Thermal management - Everything you need to know about fans

03/13/2024 Know-How

For the proper functioning and longevity of electronic systems, the maximum operating temperature of any component must not be permanently and/or significantly exceeded. To ensure this, thermal management is often necessary, for example with a fan. The decisive factor here is the selection of the optimum model for the application in question.

A fan uses a motor to generate a rotation of the fan blades and thus a pressure difference, which in turn causes a continuous airflow. The fan consists of a rotating part, the impeller, and a fixed part, the package.

The different types of fans

There are numerous types of fans. If it is a question of using them to cool an electronic device, the most important criterion is the direction of the airflow. Accordingly, a distinction is made between:

  • Axial fans (the airflow is parallel to the axis)
  • Radial fans (the airflow is perpendicular to the axis)
  • Tangential fans or cross-flow fans (their long package produces a wide, flat airflow perpendicular to the axis and tangential to the package)
  • Spiral radial fans (the blades in the impeller are not straight as in radial fans, but have a spiral or helical structure; this allows them to produce an airflow that is somewhere between that of an axial fan and a radial fan (Fig. 1))

In addition, fans are classified by their supply voltage as DC voltage or AC voltage fans. Current AC models are primarily electronically commutated (EC) fans. They achieve higher energy efficiency due to their brushless DC motor and electronic control.

The most important selection criteria

Airflow is also an important indicator for selecting a fan. It is characterized by the amount of air that is discharged from or introduced into an electronic system within a given period. This flow rate is usually expressed in cubic feet per minute (CFM) or cubic meters per hour (CMH; m3/h). The relationship between CFM and CMH is 1 CFM = 1.699 CMH.

A certain force is required to move the volume of air. This force per unit area is called pressure. To create a specific airflow in a system, its airflow resistance must be known. This is caused by the friction of the air against the duct walls, bends, grilles, filters, slats, and other elements that may restrict air movement. This resistance, called pressure drop or pressure loss, is expressed in pascals (Pa) or millimeters or inches of water column (mm H2O or inAq). These relate to each other as follows:

1 pascal = 0.102 mm H2O = 9.8692 · 106 atmosphere

1 pascal = 0.0040146 inch of water (4 °C)

This makes it important to know the pressure drop in the system in order to select a fan that can provide the necessary pressure to overcome this resistance and maintain the desired airflow.

Fans impart the necessary pressure to an air mass to create a pressure differential and thus airflow. Three types of pressure are involved in this process:

  • Static pressure (PE) is the force exerted by the stationary air on the system walls perpendicular to it.
  • Dynamic pressure (PD) is the force per unit area and is used to overcome the resistance of the airflow in a system. It therefore ensures that the air moves and is generated by the rotational speed of the fan. It is always positive and has the same direction as the airflow.
  • Total pressure (PT) is the sum of PE and PD at a given point in the system. This is the pressure exerted by the air on a body resisting its motion at that point. It is important to note that the total pressure at different points in a system can vary due to the velocity and flow conditions of the air.

A fan’s characteristic curve

Fan suppliers perform tests on their equipment to determine how much power the fan can transfer to the air it moves. This is done by operating it at a constant speed. Depending on the pressure drop to be overcome, different values for the airflow are achieved.

Plotting the various values for the airflow and the pressure determined in the laboratory tests on a coordinate axis yields the fan’s characteristic curve. It is provided by the fan suppliers, usually with several curves for different constant speeds.

Such characteristic curves are shown in Fig. 2. The airflow is plotted on the X-axis and the pressure (in units Pa and in Aq) on the Y-axis. The pressure is highest when the airflow is zero. In this case, the fan works with high resistance to the airflow, and the maximum static pressure (PE) is created. At the same time, the dynamic pressure (PD) is zero, which means that no airflow is generated. At this point PT = PE.

Figure 2 also shows that when the pressure in the fan is zero, the maximum airflow is achieved. Since there is no resistance to the airflow – i.e. an obstacle-free environment (PE = 0) – the fan delivers the greatest possible airflow. The total pressure corresponds to the maximum dynamic pressure (PT = PD) generated by the corresponding speed.

Calculating a fan’s operating point

To calculate a fan’s operating point (OP), it is recommended to consult with the supplier, as they have the necessary technical means to perform simulations. However, the approximate OP can also be calculated. To calculate the operating point with the lowest power consumption, the power consumption and current values listed in the table (Fig. 2) are important data.

To calculate the operating point, the resistance conditions of the system, represented by the S curve (Fig. 3), must be known (OP is marked here as Q1 and Q2). A complex calculation using thermal equations is required to determine the optimum pressure and airflow values for cooling the system. It is recommended to measure the pressure drops in the system at different airflow rates, for example with pressure sensors and/or manometers. For each airflow rate, the pressure drop values are then recorded and plotted as in Figure 3 (airflow rate on the X-axis, pressure drop on the Y-axis). In addition, it is also important to continuously measure the temperature inside the system. For this purpose, temperature sensors are placed at strategically favorable locations. The optimum operating point is determined based on the airflow that cools the system most effectively.

If the operating point is at the intersection of the S curve and one of the fan’s three characteristic curves (Fig. 3, Q1), the fan’s speed, airflow, pressure, and power consumption (W and I) can be calculated using the measurement data provided by the supplier in Figure 3 (blue, 3,500 rpm).

If the operating point is not at an intersection with one of the three curves (Q2), the curve can be extrapolated (dashed curve) to determine the intersection with Q2. Alternatively, the data can be provided to the fan supplier to obtain the corresponding characteristic curve (dashed characteristic curve) for the Q2 operating point. The aim is to determine the fan’s speed, airflow rate, pressure, and consumption values (W and I).

Factors influencing life expectancy

The most important factors influencing a fan’s service life are its temperature profile and its type of bearing. In the case of a plain bearing, it again depends heavily on the lubricants used. A two-ball bearing consists of small metal balls in a raceway, allowing for lower friction and higher efficiency. More detailed information on the fan's operating temperature profile can typically be found in its data sheet.

Main markets

Fans are used in numerous industries to remove heat and maintain optimal temperatures. Major markets include:

  • Electronics: For cooling internal components and preventing overheating.
  • HVAC (heating, ventilation and air conditioning): For air circulation, indoor temperature regulation, and for generally improving air quality in buildings, homes, offices, and industrial facilities.
  • Motor vehicles: For regulating the engine temperature and preventing overheating.
  • Renewable energy: For use mainly in wind turbines and solar inverter cabinets.
  • Industry: For keeping machines and equipment, e.g. in manufacturing, power generation, or petrochemical industries, at their optimal operating temperatures.
  • Data centers: For ensuring cooling to remove heat generated by servers and other IT infrastructure. Fans are an important component of data center cooling systems.
  • Consumer electronics: For refrigerators, air cleaners, or game consoles.
  • Aerospace: For cooling systems and components.

Innovations – where do we start?

Further development of fans focuses on the following aspects:

Energy efficiency: Fan suppliers are working to improve the aerodynamic design of fan blades, reduce friction losses, and optimize motor efficiency to achieve higher overall energy efficiency. This includes the use of advanced materials and manufacturing techniques to reduce weight and increase performance.

Improved ball bearings: Suppliers are using newly developed bearing technologies to increase the reliability and service life of their fans. For example, fluid dynamic bearings (FDBs) and magnetic levitation bearings (MLBs) offer a longer service life and lower noise compared to conventional plain or ball bearings.

Noise reduction: An important issue in fan applications, especially in environments where low noise levels are required. Technical development focuses on improved blade designs, optimized motor controls, and the use of noise-absorbing materials. Computational fluid dynamic (CFD) simulations and other modeling techniques are used to study and minimize noise.

Fan control systems: These play a major role in optimizing fan performance and overall system efficiency. Intelligent control algorithms that dynamically adjust the fan speed based on temperature, humidity, and other environmental factors ensure that fans operate at maximum efficiency while creating optimal cooling conditions.

Integration with cooling systems: Aiming to maximize the heat dissipation of the entire cooling system, fans are integrated with other cooling technologies such as heat sinks, radiators, and liquid cooling systems.

Smart and IoT-enabled fans: The Internet of Things (IoT) has enabled the development of smart fans with advanced features and connectivity. They can be monitored and controlled remotely and enable real-time adjustments based on environmental conditions and user preferences. In addition, IoT-enabled fans can provide valuable data on performance, energy consumption, and maintenance needs for their optimization and predictive maintenance.

All of these advances aim to further improve the cooling performance of fans, further reduce their energy consumption, and provide more reliable and efficient cooling systems for a wide range of industries.


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Fan types by direction of airflow

uppliers usually specify several characteristic curvefor their fans for operation at different speeds.

Resistance conditions within the system

Manufacturers usually specify several characteristic curves for their fans for operation at different speeds. (Image: DELTA ELECTRONICS)

Resistance conditions within the system (Image: DELTA ELECTRONICS)