Air-Cooled Fans Reduce Maintenance Issues
High-temperature fans used in industrial thermal-processing furnaces basically provide a mechanical means to move atmosphere gases around inside the enclosed space at operating temperatures higher than about 1200 F (650 C). Fans used in heat-treating furnaces contribute to good uniformity of heating throughout the workload by convection and mix and circulate the special atmosphere around the work being processed. The working atmosphere in a carburizing furnace is continually flushed to remove water formed by combining with oxygen by adding natural gas, for example, to maintain carbon potential. This requires a dynamic atmosphere condition that is facilitated by the rotating fan element.
High-temperature fan design
Furnace fans look similar to some common fans seen outside of furnaces. Two general categories of fans are axial and radial/paddle type. There are many fan-blade designs, some of which can be seen on the Illinois Gas Equipment website www.igefans.com, a major manufacturer of high-temperature fans.
Axial fans, which look similar to an airplane propeller, are not suitable to build up pressure, and, therefore, are not the preferred fan for driving gases through ductwork. By comparison, paddle-type fans create a higher static pressure and can more readily overcome pressure losses traveling over a distance. Therefore, with the proper shrouding, paddle fans can duct gases from one part of a furnace to another. Both fans work almost equally as well in open box applications where simple stirring is being performed.
The selection of the material of construction for a fan used in high-temperature applications is crucial for satisfactory fan service life. Elevated temperatures and the dynamic nature of fans limits material selection to those having sufficient high-temperature tensile strength and creep resistance, such as the cast and wrought fabricated heat-resistant nickel-base alloys.
At high temperatures, metals are subject to a marked drop off in tensile strength, as well as creep deformation. AHTF uses a design parameter of 1% creep in 10,000 hours at a stress range of 300-700 psi for the heat-resistant metal alloys used in its fans.
The creep resistance of the material has a significant influence on determining fan-shaft diameters and blade thickness. Blade thickness also has an impact on overall fan element weight, which directly affects what diameter a shaft should be. Creep is the factor that results in fan designs having 2 to 3 in. (50 to 75 mm) diameter shafts. The weight of the blades in horizontal or vertical orientation at room temperature is irrelevant, but is a significant factor at high operating temperatures to 1700 F (930 C) or above, where they are subjected to creep deformation. A fan rotating at 900 to 1500 rpm results in a centrifugal force on the blades of 200 to 800 lbf (890 to 3,560 N). A material having insufficient creep strength can be subject to a quick failure once the fan is at operating temperature. Creep deformation also is the reason why a fan should not be shut off until it is almost at room temperature. In a vertical installation, the blades will bend, and in a horizontal installation, the shaft will bend if the fan is turned off for a length of time. Generally, compromises in design, such as larger cross sections, are made to compensate for the problems associated with lower high-temperature strength and creep resistance.
While the fan is designed to operate at the high operating temperatures in the furnace, the fan bearing generally cannot handle temperatures greater than about 400 F (205 C). The large diameter shaft functions as a significant heat sink and can transfer much more heat to the outside of the furnace shell than the furnace refractory, which can be sufficient to start a fire outside the furnace.
Traditionally, this problem is solved by using a cooling jacket around part of the shaft. The lower bearing also can be mounted to the cooling jacket. The excess heat is carried away by means of some medium and is dissipated using an external cooling apparatus.
High temperature fans of this design-incorporating a proper style of blade, proper alloy selection and provisions for cooling the lower bearing-essentially have been unchanged for the past 50 years.
High-temperature fans eventually deteriorate due to creep deformation, cracking due to thermal cycling and carburization, and other factors. However, service life can be prolonged by minimizing these problems through proper maintenance including:
- Following the recommended manufacturers practice on shutdown
Rotating the fan at the slowest speed that will provide the desired quality and heating uniformity
- Checking the fan balance to correct in both planes via a reliable company that uses modern digital equipment that has been calibrated in a traceable manner.
The reason that proper shutdown is required was explained above. Experience shows that using slower (as is practical) fan speeds is a factor in extending fan service life. An improperly balanced fan suffers from the same creep problems as fans subjected to incorrect shutdown procedures. Some fan mounts are made of very large castings, the mass of which can hide significant imbalance problems that reduce fan life.
Fans can fail in several spectacular ways. For example, a blade can crack off or the weld holding the cast blade to the shaft can fail resulting in a severely unbalanced fan, which can damage refractory and create other hazards. The fan eventually can work itself into such a severe state of unbalance that it can no longer be used.
Fans that incorporate water-cooling jackets require significant maintenance and pose some risk. A particularly catastrophic failure can occur if the fan cooling-water jacket fails. If the cooling-water jacket starts to leak, it will become progressively more difficult to maintain carbon potential inside the furnace. Eventually, the furnace must be shut down to replace the problematic fan. In certain cases, the water leak can flood the furnace while the furnace is coming down in temperature (because the fan should be left rotating with the flow of cooling water continuing for one day or more depending on how long it takes to cool down the furnace). Some manufacturing facilities have critical furnaces that cannot be off line for any significant period of time, and a catastrophic water leak can sideline a furnace for weeks or even months. Water-cooling jackets also can plug up with sediment. In a closed loop system, holding tanks and cooling towers present additional maintenance issues.
Other cooling methods, such as using quench oil instead of water, can result in a fire hazard in the case of a leaky cooling block, and cooling using nitrogen and compressed air systems have significant expenses associated with them.
Most users monitor the condition of their fans and make replacements prior to failure, but this approach also is dictated by furnace downtime availability and budget restraints.
An alternative design approach
American Heat Treat Furnaces Corp. developed a new fan design (U.S. Patent #6,454,530) to solve several engineering problems of traditional designs. Specifically, while a thicker fan shaft is required to deal with creep deformation problems in the red heat zone, once you travel 6 inches (about 150 mm) into the refractory, creep is no longer an issue. Therefore, you essentially create a thermal discontinuity in the shaft by reducing the shaft diameter where creep is not a problem. This thermal discontinuity blocks a significant portion of the conducted heat inside the furnace, which allows for the use of standard bearings outside the furnace without the need for a cooling jacket.
This design requires no active cooling-no space-age bearings or lubricants. No active cooling means no water, oil, hydrogen, compressed air, combustion air, etc. It is a passive cooling method, so the fan does not need to be rotating for the method to function. The elimination of water jackets not only prevents the problem with water leaks, but also eliminates the need for piping, water towers, water dumping permits, etc. This method blocks the heat in side the furnace compared with competing methods that use heat slingers to dissipate the heat outside the furnace.
This simplified design uses lighter wrought interlocked welded fan elements and has no mount castings. Wrought heat-resistant alloys can be used in thinner, lighter weight cross sections than cast materials in fans. In addition, slots and tabs are readily cut out via plasma torch, allowing for a mechanically interlocked and welded lightweight, durable fan assembly. Bearings are standard and can be replaced while the furnace is hot. Lifting eyes are incorporated for both vertical and horizontal lifting.
The design has been incorporated into standard plug fans dimension and can be used to replace existing fans. Pit-furnace and static roof-mount fans also can be replaced. Using a standard template method for shafts and fabricated blades, lead times are in the four-week range. It also is possible to fabricate equivalent replacements to existing fans.