HVAC System

With root cause analysis and better materials, fans can excel

Combine root cause analysis with better materials and you get a fan that is a pleasure to watch

By Thomas J. Kuli and Tim Rape





When a fan or blower fails or wears excessively, users can either replace it or repair it. In the past, replacement was the preferred option, especially in cases of severe damage. Now, however, economics favor repairing or rebuilding it. Even when the cost exceeds 60% of the price of a new fan, many choose the repair option, where savings can range from 20% to 80%.

In many cases, the quality of the repaired component can exceed the quality of the original because repair engineers have an advantage over the original designers. They can see how the fan performed , or failed to perform , in a specific application. Based on the condition of the fan and its performance history, fan engineers can rework the design to prevent that type of wear, corrosion or other problem from recurring. Repair engineers are free to choose the most appropriate metal or alloy for the application.

Inspect regularly
Control repair cost by keeping up with needed maintenance. The importance of regular inspection to document wear can't be stressed enough. The earlier a fan repair expert can address a repair or design problem, the lower the cost. Here are a few reminders about regular inspections, maintenance and documentation.

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Perform routine in-house visual inspections on fan wheels to ensure that erosion hasn't reduced metal thickness to a dangerous degree, that stress hasn't cracked any components, and that coatings and liners are still in good condition. Monitor airflow, pressure, air stream temperature and power consumption for changes that could be early warning signals.

Monitor bearing temperature and vibration continuously. Inspect motors, V-belts, expansion joints and dampers every four to six months. Include coupling alignment as part of a regular maintenance program.

Balance as needed
Field service work commonly includes fan rotor balancing to reduce vibration. A balancing technician using a portable dynamic balancing unit usually can do the work without dismantling the rotor. Because fan balance isn't expected to change under normal operating conditions, investigate carefully the cause for any unbalance you find. In more serious cases, the contractor can provide shop-inspection and shop-balancing (photo above). The Air Movement and Control Association (AMCA) publishes AMCA Standard 204-96, which provides recommended fan bearing vibration levels for many fan applications.

Several phenomena can unbalance a fan rotor. Normally, the buildup of particulate matter on the wheel is evenly distributed. Imbalance results when a piece dislodges.

A temperature differential between the top and bottom of an idle fan produces differential thermal expansion, which can cause high vibration at start-up. Stopping to add correction weights can compound the vibration problem because the fan eventually returns to a uniform temperature.

Damaged parts, such as a bent shaft, a cracked or missing section of a blade or vane, non-uniform erosion or corrosion, or a dislodged balance weight can produce imbalance.

Hub-to-shaft connections can loosen. If the fan is held in place by setscrews, over time, wear or corrosion may loosen the fasteners, resulting in movement between hub and shaft. The best hub-to-shaft connection is an interference fit or some type of taper.

An unbalanced air foil blade is another source of imbalance. Some centrifugal fans have hollow airfoil blades, a design offering maximum efficiency in clean operating systems. Extended operation in wet or dirty environments, however, can produce pinholes in the skin, with dirt or fluid accumulating inside the blades. The result is a weight shift during start-up, which makes the fan nearly impossible to balance. Select solid blade shapes (backward curved, backward inclined, radial blades) for centrifugal fans operating in dirty or wet environments.


Figure 1. A 179-in.-diameter cement plant backward-curve fan undergoes a complete rehab and makeover before going back into service. Notice the wear pads that will help the wheel defy the erosion that destroyed its previous incarnation.

Repair or replace?
Even with the best maintenance, a fan or blower someday will need replacement or repair. Most industries prefer repair because of potential cost savings and an opportunity to have a better fan. Rebuilding , an extensive form of repair , also is an option. It may be possible to rebuild to like-new condition what appears to be a destroyed or unredeemable fan wheel. Even one whose blades have been worn through can be rebuilt, frequently with superior materials that reduce future wear (Figure 1).

Rebuilding presents an opportunity for design modifications that can improve strength and efficiency. Reworking can move the fan's critical speed farther away from the operating speed to reduce vibration and sensitivity to imbalance. Reducing rotor inertia can reduce both the time needed to bring the wheel to full speed and the stress on the power train. Material thickness can be adjusted to improve resistance to cracking.

Increase wear resistance
Wear occurs because the blade or blade liners aren't hard enough to resist erosion from a dirty air stream. Dozens of new alloys, steels and wear-resistant materials are available. Finding the right one for the application depends on cooperation among the plant engineer, fan engineer and the manufacturer of the metal or alloy. Chromium carbide liners, for instance, have proved to be a cost-effective means of reducing wear and extending wheel life. Blade liners, web liners, wear strips and bar liners are other options to combat erosion.

Fan repair begins with an analysis that determines which variables are responsible. The fan then can be redesigned and constructed to fit the application precisely. In some cases, only the shaft and hub (and, possibly, part of the wheel) will be preserved, while the rest of the fan is rebuilt. The fan engineer has a spectrum of design options, including radial, backward inclined, airfoil, radial tip, backward curved, forward curved or axial flow blades, to address operational problems. For instance, in certain applications, backward-curved fans can resist material buildup. Backward-curved fans should not be designed with too much curvature, however, because dust can accumulate in the pockets. Backward-curved centrifugal fan designs with steeply sloped blades minimize buildup of this type.

Stop corrosion
With corrosion, the repair professional begins, again, by identifying the variables responsible for the condition. An air-stream analysis is a good place to start. The air stream's composition may have changed in some way since the fan was installed. Or, the fan may have been ill equipped to handle the air stream from the outset. Manufacturers of new stainless steels and exotic alloys provide data concerning which materials can handle potentially corrosive situations. In addition, fan experts test these materials under simulated conditions. Still, there is no substitute for experience and documentation. Hence, the importance of regular inspections and cooperation among stakeholders.

Withstand temperatures
Steels and alloys last only so long at temperatures above their design limitations. If the original materials weren't appropriate for the application, the fan can be repaired or rebuilt using an alternative material of construction. In high-temperature applications, anti-friction bearings are vulnerable to heat from internal and external sources, improper lubrication and vibration.

L10 life is defined as the number of operating hours beyond which 90% of a group of apparently identical bearings will continue to operate. It's a function of radial load, axial load, speed and the bearing's basic load rating. L10 life should be 50,000 hours to 100,000 hours for bearings used in high-temperature fan applications. Anything less is a compromise.

Operating temperatures for anti-friction bearings should not exceed 200 degrees F to avoid degrading the lubricant and damaging bearing components, particularly the cages and seals. Fabricating the fan shaft from a material with low thermal conductivity minimizes heat conduction to the bearings. In addition, a heat flinger , a small centrifugal fan made of aluminum or some other material with high thermal conductivity , attached to the shaft between the fan casing and the bearing also will help to minimize heat conduction.

Insulating the fan casing and adding a radiation shield reduces radiant heating of the bearing. Improperly maintained or missing insulation is one of the more common problems in high-temperature applications. When temperatures inside the fan housing exceed 1,000 degrees F, proper insulation is imperative for maintaining proper bearing temperature.

The value of a precise analysis of a fan's degradation , whether from wear, corrosion or temperature , appears over the long term in capital expenditure savings. To replace a fan that has performed below par with another just like it, hoping for another short period of merely just-adequate performance doesn't make sense. The sensible approach , and appropriate approach from an engineering standpoint , is to assess the root cause and use the information to repair or replace the fan with an improved model. In many cases, repair proves the more economic solution in the short and long terms. 

Thomas J. Kuli is chief engineer and Tim Rape is repair/rebuild manager for Robinson Industries Inc., Zelienople, Pa. Contact them at (724) 452-6121.

Figures courtesy of Robinson Industries Inc.