“Structural Dynamics of Fan Wheels:

Fans are used for a wide variety of industrial and commercial applications. There are two primary types of fans, centrifugal and axial. This paper concentrates on the structural dynamics of centrifugal fans. A centrifugal fan system consists of a rotor (fan wheel and shaft), stationary components (fan housing, scroll, inlet cone, dampers, pedestals and foundations) and interface components (bearings).

Centrifugal fans can be divided into two groups depending upon the wheel type. Figure 1 contains a sketch of a double-wide, double inlet (DWDI) fan wheel in which air enters the fan on each side of the center plate flowing radially from the hub to the trailing edge of a blade. Another type of fan wheel is a single-wide, single inlet (SWSI) wheel. It is similar to the DWDI type except it has only one set of blades and air enters the fan from only one side of the fan wheel.

Centrifugal fans can be divided intoCentrifugal fans have modes of natural frequencies in which the fan wheel participates along with the shaft and modes in which the fan wheel participates independent of the shaft. This paper concentrates on the structural dynamics of the fan wheel independent of the shaft and the structural dynamics of the supporting systems. Other papers [1]-[3] are available that address the rotor dynamics of the fan wheel that include the shaft. The natural frequencies of the fan wheel can be very sensitive to dynamic pressure pulsations produced either as a part of normal operation or by unusual aerodynamic circumstances. 

Catastrophic failures have been attributed to the excitation of natural frequencies of fan wheels by dynamic pressure pulsations.two groups depending upon the wheel type. Figure 1 contains a sketch of a double-wide, double inlet (DWDI) fan wheel in which air enters the fan on each side of the center plate flowing radially from the hub to the trailing edge of a blade. Another type of fan wheel is a single-wide, single inlet (SWSI) wheel. It is similar to the DWDI type except it has only one set of blades and air enters the fan from only one side of the fan wheel.

A finite element analysis of an eleven bladed DWDI centrifugal fan was performed to illustrate the various modes of natural frequency of a fan wheel that are independent of the shaft. The fan wheel used for this example was a type that is typically employed for large induced draft (ID) fans. Experimental test methods used to identify these modes are included in another paper [4].

“Nodal Circle Mode:

This mode shape is sometimes referred to as the “umbrella” mode because the general form of this mode resembles an umbrella opening and closing. It is dominated by axial deformation of the fan wheel, although there is some axial shaft movement that is out-of-phase with the fan wheel. All points in the fan wheel move axially in-phase with each other. The amount of shaft movement is dependent upon the stiffness of the thrust bearing and the structures supporting the bearings.”