Fan Types

This is a subsidiary post to the Fan Selection & Application Guide, elaborating on the different values for the Fan Type attribute used to characterize AC and DC fans at Digi-Key.

Axial/Tubeaxial fans

The mental image summoned by the word “fan” for most folks is likely that of an axial fan; a device which uses blades that are radially attached to a rotating axle to effect air movement along the axis of said axle through a screw-like action. The typical ceiling fan is an axial fan.

Tubeaxial fans are essentially just axial fans. In a tube… By providing a shroud that fits closely to the blade tips of an axial fan, the resulting airflow is focused and efficiencies are improved for applications requiring a more controlled, localized airflow rather than a simple stirring of the air mass in loosely-confined quarters such as one’s living room.

Tubeaxial fans are well-suited to applications calling for high volumes of airflow delivered into relatively low static pressures; they’re the air-handling equivalent of a low-voltage, high-current power supply.

Tubeaxial (Dual)

Qualification of a tubeaxial fan as a ‘dual’ type implies a fan of tubeaxial style incorporating hub & blade assemblies, usually in a series configuration. Such an arrangement can be used to achieve relatively high maximum static pressures more commonly found among centrifugal type fans, while retaining the straight-through flow characteristic of a tubeaxial fan. A common characteristic of such devices is a relatively high noise level.

Tubeaxial, Case Fan

The “case fan” qualification implies a fan equipped with a connector of a style commonly found in desktop computers.

Centrifugal fans/blowers

Centrifugal fans, also referred to as blowers or squirrel-cage fans in reference to the shape of their moving parts, move air in a radial direction relative to the axis of rotation, with the air intake to the device occurring axially. By enclosing the moving element in a close-fitting, scroll-shaped shroud, the radial motion of the airflow is converted into linear air movement, and directed as needed. Depending on the shape and orientation of the blades, different characteristic curves can be realized, allowing for a high degree of adaptability to different applications. Generally speaking, centrifugal fans/blowers are more effective than axial fan types at delivering airflow into relatively high static pressures, and are commonly used in building air handling systems for this reason.

Cross Flow

Cross flow blowers, also called tangential blowers, are a centrifugal blower variant characterized by intake and exhaust flows that are both approximately tangential to the direction of the rotating element. This axial dimension of this element is commonly much larger compared to its diameter than is common for other centrifugal blower styles, leading to devices of this type being useful for creating thin regions of airflow along broad linear dimensions, often at lower static pressures than is common for blowers of other types.

Motorized Impellers

Motorized impellers are essentially a form of centrifugal fan which leaves the matter of shroud construction up to the end user. Consisting of a motor and impeller (no surprise, this…) such devices permit a greater degree of flexibility with regard to direction of exhaust airflow relative to other types. While the intake remains axial, shrouds can be designed to permit radial exhaust flow along the device’s full circumference or a portion thereof, axially to the intake but in the opposite direction, or other configurations as needed. Devices of this type tend to be very space-efficient, and are often used in space-constrained applications for this reason.

Synthetic jets

Synthetic jets are devices which create jet flows (volumes of fluid traveling through a more stationary body of fluid) by alternating intake and expulsion of ambient air through an orifice. Conceptually, it’s not far from the mark to regard synthetic jets as high-frequency smoke ring generators, absent the smoke part. Unless of course, things go very, very wrong…

The functional mechanism of such devices is in fact quite similar to the process by which smoke rings are blown; an electromagnetically-driven diaphragm forming one wall of a small chamber is used to draw air in and out of the chamber through a small opening. Ambient air is drawn into the chamber during the intake portion of the operating cycle, and expelled at high speed during the exhaust portion. The expelled volume of fluid is relatively small, but travels through the ambient fluid as a fast-moving, turbulent mass.

The turbulent character of that motion makes the resulting flow very effective at aiding transfer of heat from a solid surface to the surrounding air, without the necessity of the rotating bearing system that typically limits the service life of other air movers. Because they do not create a continuous flow however, synthetic jets cannot effectively develop a static pressure in order to drive airflow over long distances through an enclosure. Put differently, synthetic jets are very effective at scrubbing heat off a surface, but not so much at rinsing it away.

A line of synthetic jet devices marketed for LED lighting applications was developed by Nuventix in years past. That entity was subsequently acquired by Aavid Thermalloy, which was in turn acquired by Boyd Corp, and at the time of writing the Nuventix-designed products have been discontinued.