In recent years, manufacturers of centrifugal blowers for ventilation systems have abandoned the traditional arrangement of radial vanes shown in Fig. 1(a) for the “backward-aerofoil” arrangement shown in Fig. 1(b), which is the same as a conventional centrifugal water pump. Unlike the older type, the aerofoil-type blower does not depend on “centrifugal force” to produce a pressure rise despite the misleading accounts given in some hydraulics textbooks (the circumferential component of the air velocity is zero at entry and small at exit) although it does depend on the increase in blade peripheral velocity from the entry to the exit (which explains the curious blade arrangement).