HVAC Service Call: Belt Driven Centrifugal Exhaust Fan Not Working (Broken Pillow Block Bearings)

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A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such a fan is also called a blower, blower fan, biscuit blower[citation needed], or squirrelcage fan (because it looks like a hamster wheel). These fans increase the speed and volume of an air stream with the rotating impellers.

Centrifugal fans use the kinetic energy of the impellers to increase the volume of the air stream, which in turn moves against the resistance caused by ducts, dampers and other components. Centrifugal fans displace air radially, changing the direction (typically by 90°) of the airflow. They are sturdy, quiet, reliable, and capable of operating over a wide range of conditions.

Centrifugal fans are constantdisplacement or constantvolume devices, meaning that, at a constant fan speed, a centrifugal fan moves a relatively constant volume of air rather than a constant mass. This means that the air velocity in a system is fixed even though the mass flow rate through the fan is not.

Centrifugal fans are not positivedisplacement devices and centrifugal fans have certain advantages and disadvantages when contrasted with positivedisplacement blowers: centrifugal fans are more efficient, whereas positivedisplacement blowers may have a lower capital cost.

The centrifugal fan has a drum shape composed of a number of fan blades mounted around a hub. As shown in the animated figure, the hub turns on a driveshaft mounted in bearings in the fan housing. The gas enters from the side of the fan wheel, turns 90 degrees and accelerates due to centrifugal force as it flows over the fan blades and exits the fan housing.

Each blade type varies in terms of efficiency – in other words, how fast the fan moves air through the duct or system where it’s installed. The most important thing to know is that backward curved blades are the most efficient, whereas forward curved blades are one of the least efficient types of fans.

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A pillow block usually refers to a housing with an included antifriction bearing, wherein the mounted shaft is in a parallel plane to the mounting surface, and perpendicular to the center line of the mounting holes, as contrasted with various types of flange blocks or flange units. A pillow block may contain a bearing with one of several types of rolling elements, including ball, cylindrical roller, spherical roller, tapered roller, or metallic or synthetic bushing. The type of rolling element defines the type of pillow block. These differ from "plummer blocks" which are bearing housings supplied without any bearings and are usually meant for higher load ratings and a separately installed bearing. Plummer block bearings are designed for more corrosive environments.

The fundamental application of both types is the same, which is to mount a bearing safely enabling its outer ring to be stationary while allowing rotation of the inner ring. The housing is bolted to a foundation through the holes in the base. Bearing housings may be either split type or solid type. Split type housings are usually twopiece housings where the cap and base may be detached, while solid are singlepiece housings. Various sealing arrangements may be provided to prevent dust and other contaminants from entering the housing. Thus the housing provides a clean environment for the environmentally sensitive bearing to rotate free from contaminants while also retaining lubrication, either oil or grease, hence increasing its performance and duty cycle.