External static pressure calculation Calculator
External static pressure calculation
Determining the capacity of an exhaust fan or fresh air fan is not just by calculating the CMH (Airflow), we also have to do the external static pressure calculation or calculating the resistance along the duct channel.
Therefor we provide you external static pressure calculator & external static pressure calculation excel format free download.
What is external Static Pressure (ESP) and how is it calculated?
External Static Pressure (ESP) is the amount of pressure a fan must overcome to move air through the external components in the HVAC system, everything outside the unit itself (for examples: duct surface, filters).
So, external static pressure is the pushing power left over after the air has already fought its way through the internal parts of the Aircon unit.
External static pressure formula
External static pressure (ESP) = P Base + Safety Factor
External static pressure (ESP) = Duct Friction + Fittings Loss + Accessories Loss + Terminal Pressure + Safety Factor
Where:
- Duct Friction: Resistance from straight duct (Pa / meter).
- Fittings Loss: Resistance from elbows, tees, and reducers.
- Accessories Loss: Resistance from filters, volume dampers and fire dampers.
- Terminal Pressure: The pressure is required by the diffuser to blow air effectively.
- Safety Factor: Usually 10% to 20% added for leakages or construction defects.
Case examples:
HVAC system is being designed for a commercial building by engpocket team. Our engineer has calculated the following pressure losses:
- Duct friction loss : 45 Pa
- Fittings loss : 80 Pa
- Accessories loss (dampers, filters, and cooling coils) : 62 Pa
- Required terminal pressure at the farthest supply diffuser : 25 Pa
- Safety factor : 20%
Let’s find and do the external static pressure calculation using the manual formula:
Step 1, we calculate the base pressure loss (without safety factor):
P base = 45+80+62+25 = 212 Pa
Step 2 we calculate 20% safety factor:
Safety Factor = 0.20 × 212 = 42.4 Pa
Step 3 we calculate the total ESP:
ESP=P base + Safety Factor:
212 + 42.4 = 254.4Pa
So, the total external static pressure is 254.4 Pa
Standard Pressure Drop Reference Table (SMACNA/ASHRAE Guidelines)
Engpocket friends can use the table below as a cheat sheet to fill in the external static pressure calculation in our free excel format.
| Component Name | Est. Pressure Drop Range |
|---|---|
| 1. Ductwork Friction Loss | |
| Straight Duct (Low Velocity) | 0.8 – 1.2 Pa/m |
| Straight Duct (Medium Velocity) | 1.5 – 2.5 Pa/m |
| Flexible Duct | 3.0 – 5.0 Pa/m |
| 2. Fittings & Connections | |
| Elbow 90° (Smooth Radius) | 4 – 8 Pa |
| Elbow 90° (Square Throat) | 10 – 15 Pa |
| Tee / Branch Fitting | 5 – 10 Pa |
| Reducer / Enlarger | 2 – 5 Pa |
| 3. Dampers & Accessories | |
| Volume Control Damper (VCD) | 15 – 25 Pa |
| Fire Damper (Curtain Type) | 20 – 40 Pa |
| Motorized Damper (MD) | 20 – 30 Pa |
| Sound Attenuator | 30 – 60 Pa* |
| Weather Louver (Fresh Air) | 30 – 50 Pa |
| 4. Filtration (Initial Resistance) | |
| Pre-Filter (G4) | 40 – 60 Pa |
| Medium Filter (F7/F8) | 80 – 120 Pa |
| HEPA Filter (H13) | 250 – 350 Pa |
*Note: Values are estimates based on SMACNA/ASHRAE standards. Always check specific manufacturer data for final selection.
External static pressure calculation excel
This excel workbook provide more details that can help you calculate the external static pressure.
Detailed External Static Pressure Calculator Excel
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