A centrifugal fan delivers 2.0 m3 s−1 when running at 960 rev min−1. The impeller diameter is 70cm and the diameter at blade inlet is 48 cm. The air enters the impeller with a small whirl component in...

A centrifugal fan delivers 2.0 m3 s−1 when running at 960 rev min−1. The impeller diameter is 70cm and the diameter at blade inlet is 48 cm. The air enters the impeller with a small whirl component in the direction of impeller rotation, but the relative velocity meets the blade tangentially. The impeller width at inlet is 16cm and at outlet, 11.5 cm. The blades are backward inclined making angles of 22.5° and 50° with the tangents at inlet and outlet, respectively. Draw to scale the inlet and outlet velocity triangles and from them determine the theoretical total head produced by the impeller. Assuming that the losses at inlet in the impeller and in the casing amount to 70 per cent of the velocity head at impeller outlet and that the velocity head at fan discharge is 0.1 of the velocity head at impeller outlet, calculate the fan static pressure in millimetres of water column. Assume the air density to be 1.2 kg m−3 and neglect the effect of blade thickness and inter blade circulation.