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Key concepts
Detecting imbalance early reduces damage and lowers repair costs.
Imbalance and vibration are not the same.
Dirt, temperature, and loose fits can cause imbalance.
Fans play a key role in most manufacturing processes by recirculating air, ventilating hazardous gases, and cooling machinery. One problem that can develop in all of these applications is imbalance. Since imbalance is a potentially dangerous condition and can result in breakdowns and costly plant shutdowns, it is imperative that plant engineers understand what it is, how to detect it, what causes it, and how it can be addressed.
Detecting imbalance early can save large amounts of money. The less damage, the less the cost of repair. In some cases, imbalance results from an improper manufacturing process.
It is important to clarify the difference between "imbalance" and "vibration." A fan rotor generally consists of a welded, riveted, or cast fan impeller mounted on a shaft. Even if the manufacturer takes care in locating blades and weighing component parts, the weight center is separated from the axis of rotation. This difference between the weight center and axis of rotation is referred to as "imbalance."
Imbalance is not a function of rotational speed and therefore can be assessed and measured when the fan is not in operation. Imbalance can be quantified by multiplying the weight of the fan rotor by the radial distance between the weight center and axis of rotation (Fig. 1).
Vibration occurs during fan operation and may
have many causes, one of which could be imbalance. Other causes of vibration include mechanical looseness, coupling misalignment, defective bearings, insufficient flatness of bearing mounting surfaces, rotor cracks, driver vibration, and V-belt slippage.
Buildup on blades
Imbalance may be the result of a manufacturing process or operating conditions. For example, many fan rotors are used on wet scrubber systems where wet and sticky particulate matter sometimes passes through and adheres to the surfaces of the fan impeller. Usually this buildup of particulate matter is evenly distributed over all surfaces and the resulting imbalance is minimal. However, if a piece of the built-up material flies off due to centrifugal force, then significant imbalance can occur.
In some cases, backward-curved fan blades have proven effective in controlling buildup, particularly on preheater ID fans...