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.
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.
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