Double stage axial fans is a two-stage axial flow fan, which develops double the pressure rise compared to that of a conventional single stage fan. The fan has two impellers mounted on a common shaft with a guide vane arrangement between the two impellers. The fans operating point is quickly adjusted to the system load demand by varying the pitch of the hydraulically adjustable impeller blades. These fans are applied where high static pressure is required or where the static pressure per stage should be reduced so that a smaller hub diameter can be used. There are two ways to design these fans: (1) with two fan wheels rotating in the same direction and with guide vanes placed between the two stages or (2) with two counter-rotating fan wheels and no guide vanes at all. By either method, the two-stage unit will approximately double the static pressure.