Ductwork Sizing Calculator

Understanding Ductwork Sizing

Proper duct sizing is crucial for HVAC system efficiency.

  • Airflow (CFM) Cubic feet per minute.
  • Velocity (FPM) Feet per minute speed.
  • Friction Loss Pressure drop per length.
  • Equivalent Length Including fittings.

Duct Types

Round

Most efficient

Rectangular

Space-saving

Flexible

Easy installation

Oval

Hybrid solution

Fluid Dynamics

  • Flow Parameters Reynolds number: 4,000-10,000. Friction factor: 0.015-0.025. Dynamic pressure: 0.4-1.0 iwg. Velocity pressure: 0.1-0.5 iwg. Static regain: 0.5-0.7.
  • Velocity Limits Main trunk: 1,000-2,000 fpm. Branch ducts: 600-1,200 fpm. Terminal units: 400-800 fpm. Return air: 800-1,500 fpm. Outside air: 500-1,000 fpm.
  • Pressure Design Total pressure: 0.08-0.25 iwg/100ft. Static pressure: 0.7-2.5 iwg. Fan pressure: 1.5-4.0 iwg. System effect factor: 0.5-1.0. Leakage class: 6-12.

Material Engineering

  • Sheet Metal Properties Galvanization: G60-G90. Gauge range: 16-26. Tensile strength: 30-45 ksi. Yield strength: 20-35 ksi. Elongation: 20-30%.
  • Insulation Requirements R-value: 4.2-8.0. Thickness: 1-2". Density: 1.5-3.0 pcf. Vapor barrier: 0.02 perm. Surface burning: 25/50.
  • Joint Systems SMACNA class: A-D. Sealing level: A-C. Pressure class: ±2-10" wg. Reinforcement spacing: 4-8'. Connection efficiency: 85-95%.

Design Standards

  • Sizing Methods Equal friction: 0.08-0.15 iwg/100ft. Velocity reduction: 20-30%. Static regain: 0.5-0.7 recovery. T-method: β=0.35-0.45. Optimization factor: 0.8-0.9.
  • Aspect Ratio Rectangular: 1:1-4:1. Oval: 1:1-3:1. Equivalent diameter: 0.785(a+b). Hydraulic diameter: 4A/P. Shape factor: 0.85-1.0.
  • System Components Elbow radius: 1.5D-2.0D. Transition angle: 15-30°. Branch entry: 30-45°. Damper leakage: Class 1A-3. Terminal accuracy: ±5%.

Performance Criteria

  • Acoustic Requirements NC rating: 25-35. Insertion loss: 5-15 dB. Breakout transmission: TL 40-50. Regenerated noise: NC+5. Room criteria: RC 25-35.
  • Energy Efficiency Fan efficiency: 60-75%. System effect: <0.5". Leakage rate: <5%. Heat gain/loss: 0.5-1.0 BTU/hr·ft². Power factor: 0.85-0.95.
  • Maintenance Standards Access doors: 200-300 ft². Cleaning interval: 3-5 years. Inspection points: 50-100 ft². Balance tolerance: ±10%. Sensor calibration: annual.

Installation Tips

  • Support Every 4-6 feet.
  • Sealing All joints and seams.
  • Insulation R-value requirements.
  • Access For maintenance.