Sheet Metal Duct Geometry, CFM & Pattern Stretchout

Airflow velocity, duct cross-sectional area, bend allowances, and metal gauges

SMART (International Association of Sheet Metal, Air, Rail and Transportation Workers) craftspeople design, fabricate, and install architectural metal, industrial ventilation, and HVAC duct systems. The aptitude exam tests trade geometry, spatial 2D-to-3D visualization, airflow formulas, and sheet metal fabrication math.

On the Test

SMART apprenticeship aptitude exam format. Focuses heavily on rectangular and round geometry, area conversions, air volume (CFM), and pattern development.

•Airflow Equation: Q = A * V (Airflow CFM = Cross-Sectional Area in sq ft * Velocity in FPM).
•Area of Rectangular Duct: A (sq ft) = (Width in inches * Height in inches) / 144.
•Area of Round Duct: A (sq ft) = (pi * Radius in inches^2) / 144 = (pi * Diameter^2) / (4 * 144).
•Sheet Metal Gauge Rule: Higher gauge number = thinner metal (e.g. 16 ga is significantly thicker than 26 ga).
•Stretchout Length: The flat pattern perimeter dimension plus seam allowances (Pittsburgh lock, drive cleat tabs).
⚠Common trap: Forgetting to divide cross-sectional area by 144 when calculating airflow CFM. Velocity is measured in feet per minute (FPM), so duct dimensions in inches must be converted to square feet.

HVAC Airflow Calculations (CFM, Area, and Velocity)

Heating and air conditioning design relies on continuous volume balance. Airflow volume is measured in Cubic Feet per Minute (CFM), and air speed is measured in Feet per Minute (FPM).

Fundamental Equation: CFM = Area (sq ft) * Velocity (FPM).

Example: A 24" x 12" rectangular duct carries air at an average velocity of 800 FPM. First calculate duct area in sq ft: (24 * 12) / 144 = 288 / 144 = 2.0 sq ft. Total airflow: CFM = 2.0 * 800 = 1,600 CFM.

To determine required duct size for 1,200 CFM at 600 FPM: Area = 1,200 / 600 = 2.0 sq ft = 288 sq inches. A 16" x 18" or 12" x 24" duct satisfies this volume.

Round vs Rectangular Duct Equivalent Geometry

Round duct is more aerodynamic and requires less sheet metal per volume of air compared to rectangular duct. Friction losses are lower because the ratio of perimeter to area is minimized.

Round Duct Example: A 14-inch round duct carries air at 900 FPM. Radius = 7 inches. Area = (pi * 7^2) / 144 = (3.1416 * 49) / 144 = 153.94 / 144 = 1.069 sq ft. CFM = 1.069 * 900 = 962 CFM.

Sheet Metal Pattern Stretchout & Seam Allowances

Fabricating a three-dimensional rectangular duct fitting requires unrolling the 4 sides into a flat metal sheet: Stretchout = 2 * (Width + Depth) + Seam allowances.

Common Seams: Pittsburgh Lock requires a 1-inch allowance for the pocket side and 1/4-inch for the flanged insert side. Drive cleats and S-cleats connect duct sections end-to-end.

Sheet Metal Thickness Gauges: In galvanised sheet steel: 26 ga = .0217", 24 ga = .0276", 20 ga = .0396", 16 ga = .0635", 10 ga = .1382". As gauge increases, metal thickness and weight decrease.

Key Takeaways

✓Always convert inches to square feet by dividing by 144 before multiplying by FPM.
✓Remember: Higher gauge number = thinner sheet metal.
✓Circumference of round duct = pi * Diameter (approx 3.1416 * D).

Key Terms

CFM

Cubic Feet per Minute; standard imperial measurement of volumetric airflow.

FPM

Feet per Minute; air velocity speed inside a duct system.

Static Pressure

Outward burst pressure exerted on the interior duct walls, measured in inches of water gauge (in. w.g.).

Pittsburgh Lock

Interlocking longitudinal mechanical seam widely used in rectangular duct construction.

Sheet Gauge

Standard numbering system for sheet thickness; inverse scale (higher number = thinner).

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