Airflow calculation in HVAC ducts is a core in the design and operation of ventilation systems, whose basic principle is to calculate the volume of air passing through a specific cross-section of the duct per unit time. The core formula for calculation is: Airflow (Q) = Cross-sectional Area (A) × Average Air Velocity (V). Among them, the unit of airflow (Q) is usually cubic meters per second (m³/s) or cubic meters per hour (m³/h), the unit of cross-sectional area (A) is square meters (m²), and the unit of average air velocity (V) is meters per second (m/s). For circular ducts, the cross-sectional area is calculated as π × (radius)²; for rectangular ducts, it is length × width.

 

The calculation process must follow rigorous steps. First, accurately measure the inner diameter or side length of the duct, considering changes in the actual effective area caused by pipeline deformation, dust accumulation, or insulation layers. If necessary, multiply by a correction factor (e.g., when there is a 5% area loss in the pipeline, the effective area should be multiplied by 0.95). Second, when measuring the average air velocity, professional instruments such as pitot tubes or thermal anemometers should be used to perform multi-point measurements (at least 5 points) in the straight pipe section with stable airflow, avoiding disturbance sources such as elbows, reducers, or valves, and finally take the arithmetic mean as the calculated value. Substitute the measured cross-sectional area and average air velocity into the formula to obtain the basic airflow. If it is necessary to unify the time unit (e.g., converting seconds to hours), multiply by 3600.

 

In practical applications, operating condition correction and system design considerations must be carried out. Air density is affected by temperature and altitude. In high-temperature or high-altitude areas, the decrease in density will lead to deviations in mass flow. In this case, the mass flow formula should be used: Q_m = A × V × ρ (ρ is the air density under actual operating conditions). During system design, the theoretical airflow needs to consider the air leakage loss of the pipe network (about 3% for metal pipes and up to 8% for hoses) and the local resistance generated by components such as elbows and filters. Therefore, it is recommended to add a margin of 15%-20% to the designed total airflow. The final fan selection must be based on its performance curve to ensure that it meets the corrected airflow requirements while overcoming system resistance.

 

Take a circular duct with a diameter of 0.5 meters as an example: Cross-sectional area A = π × (0.25)² ≈ 0.196 m²; Measured average air velocity V = 8 m/s; Airflow Q = 0.196 × 8 ≈ 1.568 m³/s; Converted to hourly airflow: 1.568 × 3600 ≈ 5644.8 m³/h. This case demonstrates the practical application of the calculation process, but attention should be paid to error control, such as instrument accuracy (at least ±2%), calibration cycle (no more than half a year), and environmental factors (density correction is required when the temperature change exceeds 10°C).

 

In conclusion, airflow calculation is a rigorous process that requires multi-factor correction, involving measurement accuracy, operating condition adaptability, and system design optimization to ensure the efficient and reliable operation of the ventilation system. Following standard methods combined with actual corrections can effectively improve system performance and energy efficiency.