TOOL GUIDE
Pressure loss & fan sizing
Sum one critical flow path to calculate fan total/static pressure, shaft power and electrical input.
Purpose & scope
Sum one critical flow path to calculate fan total/static pressure, shaft power and electrical input.
No network balancing or fire simulation. Fan static pressure is not AHU external static pressure. Fan type does not automatically change efficiency.
Inputs
Use drawings, design conditions and manufacturer data. Units and example values follow the calculator; example values are not universal design requirements.
| Parameter | Value / interpretation |
|---|---|
| Proposed fan type | Not specified / Centrifugal fan / Axial fan / Mixed-flow fan |
| Application | Supply air / Exhaust air / Pressurization / Smoke extraction / Kitchen exhaust |
| Fan airflow · m³/h | Volumetric flow at the calculation location; check the selected unit. |
| Air density · kg/m³ | Density at design conditions, distinct from mass flow rate. |
| Dynamic viscosity · µPa·s | Dynamic viscosity; this tool uses µPa·s. |
| Duct roughness · mm | Absolute roughness of the internal pipe/duct surface, in mm. |
| Additional loss / system effect · Pa | Additional losses not included in path rows; avoid double counting. |
| Pressure allowance · % | Allowance applied to the result; not a substitute for entering all losses. |
| Fan total-pressure efficiency · % | Fan/pump efficiency at duty, from manufacturer data. |
| Motor efficiency · % | Motor efficiency used to estimate electrical input power. |
| Fan outlet area · m² | Fan outlet area used for velocity pressure and fan static pressure. |
Steps
- Enter fan airflow, air properties, duty efficiencies and outlet area.
- Enter consecutive rows: length, section, flow, summed K and equipment Δp. Do not sum parallel branches.
- Match total/static pressure to the catalogue basis, then check Q–P curves, efficiency and shaft power.
Worked example
This reproducible training example uses the values below. Select the appropriate mode and overwrite restored project values. Example rates, K values and equipment losses are assumptions, not mandatory design criteria.
Examples use a decimal point and no thousands separator.
All example inputs
| Parameter | Value / interpretation |
|---|---|
| Proposed fan type | Not specified |
| Application | Supply air |
| Fan airflow | 2400 m³/h |
| Air density | 1.2 kg/m³ |
| Dynamic viscosity | 18.1 µPa·s |
| Duct roughness | 0.09 mm |
| Additional loss / system effect | 50 Pa |
| Pressure allowance | 10 % |
| Fan total-pressure efficiency | 65 % |
| Motor efficiency | 90 % |
| Fan outlet area | 0.15 m² |
Replace the default path with exactly this row; delete the other sample rows.
| Parameter | Value / interpretation |
|---|---|
| Section | Example calculation path |
| Section flow | 2400 m³/h |
| Section dimensions | 500 × 300 mm |
| Section length | 20 m |
| Total fitting K | 1 |
| Equipment pressure loss | 150 Pa |
Expected results (rounded)
| Parameter | Value / interpretation |
|---|---|
| Fan total pressure | 246.17 Pa |
| Fan static pressure | 234.32 Pa |
| Shaft power | 0.252 kW |
Small differences in displayed rounding are normal. A result that passes the entered limits does not establish suitability for the complete system.
Method & limitations
No network balancing or fire simulation. Fan static pressure is not AHU external static pressure. Fan type does not automatically change efficiency.
Enter consecutive sections on one critical path; do not add parallel branches. Section flows can differ from the total fan airflow.
Δp = f·L/Dh·ρv²/2 + ΣK·ρv²/2 + equipment loss. K uses the entered section velocity; obtain coefficients for the actual fitting geometry, flow split and reference area.
The duty is fan total pressure; fan static pressure = total pressure − outlet velocity pressure. This is not AHU external static pressure. Select using the Q–P curve and efficiency at duty. Thermal buoyancy and branch-network balancing are excluded.
Common mistakes
Adding parallel losses; using K with the wrong reference section; confusing total/static curves; retaining sample equipment losses.
Projects & reports
- Choose or create an engineering project and enter your actual design inputs.
- Project inputs/results are autosaved in this browser. Where a saved-list panel is available, name and add the selected section or alternative to compare multiple options.
- Review the report before exporting PDF. In the print dialog select Save as PDF. Export JSON for backup and use Open file to restore it; PDF cannot restore project data.
Saved results do not automatically recalculate when another tool changes. Review any dependency warning, recalculate and update the affected results.
References & related tools
The tool implements the methods and limits stated above; linking a standard does not establish complete compliance with that standard.