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

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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).

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