What happens to the velocity of air when ductwork has a converging cross-sectional area?

When ductwork has a converging cross-sectional area, the velocity of air increases. This phenomenon is rooted in the principles of fluid dynamics, specifically the concept of continuity which states that for an incompressible fluid, the mass flow rate must remain constant from one cross-section of a duct to another.

As the air flows from a larger cross-sectional area to a smaller one, the same volume of air must pass through the narrower section in the same amount of time. To accommodate this flow rate without any change in mass or density, the air must speed up, resulting in an increase in velocity. This behavior is also described by Bernoulli's principle, which indicates that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or potential energy in that fluid.

In the context of HVAC systems and ductwork design, understanding how air velocity changes in converging sections is crucial for calculating air distribution, ensuring adequate ventilation, and maintaining system efficiency. This principle is foundational for millwrights and engineers who work with pneumatic systems.