Ventiladores Y Exti

5. FANS AND BLOWERS
Syllabus Fans and blowers: Types, Performance evaluation, Efficient system operation, Flow control strategies and energy conservation opportunities

5.1 Introduction
Fans and blowers provide air for ventilation and industrial process requirements. Fans generate a pressure to move air (or gases) against a resistance caused by ducts, dampers, or other components in a fansystem. The fan rotor receives energy from a rotating shaft and transmits it to the air. Difference between Fans, Blowers and Compressors Fans, blowers and compressors are differentiated by the method TABLE 5.1 DIFFERENCES BETWEEN FANS, BLOWER used to move the air, and by the AND COMPRESSOR system pressure they must operate against. As per American Equipment Specific Ratio Pressure rise (mmWg)Society of Mechanical Engineers Fans Up to 1.11 1136 (ASME) the specific ratio - the Blowers 1.11 to 1.20 1136 – 2066 ratio of the discharge pressure more than 1.20 – over the suction pressure – is Compressors used for defining the fans, blowers and compressors (see Table 5.1).
TABLE 5.2 FAN EFFICIENCIES
Type of fan

5.2 Fan Types
Fan and blower selection depends on the volume flow rate, pressure,type of material handled, space limitations, and efficiency. Fan efficiencies differ from design to design and also by types. Typical ranges of fan efficiencies are given in Table 5.2. Fans fall into two general categories: centrifugal flow and axial flow. In centrifugal flow, airflow changes direction twice - once when entering and second when leaving (forward curved, backward curved or inclined,radial) (see Figure 5.1). In axial flow, air enters and leaves the fan with no change in direction (propeller, tubeaxial, vaneaxial) (see Figure 5.2).
Bureau of Energy Efficiency 93 Centrifugal Fans Airfoil, backward curved/inclined Modified radial Radial Pressure blower Forward curved Axial fan Vane axial Tube axial Propeller

Peak Efficiency Range

79–83 72–79 69–75 58–68 60–65

78–8567–72 45–50

5. Fans and Blowers

Figure 5.1 Centrifugal Fan

Figure 5.2 Axial Fan

Centrifugal Fan: Types The major types of centrifugal fan are: radial, forward curved and backward curved (see Figure 5.3). Radial fans are industrial workhorses because of their high static pressures (upto 1400 mm WC) and ability to handle heavily contaminated airstreams. Because of their simple design,radial fans are well suited for high temperatures and medium blade tip speeds. Forward-curved fans are used in clean environments and operate at lower temperatures. They are well suited for low tip speed and high-airflow work - they are best suited for moving large volumes of air against relatively low pressures. Backward-inclined fans are more efficient than forward-curved fans. Backward-inclined fansreach their peak power consumption and then power demand drops off well within their useable airflow range. Backward-inclined fans are known as "non-overloading" because changes in static pressure do not overload the motor.
Paddle Blade (Radial blade) Forward Curved (Multi-Vane) Backward Curved

Figure 5.3 Types of Centrifugal Fans

Axial Flow Fan: Types The major types of axial flow fansare: tube axial, vane axial and propeller (see Figure 5.4.) Tubeaxial fans have a wheel inside a cylindrical housing, with close clearance between blade and housing to improve airflow efficiency. The wheel turn faster than propeller fans, enabling operation under high-pressures 250 – 400 mm WC. The efficiency is up to 65%.
Bureau of Energy Efficiency 94

5. Fans and Blowers

Vaneaxial fans aresimilar to tubeaxials, but with addition of guide vanes that improve efficiency by directing and straightening the flow. As a result, they have a higher static pressure with less dependence on the duct static pressure. Such fans are used generally for pressures upto 500 mmWC. Vaneaxials are typically the most energy-efficient fans available and should be used whenever possible. Propeller fans...