Centrifugal Fan Arrangements (2)


As mentioned above, arrangement 1 fans and arrangement 3 fans show many common features. Arrangement 9 and 10 fans are also similar in many ways. From the perspective of fan appearance, the difference is the motor of an arrangement 10 fan is installed under the bearing base while that of arrangement 9 fan on the left or right side.  

Usually the most commonly applied is arrangement 10, which has compact structure and has a constant overall size and is easy for protection of motors, drives and bearings when used outdoors. Due to the limited space under the bearing base, this type of arrangement has more restrictions on the size of the motor frame compared to arrangement 9. In both arrangements, the motor is directly connected to the fan, which is very compact and easy to install. In addition to the motor size, the operating temperature is another limiting factor for arrangement 9 and 10 because the motor is very close to the air flow. Typically, fans with arrangement 9 and 10 are limited to operating at airflow temperatures of 500 degrees Fahrenheit or less. For arrangement 10, an insulating plate can be placed between the motor and the fan scroll to reduce heat entering the motor area.

Direct Drive Arrangement

Different from the arrangements discussed above that use belts and pulleys to drive the wheel, fans of direct drive type may be used for low maintenance. Arrangement 4 has the best structural simplicity where the wheel is connected directly to the shaft. It means that pulleys, belts, and shaft bearings are no longer needed, which makes fan operation more stable.  

The drawback of arrangement 4 is very clearly seen if there is a big fan operating at low speeds. There may be low fan horsepower, but the motor must be designed big to hold the wheel weight. For instance, a fan sized 36 inch may need only 5 bhp to run, but a 15 to 20 hp motor will be needed to guarantee that there is large frame size for right shaft diameter and length. This may entail more purchasing costs at first and the motor operates costly in the life cycle. One recommended method is to have the wheel constructed of aluminum for less weight if it meets requirements for application in the airstream.

For arrangement 4 applications, some items still need to be considered. A proper L10 bearing life for the motor bearings must be offered. Besides the rotor weight, bearings shall support the radial and axial thrust load of the wheel. The size of the motor shall be able to meet the wheel inertial requirement for startup.

As the wheel is directly mounted to the shaft, the fan runs at 60 cycle (or 50 cycle) synchronous speed. You can consider outlet damper, inlet guide vanes, percent width wheel or VFD to be added as a method to achieve the designed performance ratings.

Another method can be used, which is to choose arrangement 8. Arrangement 8 is also of direct drive type through coupling connecting with fan shaft and bearing. The good point is the motor horsepower can be made to match the fan horsepower. Higher airstream temperatures can be allowed in arrangement 8 as the motor is positioned further from the scroll.