Figure 3.68 Spiral inlet box losses Non g. s. s. (galvanised steel sheet) ducting

The friction loss in ducting manufactured in other materials is best obtained from the absolute roughness and relating it to its size to calculate the relative roughness and hence the friction factor. Aluminium and PVC ducts will then be seen to have lower friction. Spiral wound ducting may have a higher friction depending on the smoothness of the internal surface.

In the past, large ducts in public buildings were often built into the masonry fabric and finished with glazed tiles. This was when the pressure loss due to friction was as good as that for g. s.s. Those days are unlikely to return, but underground air ducts are still used in applications such as grain drying. In these cases the approximate “correction factors” given in Table 3.7 may be used.

Dered from the fan manufacturer. The manufacturer will usually be able to supply a box which is tapered to suit and has an inter­nal swirl baffle. If having said all this, the ductwork designer still wishes to be responsible for their supply he should be aware that simple box pressure losses can be very dependent on their orientation. Figure 3.67 gives some information of a very ap­proximate nature — the loss is also very dependent on the fan design. The spiral design type E should be avoided at all costs

— the volumetric flow is seriously reduced. See Figure 3.68 which is a typical example.

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