Air duct design

There are two essentially different principles used in the design of air ducts.

• Graduated velocity with duct friction per metre maintained constant

• Velocity maintained approximately constant

The graduated velocity method is used for ventilating plants and as duct sizes are reduced in mains and branches, the ve­locity is also reduced, maintaining friction approximately con­stant per metre. This results in economy of power consumption of the fan.

In industrial schemes the initial velocity at fan discharge may be relatively high, but in public building schemes the duct velocity is limited by noise, which is an initial factor. Not only must air noise in ducts be eliminated, but the design of all sections of the plant must be for low resistance in order that a slow speed, quiet-running fan can be installed.

The velocity maintained approximately constant method is used for pneumatic collecting plants as the requirement is to provide the velocity to keep the particles in suspension through­
out the system. Too high a velocity means excessive resistance with consequent high power consumption. Too low a velocity means a risk of choking at bends etc, with consequent com­plaints. Velocity may be varied slightly in different branches ac­cording to the ideas of the designer, but in principle the basis is constant velocity.

Posted in Fans Ventilation A Practical Guide

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