In addition to the general construction material of the fan, other considerations can be made to increase the limiting temperature. One of the most critical components of the fan are the bearings. So naturally if the bearings are kept out of the hot airstream they are less susceptible to the effects of the high temperature, and therefore the air temperature can be higher than the limiting temperature of the bearings. Inline fans inherently have drive components in the airstream. In such cases, the unit can sometimes be designed so that cooler outside air is drawn over the drive components, providing some additional cooling. Incorporating this type of external cooling is one of the main reasons GreenheckÕs medium pressure tube axial inline fan (TBI) can be used for high temperature exhaust applications. There are, however, some arrangements that donÕt allow for the incorporation and benefits of external cooling. For instance, fans in arrangement 3 have the wheel suspended in-between the two bearings. Consequently, one or both of the bearings will always be located in the air stream. Similarly, direct drive fans are limited by the maximum operating temperature of the motor, since the motor is located directly in the hot air stream. That is why most often high temperature exhaust fans are required to be belt driven units; typically the motor is the temperature-limiting component of the unit when it is installed in the airstream. Other than the installation location of the bearing within the fan assembly, other critical points to consider include the type of lubrication in the bearing, as well as the construction of the bearing itself. There is a seemingly endless list of lubricants available for most any application, but the appropriate lubricant can add considerable life to bearing operation. In addition, high temperature bearings can be constructed with inherent construction modifications, such as a larger grease cavity to hold more lubricant.
Another common variation to the standard fan design is the addition of insulation. The bearing mounting plate, bearing cover and belt tube are the most likely components to be insulated for a high temperature application. Canada Blower automatically includes insulation for these components on all fans specified with either HT Option III or HT Option IV.
It is important to understand that emergency smoke exhaust fans are designed to operate effectively for the temperature and time limits stated as long as the power supply to the fan is not terminated. Since they are not designed to sustain higher temperatures for continuous use, exposure to the extreme high temperatures caused by a fire likely would destroy some or all parts of the fan once the time design limit has elapsed, rendering it incapable of future operation.
Fans that are built for continuous high temperature ventilation have many of the same construction features and options as those built for emergency smoke exhaust. For example, both are typically constructed of higher temperature ferrous materials, have high temperature bearings, have the motor and drive installed either out of the airstream or with some type of additional cooling capabilities, and incorporate some heat shielding or insulation. There are, however, some distinct differences between the fan designs for the two application categories. One of the most common applications for which continuous duty high temperature exhaust fans are specified is commercial kitchen ventilation.
Fans that are specified for continuous operation at higher temperatures need to be carefully selected taking into account air density corrections. Although a fan will move the same amount of air at two different temperatures, the static pressure and horsepower requirements can change drastically as the density of the air varies with changing temperatures. For example, if a fan moves 3,000 CFM at 70 F it will also move 3,000 cfm at 500 F. However, since the hotter air weighs much less than the 70¼F air, the fan will require less brake horsepower and will create less static pressure.
There are also some accessories and construction options that are incorporated into high temperature process fans to increase the limiting temperature. For example, a heat slinger can be installed on the shaft between the fan housing and the inboard bearing. A heat slinger is an aluminum disc that absorbs and dissipates some of the heat conducted along the shaft before it travels to the bearings. Many heat slingers incorporate fans that promote additional inboard bearing cooling because it circulates air with every rotation of the shaft.
Another common construction technique is the use of a heat gap. A heat gap is a physical gap between the bearing support structure and the hot fan housing. This separation slows the process of heat conduction to the bearings and drive components through the housing and support structure. An expansion bearing should be installed where the fan shaft is predicted to conduct enough heat that it could expand to a length greater than that of the structure to which it is secured. This allows for expansion of the shaft within the bearing due to the higher temperatures.
High temperature applications can be found in many different areas of the air movement and control industry. It is important to remember that one high temperature fan design is not suited to all applications.
HVAC Ventilation Engineer
Canada Blower Co.
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HVAC Ventilation Engineer
Canada Blower Co.
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