Fan repair blows replacement away

Traditionally, as fans and blowers in process equipment would break down, replacement used to be standard practice, especially for severely damaged or worn equipment. But that may not be the best case for manufacturers today. By simply replacing a fan or blower, end users may lose an opportunity to enhance the quality of their equipment.

Fan repair specialists have an advantage over original fan designers. They can observe the performance history of an existing fan and rework its design to prevent a particular type of wear, corrosion or other problem. Analysis allows repair specialists to select the most appropriate metals or exotic alloys for an application and make adjustments to enhance equipment life. In many cases, the quality of the repaired part or component will exceed that of the original.

Determine whether to repair or replace

Fans and blowers will inevitably need to be replaced or repaired, even with the best maintenance in place. Plant engineers also have the option of rebuilding, an extensive form of repair. What may look to be an irreparable fan destined for the scrap heap may actually be able to be rebuilt to look and function like new (Fig. 1). Even fans with worn-through blades or other components can be rebuilt and improved to reduce wear in the same locations.

Rebuilding allows specialists to improve efficiency or strength and reduce a fan’s vibration and sensitivity to imbalance. A design modification such as reducing rotor inertia can shorten start-up time and lessen stress on the motor. If cracking has been a problem, specialists can adjust material thickness and may incorporate FEA stress analysis to confirm the fan wheel design is structurally robust and will last a long time.

In redesigning a fan, specialists may address three common causes of damage: wear, corrosion and temperature.

Wear, corrosion and temperature

Wear, or erosion, is sometimes evident when some fan parts or components have given way or have been unable to perform. A fan repair specialist can determine the variables responsible for wear and suggest a redesign when appropriate. In some cases, just the shaft and hub — and, possibly, part of the wheel — can be preserved while the rest of the fan is rebuilt.

Blades or blade liners that aren’t hard enough to resist erosion from a dirty air stream are bound to wear. Numerous new alloys, steels and other wear-resistant materials may help. For example, chromium-carbide or tungsten-carbide blade liners are cost-effective solutions for reducing wear. In severe instances of wear, repair specialists may use computational fluid dynamics (CFD) to simulate the interaction of fluids and gases with fan components. A CFD analysis can provide accurate data to improve wear resistance.

Fan corrosion may be the result of a change in the gas stream’s composition since the fan was purchased and installed. Or, perhaps the fan was not equipped to handle the gas stream in the first place. Repair specialists may begin with a gas-stream analysis to identify variables responsible for corrosion. New stainless steels and exotic alloys that can handle potentially corrosive situations may be the answer. Fan specialists can test these materials under simulated conditions to select the best one for an application.

Temperature can also be a critical factor in fan failure. Steels and alloys can last only so long at temperatures above their design limitations. If materials used in a fan’s original design did not last as long as they should have, alternatives can be used for the repaired or rebuilt fan.

One of the most common points of breakdown in high-temperature applications are anti-friction bearings. Causes for bearing breakdown include:

• Low inherent L10 life: The calculated life of a bearing, or L10 life, is the number of operating hours a group of apparently identical bearings will exceed under a given set of conditions; it is a function of radial load, axial load, speed and bearing basic load rating

• Heat buildup and improper lubrication: Excessive temperatures can damage bearing components and cause breakdowns in lubricant. Heat conduction can be reduced by using materials with low thermal conductivity, adding a heat flinger or installing a radiation shield.

• Broken down or worn fans or blowers may have a greater useful life than most think. Analyzing the precise cause of a fan’s degeneration can provide long-term capital expenditure savings. The cost savings realized by repairing or rebuilding an existing fan can range from 20% to 80%.

  Before rushing out to order a new fan, consider the benefits of repairing or rebuilding the existing unit. Replacing an underperforming fan with another just like it will be a temporary fix at best. The sensible approach is to assess the cause of the fan’s degeneration; then use that information to either repair it or replace it with an improved model. In many cases, repair will be the more economic solution for both the short- and long-term.

  Common heat sources that affect bearings include heat conducted through the shaft, radiation from the fan casing, self-generated heat due to friction and hot ambient air.

  Even fans with excessive wear or corrosion (top) may be able to be rebuilt to look and function like new (bottom).

  Author Information

  Thomas J. Kuli is chief engineer, and Tim Rape is repair/rebuild manager, both for Robinson Industries Inc.

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