ductdiameter

Duct diameter is the internal circular cross-section dimension of a duct, used to characterize the capacity of air-handling and exhaust systems. It is typically specified by the internal diameter in inches or millimeters and serves as a primary geometric parameter for design and performance assessment.

For a circular duct, the cross-sectional area is A = π d^2 / 4, where d is the diameter. The volumetric flow rate Q is the product of average air velocity v and area: Q = v A. Therefore, for a given flow rate, increasing diameter lowers velocity, which can reduce noise and friction losses but increases material costs and space requirements.

Designers select duct diameter to balance energy use, noise, and pressure drop. A too-small diameter raises velocity, increases friction and noise, and can strain fans; a too-large diameter wastes space and may reduce system efficiency if fan performance is not matched. Design methods commonly relate velocity, pressure drop, and flow rate, with equal-friction and friction-based approaches among the typical practices.

Non-circular ducts can be approximated by an equivalent diameter De, defined as De = 4A/P, where A is the cross-sectional area and P is the wetted perimeter. This allows the use of circular-duct design equations for rectangular or elliptical ducts.

Diameter is specified alongside other factors such as duct material, insulation, and fittings. In practice, standards from organizations like SMACNA and ASHRAE guide duct diameter decisions to ensure safety, reliability, and performance. Real-world systems may deviate from ideal calculations due to fittings, transitions, and leakage, which can affect effective diameter and overall performance.