HVAC Ventilation Duct-System Resistance

Air ducts may be designed with either a round or a rectangular cross section. The radius of elbows should preferably be at least 1.5 times the pipe diameter for round pipes, or the equivalent round pipe size in the case of rectangular ducts. Accuracy in estimating the resistance to the flow of air through the duct system is important in the selection of blowers for application in duct systems (Fig. 3-6).

Resistance should be kept as low as possible in the interest of economy, since underestimating the resistance will result in failure of the blower to deliver the required volume of air. You should carefully study the building drawings with consideration paid to duct locations and clearances. Keep all duct runs as short as possible, bearing in mind that the airflow should be conducted as directly as possible from the source to the delivery points. Select the locations of the duct outlets to ensure proper air distribution. The ducts should be provided with crosssectional areas that will permit air to flow at suitable velocities. Moderate velocities should be used in all ventilating work to avoid waste of power and to reduce noise. Lower velocities are more frequently used in schools, churches, theaters, and so on, instead of factories and other places where noise caused by airflow is not objectionable.

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