Date: 2020-10-01 15:04:00
In a high-temperature manufacturing environment, failure of a fan can have a devastating effect on operations. To specify a fan for maximum service life, it is important to understand factors influencing fan design and construction.
Note: This article appears in the 2020 edition of AMCA inmotion magazine.
By AARON SALDANHA
Whether it is optimizing the mix and temperature of a chemical composition in a petrochemical plant, regulating process conditions in a metal furnace, or simply circulating hot air through an industrial bakery oven, fans play a critical role in many manufacturing operations. It should come as no surprise, then, that failure of these fans can have dire consequences. This is especially true with high-temperature applications, given that, when a fan fails, the system typically needs to be shut down. Once a replacement fan is installed, the system may take a few days to reach its optimum temperature, resulting in significant productivity loss.
The key to ensuring proper selection and construction of a fan for a high-temperature environment is good communication between the consulting engineer or end user and the manufacturer by way of the specification. This article will discuss important considerations that go into the selection and construction of a fan for a high-temperature environment.
There is no formal definition of high-temperature fan. Most industrial-fan manufacturers, however, consider a high-temperature fan to be a fan capable of withstanding operating air-stream temperatures 250°F (approximately 120°C) and higher. Air-stream temperature is the temperature of air/gas inside of a fan; it is different from ambient temperature, or the temperature of the air surrounding the motor, bearings, and external accessories of a fan (Figure 1). It is important to understand how these two temperatures impact the design of a fan.
High-Temperature Air Streams, Continuous Operation
A high-temperature fan can fail in a number of ways.
Wheel failure. Wheel failure can be evidenced by not only breakage, but distortion. One of the main contributing factors is thermal creep. Fan manufacturers account for thermal creep through proper selection of the materials of construction of air-stream components (Table 1).
For temperatures greater than 1,550°F (843°C), many manufacturers use special casting alloys to reduce welding and mitigate issues caused by thermal creep. Often, this increases both costs and lead time.
Though shrouded wheels with airfoil, backward-curved, or backward-inclined blades are the most efficient, they typically have a maximum-temperature capability of 900°F (482°C). For higher temperatures, forward-curved (Figure 2), propeller (Figure 3), or radial-blade (Figure 4) fans normally are recommended, depending on the application. As temperature increases, the maximum speed of these fans often is derated. If you plan to increase operating temperature or fan speed, it is important to check with the fan manufacturer to avoid wheel or shaft failure.
Shaft failure. As with wheels, the most common mode of failure with shafts is bending or breaking. Shaft materials of construction vary based on temperature, fan speed, fan arrangement, and load. The method by which a shaft is cooled also is important. The most commonly used types of shafts are:
- Solid shafts with heat slingers, for temperatures up to 900°F (482°C) (Figure 5).
- Air-cooled shafts with heat slingers, for temperatures from 900°F to 1,850°F (482°C to 1,010°C) (Figure 6).
- Water-cooled shafts with heat slingers, for temperatures above 1,850°F (above 1,010°C) (Figure 7).
Often, the bending of a shaft will cause bearing failure and vibration before the shaft fails completely.
Bearing failure. Depending on the load and fan speed, the selection of bearings for high-temperature fans is similar to that for traditional fans—that is, self-aligning ball bearings or spherical roller bearings. The main difference is that, in the case of most high-temperature fans, bearings with C3 internal clearance and appropriate lubrication are used. Lubrication quantity and type are dependent on bearing temperature. If bearing temperature exceeds a lubricant’s rated temperature, the lubricant will lose viscosity and cause bearing failure. When replacing lubricant, check with the fan or bearing vendor.
Drive arrangement. ANSI/AMCA Standard 99, Standards Handbook, prescribes drive configurations for different types of fans. With high-temperature fans, the bearings must be outside of the air stream. For these fans, then, Drive Arrangement 1 (two bearings mounted on a pedestal and the wheel overhung to one side), 2 (similar to Arrangement 1, except the bearing pedestal is supported by the fan housing), 8 (similar to Arrangement 1, but with a smaller “outrigger” motor or turbine base connected to the bearing pedestal), or 9 (similar to Arrangement 1, but with the motor mounted on the fan, rather than on the “floor”) typically is recommended. For double-width fans or applications requiring Drive Arrangement 3 (a bearing bracket-mounted on each side of the housing or wheel), an inlet box with the bearings outside of the air stream is recommended (Figure 8). The extended shaft length needed to account for the inlet boxes often presents a design challenge, however.
Drive Arrangement 4 (the wheel directly mounted on the motor’s shaft and bearings) is restricted by the maximum allowable temperature at the motor shaft, which usually corresponds to an air-stream temperature of 250°F (approximately 120°C). Some manufacturers use a custom motor to achieve air-stream temperatures of 450°F (approximately 230°C) for a Drive Arrangement 4 fan. For certain high-temperature applications, such as tunnel ventilation, axial fans with the motor in the air stream are used. These are rated for use for only a short period (i.e., 1.5 to 2 hr), however. This will be covered further in the “Applications of High-Temperature Fans” section.
Housing design. Expansion of housing material needs to be accounted for in housing design. With centrifugal fans, this is particularly important in defining the axial and radial gap between wheel and inlet. With axial fans, tip gap (Figure 9) is increased to account for thermal growth attributed to elevated temperature. When these fans are run at standard temperature, their efficiency will be lower, as the higher the tip gap, the lower the efficiency.
Housing insulation. Fan-housing insulation requirements come down to a few considerations:
- Surface temperature—The surface temperature of a fan is dependent on the temperature of the air stream and the amount of heat that dissipates through the surface of the fan housing.
- Location—If a fan is located near workers, insulation often will be required to prevent burns. Apart from that, heat dissipating from the fan’s surface can cause a rise in ambient temperature, which might make working conditions uncomfortable. This is especially true when multiple high-temperature fans are present.
- System efficiency—Heat dissipating through the surface of a fan’s housing can cause a reduction in system temperature and, thus, a reduction in system efficiency, depending on the application. Insulating the housing will reduce this impact.
There is more than one way to insulate a fan housing. The most robust, but also most expensive, is double jacketing (Figure 10). Double jacketing involves building a second housing around a fan with insulating material (ceramic fiber for temperatures greater than 1,000°F [537°C]) between the two housings. The other method of insulating a fan housing is to ask the fan vendor to provide insulation pins (Figure 11) on the housing so that an insulation jacket or insulating material can be put onto the housing at the building site. It is important to also request that the fan vendor add an extended inlet funnel, a raised access door, and an extended drain pipe, if applicable, to the design to account for the insulation that will be added at the building site.
Sealing/semi-gastight construction. Although, depending on their size and method of construction, many fans cannot be made 100-percent gastight, precautions can be taken during design to reduce the impact of hot-gas leakage. For higher temperatures, use of a mechanical shaft seal is recommended. The body and internal components of the seal must be designed to handle the air-stream temperature. In many cases, a carbon ring seal with nitrogen purge (Figure 12) is used not just to improve sealing efficiency, but to cool the seal.
When using a gasket on a high-temperature fan, ensure the gasket is rated for the air-stream temperature. This is especially important when replacing gaskets during fan maintenance.
Motors and VFDs. In many cases, there is a power-saving advantage to using a variable-frequency drive (VFD) on a high-temperature fan. Consider, for example, a requirement to move 33,000 cfm (15.57 m3/s) of air with 10.2 in. w.c. (2,538 Pa) of static pressure at an air-stream temperature of 450°F (232°C), which corresponds to an operating density of 0.044 lb/ft3 (0.705 kg/m3). At this operating condition and density, a 40.2-in.- (1,021 mm) diameter centrifugal fan operating at 1,770 RPM would absorb 67 hp (50 kW) of power and require a 75-hp (55 kW) motor if used with a VFD. The fan would start at approximately 30 Hz when the system is at ambient temperature (70°F/20°C) and slowly speed up as the system temperature increased. At the rated operating temperature, the fan speed would be the rated speed of 1,770 RPM (Figure 13). If a VFD were not used, the power needed to start the fan when the system is at ambient temperature would be 114 hp (85 kW), and a 125-hp (93 kW) motor would be required. Apart from the motor cost, the owner would need to invest in panels/cables rated for 125 hp, as opposed to 75 hp. In many cases, this would be more expensive than a VFD. That being the case, it always is beneficial to do a cost-benefit analysis when investing in a high-temperature fan.
High-Temperature Ambient Conditions, Continuous Operation
As mentioned earlier, the temperature of the air surrounding motors, bearings, belts (drive arrangements 1, 3, 9, and 10), and all other outside-the-air-stream accessories requires precautions.
Motors. The temperature of the air surrounding a totally enclosed, fan-cooled motor should not exceed the temperature for which the motor was rated. If it does, the motor’s temperature can be lowered through the use of a water jacket or an external cooling system. Ensure the motor’s bearings and lubrication are designed for the temperature requirement.
Bearings. In many high-temperature applications, an oil-circulation system is used to ensure bearing temperatures are maintained (Figure 14).
Applications of High-Temperature Fans
Depending on a fan’s intended use, the amount of time the fan can be expected to be exposed to high-temperature air can vary. Thus, a fan can be rated for a short, defined duration or it can be rated for continuous operation.
Take, for example, main ventilation fans and jet fans used in a roadway or transit tunnel (Figure 15). Because their purpose is to evacuate air in the event of a fire, these fans are rated for a short duration, normally 1 or 2 hr. Most other fans, such as those for petrochemical processing (Figure 16) and those for forced circulation or recirculation of air or gas in furnaces, ovens, kilns, and dryers, are rated for continuous operation.
Most high-temperature applications have unique requirements that can make fan specification complicated. The key is identifying the system information the fan manufacturer needs to design and supply a product optimized for the application.
About the Author
As director of product management, fans, for Howden, Aaron Saldanha is responsible for driving growth strategy through product development and optimization based on customer needs in the Americas. He has more than 11 years of experience in the fan industry in the United States and India, with roles including engineering/research and development, testing, and sales. He has a bachelor’s degree in mechanical engineering from the College of Engineering, Anna University, India, and a master’s degree in engineering from RMIT University, Australia. He is a member of the technical committees for AMCA Publication 211, Certified Ratings Program Product Rating Manual for Fan Air Performance; ANSI/AMCA Standard 250, Laboratory Methods of Testing Jet Tunnel Fans for Performance; and the in-development AMCA Standard 214, Model Fan Efficiency Regulation for Stand-Alone Commercial and Industrial Fans.