Initial Sizing of Centrifugal Fans

Centrifugal fans are a type of turbomachine equipment widely used in all kinds of modern and domestic life. Centrifugal fans were developed as highly efficient machines, and the design is still based on various empirical and semi empirical rules proposed by fan designers. Due to these various rules, there are different methodologies used to design impellers and other components.

Centrifugal fans consist of an impeller in a casing with a spirally shaped contour, shown in Figure 1 (left side). The air enters the impeller in an axial direction and is discharged at the impeller outer periphery. The air flow moves along the centrifugal direction (or radial direction). Centrifugal fans can generate relatively high pressures, as compared with axial flow fans. For axial flow fans, the pressure rise is small, about be few inches of water.

Radial Fan and Static Pressure
Figure 1 Radial Fan and Static Pressure, Shaft Power V/s Volume Flow Curves for Different
Types of Blades

Generally centrifugal fans have three types of blade: forward blade, backward blade and radial blade. The characteristic curve of these three kinds of centrifugal fans is shown on right side in Figure 1.

Sizing Using Cordier Diagram

Centrifugal fans (most turbomachines) can be classified based on specific speed (Ns) and specific diameters (Ds) as shown in Figure 2. Specific speed is a criterion at which a fan of unspecified diameter would run to give unit volume flow and pressure. The correlation for specific speed and specific diameter can be seen here:

Pump Formula

where, ‘N’ is rotational speed (RPM), ‘Q’ is flow rate (ft3/sec), ‘H’ is head (ft), ‘D’ is diameter (ft)

Designers can select the specific speed and specific diameter (see Figure 2) based on the given requirement of flow rate, discharge pressure, rotation speed etc., and then identify the other stage parameters like flow coefficient and work coefficient, etc.. Based on certain recommendations and assumptions, designers can easily generate the initial size of the centrifugal fan geometry.

Cordier Diagrams
Figure 2 Cordier Diagrams Correlating Specific Speed Ns and Specific Diameter for Pump and Fan Selection.

Initial Sizing Based on Inverse Calculation

In a new trend, initial sizing of the centrifugal fans or other turbomachines can be easily performed by using the Inverse calculation, in which a few geometrical parameters can be specified along with the boundary conditions like pressure, flow rate, etc.. Here, designers have flexibility to specify a few geometrical parameters in the range from a minimum to a maximum value. And then can use quasi-random, low-discrepancy sequences like the Sobol sequence to create the different schemes from a specified range of parameters. With some additional assumptions, different schemes can be analysed using the Through flow solver or any other appropriate solver. This type of designing technique is quite fast and designers can generate different designs for the same specified boundary conditions as seen in Figure 3.

Efficiency vs. Rotor diameter
Figure 3 (a) Efficiency vs. Rotor diameter (b) Work coefficient vs. flow coefficient

Such algorithms are used in AxSTREAM® software platform, where designers can easily work from the initial sizing of any turbomachine through 3D model generation and analysis within very short duration of time.

In this method of initial sizing or preliminary design, designers can select best design based on different parameters like efficiency, power, manufacturing constraints, etc.. Here, designers can also account for different loss mechanisms like profile loss, secondary loss, blockage, etc. associated with fan operations by using the loss correlations suggested by various researchers. So, within the initial phase of design, designers can select a feasible geometric model of fan along with the initial performance prediction.

In current turbomachinery environment, where competition in the market is high, designers always need a criterion to create new and efficient designs within shorter and shorter product development times. So, algorithms based on the Inverse calculation are a great option to design efficient fans or any other turbomachines.

Interested in learning about AxSTREAM® for inverse design? We’d love to help! Request a software trial here.