5 Fans and Blowers 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 fan system 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 Table 5 1 Differences Between Fans Blower And Compressor are differentiated by the method used to move the air and by the Equipment Specific Ratio Pressure rise mmWg system pressure they must operate against As per American Society Fans Up to 1 11 1136 of Mechanical Engineers ASME the specific ratio the ratio of the 1 11 to 1 20 1136 2066 discharge pressure over the suction Blowers pressure is used for defining the fans blowers and compressors see Compressors more than 1 20 Table 5 1 5 2 Fan Types Fan and blower selection depends on the Table 5 2 Fan Efficiencies volume flow rate pressure type of Type of fan material handled space limitations and efficiency Fan efficiencies differ from Centrifugal Fan design to design and also by types Airfoil backward Typical ranges of fan efficiencies are curved inclined given in Table 5 2 Modified radial Radial Fans fall into two general categories Pressure blower centrifugal flow and axial flow Forward curved In centrifugal flow airflow changes Axial fan direction twice once when entering and Vanaxial second when leaving forward curved Tubeaxial backward curved or inclined radial Propeller see Figure 5 1 Peak Efficiency Range 79 83 72 79 69 75 58 68 60 65 78 85 67 72 45 50 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 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 Backwardinclined fans reach their peak power consumption and then power demand drops off well within their useable airflow range Backward inclined fans are known as nonoverloading 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 fans are 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 Vaneaxial fans are similar 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 Bureau of Energy Efficiency 94 5 Fans and Blowers pressures upto 500 mmWC Vaneaxials are typically the most energy efficient fans available and should be used whenever possible Propeller fans usually run at low speeds and moderate temperatures They experience a large change in airflow with small changes in static pressure They handle large volumes of air at low pressure or free delivery Propeller fans are often used indoors as exhaust fans Outdoor applications include air cooled condensers and cooling towers Efficiency is low approximately 50 or less Tube Axial Vane Axial Propeller Figure 5 4 Types of Radial Fans The different types of fans their characteristics and typical applications are given in Table 5 3 Common Blower Types Blowers can achieve much higher pressures than fans as high as 1 20 kg cm2 They are also used to produce negative pressures for industrial vacuum systems Major types are centrifugal blower and positive displacement blower Centrifugal blowers look more like centrifugal pumps than fans The impeller is typically gear driven and rotates as fast as 15 000 rpm In multi stage blowers air is accelerated as it passes through each impeller In single stage blower air does not take many turns and hence it is more efficient Centrifugal blowers typically operate against pressures of 2 0 35 to 0 70 kg cm but can achieve higher pressures One characteristic is that airflow tends to drop drastically as system pressure Bureau of Energy Efficiency 95 5 Fans and Blowers Table 5 3 Types of Fans Characteristics and Typical Applications Centrifugal Fans Type Characteristics Axial flow Fans Typical Applications Type Characteristics Typical Applications Radial High pressure medium flow efficiency close to tube axial fans power increases continuously Various industrial applications suitable for dust laden moist air gases Propeller Low pressure high flow low efficiency peak efficiency close to point of free air delivery zero static pressure Air circulation ventilation exhaust Forwardcurved blades Medium pressure high flow dip in pressure curve efficiency higher than radial fans power rises continuously Low pressure HVAC packaged units suitable for clean and dust laden air gases Tube axial Medium pressure high flow higher efficiency than propeller type dip in pressure flow curve before peak pressure point HVAC drying ovens exhaust systems Backward curved blades High pressure high flow high efficiency power reduces as flow increases beyond point of highest efficiency HVAC various industrial applications forced draft fans etc Vane axial High pressure medium flow dip in pressure flow curve use of guide vanes improves efficiency High pressure applications including HVAC systems exhausts Airfoil type Same as backward curved type highest efficiency Same as backward curved but for clean air
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