In Local Exhaust Ventilation (LEV) systems, dry dust collectors are suitable for most conventional dust scenarios. However, when waste gases exhibit specialized characteristics—such as high temperature, high humidity, flammability/explosivity, or the simultaneous presence of gaseous pollutants —dry filtration methods face risks such as filter media combustion, dust explosions, or filter bag clogging and failure. In such instances, it becomes necessary to employ alternative technological approaches: wet scrubbers  or electrostatic precipitators.

I. Wet Scrubbers: Capturing Pollutants Using Water

Basic Principles

The core principle of a wet scrubber is gas-liquid contact. After pollutant-laden air enters the device, it mixes thoroughly with sprayed water or other scrubbing liquids. Pollutants are captured through two primary mechanisms: gaseous pollutants dissolve directly into the scrubbing liquid; meanwhile, solid dust particles collide with water droplets and become encapsulated within a liquid film, forming dust-laden droplets. Subsequently, a mist eliminator located inside the device separates the clean air from these dust-laden droplets, and the purified gas is discharged from the top of the unit.

The droplet size determines the collection efficiency: fine water droplets (10–50 microns) are more effective at capturing sub-micron dust particles but result in a higher pressure drop; conversely, coarse water droplets (100–300 microns) produce a lower pressure drop but offer limited effectiveness against fine dust. This characteristic serves as a critical basis when selecting the specific type of wet scrubber to be deployed.

Main Equipment Types

  • Spray Towers:  Featuring the simplest structural design, these towers operate by spraying scrubbing liquid downward while dust-laden air flows upward, creating a counter-current gas-liquid contact. They exhibit a relatively low pressure drop (250–500 Pa) but offer limited efficiency in capturing fine dust particles; consequently, they are best suited for the removal of coarse particles and for gas absorption applications.

(2) Venturi Scrubber: The most efficient type of wet scrubber. As dust-laden air passes through the throat section at high velocity, the scrubbing liquid is sheared and atomized into extremely fine droplets. This process achieves dust collection efficiencies exceeding 95% for sub-micron particles; however, it entails a high pressure drop (ranging from 5 to 15 kPa) and consequently high energy consumption. It is ideally suited for applications with extremely stringent emission concentration requirements—such as the capture of active pharmaceutical ingredient (API) dust in the pharmaceutical industry.

(3) Packed Tower: The tower is filled with packing materials—such as Raschig rings or Pall rings—to maximize the contact surface area between the gas and liquid phases. This technology is suitable for waste gas streams where gaseous pollutants predominate and dust concentrations are relatively low; however, the packing layer is prone to clogging, necessitating upstream pre-treatment.

Applicable Scenarios 

Wet scrubbers offer distinct advantages when treating the following four categories of waste gas:

  • -High-Temperature Waste Gas:For instance, flue gas generated at casting stations in foundries, typically ranging from 300°C to 500°C. While dry filter bags would rapidly carbonize under such conditions, the scrubbing liquid simultaneously cools and removes dust during contact, effectively reducing the outlet temperature to between 60°C and 80°C.
  • –Flammable and Explosive Dusts : Such as aluminum powder, magnesium powder, or pulverized coal. Wet scrubbers maintain the dust in a constantly wetted state, thereby eliminating the conditions required for dust cloud formation and blocking potential ignition propagation pathways; this makes them the preferred safety choice in industries such as metal powder processing.
  • -High-Humidity Waste Gas: For example, paint overspray mist generated in paint booths. High humidity causes dry filter media to become clogged and ineffective, whereas wet scrubbers do not rely on filter media and remain unaffected by humidity levels.
  • -Waste Gas Containing Both Dust and Gaseous Pollutants: Chemical additives can be introduced into the scrubbing liquid to enable the simultaneous removal of dust alongside acidic or alkaline gaseous pollutants, thereby simplifying the overall process workflow.

Engineering Case Studies 

Within the chemical and pharmaceutical industry cluster in Frankfurt, wet scrubbers are widely Used for the treatment of waste gas from chemical reactors. A typical configuration consists of a Venturi scrubber connected in series with a packed tower: the former captures fine dust particles with an efficiency exceeding 95%, while the latter is responsible for absorbing gaseous pollutants. The dust emission concentration for the entire system can be controlled to below 5 mg/m³, and the sulfur dioxide removal rate exceeds 98%. This configuration has been operating stably for many years within the waste gas treatment systems of GMP workshops at numerous enterprises.

Limitations

The primary limitations of wet scrubbers include: wastewater requires further treatment, which increases capital investment in facilities; corrosion-resistant materials (such as stainless steel or FRP) must be used when treating acidic or alkaline gases; exhaust gas may carry entrained water droplets if the mist eliminator fails; and in cold regions, the discharge of saturated humid air may result in visible water plumes, necessitating heating or dilution of the exhaust stream.

II. Electrostatic Precipitators: Using Electric Fields to Capture Dust

Basic Principles

The operating principle of an electrostatic precipitator is based on corona discharge and electrostatic adsorption  within a high-voltage electric field. The discharge electrode (cathode) is connected to a high-voltage DC power supply (30–100 kV), while the collecting electrode (anode) is grounded. When the voltage rises to the corona inception threshold, air molecules near the discharge electrode become ionized, generating a large number of free electrons. As these free electrons migrate toward the collecting electrode, they are captured by dust particles, causing the particles to acquire a negative charge. Driven by the electrostatic force, these charged dust particles move toward the surface of the collecting electrode and accumulate there. A rapping mechanism is then employed to dislodge the dust, allowing it to fall into a hopper, thereby completing the dust removal process.

Main Equipment Types

  • (1) Horizontal Electrostatic Precipitators: The airflow passes horizontally through the electric field, and the collecting electrodes are arranged in parallel as flat plates. This design facilitates maintenance and dust removal via rapping; it is capable of handling large air volumes and is widely utilized in thermal power plants and cement factories. Single-unit… The equipment can be designed with multiple electric fields arranged in series (typically 2 to 5) to enhance overall efficiency.
  • (2) Vertical Electrostatic Precipitator: The airflow moves vertically upward, and the collecting electrodes take the form of circular tubes. Featuring a compact structure and a small footprint, this design is suitable for treating waste gas streams with small to medium flow rates and is commonly found in the chemical and carbon black industries.

Technical Features

Electrostatic precipitators possess three distinct technical characteristics:

  • – Low Resistance: The pressure drop typically ranges between 100 and 300 Pa—approximately one-tenth that of a baghouse filter—resulting in significantly reduced energy consumption for the fan. For large-scale thermal power plants, this translates into substantial annual savings on electricity costs.
  • – High Temperature Resistance: The equipment can process flue gas at temperatures ranging from 300°C to 400°C; with specialized designs, it can handle temperatures exceeding 500°C. This makes it ideal for high-temperature flue gas applications—such as power plant boilers and cement kiln outlets—and allows for direct installation downstream of waste heat boilers.
  • – Large Processing Capacity: A single unit can process flue gas volumes ranging from hundreds of thousands to over a million cubic meters per hour. For instance, an electrostatic precipitator paired with a typical 300 MW coal-fired power unit can handle a flue gas flow rate of up to 1.2 million cubic meters per hour.

Key Parameters Affecting Efficiency

Dust resistivity  is the most critical influencing factor. Dust with a resistivity within the range of 10⁴to 10¹¹ ohm-centimeters is most amenable to electrostatic collection. When resistivity is too low (e.g., carbon-rich dust), the dust particles discharge their electrical charge too rapidly and are easily re-entrained by the airflow; conversely, when resistivity is too high (e.g., fly ash), a “back corona” phenomenon occurs, severely compromising efficiency. For dust with unsuitable resistivity levels, flue gas conditioning measures can be implemented—such as injecting steam, sulfur trioxide (SO₃), or ammonia—to alter the surface properties of the dust particles and bring their resistivity within the optimal range.

Additionally, factors such as gas velocity (typically maintained between 0.8 and 1.5 m/s), electrode spacing, and voltage stability also significantly influence dust removal efficiency. A properly designed electrostatic precipitator (ESP)  can achieve an efficiency of over 99% for sub-micron dust particles, with outlet emission concentrations controllable to below 20 mg/m³.

Engineering Case Study 

In a waste-to-energy incineration plant, an ESP is employed to treat flue gas at temperatures ranging from 200°C to 250°C. Operating with four electric fields arranged in series, the system achieves a total dust removal efficiency exceeding 99.8%, with outlet dust concentrations consistently maintained below 5 mg/m³. Due to its low pressure drop, the fan power consumption—at an airflow rate of 200,000 m³/h—is only approximately 30% of that required by conventional baghouse filters, resulting in annual electricity cost savings of over €100,000.

Limitations 

The primary limitations of electrostatic precipitators include: high initial investment costs driven by high-voltage power supply equipment, rendering them economically unviable for small-scale applications; sensitivity to dust resistivity, where efficiency can drop significantly if conditions are unsuitable; the generation of trace amounts of ozone via corona discharge; and an inability to treat gaseous pollutants, necessitating their integration with desulfurization and denitrification equipment.

Wet scrubbers capture pollutants using liquid media, making them suitable for exhaust streams that are flammable, explosive, high-temperature, high-humidity, or contain gaseous contaminants. Electrostatic precipitators, conversely, capture dust through electrostatic adsorption; they offer low airflow resistance and are capable of treating large volumes of high-temperature flue gas. A thorough understanding of the physical mechanisms and operational boundaries of both technologies is crucial for making the correct equipment selection during the design phase of Local Exhaust Ventilation (LEV) systems. As a specialized supplier of air filtration solutions, TrennTech offers wet scrubbers, electrostatic precipitators, and comprehensive system solutions that meet the aforementioned technical requirements, thereby assisting enterprises in achieving the dual objectives of regulatory compliance and safe production.