High temperature process heat is a fundamental tool in Industrial America’s toolbox. While some operations are accomplished in a static air environment, many high temperature operations — heat treating and stress relieving, for instance — require convection to achieve the necessary results. In such cases, high temperature industrial fans are used to circulate heated air within the chamber to achieve predictable, close-tolerance results.
Regardless of the operating temperature, every industrial fan is subject to fatigue and, therefore, has a finite life. (Defined as the weakening of a material caused by repeatedly applied or cyclical loads, fatigue is progressive, localized structural damage.) The normal life of an industrial fan can be cut short by use in an improper application or inadequate maintenance, resulting in catastrophic failure.
High temperature fans are particularly susceptible to such catastrophic failures, causing unexpected downtime and interruption of the just-in-time supply chain. A fan failure can result in damages that are far in excess of the cost of the actual fan. Understanding the limitations of the fan, operating within the fan’s design limits and following a preventive maintenance plan are the best insurance against a catastrophic failure.
Avoid Fan Bearings Failures at High Temperatures
In many instances, a post-failure inspection cites bearing failure as the first cause of a catastrophic fan failure that occurs in a high temperature application. While technically, this may be correct, the full story sometimes points to other root causes.
Such was the case with one fan that had been in operation for many years. The fan was belt driven with the wheel in the overhung position. For investigation, the user presented the remains of the 49” diameter fan wheel, a 3.9375” diameter shaft and a bucket containing the pieces of both bearings.
The shaft was bent to a 20° angle. A range of temper colors easily were observed within 3” on either side of the inboard bearing race, which was seized to the shaft. Little more than the wheel hub was remaining on the end of the shaft.
Following an investigation, we concluded that the inboard bearing had suffered from insufficient lubrication. (This is a broad category that encompasses times when the bearing grease was insufficient in quantity, incorrect for the application, or had broken down due to overuse.) The result was that, after years of trouble-free operation, the bearing temperature quickly increased and heated the shaft to the point that it could no longer bear the weight of the overhung wheel. Fortunately, no personnel were injured. The fan was completely destroyed. Collateral damage was extensive, resulting in thousands of dollars of damage and lost production — all for the want of a few dollars’ worth of grease.
The lesson here is that a disciplined preventive maintenance program is a wise financial investment. Important factors to keep in mind include:
- Note and follow the fan manufacturer’s specification regarding bearing lubrication and schedule for relubrication.
- In some applications, high temperature grease is required. The schedule for relubrication for fans using high temperature grease often is more frequent than is common for industrial fans in low temperature applications.
- Bearing vibration and operating temperature are important clues to the health of the bearings.
Ideally, vibration and temperature detectors should be installed on each bearing. Regularly analyzing the data for trends can help identify degenerations in the bearing condition. If temperature and vibration detectors are not installed, the same data can be collected using hand-held devices.
Careful attention must be given to collect this data on a regular schedule. Efforts expended to monitor bearing health may not always extend bearing life, but they will allow a timely replacement of sick bearings during a planned outage.
Avoid Fan-Wheel Failure When Used at High Temperatures
Every fan wheel is selected for a specific running speed and operating temperature. It is designed to be operated within maximum speed and temperature limitations. Failure to specify a fan wheel while accounting for these specific conditions can lead to fan-wheel failure.
For instance, a customer purchased a replacement wheel for a fan installed on a stress-relief furnace. The original fan had been in service without incident for more than 15 years. Within 48 hours of installing the new wheel, the customer called to report that the new fan was experiencing high vibration. To deal with this, the fan wheel was rebalanced on-site and the furnace returned to service.
The following night, the third-shift foreman reported that reoccurring high fan vibration made it impossible to operate the furnace. To address the problem, the replacement fan wheel was returned to the factory for inspection.
Upon inspection, clear signs were evident that indicated the wheel had been operated at a temperature exceeding the 1350°F (732°C) design limit. The customer strongly disagreed and responded by sending us their furnace strip chart, which showed an acceptable operating temperature for the last 48 hours of service.
The evidence presented by the strip chart prompted further investigation. While unlikely, it was possible that an incorrect material had been used in the construction of the fan wheel. Laboratory analysis using spark spectroscopy to determine the wheel material proved that the materials of construction were appropriate for a 1450°F (788°C) fan.
The next step was to investigate the furnace further. A thorough examination of the temperature-control system in the furnace showed that the temperature-control probe in the furnace was encased in an insulating crust of baked-on grease and foundry dust. This crust caused the probe to misread the furnace temperature by several hundred degrees.
A new temperature probe and new wheel were installed. They have been in operation for more than five years without failure. A new preventive maintenance program includes a routine inspection of the fan wheel and the temperature probe.
Rate of Temperature Change and Thermal Shock
High temperature fans have the additional consideration of a maximum rate of temperature change (both increasing and decreasing). Typically, the maximum rate of temperature change is 15°F (8.3°C) per minute. (A higher rate of temperature change is often available with custom construction and materials.)
Take care to avoid thermal shock, which can cause development of stress cracking in the parent material of the wheel. This effect is unavoidable in certain circumstances like batch anneal operations, where the furnace door must be opened to remove the payload. Frequent inspection of the fan wheel is recommended. The fan wheel should be replaced upon the first indication of stress-related cracking.
On-Hand Replacement Parts in a High Temperature Installation
The high temperature fan is a critical component of an end user’s production cycle. Due to the custom nature of the high temperature industry, high temperature fans are usually made to order. The fan manufacturer will rarely stock replacement parts. For most end users, it is good business to retain spare fan parts in the warehouse.
The minimum recommended spare parts inventory for a high temperature fan is a wheel/shaft assembly and a spare set of bearings. For time-sensitive operations where a fan outage would interrupt a just-in-time process, a complete, service-ready warehouse spare fan is recommended.
With proper maintenance and attention to the manufacturer’s instructions, a high temperature fan can be expected to provide a long and healthy life.