For industrial fans applications, it is necessary to consider high cycle fatigue, low cycle fatigue in cyclical operation with speed changes, and numerous start-stop cycles. If rapid temperature changes are encountered, transient analysis may be required due to thermal stresses. When fans are exposed to elevated temperatures, many aspects of the fan design must be reviewed to make sure the fan will withstand the extreme temperatures and that the fan is appropriately designed for the application. Depending on the exposed temperature levels, the following are to be considered:
� Materials of construction for the wheel, shaft, and hub
� Wheel to hub attachment
� Materials of construction for the housing including insulation
� Bearings and lubrication
� Shaft seals
� Shaft cooling (air-cooled, water-cooled, etc.)
Limits on the operating temperature for standard fans are defined by the arrangement of the fan. For example, the maximum temperature for a standard fan that does not have a bearing in the airstream can run up to 300F. For standard fans that have a bearing in the airstream, the maximum operating temperature is limited to the maximum temperature that the bearing can handle, which is typically about 130F.
Carbon steel, low alloy high strength steels, and chrome moly steels are satisfactory for a temperature range of 900 to 1000F, depending on stress levels. Heat resistant alloys such as stainless steels (304, 316, 347), Incalloy 800HT, 230, Inconel (600, 625, 617), Multimet, Haynes 25, etc. should be used for higher temperatures up to 1800F. Again, the material chosen also depends on the stress levels and the material properties at elevated temperatures. Temperature derates lower the maximum speed of a wheel to account for the lower material strength at elevated temperatures. The wheel must sustain the highest stress of all the fan components. It is very important that the material used on the wheel is strong enough to handle the stress at high temperatures. At higher temperatures, material eep must be considered.