Axial flow fans move air parallel with the fans shaft, the air moves with a swirling motion which is created by the rotating blades. The velocity of the air is increased as the fan blades turn, these types of fans are ideal in applications where the objective is to move large volumes of air with very low static pressure demands. Typical examples are condensers, spot cooling and blowing large volumes of air through long ducting.
A centrifugal fan moves air in a radial direction at right angle to the fan shaft. As the air leaves the tips of the fan blade velocity pressure is produced. Centrifugal fans are well suited for moving air which needs static pressure capabilities and low noise; examples are supply and return fans and are common in the HVAC industry. The common impeller designs are forward curved fans and backward curved fans. Backward curved fans, backward inclined fans have similar performance characteristics.
One attractive feature of backward curved fans is the fact that it is non-overloading on the horsepower curve. The motor horsepower increases to the maximum as the airflow increases, the power then drops off. Under these operating circumstances, the motor will never overload, overheat or burn out despite variations in resistance as long as the speed remains constant.
Backward curved fans rotate at higher speeds than forward curved fans for a given pressure. Because of this fact, the construction of these configurations must be more robust. A combination of high rotational speeds and sturdy construction makes backward curved fans ideal for applications which demand high static pressure. Although the heavy construction makes this style fan somewhat more expensive to buy when compared to a forward curved fan, the higher initial cost is easily offset by the higher operating efficiency.
When one considers identical operating demands, backward curved fans require about 15 percent less power to run than forward curved fans. The operating efficiency of a forward curved fan is about 65 percent; a backward curved fan offers operating efficiencies as high as 85 percent. With differences in efficiency to this degree, fans which must run for many hours will save the operator considerable amounts of money in electricity despite the higher initial cost of the fan.
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