Duct System Design - Complete Guide

Duct systems begin with fundamental airflow physics: air behaves as an incompressible fluid at low Mach numbers (<0.3), allowing application of Bernoulli’s principle and continuity equation. Design starts by assigning required airflow (CFM or L/s) to each space based on thermal load and ventilation needs—this forms the boundary condition for all downstream calculations.

Pressure loss is dominated by two components: major (frictional) loss along straight duct runs and minor (dynamic) loss across fittings (elbows, tees, transitions). The Colebrook-White equation—or its Moody chart approximation—is used for turbulent flow in round ducts, while the ASHRAE Duct Fitting Database provides empirically derived C-coefficients for complex geometries. Real-world ducts rarely match idealized assumptions: seam leakage, poor sealing, uninsulated surfaces, and unaccounted flex duct compression drastically degrade performance.

Advanced design incorporates computational fluid dynamics (CFD) for critical zones (e.g., surgical suites), dynamic balancing with motorized dampers and BMS integration, and lifecycle energy modeling that weights fan kW against duct material cost and maintenance access. Modern practice also embeds commissioning-by-design principles—specifying test ports, permanent pressure taps, and accessible balancing dampers—ensuring verifiability without retrofitting.