Calculate the friction loss per unit length using the Darcy-Weisbach equation or extract it directly from an ASHRAE friction chart or Ductulator tool .
Understanding and calculating is vital for selecting the right fan or Air Handling Unit (AHU). Think of ESP as the total resistance air encounters while traveling through every component of your ductwork—from the straight runs to the smallest fittings. What is External Static Pressure (ESP)? esp calculation for duct
| Component | Calculation / Value | Pressure Drop (in.w.g.) | | :--- | :--- | :--- | | | 100 ft total length $\times$ (0.10 / 100 ft) | 0.10 | | 2. Fittings (Elbows) | Four 90° elbows. Est. Equivalent Length: $4 \times 25 \text ft = 100 \text ft$. $100 \text ft \times (0.10 / 100 \text ft)$ | 0.10 | | 3. Fire Damper | Manufacturer data | 0.10 | | 4. Flexible Duct | 10 ft of flex at terminal (high resistance). Est. equivalent to 30 ft straight duct. | 0.03 | | 5. Supply Diffuser | Manufacturer data | 0.03 | | Total Supply ESP | Sum of above | 0.36 in.w.g. | Calculate the friction loss per unit length using
Common fittings include:
| System Type | Safety Factor | |-------------|----------------| | Simple residential | +0.05 to 0.10 in. w.g. | | Commercial VAV | +10% of calculated ESP | | Dirty filters / future use | +0.15 to 0.25 in. w.g. | | Poorly detailed design | +25% | What is External Static Pressure (ESP)
$$ESP = (P_supply + P_return) + P_accessories$$