Discussions around fan selection can often contain many professional words and expressions that are important to understand. Below is a selection of words and their context as well as some common units used for fans.
Fan curve
A fan curve describes a fan’s performance at different flow and pressure settings. Each fan model has its own fan curve and the curves look different depending on the design and size of the fans.
System curve
A system curve describes the system’s frictional resistance and pressure set-up at a certain volume flow.
The system curve is unique for each application and depends on the design of the system where the fan is to be mounted.
Operating point
The operating point is the point where the fan curve and the system curve meet.
Connected to a system, the fan will work at a point where equilibrium prevails between the fan and the fan system. In the equilibrium state, the energy supplied by the fan is as strong as the braking of the system.
When choosing a fan, different fan curves are matched with the system curve to find the fan that best meets the system’s requirements. Important parameters to adjust are efficiency, power requirements and noise levels.
Pump area (“Stall”)
The pump area or area is the part of the operating curve where the fan has no stable operation. For axial fans, this area can be found on the left in the fan diagram.
Fans designed for higher pressures also have clearer pumping areas and therefore it is extra important that these are chosen correctly.
The definition of a “stall” on an axial fan is when there is an uneven flow over the impeller blade, it is referred to as a flow separation from the blade.
This means that the fan does not deliver the performance needed, while there is a risk of damage to the fan.
Therefore, running the fan in stall areas should always be avoided.
If a fan must be pump-free, it can be fitted with an “anti-stall” ring.
Axis effect
Shaft power is the energy that needs to be supplied to the fan wheel to reach the operating point.
Shaft power is calculated according to:
Where:
V is the volume flow in m3/h,
Δpt is total pressure increase in Pa
η is the efficiency of the fan.
Peripheral speed (on fan blades)
The peripheral speed of the fan blade is the distance that any selected point on the periphery of the fan wheel has traveled during a certain time. In other words, the “velocity” of that point.
Most of our impellers have been optimized with finite element simulations.
Depending on the impeller material used, the peripheral speed is often the limiting factor for the maximum allowable fan speed.
Units
| Symbol | What | Example unit |
| q | Flow (Volume or Mass) | m3/h or kg/h |
| pt | Total pressure | Pa |
| ps | Static pressure | Pa |
| pd | Dynamic pressure | Pa |
| T1 | Inlet temperature | ° C |
| ρ | The density of the gas | Kg/m3 |
| n | Fan speed | RPM |
| Pw | Impellerpower | kW |
| η | Fan efficiency | % |
| u2 | Peripheral speed (blades) | m/s |
