A cross-flow fan produces a wide, flat, remarkably even sheet of air across its full length. That single property is why it dominates air curtains and fan coil units — and its pressure ceiling is why putting one in a duct is a mistake that cannot be corrected by choosing a bigger model.
How it works
The impeller is a long drum with forward-curved blades around its circumference and no inlet at the ends. Air enters through one side of the drum, passes through the blade row, crosses the interior, and exits through the blades on the opposite side. The flow is transverse to the shaft rather than axial or radial, which is where "cross-flow" and the alternative name "tangential" come from.
Because air passes through the blade row twice, the velocity profile along the drum length is unusually uniform. A 1.2 m drum delivers essentially the same velocity at both ends and in the middle, which is exactly what an air curtain over a doorway needs and what no axial or centrifugal fan naturally provides.
What it is good at
- Air curtains. A continuous flat jet across a door opening, from a fan that fits inside a shallow housing above the door.
- Fan coil units and split indoor units. The drum sits behind the heat exchanger, drawing air evenly across the whole coil face rather than concentrating flow at the centre.
- Convector heaters and drying units. Wide, low-velocity flow over a heated surface.
- Equipment cooling where the outlet must be a slot. Display cabinets, laminating machines, some office equipment.
In all of these the fan discharges directly into open air or across a short heat exchanger. That is the pattern.
The pressure ceiling
Cross-flow fans develop low static pressure — a small fraction of what a comparable centrifugal fan produces. The reason is structural: the flow path is short, the blades are shallow, and much of the energy goes into the wide flow pattern rather than into pressure rise.
The consequence is absolute. Connect a cross-flow fan to even a modest length of duct and airflow collapses. It does not reduce proportionally; the operating point runs off the end of a very steep curve. There is no model selection that fixes this, because the limitation is inherent to the geometry rather than to sizing.
If a design needs the wide flat discharge and any meaningful ducted resistance, the correct answer is a centrifugal fan with a plenum and a slot diffuser, not a larger cross-flow fan.
Noise
Cross-flow fans are relatively quiet at their design condition because tip speed is low and the flow is steady. Two things spoil that:
Obstruction near the outlet. Anything within a short distance of the discharge slot disturbs the flow and produces broadband noise disproportionate to the obstruction's size. Decorative grilles are a frequent culprit.
Drum imbalance. The impeller is long and light, so it is sensitive to imbalance from dust build-up or from a manufacturing defect. A cross-flow fan that develops a rumble in service is usually telling you the drum needs cleaning, not that the bearings are failing.
Practical points at specification
- Drum length is the primary dimension. Airflow scales roughly with length, so a doorway width dictates the fan, not the other way around.
- Both ends need proper support. Long drums running on a bearing at one end only will deflect and rub.
- Cleaning access matters. Blades are small and closely spaced, so they load with dust and lint quickly in retail and kitchen environments. A unit that cannot be opened for cleaning will lose airflow within a year.
- Direction of rotation is not reversible. The blade curvature and the housing scroll are matched; running one backwards produces a fraction of the airflow with considerably more noise.
When to choose something else
Any ducted application. Anything needing static pressure beyond a coil face. Any airstream carrying significant particulate, because the closely-spaced shallow blades block quickly. And any high-temperature airstream, since the long plastic drums common in this product family have modest temperature limits — metal drums exist but the selection is narrower.