In Local Exhaust Ventilation (LEV) systems, traditional designs typically follow a specific pathway: “Capture—Conveyance—Purification—Discharge.” However, when laboratory space is limited, building structures preclude wall penetrations, or the concentration of hazardous substances being handled is relatively low, a more flexible solution presents itself: filtration-based purification devices—most notably, the ductless fume hood.

I. Filtration-Based Purification Devices: An Underestimated Equipment Category

What Are Filtration-Based Purification Devices?

Filtration-based purification devices constitute a broad technical category, generally referring to any LEV purification equipment centered around filtration media that removes airborne contaminants through physical interception or chemical adsorption. This category encompasses a wide array of dry dust collectors widely utilized in industrial settings—including filter bags, cartridge filters, and High-Efficiency Particulate Air (HEPA) filters. However, within the context of LEV systems, filtration-based purification devices also include a specialized sub-branch: self-circulating purification equipment designed for low-concentration, intermittent operations in environments with stringent indoor air quality requirements. The quintessential example of this sub-branch is the ductless fume hood.

The Technical Principles of Filtration-Based Purification

All filtration-based purification devices adhere to the same fundamental process:

  • Step 1: Intake. A fan draws contaminated air from the operational zone into the device’s inlet.
  • Step 2: Filtration. The air passes through one or more layers of filtration media. For particulate matter, the filtration media captures dust particles through physical mechanisms such as interception, inertial impaction, and diffusion; for gaseous chemical contaminants, the media removes them via adsorption—utilizing common adsorbent materials such as activated carbon, modified alumina, and molecular sieves.
  • Step 3: Discharge. The purified air is expelled from the device’s outlet. In traditional industrial dust collectors, the purified air is typically discharged outdoors; however, in self-circulating filtration devices, the purified air is discharged directly back into the indoor environment.

This characteristic of “discharging back indoors” endows filtration-based purification devices with unique value in settings where the installation of external exhaust ductwork is impractical—such as in building renovation projects, historic structures, and clean laboratories.

Types and Selection of Filtration Materials

The performance of filtration-based purification devices relies heavily on the filtration materials employed. Depending on the target pollutants, appropriate filtration media are selected accordingly:

  • Particulate Filtration Materials: Primary filters utilize non-woven fabrics or metal mesh to intercept coarse particles larger than 10 microns; medium-efficiency filters demonstrate effective capture efficiency for particles ranging from 1 to 10 microns; and High-Efficiency Particulate Air (HEPA) filters achieve a capture efficiency of over 99.97% for 0.3-micron particles, making them widely used in the pharmaceutical and medical industries.
  • Gaseous Pollutant Adsorption Materials: Activated carbon is the most commonly used adsorbent; with a specific surface area ranging from 500 to 1500 square meters per gram, it exhibits excellent adsorption capabilities for organic vapors, chlorine gas, sulfur dioxide, and similar substances, though its adsorption efficiency for low-boiling-point compounds—such as formaldehyde —is relatively low. Chemical filters employ adsorption materials impregnated with chemical reagents to remove specific gases through acid-base neutralization or oxidation reactions.

II. Ductless Fume Hoods: A Typical Application of Self-Circulating Purification

What is a Ductless Fume Hood?

A ductless fume hood is a self-contained piece of laboratory equipment featuring an integrated filtration system. Its design objective is to provide effective respiratory protection for laboratory personnel performing experiments without requiring a connection to any external ductwork. While its exterior appearance resembles that of a traditional ducted fume hood, its internal structure is fundamentally different: the upper or rear section of a ductless fume hood houses an integrated fan unit and a multi-layer filtration assembly.

Operation Process: The fan draws contaminated air generated within the work zone into the interior of the fume hood. The air first passes through a primary filter to remove coarse particles, then flows through an activated carbon filter or chemical filter to adsorb gaseous hazardous substances, and finally passes through a HEPA filter to ensure the thorough interception of particulate matter. The purified, clean air is then discharged back into the laboratory environment from the top or the upper-front section of the fume hood.

Advantages of Ductless Fume Hoods

Compared to traditional fume hoods that require external ductwork connections, ductless fume hoods offer the following advantages:

Installation Flexibility: Traditional ducted fume hoods necessitate the installation of an internal ductwork system within the building—a process that entails prohibitively high engineering costs for building renovations, leased laboratory spaces, or historically protected structures. In contrast, a ductless fume hood requires only a connection to a standard electrical outlet to become operational, and relocating the unit requires no additional construction work.   

Significant Energy Savings: A standard 1.5-meter-wide traditional fume hood, when fully open, can exhaust 1,000 to 1,500 cubic meters of indoor air per hour. This air must then be reconditioned by the HVAC system, resulting in substantial energy consumption. Ductless fume hoods, conversely, return purified air directly back into the room; by eliminating the loss of conditioned air, they typically achieve energy savings of over 70%.

Ideal for Intermittent and Low-Concentration Operations:For experimental procedures involving small chemical volumes, low-toxicity substances, or the generation of low-concentration contaminants, ductless fume hoods represent an economical and efficient choice. In a chemistry teaching laboratory in Frankfurt, Germany, dozens of ductless fume hoods are utilized for student organic chemistry experiments; operating independently, these units saved significant ductwork space and capital investment compared to a centralized external exhaust system.

Limitations of Ductless Fume Hoods 

Ductless fume hoods are not suitable for all applications; prior to use, the following limitations must be thoroughly evaluated:

Adsorbent Saturation and Replacement:Activated carbon and chemical filters possess a finite adsorption capacity. Once the adsorbent material reaches saturation, it loses the ability to effectively capture further contaminants. Consequently, filter status requires rigorous monitoring and periodic replacement; replacement timing is typically determined using methods such as weight sensors, gas sensors, or cumulative operating time logs.

Unsuitable for High-Toxicity or High-Concentration Operations:For experiments involving highly toxic substances, high-concentration volatile chemicals, or the generation of substantial dust, the safety margin provided by ductless fume hoods is insufficient to ensure adequate protection; in such cases, traditional ducted fume hoods remain the necessary choice.

Chemical Compatibility Limitations:Not all chemical substances can be effectively adsorbed by activated carbon. Compounds with low boiling points, high volatility, or low molecular weights exhibit a strong tendency to “break through” (penetrate) the activated carbon bed at ambient temperatures; therefore, users must consult the filter’s chemical compatibility  list prior to use.

III. Key Selection Criteria for Filtration-Based Purification Devices 

Whether selecting an industrial-grade filtration dust collector or a laboratory-scale ductless fume hood, attention must be focused on the following core technical parameters:

  • Filtration Efficiency :For particulate matter, the specific filter classification (grade) should be clearly identified; for gaseous contaminants, it is essential to understand the adsorption material’s breakthrough time and adsorption capacity data relative to the target chemicals.
  • Airflow and Face Velocity: The face velocity at the sash opening of a ductless fume hood is typically designed to range between 0.4 and 0.6 meters per second. If the velocity is too low, it fails to effectively contain escaping contaminants; conversely, if it is too high, it may accelerate the saturation of the filters.
  • Filter Monitoring System: High-quality equipment should be outfitted with sensors and an alarm system capable of monitoring filter status in real time. These systems provide alerts when the filters approach saturation, thereby preventing operators from continuing to use the unit under unsafe conditions.

Chemical Compatibility: Users are required to provide a list of the chemicals routinely used—including their specific quantities—so that the supplier may verify the suitability of the filtration system. By integrating high-efficiency particulate filtration with chemical adsorption, ductless fume hoods offer a novel solution for local exhaust ventilation. A thorough understanding of the filtration media’s characteristics, the equipment’s operational scope, and the requirements for maintenance and replacement is a prerequisite for ensuring the long-term, safe operation of such devices. As a specialized supplier of air filtration solutions, TrennTech Frankfurt offers customized ductless fume hoods and system solutions, empowering enterprises to achieve their goals of energy conservation and cost reduction while simultaneously safeguarding the health of their personnel.