A duct axial fan is an inline ventilation fan that moves air axially through a cylindrical duct at low to medium static pressure, typically 50 to 500 Pa, while delivering high volume from 1,000 to 60,000 m³/h. It is the right choice when you need to push or extract large amounts of air through relatively short, low-resistance duct runs in commercial buildings, warehouses, parking garages, and industrial halls. Unlike centrifugal fans that trade volume for pressure, the duct axial design keeps the airflow parallel to the shaft, making it compact, lightweight, and energy-efficient for straight-through duty.
What Is a Duct Axial Fan
A duct axial fan, sometimes called an inline axial fan or tube axial fan, places the impeller inside a cylindrical casing that bolts directly between two duct flanges. Air enters axially, passes over the blades, and leaves axially, so the unit fits in line with the ductwork without changing flow direction. The most common configuration uses an external-rotor motor directly coupled to the impeller, which removes belt losses and simplifies maintenance. Blade designs range from simple flat plates for low-cost extract to contoured aerofoil blades for quieter, more efficient operation. Typical impeller diameters run from 200 mm to 1,250 mm, and the fan can be mounted horizontally or vertically depending on the duct layout.
How a Duct Axial Fan Works
The impeller accelerates air along the axis of rotation, converting motor power into kinetic energy and a modest pressure rise. Because the flow path is straight, the fan achieves high volumetric flow with relatively low energy input. The operating point is set by the intersection of the fan pressure curve and the system resistance curve: as duct length, elbows, and filters add static loss, the delivered volume drops. In practice, duct axial fans perform best when system resistance stays under 500 Pa; beyond that, a centrifugal or mixed-flow fan becomes more efficient. For clean air at up to 60°C, standard units suffice, while high-temperature variants extend the limit to 280°C or 300°C for fire duty.
Duct Axial vs Centrifugal vs Mixed-Flow
| Fan type | Typical static pressure | Typical volume range | Best application |
|---|---|---|---|
| Duct axial (inline) | 50 to 500 Pa | 1,000 to 60,000 m³/h | Short duct runs, high-volume extract or supply |
| Mixed-flow (inline) | 200 to 1,500 Pa | 1,000 to 40,000 m³/h | Medium-pressure ducted systems, booster fans |
| Centrifugal (forward/curved) | 250 to 3,000 Pa | 500 to 50,000 m³/h | Long ducts, filtration, cleanrooms |
| Centrifugal (high-pressure 9-19) | up to 15,000 Pa | 300 to 20,000 m³/h | Pneumatic conveyance, kilns, combustion air |
The table shows why duct axial fans dominate straightforward ventilation: they move the most air per watt at low pressure. When your ductrun exceeds about 30 m or includes several filters, move up to mixed-flow or centrifugal.
Key Performance Numbers
Three figures matter most when specifying a duct axial fan. First, the design volume flow, commonly 3,000 to 30,000 m³/h for a single commercial unit. Second, the system static pressure, usually 50 to 500 Pa for inline duty. Third, the sound power, which typically lands between 45 and 65 dB(A) at 1.5 m from the casing. A well-matched unit reaches total efficiency of 55 to 75 percent depending on blade profile, and motor power ranges from 0.18 kW on a small 200 mm fan to 30 kW on a 1,250 mm high-flow model. Impeller tip speed should stay below about 60 m/s to control noise and erosion.
When to Choose a Duct Axial Fan
- Choose a duct axial fan when duct resistance is low (under 500 Pa) and you need high volume in a straight run.
- Choose a mixed-flow or centrifugal fan when the duct exceeds 30 m, includes multiple filters, or needs pressure above 500 Pa.
- Choose a high-temperature axial (280°C or 300°C rated) only for certified smoke extract in fire scenarios.
- Choose a centrifugal fan with belt drive when you need flexible speed control and easy motor service.
- Choose an aerofoil-blade inline fan when the space demands noise below 55 dB(A).
Installation and Sizing Guidance
Size the fan from the required air changes per hour. For a warehouse needing 6 ACH and a volume of 10,000 m³, you need 60,000 m³/h. Add the estimated duct, elbow, and filter losses to get total static pressure, then pick the model whose curve passes through that point near its best efficiency. Allow at least 1.5 diameters of straight duct upstream and downstream of the impeller to avoid swirl and uneven velocity, which can drop efficiency by 10 to 15 percent. Use flexible anti-vibration connectors at both flanges and a drain point if condensation is expected. For speed control, fit a variable frequency drive so the fan tracks the actual load instead of running at full speed continuously.
Keruite manufactures this 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.
Blade Geometry and Efficiency
Blade shape strongly affects both pressure and noise. Flat-plate blades are the cheapest and suit clean, low-duty extract where an efficiency of 45 to 55 percent is acceptable. Curved or aerofoil blades reach 65 to 75 percent efficiency and cut broadband noise by 5 to 10 dB(A) at the same flow, which matters in offices and retail where the fan runs near occupants. Blade pitch is often fixed at the factory, but adjustable-pitch hubs let one frame cover 2,000 to 8,000 m³/h by changing the angle from 15 to 35 degrees. More blades raise pressure but also solidity and tonal noise, so 4 to 8 blades is the usual compromise for 200 to 800 mm fans.
Control, Energy, and Lifecycle
Pairing the fan with a variable frequency drive scales speed from 20 to 100 percent, and because fan power varies roughly with the cube of speed, a 30 percent speed reduction saves about 65 percent of fan energy. For a 5.5 kW inline fan running 4,000 hours per year, that is 8,000 to 12,000 kWh saved annually. Soft starting also reduces inrush current from 5 to 7 times rated down to 1.2 times, protecting contactors and generators. In buildings with CO or occupancy sensors, staged control keeps ventilation matched to need instead of running flat-out. The most frequent faults are imbalance from dust, worn bearings after 20,000 to 40,000 hours, and motor overload when system resistance drifts. A fan balanced to G2.5 with sealed bearings typically runs 8 to 15 years; vibration above 4.5 mm/s RMS signals impending failure, and washing blades annually can recover 3 to 8 percent of lost efficiency in dusty air.
FAQ
What is the difference between a duct axial fan and a centrifugal fan?
A duct axial fan moves air parallel to the shaft and is best for high-volume, low-pressure duty up to about 500 Pa, while a centrifugal fan generates higher static pressure up to 15,000 Pa by redirecting flow 90 degrees. Choose axial when duct runs are short and pressure loss is low, and centrifugal when long ducts or filters add resistance.
How do I size a duct axial fan for a room?
First calculate the required air changes per hour, then multiply by room volume to get m³/h. Select a model whose operating point sits near the peak of its pressure curve at your duct static loss, typically 50 to 500 Pa. Oversizing wastes energy and raises noise above 60 dB, while undersizing leaves the space under-ventilated.
Can a duct axial fan be used for smoke extraction?
Only if it is a dedicated fire-rated axial fan certified for high temperature, usually 280°C or 300°C for 60 to 120 minutes. Standard inline ventilation axial fans use motors and bearings that fail well below those temperatures, so they must never be substituted for certified smoke exhaust units in escape routes.
What noise level should I expect from an inline axial fan?
A well-selected duct axial fan typically produces 45 to 65 dB(A) at 1.5 m, depending on tip speed and blade count. Mounting flexible connectors, using lower RPM, and specifying aerofoil blades can keep sound within 55 dB(A) for offices and retail spaces where acoustic comfort matters.