A fan is a device for moving air in a given direction. Hand fans usually consist of a single vane or blade, which is moved back and forth by hand to create an air current. Mechanical fans consist of several blades, usually rotated by an electric motor. The direction of the airstream is determined by the shape of the blades or the fan housing. The two general types of mechanical fans are the centrifugal fan and the propeller fan.
A centrifugal fan consists of many thin blades mounted in such a way as to form a hollow cylinder. The blades are usually curved, although on some fans they are flat. When the cylinder rotates, air is drawn into it through the open ends and then driven outward, through the blades. The moving air is directed into a single current by the scroll-shaped casing of the centrifugal fan.
A propeller fan consists of several blades radiating from an axis. When the fan rotates, air is driven in the direction of the axis. The blades of the fan are shaped so as to draw air from one end of the fan and propel it in the other direction. The blades of a propeller fan may be simply twisted, or shaped like an airfoil, like the blades of an airplane propeller.
Fans are used wherever a movement of air is desirable, as in heating, cooling, or ventilating systems. A common use for portable fans of either the centrifugal or propeller type is for room ventilation in warm weather.