Centrifugal Fan Design

Centrifugal fan design is similar to other industrial fan designs but as a bit of warning; this article includes the complex equations that make up the design. All the industrial fans work merely on the same principle of action. The fluid strikes on the moving (rotating) impeller and the impeller transfers it’s kinetic and dynamic energy to the fluid. As a result of this action the fluid moves to certain distance with greater pressure.

Centrifugal fans work on the centrifugation action. When fluid (usually air) strikes on the impeller of fan, the impeller transfers its rotational energy to the fluid by centrifugal force. Thus, the rotational energy of the impeller is transferred to fluid in form of radial energy under the action of centrifugal force.

Design Variables of Centrifugal Fan:

  • Number of blades
  • Blade shape
  • Blade curvature
  • Axial length of blades
  • Radial depth of blades
  • Impeller diameter
  • Guiding vanes
  • Fan width
  • Impeller clearance and exposure
  • Material of construction

There are 3 important factors needed to be taken care of while designing the blade curvature for the centrifugal fans. These include the direction of action the impeller imparts in its rotational energy to air, like a radial tip, backward curve or forward curve.

In the diagram, the subscripts 1 and 2 represent influent and effluent streams of fluid. Rotating fan impeller when comes in contact with the fluid, it develops momentum in the fluid therefore we can conclude that force developed by the fan is equal to the rate of change of momentum. The ideal power, assuming there is no loss in power, the power can raise some mass M to some height H. all of the kinetic energy is transferred to gravitation potential energy. This kinetic energy of the moving fluid is entirely recoverable inform of potential energy equal to the height of column fluid for unit weight. Following equation represents this phenomenon:Design variables for the centrifugal fans include blade angles, absolute velocity of the fluid, tangential velocity of the fluid, radial velocity of the fluid, radial velocity of the blade, and the velocity of the fluid relative to the blade.

As the radial velocity component of fluid (air) is constant throughout the impeller, the rate of change momentum is directly proportional to the rate of change of tangential velocity. For the ease of designing a centrifugal fan, it is better to resolve the components of absolute velocity (V) into Va and Vr. Subscripts a and r depicts the axial and radial components of velocity–both perpendicular to each other.

In the diagram, the subscripts 1 and 2 represent in-fluent and effluent streams of fluid. Rotating fan impeller when comes in contact with the fluid, it develops momentum in the fluid therefore we can conclude that force developed by the fan is equal to the rate of change of momentum. The ideal power, assuming there is no loss in power, the power can raise some mass M to some height H. all of the kinetic energy is transferred to gravitation potential energy. This kinetic energy of the moving fluid is entirely recoverable inform of potential energy equal to the height of column fluid for unit weight. Following equation represents this phenomenon:

centrifugal fan calculations

Pressure induced by the air of column of height H can be found by a well known equation i.e. P=ρ.g.h. Pressure can be found by finding the product of density, gravitational acceleration and height of fluid in column. So we can develop an equation for the pressure developed by the centrifugal fan:

centrifugal fan calculations

We can ignore the Va1 because air enters the centrifugal fan in the radial direction. So the equation of the total theoretical fan pressure and the pressure head can be represented by the equations:

centrifugal fan calculations

centrifugal fan calculations

We have a correlation to relate fan of speed, diameter of the impeller with the velocity.  Now it is desirable in practice to relate theoretical pressure developed by the fan in terms of flow rates and geometry of the blades. This can be represented by the equating:

centrifugal fan calculations

Here in this equation: n represents the number of revolutions in (RPMs) and D represents the diameter of the impeller. And finally we can find the radial velocity of centrifugal fan blade by the following correlation:

centrifugal fan calculations

This explanation of the centrifugal fan design is a bit technical, but for those that comprehend equations, you no doubt understand this theory;but if you still need assistance Custom Fans of Australia would be glad to help.