Which is the best grease for fans working at high temperatures?
Industrial furnace or fume extraction fans handle gases at high temperatures. Due to a design more aimed at saving money rather than machine reliability, failures due to overheating of the bearings are common.
In fact, heat from the hot gases inside the fan is transmitted through the shaft to the bearings, resulting in a rise in bearing temperature. Grease working at high temperatures degrades quickly, to the point where it can lead to seizure within a few weeks of operation.
Usually after several bearing failures, someone in the company decides to contact the grease supplier to find a lubricant that can reduce shutdown.
High-temperature greases exist, of course, and they have not inconsiderable costs, but unfortunately they often do not eliminate failures.
Several solutions could prevent the heat from the flue gases from reaching the fan bearings.
For average fan operating temperatures, it may be sufficient to install a heat shield disc. This is a small aluminum disc installed on the fan shaft between the impeller and the first bearing. By rotating with the shaft, it allows heat from the impeller to be dispersed into the air and keeps the bearing temperature below 80°C.
If the fan works with fumes between 500°C and 650°C the heat shield disc alone is no longer sufficient to dissipate enough heat. A hollow shaft, inside which flows a flow of air attracted just by the heat shield disc, increases the heat removal from the shaft and thus preventing overheating of the bearing.
Above 650 °C necessary to switch to a water cooling inside the shaft to dissipate larger amounts of heat. The water can be disposable or-through a closed cooling system-recirculated.
Several years ago, we modified fans installed on a furnace in an aluminum smelter, allowing the customer to drastically cut operating and maintenance costs.
I briefly summarize what was the situation that led this smelter to contact us.
The fans installed on the furnace vault were going down every 2-3 weeks. Over the years they had installed special bearings to prolong their life, started using a grease suitable for operating temperatures above 150°C, and finally installed a system with nozzles to cool the outer surface of the bearings with compressed air.
Despite all these interventions and this cost explosion, they could not keep the bearings at a temperature below 130°C, and every 2 weeks they disassembled the fans to assess the condition of the bearings and replace the grease.
Upon receiving one of these fans at our plant, we immediately noticed two design errors .
First, the heat shield disc was small and thin for the smoke temperature values (570°C) inside the furnace. In addition, the shaft was in standard version, without any internal cooling channel.
Therefore, we replace the shaft with one provided with internal cooling and at the same time we installed a bigger heat shield disc.
These were the customer’s words about the results obtained as a result of the modification:
“The results are relative to 2-3 weeks of use: at installation the fan was already practically balanced, in process the temperature at the bearing ranges from about 65 to 80 °C, without the use of compressed cooling air.”
