DUCTING – STATIC PRESSURE CALCULATION

Static pressure in ducting systems refers to the resistance airflow encounters as it moves through ducts, fittings, filters, and other components. It is a crucial factor in HVAC design, ensuring efficient airflow distribution while minimizing energy consumption. Proper static pressure calculation helps select the right fan and maintain system performance.

Key Factors Affecting Static Pressure

1. Duct Material & Size – Smaller or rough ducts increase resistance.
2. Air Velocity – Higher speeds raise pressure drop.
3. Duct Length & Layout – Longer ducts and sharp bends increase resistance.
4. Components – Filters, dampers, and grilles add to pressure loss.

Static Pressure Calculation Method

1. Determine Airflow (CFM) – Calculate required Cubic Feet per Minute based on HVAC load.
2. Duct Sizing – Use a ductulator or ASHRAE guidelines to select duct dimensions.
3. Velocity Pressure (VP) – Calculate using:VP=(V4005)2VP=(4005V​)2where $V$ = air velocity (FPM).
4. Pressure Loss in Straight Ducts – Use the Darcy-Weisbach equation or friction charts:ΔP=f×LD×ρV22ΔP=f×DL​×2ρV2​where $f$ = friction factor, $L$ = duct length, $D$ = diameter.
5. Fitting Losses – Refer to loss coefficients (ASHRAE tables) for elbows, transitions, etc.
6. Component Losses – Add pressure drops from filters, coils, and diffusers (manufacturer data).
7. Total Static Pressure (TSP) – Sum all losses:$$TSP=\Sigma (DuctLosses+FittingLosses+ComponentLosses)$$

Importance of Accurate Calculation

• Ensures proper fan selection (too high pressure wastes energy; too low reduces airflow).
• Balances airflow across all branches.
• Prevents noise and system inefficiency.

By accurately calculating static pressure, HVAC designers optimize duct systems for performance, energy efficiency, and occupant comfort. Tools like manual calculations, ductulators, and software (e.g., CFD analysis) aid in precise measurements.