Industrial ductwork is sized by matching the design airflow (measured in m³/h) to an allowable friction rate so that total pressure drop stays within the fan curve; most plants target 0.8–1.5 Pa/m of duct and keep transport velocity between 8 and 12 m/s to control both noise and particle fallout. A correct layout also selects sheet gauge and joint type from the duty’s temperature, moisture, and corrosive load rather than from habit, because those three variables decide whether the system lasts 5 years or 25.
Why Duct Sizing Controls the Whole System
Duct is the highway of an HVAC or process system. If it is too small, velocity rises, friction loss climbs with the square of velocity, and the fan must develop higher static pressure—often pushing the motor past its 15 kW nameplate or forcing a costly upsize. If it is too large, first cost and space grow while low velocity lets dust settle on the floor of the run. The practical compromise is a friction rate of about 1.0 Pa/m for long industrial runs and 0.8 Pa/m for clean-process ducts where low noise matters more than first cost.
Velocity also sets the acoustic signature. At 6 m/s a main is nearly silent; at 15 m/s the same air generates 10–15 dB more regenerated noise that no silencer can fully remove downstream. For this reason comfort supply air is kept at 6–10 m/s while dust extraction runs 12–18 m/s, accepting the noise because suspension of material is the priority over quiet operation in a plant room.
Step-by-Step Sizing Method
Begin with the room-by-room airflow demand, usually expressed as 1,000–50,000 m³/h per zone. Convert each branch to a velocity using the continuity equation V = Q / A. For general dust extraction keep 12–18 m/s so material stays in suspension; for comfort supply air use 6–10 m/s. Then compute friction loss with the Colebrook or equivalent roughness method and sum fittings: each 90° elbow typically adds 10–30 Pa, a branch take-off 5–15 Pa, and an entry louvre 20–50 Pa. A well-run calculation totals branch and main losses and adds them to filter and coil resistance.
Always run the numbers twice—once for summer (warm, low-density air) and once for winter (cold, dense air)—because density shifts the fan’s delivered volume by 5–10%. Ignoring this is a common reason distant rooms are starved in January but fine in July. Modern duct-design software does this automatically, but the underlying physics is the equal-friction or static-regain method applied zone by zone.
Material Selection Compared
| Material | Typical velocity (m/s) | Max service temp (°C) | Corrosion class | Best application |
|---|---|---|---|---|
| Galvanized steel (0.5–1.2 mm) | 6–15 | 120 | Moderate | Comfort air, light dust |
| Stainless 304 / 316 | 6–20 | 400 | High | Food, pharma, coastal |
| FRP (fibre-reinforced plastic) | 6–14 | 120 | Very high | Acid, scrubber exhaust |
| Aluminium | 6–12 | 200 | Low–moderate | Lightweight, non-spark |
Pressure Drop You Can Expect
A 300 mm diameter galvanized branch at 10 m/s carries about 850 m³/h and loses roughly 1.2 Pa/m. A 1,000 mm main at 9 m/s moves near 28,000 m³/h with 0.6 Pa/m friction. Over a 60 m run with 6 elbows the branch totals around 120 Pa and the main around 80 Pa, leaving margin under a typical 4-72 centrifugal fan rated to 2,500 Pa. Always reserve 15–20% of fan pressure for filter loading and summer/winter density shifts, or the system will drift out of specification within a year of operation.
Worked Example: A Paint-Booth Exhaust
Consider a booth needing 12,000 m³/h of solvent-laden air at 35°C. We select 14 m/s transport velocity, giving a 550 mm duct (area 0.237 m²). Friction is about 2.0 Pa/m; a 40 m run plus 4 elbows (30 Pa each) and a 40 Pa entry louvre totals roughly 180 Pa. A 9-19 high-pressure fan rated to 3,000 Pa easily covers it with margin, and stainless 304 resists the solvent. Without the 14 m/s rule the solvent would drop out and accumulate as a fire hazard—a real, code-relevant failure mode.
Decision Checklist: Which Duct Approach
- Choose spiral round duct when space allows and you want the lowest friction per metre—round sections need less metal for equal area and leak less at seams.
- Choose rectangular duct when ceiling height is tight or you must route around structure; accept 10–25% more pressure loss and stiffeners at large sizes.
- Choose stainless or FRP when the airstream exceeds 80°C, carries solvent, or sits in a coastal EAC-certified site exposed to salt spray.
- Choose a heavier 1.2 mm gauge when negative pressure exceeds 1,500 Pa or the duct hangs longer than 6 m between supports without bracing.
Common Installation Mistakes
The most frequent errors are undersized take-offs that starve distant rooms, missing flexible connections at fans that transmit vibration into the structure, and unsealed seams that leak 5–15% of total volume. Seal with mastic or tape rated to the duty, support round duct every 3 m and rectangular every 2.4 m, and keep a straight 3–5 duct diameters of inlet run before any fan to avoid turbulent entry that can cut efficiency by 3–8%. SMACNA construction standards give the gauge-to-pressure tables most engineers follow.
Standards and Commissioning
Design to SMACNA HVAC Duct Construction for gauge and joint rules, and to ASHRAE Fundamentals for friction data. At commissioning, measure actual velocity with a pitot traverse at 2–3 stations per main; balances within ±10% are acceptable. Log the as-built pressure drop and compare to the calculation—differences over 15% usually point to a missed fitting or a leaking damper left open.
Working With a Manufacturer
Keruite manufactures HVAC fans and air-handling equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment. Sharing your duct layout and pressure budget early lets the fan and duct be matched as one system instead of two separate purchases, avoiding the classic mismatch where a fine duct is throttled by an undersized fan or vice versa.
FAQ
What friction rate should I design to?
Most industrial supply systems use 0.8–1.5 Pa/m of equivalent length. Start at 1.0 Pa/m for general plants and drop to 0.8 Pa/m for clean rooms or offices where low noise is required. Reserve 15–20% fan margin for filter dust loading and seasonal air-density changes, or distant branches will starve as temperatures swing through the year.
How fast should dust-extraction duct move air?
Keep transport velocity between 12 and 18 m/s so chips, flour, or dust stay suspended and do not drop out at elbows. Lighter nuisance dust sits near 12 m/s while heavy wood or metal chips need 16–18 m/s. Below 10 m/s settlement and clogging become likely within months, creating both airflow loss and a fire or explosion hazard in combustible streams.
Which gauge steel is safe for negative pressure?
Use 0.7–0.9 mm galvanized for pressures under 750 Pa, 1.0–1.2 mm up to 1,500 Pa, and stiffened or stainless construction above that. Long spans between supports also demand thicker sheet. Always check the fan curve’s static-pressure rating before finalizing gauge, because an undersized wall will buckle or hum under operation.
Why does my system lose airflow at far rooms?
Undersized branch take-offs and leaky seams are the usual causes; leaks alone can waste 5–15% of volume. Verify each branch velocity, seal joints with duty-rated mastic, and confirm a straight 3–5 diameter inlet run before the fan to protect delivered airflow. A pitot traverse at commissioning usually reveals the worst offenders within an hour.