In a local exhaust ventilation system, the fan can be replaced with a larger one, the filter can be upgraded to a more efficient model, and the piping can be rearranged. However, there is one element that is basically set in stone from the design stage and is extremely costly to modify later on – that is the spatial relationship among the three zones: the location where pollutants are captured, the location where the operator works, and the location where the operator breathes. The relative layout among these three zones directly determines whether the worker is breathing clean air or acting as a human filter.
I. Definition of Three Zones
The capture zone refers to the spatial range in front of the hood opening where the airflow velocity is sufficient to overcome the momentum of pollutant diffusion and draw it into the duct. The boundary of this zone is typically defined as the isosurface of the capture velocity. Simply put, within the capture zone, pollutants will be drawn into the hood; outside this zone, pollutants are free to disperse.
The work zone is the area where the operator’s hands, tools, and workpieces are located. This is the space where the process operation actually takes place and where pollutants are generated. The work zone may overlap with the capture zone or be far apart from it – and most design problems arise from the latter situation.
The breathing zone is the hemispherical space approximately 30 centimeters around the operator’s mouth and nose. This is the ultimate point of judgment for determining occupational exposure concentration. No matter how well the capture zone is designed, if pollutants pass through this area before entering the hood opening, the ventilation system fails to protect the worker.
II. Ideal Layout: Capture Area Covers Work Area, Breathing Zone Upwind of Pollution Source
The most ideal LEV layout meets three conditions:
First, the capture area completely covers the work area. Pollutants are under airflow control from the moment they are generated, with no chance to spread outward. This means the hood must be close enough or large enough so that the airflow velocity throughout the entire operation area reaches or exceeds the required capture velocity.
Second, the breathing zone is located upwind of the pollution source. Upwind refers to the direction where air flows from behind the operator towards the pollution source and then into the hood. This way, the air the operator breathes is clean air from the workshop behind them, rather than air that has passed over the pollution source.
Third, the breathing zone is not between the capture area and the pollution source. This is often violated. Many on-site setups can be seen where the operator faces the grinding wheel and the hood is directly behind it. In this case, the operator’s breathing zone is right between the pollution source and the hood – air flows from behind the operator towards the front, passes over their face, carrying the dust from the grinding wheel, and then enters the hood. This means the pollutants have to pass through the operator’s breathing zone before being captured, which is the most typical failure case.
In an ideal layout, the operator’s head and the hood are on either side of the pollution source, or the hood is on the side of the operator for lateral suction. Either way, the core principle is: do not allow the operator’s face to be in the path of pollutants entering the hood.
III. Three Common Mistakes in Layout
Mistake One: Hood Too Far Away, Capture Zone Shrinks
This is the most common mistake. To facilitate operation, the operator pushes the hood too far away, resulting in the actual boundary of the capture zone retreating beyond the work area. Pollutants disperse before entering the airflow control range, and it’s only a matter of time before they reach the breathing zone.
The on-site judgment method is simple: Use a smoke tube to release smoke near the operation point and observe whether the smoke directly enters the hood. If the smoke first drifts towards the operator’s face and then is drawn towards the hood, or simply drifts to other parts of the workshop, it indicates that the capture zone does not cover the work area.
Mistake Two: Breathing Zone Sandwiched in the Middle
As mentioned earlier, the operator, pollution source, and hood form a straight line, with the operator in the middle. In this layout, even if the hood has a strong suction force, pollutants must pass by the operator’s face before being drawn away. The actual exposure concentration may be higher than without a ventilation system because the airflow accelerates the movement of pollutants from the source to the face.
Mistake Three: Interfering Airflows Cut Off the Capture Zone
Even if the capture zone originally covered the work area, air conditioning supply in the workshop, forklifts passing by, and personnel movement can generate lateral airflows that “blow” the capture zone off course. At this point, the work area is actually outside the protection of the capture zone, and the breathing zone is exposed to the dispersed pollutants. This situation is the most difficult to detect because workers often report, “It seems the hood is drawing air, but I still smell something.”
IV. Three-Zone Layout Strategies for Three Types of Process Scenarios
Welding Stations: The weld points are not fixed and the shapes of the workpieces vary. The most suitable hood type is an external hood with a flange or a flexible arm hood that can be moved. The layout principle is that the hood opening should be no more than 30 centimeters away from the weld point, and the operator’s head must be on the side or above the hood opening. The welding fumes should be directly drawn towards the hood opening after being generated from the weld point, without passing through the operator’s face. For large workpiece welding where it is difficult to approach, side suction slots should be set up in the welding area, and supply air outlets should be placed behind the operator to form an air curtain to separate the breathing zone from the pollution source.
Grinding and Polishing: High-speed rotation of the grinding wheel generates particles that fly off along the tangential direction. The hood opening should be placed in front of and below or on the side of the grinding wheel, with an arc-shaped opening to match the wheel surface. The operator should stand on the axial outer side of the grinding wheel, not directly in front. The breathing zone naturally avoids the main direction of particle dispersion. If wet grinding is used, the three-zone requirements remain the same, but the capture effect will be significantly improved.
Chemical Feeding: When pouring powder into the feeding port of a reactor or mixing tank, pollutants rise upwards. The most effective solution is to use an enveloping hood – three sides of the feeding port are enclosed, with a hood opening on top, and the operator operates on the side with the opening. At this time, the capture zone covers the top of the feeding port, the working zone is where the bag is emptied, and the breathing zone remains clean as the operator stands outside the opening and the air flows inward. If the process does not allow for enclosures, push-pull ventilation should be used: supply air on one side of the feeding port and exhaust on the opposite side, so that the airflow acts like an air knife to cut off the upward path of the pollutants.
V. Methods for Verifying Whether the Three Zones Meet the Standards
Verify whether the capture zone covers the work zone: Release tracer smoke at the operation point and observe whether the smoke directly and continuously enters the hood opening. If the smoke drifts in a direction other than towards the hood opening or reaches the hood opening at a significantly slower speed, it indicates that the boundary of the capture zone does not cover that point.
Verify whether the breathing zone is contaminated: Release a small amount of smoke at the operator’s chest level (simulating the breathing zone) and observe the direction of the smoke. If the smoke is drawn towards the hood opening rather than towards the face, it indicates that the position is in a clean air flow. If the smoke swirls in place or is blown towards the face, it indicates a problem with the breathing zone.
Verify the influence of interfering airflows: Fix a thin line or a light ribbon around the operation point and observe its drift direction. If the thin line points in a direction other than towards the hood opening, it indicates the presence of an unidentifiable lateral airflow.
TrennTech, a leading air filter supplier based in Frankfurt, Germany, has repeatedly found during project commissioning that problems in Zone 3 are often detected by operators within the first week of new equipment being put into use. They verify this in the most straightforward way: by standing at the workstation and smelling, or by placing a cigarette butt near the operation point to observe the direction of the smoke. Engineers should take these on-site feedbacks into account in order to optimize the entire LEV system.
When designing a LEV system, keep in mind three fundamental rules: the work area must be within the capture zone; the breathing zone must be upwind of the pollution source; and under no circumstances should the breathing zone be located between the pollution source and the hood opening. If the on-site conditions cannot simultaneously meet these three requirements, the type of hood or the process layout needs to be redesigned – rather than relying on a larger fan to cover up layout mistakes.