, the following points must be paid attention to:
Is the pollutant a particle or a gas? Particles can be filtered, washed or treated with electrostatic methods; gases can be adsorbed or washed.
How large are the particles? Coarse particles larger than ten microns can be effectively removed by almost all equipment; fine particles smaller than one micron require filter cartridges, high-efficiency filters or electrostatic methods.
What is the concentration of the pollutant? When the concentration is high, a pre-separator or a larger equipment size is required.
What is the temperature of the dusty gas? Almost all equipment can be used at room temperature; when the temperature exceeds 120 degrees, high-temperature resistant filter materials or washing for cooling are needed.
Is the humidity high? High humidity can cause filter bags to clog, making filtration methods unsuitable; washing methods or dehumidification pre-treatment are preferred.
Is the pollutant flammable or explosive? If so, electrostatic methods require special explosion-proof design, and filtration methods require anti-static filter materials and explosion-proof devices.
IV. Maintenance: The purification device is not something that can be installed and then left unattended.
Filter-type devices require regular dust cleaning or filter element replacement. The frequency of dust cleaning depends on the dust concentration and the design of the dust cleaning system. When the fan frequency increases while the hood air velocity decreases, it usually indicates that the filter is clogged.
Washing-type devices need regular water replenishment, sludge discharge, and nozzle cleaning. Nozzle blockage is the most common fault of the washing tower, which can lead to uneven local spraying.
Electrostatic devices need regular cleaning of the dust collection plates. When the dust collection plates accumulate dust to a certain extent, the efficiency will drop significantly. The cleaning cycle ranges from a few days to several months, depending on the dust concentration and stickiness.
Adsorption-type devices need to pay attention to saturation judgment. The simplest judgment method is to install a gas detector at the outlet of the adsorption tower. When the outlet concentration starts to rise, it indicates that the adsorbent is approaching saturation and needs to be replaced or regenerated.
The purification device is the only part in the LEV system that directly changes the “existence form of pollution”. The hood only transfers, the air duct only conveys, and the fan only drives – only the purification device truly removes pollutants from the air. Choosing the right purification method, making the right combination, and conducting regular maintenance will prevent this system from becoming a “moving but not treating” facade project.