Inlet boxes are provided to give air side entry to the fan inlet. This also permits the bearings to be mounted outside the airstream. The large flat faces of the box require adequate stiff-
Figure 8.6 DIDW fan with dual inlet boxes
View on shaft end Cross-section
Figure 8.7 Proportions of an inlet box
Ening to prevent drumming. Pressure losses in boxed inlets can be substantial (see Chapter 3, Section 3.5.7) and for this reason are best supplied by the manufacturer as part of the fan. The proportions of the box and internal anti-swirl baffles are critical to performance and are very much dependent on the actual fan design.
They are designed to give minimum pressure loss in the working range and to ensure an absence of swirl at the impeller entry. Atypical fan and inlet box is shown in Figure 8.6, whilst the proportions which have proved satisfactory for many fans are shown in Figure 8.7.
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