If you have the chance to step into a workshop equipped with a local exhaust ventilation system, you will see hoods, filters and fans. But there is a large part of the system that you won’t see – the ducts hidden in the ceiling, the partition walls or along the columns. They are not conspicuous, but without them, the polluted air sucked in by the hoods would have nowhere to go.
If the LEV system is compared to the human body, the hood is the mouth, the filter is the stomach, the fan is the heart, and the ductwork is the blood vessels and digestive tract that run throughout the body. They are responsible for transporting the pollutants from the capture point to the treatment equipment and finally to the safe discharge location. How this journey is made, how smoothly it goes, and whether there is any leakage along the way directly determine the success or failure of the entire system.
I. What is the duct transporting? How is it transported?
The fundamental task of a duct is simple: to convey polluted air from point A to point B without losing or blocking it. Point A is the hood, and point B is the filter or exhaust outlet.
However, this is not as simple as it sounds. Polluted air is not clear water; it contains dust, smoke, oil mist, and even corrosive gases. The duct needs to achieve three things simultaneously: maintain a sufficient flow rate to prevent dust from settling, maintain airtightness to prevent leakage, and minimize resistance to save energy.
Flow rate is the first threshold. If the air flow rate inside the duct is too low, the carried dust will settle down, gradually clogging the duct over time. Different particle sizes and densities of dust require different minimum conveying speeds. Generally, fine dust requires a speed of ten to fifteen meters per second, while coarse particles and wet dust may need more than twenty meters per second. However, the flow rate should not be too high either, as it can wear the inner walls of the duct, increase energy consumption, and generate noise.
II. Pressure Zones: Negative Pressure Zone and Positive Pressure Zone
The pressure within a duct system is not uniform. Understanding pressure zones is crucial for assessing leakage risks.
From the hood opening to the fan inlet, this section of the duct is under negative pressure. That is, the pressure inside the duct is lower than the workshop’s atmospheric pressure. The advantage of negative pressure is that if there is a small hole or poor seal in the duct wall, air will leak in from the outside rather than spray out from the inside. This means that contaminants will not escape into the workshop through the leakage point. Therefore, most sections of the LEV system – including the straight ducts, elbows, branch ducts, and the upstream of the filter after the hood – are designed to be in the negative pressure zone.
From the fan outlet to the exhaust outlet, this section of the duct is under positive pressure. The pressure inside the duct is higher than the atmospheric pressure. If there is a leakage in this section, the polluted air will be directly blown into the workshop. Therefore, the positive pressure zone is usually kept as short as possible, and all joints need to be more strictly sealed. Ideally, the positive pressure zone only includes the short section from the fan outlet to the roof exhaust outlet, and the exhaust outlet should be located in an area where no personnel are active.
A common mistake is to place the filter on the positive pressure side of the fan. Although some processes require this, if the filter body or inspection door is not sealed properly, unfiltered polluted air will directly leak into the workshop. Unless there are special process requirements, the filter should be prioritized to be placed on the negative pressure side of the fan.
III. Why Circular Ducts Are Superior to Rectangular Ones
There are mainly two choices for the cross-section of air ducts: circular and rectangular. In engineering practice, circular ducts are generally preferred for three reasons.
First, for the same cross-sectional area, circular ducts are structurally lighter. The circular cross-section is subject to uniform force and can achieve the same structural strength with thinner materials. Rectangular ducts need to add stiffeners or use thicker plates to resist the inward depression deformation caused by internal negative pressure. For the same ventilation volume requirement, circular ducts usually save 20% to 30% of materials compared to rectangular ducts.
Second, circular ducts have a stronger ability to withstand pressure differences. The circumferential stress distribution of the circular cross-section is uniform, without stress concentration at the corners. The four corners of the rectangular duct are natural weak points, which are prone to inward depression deformation under negative pressure and outward bulging under positive pressure. During long-term operation, the corner welds of rectangular ducts are also high-risk areas for fatigue cracking.
Third, circular ducts generate less noise. The flat panels of rectangular ducts vibrate like drumheads when air flows through, becoming a “secondary sound source” of noise. Circular ducts have no large flat panels, and their aerodynamic noise is also relatively low. In places where workshop noise needs to be controlled, such as laboratories and precision assembly workshops, this advantage of circular ducts is particularly obvious.
Rectangular ducts are not without merit. When the installation space is extremely limited, such as when they need to be closely attached to the ceiling or embedded in the wall, flat rectangular ducts can fit into the gaps where circular ducts cannot. However, the use of rectangular ducts requires that the wall thickness be increased or stiffeners be added, and the corners must be well sealed.
IV. Components in the Ductwork
A complete ductwork system is not just straight pipes but also includes several key components.
Dampers are adjustable valves installed inside the ducts, used to regulate or balance the air flow in each branch. In a LEV system where multiple hoods share one fan, the branches closer to the fan have a natural higher suction force, while those further away have less. The function of dampers is to slightly close the closer branches to allow more air flow to the distant ones. Without dampers, it is almost impossible to evenly distribute the air flow in a multi-branch system.
Elbows and reducers are major sources of resistance. The resistance of a sharp elbow can be equivalent to that of several meters of straight pipe. During design, elbows with large radius of curvature should be used as much as possible, and gradual expansion or contraction pipes should be used at reducer sections instead of abrupt step changes. Each unnecessary elbow and reducer consumes the power of the fan.
Access doors and test points are often overlooked but are crucial parts. After the ductwork has been in use for some time, dust will accumulate inside. Without access doors, it is impossible to clean the ducts, and long-term accumulation may lead to blockages or even fire risks. Test points are used to measure the air velocity and static pressure inside the ducts regularly, providing key data for determining whether the system is operating normally.
Markings include the location marks of test points on the inner walls of the ducts, hazard warning signs, and flow direction arrows. When inspectors need to find test points, clear markings can save a lot of time.
V. Materials and Sealing
The material of the air ducts depends on the nature of the pollutants being conveyed. Galvanized steel plates can be used for ordinary dust, while stainless steel or plastic pipes are required for corrosive gases. High-temperature flue gas needs heat-resistant materials.
Sealing is the lifeline of the air ducts. All joints, flanges, and access doors must be reliably sealed. Leaks in the negative pressure section will cause untreated workshop air to be drawn into the ducts, diluting the pollution concentration, reducing the collection efficiency, and increasing the burden on the fan. Leaks in the positive pressure section will directly discharge pollutants into the workshop. TrennTech, a high-quality air filter supplier from Frankfurt, has found in many years of project practice that many complaints about “system performance deterioration” are ultimately found to be not due to fan or filter failures, but to loose duct joints or aging sealing strips on access doors – all of which are preventable low-level problems.
To sum up, the basic principles of the LEV duct system can be summarized into four key points:
Pressure zoning: Most sections of the ducts are kept under negative pressure to prevent leakage, and positive pressure sections should be as short as possible and strictly sealed.
Circular priority: Circular ducts are superior to rectangular ones in terms of structural strength, material usage, and noise control. They should be the first choice unless space is limited.
Appropriate flow rate: A flow rate that is too low can cause dust to settle and clog the ducts, while a rate that is too high increases energy consumption and wear. The minimum conveying speed should be determined based on the type of dust during design.
Complete components: Dampers are used for air volume balancing, and elbows and reducers should be as smooth as possible. Inspection ports and test points must not be omitted.
The air duct is the part of the LEV system that is most easily overlooked because it does not directly participate in pollutant capture and does not have an obvious replacement cycle like filters. However, once it malfunctions, the entire system’s transportation efficiency will collapse. We need to remember: the hood is responsible for catching, the fan is responsible for pulling, and the air duct is responsible for safely delivering what has been caught to the destination – all three are indispensable.