Bearings usually require high temperature lubricants and sometimes circulating oil or static oil in a monoblock. Bearings can be kept cool in high temperature applications by using a shaft cooler and a shaft seal. For higher temperature fans, water cooling can be used to cool the shaft. In this case, the fan should be belt driven in order to install a rotating union to circulate the water over the shaft. Bearings should be kept out of a high temperature airstream and should never run in air temperatures hotter than 130F. The inboard bearing, which is the bearing nearest the fan housing, should be moved away from the fan housing to create some space for the heat to dissipate. The space created between the inboard bearing and the fan housing allows for the use of a shaft cooler and shaft seal. Bearing temperatures should be monitored and should not be allowed to exceed 200F at speeds higher than 2500 RPM and 220F at speeds below 2500 RPM. Bearings should be selected to allow for free movement of the shaft lengthwise due to temperature changes. One bearing should be fixed, serving as an anchor bearing to locate the shaft lengthwise. The bearing closest to the drive end is normally fixed. All other bearings should be expansion bearings to permit the shaft to move lengthwise. The thermal growth of the shaft may limit the bearing used.
A shaft cooler is a small fan that clamps on the shaft between the fan housing and the inboard bearing. The shaft cooler draws cool air over the shaft and bearings as the shaft rotates to dissipate the heat. A shaft cooler is recommended for all applications over 300F. Generally, the pedestal must be modified to allow room for the shaft cooler. This will increase the distance between the inboard bearing and the wheel. This dimension, referred to as the overhang dimension, is critical in determining the safe speed of the shaft. As the overhang dimension is increased, the shaft safe speed becomes lower. During maintenance repairs, it is very important that the overhang dimension is never increased. When the temperature exceeds 800F, it is necessary to separate the fan pedestal from the fan housing. By doing this, the amount of heat conducted through the pedestal and to the bearings and motor is greatly reduced.
For housing material, carbon steels are satisfactory for temperatures up to 800F and Corten is often used for temperatures up to 1000F. For higher temperatures, stainless steels and Inconel may be required. A customer may want to keep the heat of the airstream from heating up the fan housing and radiating out to the surrounding area. Insulating the fan accomplishes this. There a few ways to insulate a fan, but the main idea is to create a second fan housing offset from the main fan housing and then fill the gap between the two with insulation. It is common to have 2″ – 6″ of insulation depending on the temperature of the airstream and the need for the outer skin to be cool. For fans that require an access door and also have an insulated housing, the access door is raised from the fan housing to the outer insulation skin by building a box between the two housings.