Flexible Ducting

Using the simple velocity method of calculations to determine the resistance to airflow, it is generally acknowledged that flexible ductwork offers approximately twice the resistance of a smooth bore duct, even when fully stretched. Flexible duct is manufactured around a wire helix. The flexible sheath will adopt a smaller diameter than the helix, reducing the cross sectional area. A duct with a normal diameter of 100mm will reduce to an actual diameter of around 80mm when not fully stretched and at a bend. A sharp 90° bend in flexible duct has up to three times the resistance of a smooth bore duct. The reason for these higher resistance values is due to turbulence in the duct caused by the constantly varying surface.

Flexible duct should be supported when in false or suspended ceiling applications to avoid the socalled ‘Loch Ness Monster’ effect of a series of humps. Each of these humps can have the resistance of two 90° bends.

The main advantage of flexible ducting over rigid is its ‘flexibility’! The cost per metre for straight runs is higher than rigid, but these are no expensive bends - of any angle.

This can also be the potential downfall of flexible duct, because it may lead to lazy workmanship, and higher than calculated resistance. In an ideal world, flexible duct would only be used with rigid straight duct to reduce the cost of specially made bends, but as we all know, the world is not an ideal place. Therefore, err on the side of caution when calculating the resistance of a flexible duct system. Keep the diameter of the duct on the high side, at least 1.25 times the impeller diameter for simple, small diameter Axial fans and no less than the diameter of the inlet/discharge spigot for centrifugal and in-line centrifugal and mixed flow fans. The general rule of thumb for calculating the appropriate discharge, a free area of 1.5 times the duct cross-section is equally important, and should also apply to any inlet grilles.

Metal Duct (Square and Spiral Wound)

The velocity method calculations for spiral wound duct are included in the tables in the System Calculator. Square duct should be based on an equivalent cross sectional area basis.

It is not within the remit of this course to discuss specially made ductwork which is based on static regain calculations. This is the realm of the consulting engineer. There are several external courses organised by such as the Mid Career College that cover more complex duct calculations.