FANS

FD and ID fans operate continuously for long periods. up to I or I t years. So. these must be well designed, ruggedly constructed, well balanced, and highly efficient over a wide range-of outputs. Typical fans have capacities 0(700 m3/s of volume flow producing 152 mm water static pressures (about OJ 5 bar). 
There are two types of fans. viz., centrifugal and axial. In the centrifugal fan, the gases arc accelerated, radially through curved or flat impeller blades' from rotor to a spiral 'or volute casing. In the axial fan. gases arc accelerated parallel to the rotor axis. This is similar to a table fan, but here the fan is housed in a casing to develop static pressure. Axial fans have higher capital costs . 

Centrifugal fans can have forward-curved. flat or backward curved impeller blades . The velocity triangles at exit from the tip of the blades are shown, where the absolute velocity of gas V is the same in all the three cases. It is seen that for the same V the blade tip velocity Vb is the highest for the backward-curved blades (c) and the lowest for the forward-curved blades (a). Since Vb = (;rDN)J60, for the same tip diameter D, the rpm N is the highest for the backward-curved and the lowest for the forward-curved blades. The FD fans should have high Vb so as to rotate at high speeds and handle large volume flow of air. Therefore, centrifugal fans with backward-curved blading arc normally used for FD fans. The IDfans handle dust-laden flue gases and so the blades are subject to erosion by the fly ash. The erosion rate of blades is lower if the blade tip speed Vb is less and the fan rotates at lower speeds. Therefore, centrifugal fans having forward-curved or flat blading arc used for ID fans. Low-speed fans with flat blades are used for particularly dirty or corrosive gases.