An axial flow fan comprising: a generally hollow casing defining an inlet end and an outlet end of the axial flow fan, wherein a diameter of the casing increases from the inlet end to the outlet end; an impeller rotatably mounted within the casing, the impeller including: a hub , wherein an outer diameter of the hub increases from the inlet end to the outlet end , and wherein the increase in the diameter of the hub from the inlet end to the outlet end is generally equal to the increase in the diameter of the casing from the inlet end to the outlet end, and wherein the hub has a radially stepped configuration extending in the circumferential direction; a plurality of fan blades extending from a root at the hub to a blade tip , wherein a diameter of the blade tip of each of the plurality of fan blades increases from the inlet end to the outlet end; and a tip fence located at the blade tip of at least one of the plurality of fan blades, wherein an axial length of the tip fence measured perpendicular to the fan blades is greater than a thickness of the at least one of the plurality of fan blades; wherein an axial clearance , measured parallel to the diameter of the casing , is defined between the tip fence of each of the plurality of fan blades and an adjacent surface of the casing and the axial clearance remains constant between the inlet end and the outlet end of the axial flow fan, with a tolerance of +/- 1mm. a generally hollow casing defining an inlet end and an outlet end of the axial flow fan, wherein a diameter of the casing increases from the inlet end to the outlet end; an impeller rotatably mounted within the casing, the impeller including: a hub , wherein an outer diameter of the hub increases from the inlet end to the outlet end , and wherein the increase in the diameter of the hub from the inlet end to the outlet end is generally equal to the increase in the diameter of the casing from the inlet end to the outlet end, and wherein the hub has a radially stepped configuration extending in the circumferential direction; a plurality of fan blades extending from a root at the hub to a blade tip , wherein a diameter of the blade tip of each of the plurality of fan blades increases from the inlet end to the outlet end; and a tip fence located at the blade tip of at least one of the plurality of fan blades, wherein an axial length of the tip fence measured perpendicular to the fan blades is greater than a thickness of the at least one of the plurality of fan blades; a hub , wherein an outer diameter of the hub increases from the inlet end to the outlet end , and wherein the increase in the diameter of the hub from the inlet end to the outlet end is generally equal to the increase in the diameter of the casing from the inlet end to the outlet end, and wherein the hub has a radially stepped configuration extending in the circumferential direction; a plurality of fan blades extending from a root at the hub to a blade tip , wherein a diameter of the blade tip of each of the plurality of fan blades increases from the inlet end to the outlet end; and a tip fence located at the blade tip of at least one of the plurality of fan blades, wherein an axial length of the tip fence measured perpendicular to the fan blades is greater than a thickness of the at least one of the plurality of fan blades; wherein an axial clearance , measured parallel to the diameter of the casing , is defined between the tip fence of each of the plurality of fan blades and an adjacent surface of the casing and the axial clearance remains constant between the inlet end and the outlet end of the axial flow fan, with a tolerance of +/- 1mm. <EOS>