The differences in fan performance according to installation category are as much a function of the fan type and design, as
Figure 4.21 Typical performance curves for a tube axial fan to different installation categories
Of the position of duty point on the particular characteristic curve. In practice the type B and type D characteristics for most fan types will be nearly the same at the best efficiency point, provided the fan is supplied, in its free inlet form, with a properly shaped entry cone or bellmouth. With the same proviso, type A performance will coincide with type C.
The essential difference remaining is that between free outlet and ducted outlet performance, which is significant for fans of all kinds though it diminishes as the ratio of fan velocity pressure to fan total pressure falls. It will also be affected by tongue pieces in a centrifugal fan outlet. In the latter, a length of ducting is desirable to enable some recovery of dynamic pressure to useful static pressure to be achieved from the distorted velocity profile.
Typical performance curves for a forward curved centrifugal, a backward inclined centrifugal and a tube axial fan are shown in Figures 4.19, 4.20 and 4.21.
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