Bends

The hard bend throws the air to one side as it turns the corner and so causes higher resistance. This loss can be reduced by the inclusion of splitters. Figure 3.57 gives equivalent lengths in diameters for a number of different bends, including those with splitters.

The equivalent length in diameters is based upon the assump­tion that one velocity pressure is lost in 55 diameters of ducting. Or to be precise the equivalent of one velocity pressure is lost in frictional resistance. Extensive tests have been made on bends of various designs and their losses measured.

These were then converted into fractions of velocity pressure. This factor is then independent of velocity over a limited work­ing range. For example a bend with a resistance of 50 Pa at 10 m/s (velocity pressure 60 Pa) therefore has a loss factor of 0.83. As resistance may be taken as the square of velocity over this limited range, at 20 m/s the loss would be 200 Pa and the velocity pressure would be 240 Pa and the loss factor would still

Be— i. e. 0.83.

240

To repeat, it is convenient in estimating the resistance, or pres­sure loss of a ducting system to calculate assuming that bends are equivalent to so many metres of straight ducting.

3.5.1.1 Reducing the resistance of awkward bends

When ducting is to be arranged in large buildings it is often im­possible to find the space to incorporate bends of a reasonable radius. It is then possible to insert vanes or splitters to reduce the pressure loss. See Figures 3.58 to 3.61.

Figure 3.55 Recovery in duct after a bend

It should be noted that the factors are in diameters. For exam­ple a 355 mm diameter single radius 90° bend is equivalent in resistance to 9 diameters of straight duct. Its equivalent length 355

In metres is then——- x9 = 32 metres. When dealinq with rect-

1000

Angular bends, the equivalent is taken on the “way” of the bend i. e. on dimension W (see Figure 3.56). Two 90° bends of exactly the same cross-section will have different pressure losses ac­cording to the “way”. One is an easy bend and the other a hard bend.

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Figure 3.58 Bend with splitters

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Figure 3.56 Easy and hard bends

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Figure 3.59 Detail of splitter

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Figure 3.64 Duct resistance equivalent lengths for branches and junctions

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2000

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Figure 3.65 Duct resistance equivalent lengths for branches and junctions