Correct material and design choices for high temperatures
When dealing with air or gases at high temperatures, we naturally take material selection into account. Although the temperature isn’t high enough to melt steel (which melts around 1375°C), the material quickly loses strength.
In a recent project, gases with temperatures up to 650°C were extracted. The fan impeller for this project can be seen in the photo next to this text. Due to the high temperatures, stainless steel 321 (RVS 321) was chosen as the base material. Even with the use of specialized materials, the high temperature still poses challenges. As shown in the table, RVS 321 loses almost half of its strength!
Because the impeller is in the gas stream, there’s no way to prevent it from heating up. Therefore, we reinforce the impeller in such cases to compensate for the loss of strength.
Design measures to strengthen the impeller include:
- Using thicker materials;
- Adding extra reinforcing rings;
- Fully welding the blades all around;
- Interrupting the shaft to reduce heat transfer to the bearings.
An example of a Stainless Steel impeller with a cooling coupling.
Additionally, you can see here that the bolts fastening the hub have been spot-welded in place. Due to the cyclical heating and cooling of these bolts, they can gradually loosen, and using chemical binders like Loctite is not an option at such high temperatures.
Aside from construction, various measures can be implemented to ensure the impeller stays within certain parameters during operation. For example, speed monitoring can be used to ensure the impeller never exceeds a certain RPM, or temperature sensors combined with a safety PLC can trigger various actions if temperatures get too high.
| Design temperature: | 550 °C |
| Maximum temperature: | 650 °C |
| Material: | 650 °C |
| Tensile strength of Stainless Steel 321 (20 °C) | 240 MPa |
| Tensile strength of Stainless Steel 321 (20°C) | 150 MPa |
| Tensile strength of Stainless Steel 321 (650 °C) | 135 MPa |
Proper material selection for high temperatures – cooling couplings, bearings, and shaft seals
In addition to the impeller, there are several other factors to consider. Since the impeller is made of metal, which conducts heat well, it is inevitable that the shaft will also become very hot.
Cooling couplings for bearing protection
To minimize heat transfer, we use a so-called cooling coupling. This clever geometric design effectively transmits forces while transferring very little heat. Heat transfer through this connection follows a more indirect path compared to a simple shaft. Thanks to its hollow design, through which cooling air can also flow, the heat transfer to the bearings is much lower than with a “standard” solid shaft.
A mounted shaft with a cooling coupling.
Heat-resistant bearings and lubricants
Despite this, it is still possible for the rest of the shaft to become warm. Even with a cooling coupling, the bearings may still experience temperatures up to 200°C. Therefore, we use heat-resistant bearings from reliable suppliers known for their consistent high quality, such as SKF in this case. By selecting the appropriate bearings, we can prevent the need for frequent replacement.
Regular lubricants would become too fluid at these temperatures, causing them to lose their effectiveness or even escape the bearings. Heat-resistant lubricants based on PTFE, which remain effective at temperatures up to 200°C, are an excellent solution in such cases.
Special heat-resistant shaft seals
The impeller in a centrifugal fan is almost always connected to a motor located outside the fan housing. The impellers can either be mounted directly on the motor shaft or use an intermediary shaft, as seen in the example above. Regardless of the setup, there is a hole in the housing where the (motor) shaft passes through. This hole is naturally larger than the diameter of the shaft.
From an airflow design perspective, the gap between the hole and the shaft should ideally be as small as possible to minimize leakage. Often, a shaft seal is applied, fitting snugly around the shaft.
Depending on the application, we use various types of shaft seals. Where possible, we opt for simple shaft seals. In other cases, we use seals with oil seals, carbon rings, and/or gas barrier chambers.
For high-temperature applications, several other options for shaft seals are available, depending on the specific use. Shaft seals made from graphite can withstand temperatures up to 1000°C, while seals made from compressed (stone) fibers are also possible.
Fans for furnaces and smelters
One of our clients, a manufacturer of aluminum semi-finished products, operates multiple homogenizing furnaces. To process the products effectively and consistently, it is crucial that the base material maintains a uniform temperature throughout.
In these homogenizing furnaces, hot air is blown over all the plates or rods. The impellers we supplied ensure this air circulation. These impellers are located inside the homogenizing oven, and thus, they also reach the same temperature—up to 620°C in this case.
This impeller is designed for high air movement with minimal pressure loss. When moving air within the same open space, little pressure difference is required. To keep the airflow slow and steady, the impeller is relatively large, with a diameter of about Ø1300 mm and a depth of 700 mm, rotating at a low speed of 620 RPM. The base material is stainless steel 309, which can certainly withstand these high temperatures.
A large stainless steel impeller for a homogenizing oven. This photo was taken during an intermediate stage before the impeller was pickled to remove welding residues.
Customer example: high-temperature fan for pyrolysis
The photos below are from an order for one of our valued clients. This client is an OEM that provides specialized high-temperature furnaces for applications such as material sintering. They also supply machines for pyrolysis, a process used to remove polymers from old electronic components, allowing valuable metals to be extracted.
Attentive readers will notice the significant difference in power consumption between 20°C and 550°C. This is because the hot air is much less dense at high temperatures than at lower ones, meaning the fan has much less work to do.
| Reference | 81517 |
| Flow rate m3/h | 16.000 |
| Pressure drop | 900Pa |
| Rotation speed (o/min) | 2146 |
| Motor power (550 °C) kW | 5.2 |
| Motor power (20 °C) kW | 13.9 |
| Materials | Housing: RVS 315 Impeller: RVS 321 Shaft: RVS 321 Support frame: S235JR |
Safety measures for high temperatures
Of course, we also take employee safety into careful consideration. After the fan is installed, it is wrapped in 100-150 mm thick rock wool, and the rotating parts are thoroughly shielded. In the first photo, you can clearly see the additional volume of the rock wool next to the cooling coupling.
Are you looking for a fan for high-temperature applications? We’d be happy to help you find the most suitable solution!