Fans
Fan power from flow, pressure and efficiency (and SFP)
- where
- air power (W)
- air volume flow rate (m³/s)
- fan total pressure rise (Pa)
- fan shaft power (W)
- fan total efficiency (%)
- electrical input power (W)
- motor and drive efficiency (%)
- specific fan power (W per l/s)
Explain this equation
Works out the power a fan puts into the air, the shaft power it needs and the electricity it draws, plus the specific fan power used for Building Regulations checks.
V̇ Δp is the useful power given to the air: flow in m³/s times pressure in pascals gives watts. η_f is the fan's total efficiency and η_m that of the motor and drive, both as percentages, so dividing by them gives shaft and electrical input. SFP is electrical input per litre per second.
Each loss stage divides the power by its efficiency, so overall input is air power over the product of the efficiencies. The 1000 converts m³/s to l/s.
Sense check: 1 m³/s at 500 Pa is 500 W of air power; at 70 % and 90 % efficiency the fan draws about 790 W, an SFP of about 0.8 W/(l/s).
Watch out: match total pressure with total efficiency, and add all fans in a system for SFP.
Works out the air power, shaft power and electrical input of a fan, and its specific fan power (SFP) for Building Regulations compliance.
Enter your values to see the result.
Good to know
Air power in watts is flow (m³/s) times pressure (Pa). Fan efficiencies range from about 40 % for small forward-curved fans to over 80 % for large backward-curved and plug fans. The Non-Domestic Building Services Compliance Guide sets SFP limits, in W per l/s, for each type of system.
Common mistake
Using fan static pressure with a total efficiency (or the other way round), or quoting SFP with only one fan when the system has both supply and extract fans.
Limits
Steady duty point; efficiencies must be at that point. For SFP of a whole system add the electrical input of all fans and divide by the larger of supply or extract flow.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.