Basic HVAC Duct Design Calculation

Proper duct design is essential for maintaining required airflow and system efficiency in HVAC systems. One of the key steps is calculating the required duct size based on airflow (CFM).

Step 1: Airflow Requirement (CFM)

Airflow is determined based on room cooling load.
CFM = Cooling Load (BTU/hr) ÷ (1.08 × △T)

Step 2: Select Air Velocity

Typical duct air velocity range:

* Main duct: 1200-1500 FPM

* Branch duct: 600-900 FPM

Step 3: Duct Area Calculation

Duct Area = CFM + Velocity

Example:

Required airflow = 2000 CFM

Selected velocity = 1200 FPM

Duct Area = 2000-1200

Duct Area = 1.67 ft²

Now we can select the nearest duct size from standard duct dimensions.

Example duct size:

24" x 10" duct ≈ 1.67 ft²

Accurate duct sizing ensures proper air distribution, reduces pressure loss, and improves HVAC system efficiency.

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