Even under the best maintenance program, fans and blowers inevitably will need to be replaced or repaired. Plant engineers also have the option of rebuilding, which is an extensive form of repair.
Replacement used to be standard practice, especially for severely damaged or worn equipment. However, by simply replacing a fan or blower, you may lose an opportunity to enhance the quality of your equipment. Fan repair specialists have the advantage over the original designers of being able to observe the performance history of an existing fan over time. Repair specialists can rework the fan design to prevent specific types of wear or corrosion, or other problems. Analyzing performance data 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 can exceed that of the original. In addition, the cost savings can range from 20 to 80 percent.
Even fans with worn-through blades or other faulty components can be rebuilt and improved to reduce wear in the same locations. Rebuilding allows engineers to improve efficiency or strength and reduce a fan’s vibration and sensitivity to imbalance. A design modification like reducing rotor inertia can shorten startup time and lessen stress on the motor. If cracking has been a problem, engineers can adjust material thickness and may include finite element analysis (FEA) stress testing as needed to confirm the fan wheel design is structurally robust and will last a long time.
Given all that, 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. When redesigning a fan, engineers address three common causes of damage: wear, corrosion and temperature.
WearAlso called erosion, wear typically is described as fan parts or components that have given way or become unable to perform over time. A fan repair professional will determine the variables responsible for wear and suggest a redesign when appropriate. In some cases, just the fan’s shaft and hub -- and, possibly, part of the wheel -- will be preserved while the rest of the fan is rebuilt.
In severe instances, repair engineers may use computational fluid dynamics (CFD) software analysis to simulate the interaction of fluids and gases with the fan components. A CFD analysis will give engineers accurate data to improve wear resistance.
Blades or blade liners that are not hard enough to resist erosion from a dirty airstream are likely to wear. Numerous new alloys, steels and other wear-resistant materials may help in this scenario. One way to reduce wear due to a dirty airstream is to use chromium carbide or tungsten carbide blade liners.
CorrosionFan 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.
To repair issues relating to corrosion, fan professionals may begin with a gas-stream analysis to identify the variables responsible for corrosion. New stainless steels and exotic alloys that can handle potentially corrosive situations may be the answer. In addition to reviewing data for these materials, fan experts may test them under simulated conditions to select the best material for an application.
TemperatureTemperature can be a critical factor in fan failure. Steels and alloys can last only so long at temperatures above 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. Bearing breakdown can be caused by the inherent characteristics of the bearing as well as operating conditions.
Low Inherent L10 Life. The calculated life of a bearing, or L10 life, is defined as 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 the lubricant. For this reason, some fan manufacturers recommend that operating temperatures for anti-friction bearings not exceed 200°F (93°C).
Figure 1 shows four common heat sources that affect bearings:
- Heat conducted through the shaft.
- Radiation from the fan casing.
- Self-generated heat due to friction.
- Hot ambient air.
Analyzing the precise cause of a fan’s degeneration can provide long-term capital expenditure savings. Replacing an underperforming fan with another just like it will be a temporary fix at best. A sensible approach is to assess the cause of the fan’s degeneration and use the information to repair or replace the fan with an improved design. In many cases, repair may be the more economic solution in the short- and long-term.