A Guide to Scientific Exhaust Duct Design: The Key to Efficient Kitchen Exhaust Purification

In the foodservice industry, it’s not uncommon for restaurants to fail environmental inspections—even when compliant exhaust purifiers are installed. Through extensive on-site experience, AirQuality Technology engineers have found that poor ductwork design is often the hidden culprit. As the vital channel that delivers fumes to the purification system, the performance of an exhaust duct directly impacts whether the purifier can function at full efficiency. This article outlines the critical elements of scientific duct design and the common pitfalls that hinder effective purification.

1. How Poor Duct Design Undermines Exhaust Purification

The primary function of an exhaust duct is to deliver kitchen exhaust smoothly and completely to the purifier. When the design is flawed, three main issues arise:

1.1 Exhaust Leakage

Inadequate sealing or chaotic duct layouts can cause 30%–50% of the exhaust to escape before reaching the purifier—either through gaps or from the edges of the hood. This leads to a dramatic rise in exhaust concentration. For example, one stir-fry restaurant had a leaky duct system, allowing only 60% of exhaust to reach the purifier. Even with a 98% purification rate, the final emissions still exceeded regulatory limits.

1.2 Unstable Airflow

If the duct diameter changes abruptly or includes too many bends, airflow velocity becomes inconsistent. High speeds reduce the time exhaust stays inside the purifier, compromising purification. Low speeds lead to accumulation and “dead zones,” which lower the system’s overall efficiency.

1.3 Excessive Pressure Loss

Long duct runs or excessive elbows create high resistance. If the exhaust fan doesn’t provide sufficient pressure, airflow may reverse or stagnate, rendering even high-end purifiers ineffective.

In short, poor duct design can negate the benefits of even the best exhaust purification equipment.

2. Common Design Mistakes and AQTs Recommended Solutions

2.1 Mismatch Between Duct Size and Air Volume

Common Issue:Improper matching of duct diameter with exhaust volume is a frequent problem.

• Small airflow with large ducts results in airflow speeds under 6 m/s, causing grease to settle.

• Large airflow with small ducts increases speeds to 12–15 m/s, causing grease to stick to duct walls due to centrifugal force, especially at bends. This also increases noise and condensation risk in horizontal runs.

Recommended Solution: According to the Technical Measures for Civil Building HVAC Design (2009), Clause 4.2.10: “Kitchen exhaust systems should meet the following requirements... (2) Duct airflow speed should not be lower than 8 m/s or exceed 10 m/s; hood neck speed should be 4–5 m/s.”

Thus, an optimal airflow speed for kitchen exhaust ducts is 8–10 m/s, and for airflow across the purifier’s cross-section, 2.5 m/s is ideal. In constrained conditions, keep this below 4–6 m/s. Sudden changes in duct diameter should be avoided; if necessary, use tapered transitions to ensure consistent airflow and system stability.

2.2 Inefficient Elbow Design

Common Issue: Right-angle (90°) elbows significantly reduce airflow efficiency, causing up to 30% pressure loss and chaotic turbulence. One barbecue restaurant had four such elbows, reducing the actual exhaust volume to just 65% of the rated capacity. As a result, the purifier could not perform effectively.

Recommended Solution: Use 45° angled elbows or add guide vanes inside 90° elbows to reduce resistance and stabilize flow. Horizontal sections should slope toward the grease collection point. Minimize the number of elbows to maintain airflow efficiency.

2.3 Poor Hood-to-Duct Integration

Common Issue: Improperly sized or installed hoods lead to leakage. For instance, if a hood extends less than 30 cm beyond the cooktop or is poorly sealed at the duct connection, exhaust can escape into the kitchen space. One teppanyaki restaurant experienced around 20% exhaust loss due to these issues.

Recommended Solution: Hoods should cover at least 30 cm beyond the cooking surface on all sides. All connections should be sealed with fire-resistant sealant. For enhanced monitoring, install a pressure sensor to detect air leakage. An alarm should trigger if pressure loss exceeds 10%.

2.4 Lack of Insulation on Exhaust Ducts

Common Issue: In colder climates, especially in winter, uninsulated ducts can lead to condensation. Grease condenses on the inner walls, forming thick deposits that reduce airflow and increase fire risk.

Recommended Solution: In regions with large temperature differences, ducts must be insulated with at least 50 mm thick thermal insulation. This prevents condensation, protects the purifier, and reduces cleaning frequency. For duct lengths over 15 meters, increase fan pressure accordingly—about 100 Pa for every additional 5 meters.

3. Post-Installation Inspection and Optimization

To ensure optimal performance, a thorough system inspection is essential after installation. AQT engineers recommend the following:

3.1 Airflow Speed Testing

• Duct speed should be 8–10 m/s.
• Cross-sectional speed at the purifier should ideally be 2.5 m/s.
• Speed deviation across different points should be within 10%.

3.2 Exhaust Capture Assessment

• Turn off the purifier and introduce visible fume at the cooktop.
• Observe whether all fumes are captured by the hood.
• Turn the purifier back on and verify that the exhaust is visually smoke-free.

3.3 Grease Accumulation Check

• After one month of operation, inspect the duct’s inner walls.
• Ideal performance is defined by an average grease layer thickness of ≤1 mm, indicating effective airflow and low residue accumulation.

Conclusion

Scientific exhaust duct design is essential for maximizing the performance of kitchen exhaust purification systems. A well-designed system reduces maintenance frequency, improves environmental compliance, and lowers long-term operating costs. AQT not only supplies high-efficiency purification equipment but also offers end-to-end support for design, installation, and maintenance—helping clients avoid costly retrofits and penalties.

Interested in a customized kitchen exhaust purification solution? Reach out to an AQT sales engineer today.

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