The task of the hood of a local exhaust ventilation (LEV) system is not complicated: to intercept the pollutants on their path from the process operation point to the operator’s breathing zone and direct them into the duct. However, behind the word “intercept” lies a series of engineering design judgments, including the attenuation law of air flow, the momentum ratio of the pollution source, and the assessment of interfering air flows.
I. Fundamentals of Aerodynamics: Capture Velocity and Airflow Attenuation
Capture velocity refers to the airflow speed required at the release point of pollutants in front of the hood to overcome the initial momentum of the pollutants and resist lateral interfering airflows. It is not the wind speed of the hood itself but the target speed at the control point.
The requirements for capture velocity vary greatly among different processes. For scenarios with extremely low pollutant release rates, such as the breathing valves of chemical storage tanks or electroplating baths, an airflow of about 0.3 meters per second is sufficient. For operations with slight dust generation, such as manual bag dumping or material dropping on conveyor belts, a speed of 0.5 to 1 meter per second is needed. For processes with moderate release rates, such as welding or paint spraying, a speed of 1 to 2.5 meters per second is required. For high-momentum jet processes like sandblasting, polishing wheels, or pneumatic gun cleaning, the capture velocity may need to reach 2.5 to 10 meters per second.
There is a common misconception that “a large fan volume equals good performance.” However, in reality, even if the fan volume is large, if the hood is too far from the pollution source, the actual airflow speed at the control point may still be close to zero.
Three typical cases of air flow attenuation at the hood opening
When air is drawn in through the hood opening, the velocity decreases sharply with the increase of distance. The most basic and poorest form is a circular opening without a flange. When a circular pipe is directly opened to draw in air, the air flows in from all directions, and the velocity decays extremely rapidly. At a distance of one diameter from the hood opening, the air velocity has already dropped to about one-tenth of the velocity at the hood opening. This means that for a circular pipe hood opening with a diameter of 30 centimeters, the suction force is almost negligible at a distance of 30 centimeters.
The significant improvement can be achieved with the flanged cover opening. The flange refers to a flat plate extending outward from the edge of the cover opening, typically with a width of three to five centimeters. This plate forces the air to flow in only from the front of the cover opening instead of bypassing from the sides. As a result, the airflow velocity at the same distance is approximately doubled. At a position one diameter away from the cover opening, the flanged cover opening can still maintain about 20% of the cover opening’s wind speed.
The narrow and long slot hood is another form, mainly used for linear pollution sources, such as multiple welding torches working side by side. Its airflow attenuation is much slower than that of the circular opening. Under the same exhaust volume, the velocity of the long slot hood at a distance of one equivalent diameter from the hood opening is about two and a half times that of the circular opening without a flange. However, its drawback is that it is extremely sensitive to cross-flow. Even a slight asymmetry in the connection position of the pipeline can lead to one side drawing in air while the other side leaks air.
In the engineering field, there is a rule of thumb: for every doubling of the distance between the hood and the source, the required air volume should at least quadruple. Therefore, the correct design approach is always to prioritize adjusting the process layout to place the hood as close as possible to the pollution source, rather than purchasing a larger fan when the hood is already placed far away.
II. Classification and Selection Principles of Hood
According to the relative position of the hood to the pollution source and the control method, LEV hoods are classified into three major categories.
Receiving type hoods are used for pollution sources that have an upward or directional momentum, such as hot smoke rising upward or shot blasting particles flying along a fixed trajectory. The hood only serves as a receiving opening and catches the pollutants at the end of their movement path. Typical applications include hot smoke hoods above electroplating tanks and heat treatment furnaces, and particle receiving hoods at the outlet of shot blasting machines. When designing, the hood opening area must cover the entire projected range of the pollution plume, and it is usually required to be 20% to 30% larger than the actual width of the plume.
The enclosure hood surrounds the pollution source on three or four sides, leaving only an operating opening. As the volume of captured air is confined to a very small space, this form requires the least air volume and has the highest energy efficiency. Typical applications include laboratory fume hoods, sandblasting rooms, and grinding cabinets. When designing, the air velocity at the operating opening is usually controlled between 0.3 and 0.6 meters per second. If it is too fast, small tools may be sucked away or discomfort may occur; if it is too slow, pollutants are likely to escape.
External hood: In cases where the pollution source is completely outside the hood opening, the pollutants are drawn in by the airflow generated at the hood opening. This is the type that is most prone to design failure and is also the most common source of problems in industrial settings. Typical applications include large workpiece grinding, robot welding, and bucket feeding.
When designing external hoods, the shape of the hood opening must match the form of the pollution source: circular hoods for point sources, long slot hoods for linear sources, and rectangular hoods for surface sources. Additionally, flanges are almost essential – external hoods without flanges have an effective capture distance of no more than one times the diameter of the hood opening. In many failed cases, what was installed on site was just a straight pipe with a beveled end, and the effective capture distance of this structure is almost zero.
There is another important principle for external hoods: Do not place the operator’s head between the pollution source and the hood opening. The correct arrangement is to have the hood opening behind the pollution source, with the operator standing on the upwind side, and the airflow blowing from behind the operator towards the pollution source and then into the hood.
III. Three Easily Overlooked Application Details
Interfering airflows are more troublesome than the pollution itself. Air conditioning outlets in workshops, forklifts passing by, and people walking around all generate lateral airflows of 0.5 to 1 meter per second. When the lateral wind speed exceeds the suction speed of the hood at the pollution source position, pollutants will be directly blown away from the hood. Solutions include adding flexible curtains to the edge of the hood, reinforced side baffles, or repositioning the direction of the air conditioning outlets.
Large particles are suitable for entering the hood opening by themselves through inertia. For large particles such as sandblasting grains and metal chips, the drag force of the air flow is limited. The correct approach is to let the particles fly into the hood opening relying on their own momentum, with the air flow only responsible for preventing rebound and escape. For horizontally projected particles, the hood opening should be directly facing the projection line.
The matching of the hood and the filter needs to be considered as a whole. A common mistake is to increase the air volume of the fan when poor performance is found, resulting in coarse particles in the pipeline being crushed into fine dust, increasing the burden on the filter; the face velocity of the filter cartridge exceeds the standard, shortening its lifespan; the workshop is drawn into negative pressure, and energy consumption rises. The correct matching sequence is: first determine the form of the hood, then calculate the required air volume, then calculate the pipe diameter based on the minimum conveying speed of the pipeline, and finally select the filter and fan.
IV. On-site Performance Self-check and Summary
When designing or inspecting a LEV hood, a simple four-step method can be used: First, check the distance to see if the pollution source is within one equivalent diameter in front of the hood opening; second, check the edge to see if there is a flange with a width of three to five centimeters around the hood opening; third, check for interference to see if there is any lateral airflow blowing over the area in front of the hood opening; fourth, listen for any whistling or hissing sounds.
After a LEV system is installed, the cost of adjusting the air volume and replacing the filter elements in the later stage is acceptable. However, modifying the position and shape of the hood is often costly and requires a long downtime. Industry statistics show that approximately 70% of LEV system failures are caused by incorrect hood positions or unreasonable shapes, rather than problems with the fan or filter. TrennTech, an air filter supplier based in Frankfurt, Germany, has repeatedly verified this conclusion through numerous project reviews.
The basic principles of the design of LEV hoods can be summarized in four words: “Near” – it is better to reduce the operating space than to increase the distance between the hood and the source; “Suitable” – choose the type that is suitable for receiving, enclosing or external installation; “Stable” – avoid interfering airflows and protect the suction area with flanges and curtains; “Verify” – measure with a smoke tube or fine thread instead of relying on feeling. The cost of the hood usually accounts for only 5% to 10% of the entire LEV system, but it determines over 80% of the on-site usage effect. Remember: fix the hood opening first, then consider changing the fan.