The most prestigious event in this sector worldwide, the annual Cooling Tower Institute (CTI) conference, took place in Houston, TX, from February 4th to 8th, 2024.
FanTR was present not only as an exhibitor of products and solutions for cooling towers but also as a speaker.
Check out the publication about the event at: 2024 CTI ANNUAL CONFERENCE AND CTI EXPO.
Selection of Materials for Fan Blades: A Comprehensive Guide
Marcelo Carvalho, Commercial Director, and Ricardo Costa, R&D Coordinator, presented in an educational seminar titled: "Selection of Materials for Fan Blades: A Comprehensive Guide"
During this participation, both speakers shared valuable insights, which are presented in this article.
Check below:
Picture: CTI 2024 Event
During the presentation, FanTR sought to clarify the connection between the aerodynamic performance and the structural performance of large-scale axial fans, as well as the relationship between the materials and the manufacturing processes of the blades.
In addition, important points in the choice of fasteners were addressed, with emphasis on those applied in the fan hubs.
It was possible to elucidate that the analysis of mechanical properties is just one of the characteristics for defining the best material. The design possibilities associated with each material and manufacturing process, which define the freedom to create complex aerodynamic geometries, were the central theme of the presentation.
As an example, some possible geometries of transition between the blade and the fan hubs were presented:
Figure 1: Abrupt Transition Figure 2: Smooth Transition
Note that in the first figure, the transition between the blade and the hub is made abruptly, resulting in better aerodynamic conditions in the root region of the blade. The second figure details a smooth transition which, despite being structurally more conservative, can lead to aerodynamic recirculation problems in the root region, and consequently, lower fan efficiency.
FanTR explained the origin of this aerodynamic condition and emphasized that the adoption of a seal disk, as shown in figure 2, can minimize the problem. It also pointed out that not every fan requires a seal disk. If the manufacturer can, for instance, increase the thickness of the material at the root of the blade as a means of dissipating mechanical stresses, an abrupt transition can be adopted.
In this case, the recirculation condition at the root is resolved and better efficiencies can be achieved. However, this is a condition that not all materials/processes can provide.
Analysis of Resources for Aerodynamic Projects
Several aerodynamic resources available for efficient fan blade design were presented. Among them, we can highlight the variation of the chord (measure of the length of the aerodynamic profile) between the root and the tip, and the twist of the blade.
Figure 3: Variation of the chord between the root and tip of the blade to maintain uniform flow (velocity)
Figure 4: Aerodynamic twist of the blade to compensate for the difference in velocities between root and tip
It was demonstrated that the airspeed on the fan blade is not constant. Due to the rotational movement, the air speed at the tip of the blade is greater than at the root. The valuable aerodynamic resources presented in the above examples were detailed in the presentation.
They are used to homogenize the airflow produced along the radial position of the fan, balance the aerodynamic forces, and improve the overall efficiency of the fan. However, these resources cannot be employed depending on the chosen material or manufacturing process.
At this point, through the proposed examples, it became clear the intrinsic correlation between aerodynamic and structural performance with the type of material/process chosen.
Next, FanTR began to exemplify the materials used in the manufacturing of large axial fan blades: composites and aluminum.
Comparison between Aluminum and Composite Materials and Manufacturing Processes
Regarding aluminum, it was clarified that its properties are defined by standards, usually divided by alloys, being isotropic. For large-scale fans, manufacturing processes are mainly restricted to two: extrusion or molding.
Extrusion gives rise to continuous aluminum structures, making it impossible to apply the aerodynamic design features mentioned earlier. The aluminum molding process is more versatile, being viable only with very thin sheet thicknesses. The thickness of the sheets limits the strength of the blades and their application in projects with high loads, such as Air Cooler Condensers (ACCs). Impact resistance is also compromised in this case.
Subsequently, FanTR addressed the definition of composite materials and the various composition options for these materials, highlighting that when associated with manufacturing processes, they can result in quite distinct mechanical properties. For illustrative purposes, a graph was presented outlining a comprehensive view of the weights and strengths of different materials:
Figure 5: Comparison of Weight and Strength of Material
FanTR drew attention to the "strategic position" of composites in this graph, occupying an intermediate density (weight) position. It also drew attention to the logarithmic scale of strength (vertical axis of the graph).
In this example, strengths of composites can vary from <100MPa to >1,000MPa. The difference is explained by the composition of the material itself (combination of fibers, resins, etc.) and the associated manufacturing process.
FanTR exemplified some manufacturing processes of composites, such as Hand Lay Up, RTM, and Infusion. It also brought comparisons of mechanical strengths of "equal" composites - same resin and fiber - but with different manufacturing processes. It was possible to note that the infusion process gives rise to composites with much higher mechanical strengths compared to the Hand Lay Up process and also to aluminum.
This superiority is attributed to the better-quality control of the process and the obtaining of composites with a significantly higher fiber content compared to the manual process.
Aluminum vs. Composite: Making the Right Choice.
Finally, FanTR sought to conclude its presentation using the following table:
Figure 6: Comparative Table between Aluminum and Composites
This table summarizes the presentation entirely. The materials used in the manufacture of large-scale fan blades are highlighted in two columns: aluminum and composite.
The rows discriminate the results into two categories: the possibility of full application of aerodynamic design resources or with some restriction. As a further breakdown within the table, the associated manufacturing processes are presented.
It can be observed that both aluminum and composite have limited processes, such as extrusion and pultrusion. These are continuous manufacturing processes without the possibility of employing variable chord, twist, or thickness optimization along the length of the blade, limiting the possibility of aerodynamic and structural optimization and, consequently, the performance of the fans. These processes result in more affordable structures.
On the other hand, in terms of geometry, both materials can benefit from processes with more design freedom. Molded composites can easily assume complex geometries. Molded aluminum, on the other hand, offers design freedom with some limitations.
Although aluminum can be molded into more complex geometries, it is usually done with thin sheets. Given the limitation in thickness and structural strength of the sheets, some boundary conditions are required, especially in the transition region between the hub and the blade.
This increases the cost of the fan and does not fully resolve the structural challenges, becoming a restrictive process for projects involving high loads or impact requirements.
Finally, molded composites stand out for their flexibility in aerodynamic and structural design, being able to create highly optimized structures. In this context, some manufacturing processes, such as infusion, excel in providing excellent mechanical properties, greater fatigue resistance, and more precise control of the process quality, as it is less dependent on the skill of a professional laminator.
It is worth noting that the quantity of materials required for blade manufacturing by infusion naturally raises production costs.
As can be seen in the presented context, there is no right or wrong material or process for application in axial fan blades. It is really about a commitment between aerodynamic/structural efficiency compared to cost. More efficient solutions are more expensive.
It is correct to assume, therefore, that the financial analysis of an application should be performed based on the energy consumption in perpetuity (for example, 20 years of continuous operation).
Bringing future energy savings to present value allows for a fair comparison of the costs of all available solutions in the market at the time of negotiation.
