Specific fan power calculation
Divide electrical input power by airflow using compatible units. This calculator uses watts and litres per second. Enter the power for the boundary you are assessing, such as one extract fan including its controller.
Illustrative inputs. This calculation does not select equipment or establish a safe operating limit.
For a system with several fans, establish the applicable system definition before adding powers or choosing the reference airflow. Supply and extract airflows are not automatically added together. Record the calculation boundary so another person can reproduce the result.
Specific fan power units
SFP in W/(L/s) has the same numerical value as kW/(m³/s). One thousand watts is one kilowatt, and one thousand litres is one cubic metre, so both numerator and denominator change by the same factor.
For example, 600 W divided by 500 L/s gives 1.2 W/(L/s). The same duty is 0.600 kW divided by 0.500 m³/s, again 1.2 kW/(m³/s). In W/(m³/s), the value is 1,200. Always carry the unit with the number.
SFP is not the fan's efficiency percentage. It includes the effect of the pressure duty as well as efficiency. A system moving air through greater resistance may have higher SFP even when its fan is efficient at that point.
Choose the correct power boundary
A motor nameplate rating is not a measured electrical input at the chosen airflow. A shaft-power value also excludes losses between the electrical supply and the shaft. Use measured input or a documented calculation based on relevant motor and drive efficiencies.
The AMCA power-boundary explanation separates those components. For the arithmetic from airflow and pressure through to input power, use our fan power calculator.
When comparing quotes, ask whether control losses, transmissions and auxiliary loads are included. A small apparent improvement may simply reflect different reporting boundaries.
Compare like operating conditions
Set the airflow and pressure requirement first. Then compare candidate selections at that same duty, air density and operating state. Do not compare a clean-filter figure with another system's dirty-filter design point without explaining the difference.
A variable-speed system needs more than one point if it spends substantial time at different duties. List the modes, hours and input power. Annual energy is the sum of kW multiplied by hours for each mode. This is more useful than treating one favourable SFP figure as the result for the whole year.
Use the fan curve to see how the operating point affects performance. A low-power setting still has to deliver the extraction and containment required for the task.
Where improvements can come from
Unnecessary duct resistance, abrupt transitions and a poor fan duty match can all add to input-power requirements. Review the route, fittings and operating controls with the system designer. A larger or shorter duct may change both space requirements and pressure loss, while a different fan selection may suit the duty better.
There is no single SFP threshold on this page for every New Zealand chemical-extraction system. Project specifications and applicable requirements determine the acceptance criteria. We use the comparison to support industrial fan selection, with the chemical process and required performance kept central.
Your questions
Is a lower SFP always the better selection?
Only when the systems deliver the same required task with a consistent reporting boundary. Chemical compatibility, containment, operating range and maintenance still matter.
Does SFP stay constant when fan speed changes?
No. Airflow, pressure and component efficiencies change with operating duty. Calculate or measure the relevant operating points.