Bends

In the case of bends it is important to note that much American data is categorised on the basis that the radius of a bend is to its centreline. British practice is usually to give the inside radius. When looking at data, make sure you are comparing like with like.

The loss of pressure in a bend following by further straight ducting is less than if it discharges to atmosphere. In the former case there is some recovery in the expansion of the airflow to the full duct diameter. (See Figure 3.55.)

 

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.

 

Bends

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

 

__ .Radius Easy

Figure 3.56 Easy and hard bends

 

II r

  Bends

Figure 3.59 Detail of splitter

  Bends

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подпись: figure 3.61 detail of aerofoil section vaneFigure 3.57 Duct resistance equivalent lengths for bends

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The aerofoil section vanes are cast aluminium and are less lia­ble to be noisy that sheet metal splitters. They also result in a lower pressure loss i. e.

Sheet metal splitters pLb = 0.24 x velocity pressure

Aerofoil section vanes pLb = 0.11 x velocity pressure

An alternative design of splitter which encompasses the com­plete bend may also be used. This effectively divides the bend into a number of parallel sections for which the dimensions are known. The loss for these may then be calculated and the high­est value used. See Figures 3.62 and 3.63.

Bends

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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

Bends

Bends

Figure 3.62 Splitter radius in radiused rectangular bends

 

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Bends

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Number

Figure 3.63 Chart for determining position of splitters

For example, as shown by the line drawn across the chart in Figure 3.63, a bend has an inner radius of 50 mm and an outer radius of 500 mm. If there were 2 splitters, these would be posi­tioned at radii of 112 mm and 230 mm.

If there were 3 splitters, these would be positioned at radii of 90 mm, 160 mm and 260 mm.

Posted in Fans Ventilation A Practical Guide


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