Fan Technology - How Fans Work

Fan, in mechanical engineering, is a device for circulating air or other gases in a large space, such as a room, or in a duct system. Fans are employed not only to move air into a space but also to remove, or exhaust, air from a space. They find extensive application in moving the enormous amounts of air required for the ventilation and air-conditioning systems of large buildings and for the ventilation systems of mines. In most ventilated buildings the air inside is changed by fans from one to three times an hour.

 There are two principal types of fans, axial and centrifugal. In axial fans, the air flow is essentially parallel to the axis of rotation of the blades. The ordinary portable household fan is an axial fan of the propeller design. Although fans of the propeller type are capable of moving large quantities of air in open spaces, they are unable to build up enough pressure on the air to be used in duct systems.

Vane axial and tube axial fans resemble propeller fans, but have blades built in different forms, which permit high-speed rotation and consequently can develop sufficient pressure to move gases through duct systems against the flow resistances that must be overcome. These fans are mounted in cylindrical casings. Vane axial fans differ from tube axial fans mainly in that they are provided with guide vanes on either the inlet or outlet or both. The guide vanes convert the rotational motion of the gases, which serves no useful purpose, into pressure, which helps move the gases through the system.

The other principal type of fan, the centrifugal fan, is used where still higher fan pressures are required. Gases enter on one or both sides of the fan in the direction of the axis of rotation. They then turn through 90° and pass through the impeller, leaving in a direction essentially perpendicular to the axis of rotation. The impeller consists of closely spaced, narrow, essentially radial blades. These blades impart a high velocity to the gases, and because of their rotary motion, centrifugal force also acts to send the gases into the volute or scroll housing. In the housing, the high velocity of the gases is reduced, resulting in a pressure buildup.

Centrifugal fans can deliver gases at pressures up to 0.5 pounds per square inch (0.04 kg/sq cm). The delivery pressures of most fans are lower, being equivalent to the pressure of from 1 to 5 inches (2.5 to 12.5 cm) of water. Fan tip speeds can exceed 10,000 feet (3,000 meters) per minute, but in practice they seldom exceed 5,000 feet (1,500 meters) per minute, because of the noise at higher speeds. Generally, the pressure produced by centrifugal fans with blade tips curved forward in the direction of rotation is greater than that of a similar fan with straight blades or blades curved in the opposite direction.

Fans vary in capacity from less than 100 cubic feet (2.8 cubic meters) of air, or other gases, per minute to capacities in excess of 50,000 cubic feet (1,400 cubic meters) per minute. For very high capacities it is advisable to use two or more fans side by side. The capacity of a fan varies directly as its rotational speed. The pressure produced is proportional to the square of the fan speed, and the horsepower required is proportional to the cube of the fan speed.

In choosing a fan for a particular application, it is necessary to consider pressure and capacity requirements, noise, and available space as well as initial and operating costs.