Delivered airflow — rigid vs. flex (as installed)
Airflow vs. static pressure
System curves at your current geometry. The vertical line marks your fan static pressure; where each curve crosses it is the delivered CFM.
The efficiency cost of common installation errors
| Installation condition | Friction × | Reynolds | Regime | Delivered CFM | Airflow loss |
|---|
Loss measured against the rigid metal reference at your selected diameter, length, and fan static pressure. Compression multipliers follow ACCA Manual D anchors (15% ≈ 2×, 30% ≈ 4×); field studies document up to ~10× at severe sag.
Numbers on a screen are a hypothesis. Measurement settles it.
This model tells you what your duct layout can and cannot deliver. What it can't tell you is how much of your air never reaches a room at all. A Diversified Energy assessment measures duct leakage at the registers, static pressure across the air handler, and delivered airflow run by run — so the fix is aimed at the actual restriction instead of guessed at.
How the model works
1 · Reynolds number → flow regime
Air velocity is found from the volumetric flow and the duct's cross-sectional area, then the Reynolds number determines whether flow is laminar or turbulent.
Residential duct flow is almost always turbulent (Re ≈ 50,000–500,000).
2 · Friction factor
Laminar flow uses f = 64/Re. Turbulent flow uses the Haaland explicit approximation to the Colebrook-White equation, using relative roughness ε/D.
3 · Darcy-Weisbach friction loss
Straight-run pressure drop, expressed in inches of water gauge, plus fitting loss at each bend using a loss coefficient K keyed to the bend radius.
4 · Compression & net airflow reduction
Longitudinal compression bunches the helix and collapses the core: the model scales effective roughness by the ACCA friction multiplier and shrinks the effective diameter. Delivered CFM is solved so the total duct pressure drop equals your available fan static pressure — the same physics that starves registers on real jobs.
Sources
- ACCA — Flex Duct Can Work Well If Installed Properly (compression friction multipliers)
- Energy Vanguard — The Science of Sag (Texas A&M field data)
- OSTI — Compression Effects on Pressure Loss in Flexible HVAC Ducts
- ASHRAE Fundamentals Ch.21 — Duct roughness & friction
- Darcy-Weisbach equation · Reynolds number reference