Duct Sizing Notes
In equal friction duct design, you typically select a friction target (pressure loss per length) and size ducts so
pressure drop stays uniform throughout the system. This helps balancing, keeps fan static predictable, and makes
duct sizing consistent from main to branches.
When to Adjust Your Target
- Noise issue: reduce velocity (bigger duct) or reduce friction target.
- High fan static: lower friction target in long mains and long runs.
- Space constraints: allow slightly higher friction but keep AR reasonable.
Good Practice Checklist
- Confirm total static includes fittings (elbows, transitions, dampers, coils, filters).
- Prefer standard duct sizes where possible.
- Consider insulation thickness and clearance before finalizing sizes.
Quick Checks (Design Validation)
Aspect Ratio
- Try to keep AR ≤ 4:1 for easy fabrication and lower noise.
- If space forces high AR (e.g., 6:1), consider reducing velocity.
- Very flat ducts can increase losses and noise.
Noise & Velocity
- Near diffusers: supply typically 500–900 fpm.
- Main trunks can be higher, but watch noise limits.
- If VP looks high, reduce friction target / velocity.
Pressure Drop Strategy
- Long runs → lower friction (e.g., 0.06–0.08) reduces fan static.
- Short runs / tight shafts → slightly higher friction can be acceptable.
- Also check fittings (elbows, transitions, dampers).
Practical Notes
- Prefer standard sizes; avoid odd dimensions where possible.
- Check insulation thickness (space clearance).
- Confirm access doors, turning vanes, balancing dampers.
How to Use (Short)
- Select US/Metric and Air Condition.
- Enter any 2 values (Flow + Friction is common).
- Enter Height to calculate Width; verify aspect ratio.
- Review velocity and VP; adjust friction target if needed.