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Mechanical and Natural Ventilation 85

Figure 6.3 Axial flow fan parts.

The fan consists of a number of aerofoil-sectioned blades mounted on a streamlined
hub that revolves within a cylindrical casing. The air reaches the blades at high veloc-
ity, having been accelerated in the streamlined approaches to the casing. It leaves the
blades at roughly the same velocity, and the gain in pressure is essentially one of static
pressure. The blades tend to rotate the air in a helical motion. Most of the energy of the
swirl is recovered by fixed guide vanes placed downstream of the rotor. The air then
enters the diffuser, or an evasée, where the velocity pressure is converted to static pres-
sure. Fig. 6.3 shows various parts of an axial flow fan.

6.3.1 Performance Characteristics of an Axial Flow Fan

Eq. (6.1) can again be used to calculate the theoretical head developed by an axial flow
fan, but the blade angle, q, must be replaced by actual fluid angle, a. Further assuming
that there is no inlet whirl,

V 2 VQ
Theoretical head H ¼ cot a (6.6)
g gA
Here, A is the area through which the fluid flows, that is, the area of the annulus
between the hub and tip minus the area taken up by the blades. Because a is always
less than 90 degrees, the actual head developed will always be less than the ideal
head of V 2 . The actual head is further reduced by some of the same losses as for cen-
g
trifugal fans discussed earlier except for shock losses.
Fig. 6.4 shows the characteristic of a typical axial flow fan. The nonoverloading
characteristic of the axial flow fan is noticeable and very desirable [1].