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Air moving fans and blowers utilize power from a motor to generate a volumetric flow of air at a given pressure, and are considered low-pressure air pumps. A propeller converts torque (turning force) from the motor (typically permanent split capacitor AC induction motors or brushless DC) to increase static pressure across the fan rotor and to increase the kinetic energy of the air particles. Fans are classified into propeller, tube axial and vane axial styles, and each type has different characteristics and different areas of application. Propeller fans ususally consist of only a motor and propeller and therefore are the simplest among all. Tube axial and vane axial are similar to propellers but has a venturi around the propeller to reduce the vortices. Vane axial fans are equipped with vanes that trail behind the propeller in the airflow to straighten the swirling flow. The flow and pressure properties are the main differences between a fan and a blower. While fans deliver air in an overall direction that is parallel to the fan blade axis, blowers deliver air in a direction that is perpendicular to the blower axis. Hence, fans can be configured to deliver a high flow rate, working against low pressure while blowers to deliver a relatively low flow rate against high pressure. Centrifugal blowers can be squirrel cage type, have a forward curved wheel or a backward curved wheel. Another major difference between the fans types is in the mechanism regarding restriction to the air-flow. In a blower, it has an opposite effect on the same motor driving a fan blade. For instance, the load on the motor decreases when the motor is driving a squirrel cage blower. As the system becomes clogged, the blower speeds up. On the other hand, the load on the motor increases and the fan slows down as the airflow system becomes clogged with fan blades. Commonly available practical methods of plants ventilation are: * Exhaust fans or Power Roof Ventilators (PRVs). To do a satisfactory job of eliminating excessively hot air, it is usually essential to have powered wall and/or roof exhausters. These fans should also help to control air pressure within the building whether it be negative or positive pressure. * Supply Fans or PRVs. A large number of buildings use exhaust PRVs to exhaust fumes, smoke, dust or other contaminants unavoidable in the operation of the business. As a result, these buildings are frequently under a severe negative pressure. The solution to the problem of this kind is usually found in the use of supply fans or "make-up" air ventilators. * Air Circulators. If the exhaust and supply air requirements of a building have been carefully engineered and installed, and there continues to be a high instance of worker discomfort, the problem usually relates to the matter of air circulation. In this way, in addition to amximum benefit from the fresh, cooler air, occupants receive the added comfort of air circulation over their bodies and they are not adversely affected by the superheated air being exhausted from the building. Additional information can be found at the Fans and Blowers Company web site http://www.fansandblowers.net/biblower.html Susana Terlitsky Industrial Air Handling Units Division fan@cogeco.ca Fans and Blowers Company http://www.fansandblowers.net/ForwardCurvedFans.html http://www.fansandblowers.net/backwardcurvedblower.html
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