The core of a local exhaust ventilation (LEV) system is a seemingly simple component – the exhaust hood. It is the entry point for pollutants into the entire system and also the key link that determines whether the LEV can work effectively.
I. Enclosure: The Strongest Control, the Highest Constraint
Working Principle: An enclosure completely or partially encloses the pollution source within a casing, maintaining a negative pressure inside to prevent the escape of pollutants. Once pollutants are generated in the enclosed space, they are immediately carried away by the airflow, with almost no chance to diffuse into the working environment. This is the most efficient type of exhaust hood among the three. Under the enclosed conditions, there is a physical barrier between the pollutants and the operator, and it is not disturbed by the lateral airflow in the workshop. As long as the negative pressure inside the enclosure is maintained properly, leakage can be controlled at an extremely low level.
Typical Applications: Enclosures are suitable for situations where the amount of pollutants generated is large, the toxicity is high, or the operation point is fixed and suitable for enclosure. For example: grinders, sieves, crushers, powder transfer points, reaction vessels, sandblasting rooms, etc. In the chemical and pharmaceutical industries, the weighing and mixing processes of highly active pharmaceutical ingredients usually use glove boxes – a completely enclosed exhaust hood. Operators perform operations through gloves and windows, completely isolated from the materials.
Design Points: The key to the design of an enclosure is to minimize openings. Any opening (operation port, observation window, material inlet and outlet) is a potential path for pollutant leakage. The air flow rate at the opening must be fast enough to prevent the diffusion of pollutants outward. For situations that require frequent operations, operable doors or sliding windows can be designed to maintain a negative pressure state while ensuring operational convenience.
Limitations: The main limitation of enclosures is the constraint on operations. When the pollution source is too large, has an irregular shape, or requires frequent manual intervention, effective sealing is difficult to achieve. In such cases, other types of exhaust hoods need to be considered.
II. Receiving Hoods: Utilizing the Initial Kinetic Energy of Pollutants
Working Principle: Receiving hoods do not rely on the suction airflow generated by fans to capture pollutants. Instead, they utilize the initial kinetic energy of the pollutants themselves, which is usually thermal updraft or jet force. The exhaust hood is placed along the pollutant’s movement path and passively “receives” the naturally moving pollutants. Unlike Capturing hood, receiving hoods do not actively “suck” but rather “wait” for the pollutants to move to the hood opening. Therefore, the exhaust air volume required by receiving hoods is typically less than that of Capturing hood under the same conditions, as the fan only needs to extract the pollutants that have already entered the hood, rather than generating a large-scale airflow to draw in pollutants from a distance.
Typical Applications: Receiving hoods are suitable for pollution sources with distinct thermal updraft or high-speed jet characteristics. The most common applications are thermal process equipment: furnaces, ovens, hot-dip galvanizing tanks, and casting pouring ports. Hot exhaust gases rise naturally due to reduced density, and the receiving hood is installed directly above the heat source, utilizing the upward momentum of the gases to guide them into the exhaust system. Another typical application is sandblasting and shot blasting equipment – sand particles are jetted at high speed onto the workpiece surface, rebound and carry dust, and the receiving hood is set along the rebound trajectory to effectively capture them.
Design Considerations: The position and size of the hood opening are key design parameters. For heat sources, the hood opening should be large enough to cover the diffusion range of the thermal plume. As the hot exhaust gases rise, they gradually spread; an insufficiently sized hood opening will cause some gases to escape around the edges. The distance between the hood opening and the heat source also needs to be precisely controlled – if the distance is too large, the thermal plume may be deflected by cross-ventilation in the workshop, thus escaping the receiving range.
Limitations: Receiving hoods are sensitive to external conditions. Cross-ventilation in the workshop (from air conditioning supply outlets, forklift traffic, door openings, etc.) can cause the thermal updraft to deviate, significantly reducing the receiving efficiency. In environments with strong cross-ventilation, it is necessary to install curtains or baffles around the receiving hood to minimize interference.
III. Capture Hood: An Active Approach to Collection
Principle of Operation: The capture hood uses the suction airflow generated by the fan to actively “pull” pollutants from the source to the intake of the exhaust hood. Unlike the receiving hood, the capture hood does not rely on the initial kinetic energy of the pollutants but instead uses the airflow generated by the exhaust hood to overcome the diffusion tendency of the pollutants.
The capture hood is the most common type of exhaust hood and has the widest range of applications. Side suction hoods or top suction hoods at welding stations, side exhaust ports in spray booths, dust collection ports on woodworking table saws, and semi-open laboratory fume hoods – these are all typical examples of capturing hoods.
Design Considerations: The core of the design for a capture hood is to establish a sufficient “control velocity” – at the point of pollutant generation, the airflow velocity generated by the exhaust hood must be sufficient to overcome the initial kinetic energy of the pollutants and the interference of cross airflows in the workshop, pulling the pollutants towards the hood opening.
Limitations: The efficiency of the capture hood is significantly affected by cross airflows in the workshop. Even if the designed airflow velocity meets the standards, if there are strong cross airflows in the workshop (such as nearby fans, air conditioning outlets, or people walking), the pollutants may be blown out of the capture range of the hood opening. For highly toxic or high-value pollutants, the capture hood is usually not the first choice – a closed hood can provide more reliable protection.
IV. Selection Logic for Three Types of Exhaust Hoods
There is no absolute superiority or inferiority among the three types of exhaust hoods. The choice depends on the process conditions and control objectives.
The closed hood has the highest control efficiency, but it also imposes the greatest constraints on the process. When the pollution source is small in size, regular in shape, and the operation can be standardized, the closed hood is the preferred solution, especially when dealing with highly toxic or highly carcinogenic substances. Precision manufacturing enterprises in the Stuttgart area of Germany commonly use closed hood designs in powder handling and weighing processes to physically separate operators from the pollution source.
The receiving hood utilizes the kinetic energy of the pollutants, has the lowest energy consumption, but its application conditions are limited. It can only work effectively when the pollutants have sufficient thermal buoyancy or ejection velocity. Thermal processes and sandblasting operations are typical scenarios where the receiving hood can demonstrate its advantages.
The capture hood has the widest range of applications but requires the highest design precision. The calculation of control wind speed, the determination of hood opening position, and the coordination with the air flow organization in the workshop – these factors collectively determine the success or failure of the capture hood. In most conventional ventilation scenarios such as welding, grinding, woodworking, and laboratories, the capture hood is the standard configuration.
V. Matching of Exhaust Hoods and Downstream Filtration
After the exhaust hood captures pollutants, the dusty air flow enters the air purification device through the pipeline. Different types of pollutants have very different requirements for filtration equipment. Even if the exhaust hood is highly efficient, if the downstream filtration equipment is improperly selected, the pollutants are merely transferred from the workshop to the outside, without being truly eliminated. The selection of filtration equipment should be carried out simultaneously with the design of the exhaust hood to ensure the integrity and effectiveness of the entire LEV system. Professional air filter suppliers such as TrennTech provide air filters that offer comprehensive and professional solutions for different dust separation types and characteristics.
Enclosing hood, receiving hoods and capturing hoods respectively represent three different control strategies: physical barriers, passive reception and active capture. None of these types is the “best” – only the most suitable choice for specific process conditions and pollutant characteristics. The correct selection of the type of exhaust hood, precise location design, sufficient control air velocity, and the matching back-end filtration equipment – the combination of these factors can constitute a truly effective local exhaust ventilation system.