Airflow and Velocity Calculation Formulas
Airflow = Sum of air volume at ventilation terminals
Velocity = Flow velocity within the duct
Air pressure = Pressure within the duct during exhaust
A uniform airflow duct is a type of duct that delivers equal amounts of air into a room uniformly from several air outlets (or short pipes) on the side wall of the duct at the same outlet velocity. There are two types of uniform airflow ducts: one where the cross-section of the duct changes (i.e., the cross-section gradually decreases) while the area of the side outlets (or short pipes) remains constant; and another where the cross-section of the duct remains constant while the areas of the side outlets (or short pipes) are not equal. The basic principle of calculation is to maintain equal static pressure at each side outlet. The formula for equal static pressure at each side outlet can be obtained based on the calculation of duct resistance and the energy equation. The purpose of uniform airflow duct calculation is to determine the area of the side outlets, the ductwork, and the resistance of the uniform airflow duct section. Once the number of side outlets, the spacing between them, and the airflow volume at each side outlet are determined, the dimensions of the uniform airflow duct can be calculated according to the above principle.
The calculation method for a uniform air supply duct is as follows:
(1) Determine the number of side holes, the spacing between side holes, and the air volume of each side hole.
(2) Calculate the side hole area f0 m2, where L0 is the total air volume of the uniform air supply duct (m3/h); f0 is the average outflow velocity of the side hole (m/s); and μ is the flow coefficient of the orifice.
(3) Calculate the duct diameter (or cross-sectional dimensions). First, set vd (the velocity inside the duct) according to the principle of vj/vd≥1.73, and then calculate the duct diameter D of the corresponding section.
(4) Calculate the static pressure required for the resistance side holes of the duct.
