PRODUCT SPOTLIGHT:
DC FANS

Introduction

Welcome to the CUI Devices Product Spotlight on how to select a fan for forced air cooling. This resource will provide an overview on proper fan selection, including system profiling, determining a system's cooling requirements, and an outline of fan types and features. CUI Devices' line of dc axial fans and centrifugal blowers will also be highlighted.

Objectives

Discuss the importance of proper fan selection in thermal management
Learn how to create a thermal system profile
Outline calculations for determining a design's cooling requirements
Introduce CUI Devices' line of dc axial fans and centrifugal blowers

Searching for Dc Fans? Check out CUI Devices' portfolio of Dc Fans and Blowers ▶

Why is Cooling Needed?

Heat is an inescapable fact in electronic applications. In most systems, particularly those that employ an enclosure, some form of forced air cooling will be needed to remove heat and keep electronic components running at their ideal operating temperature. While conduction of heat away from components might work for some applications (for example via heat sinks), the fact remains that designs that consume as little as 25 W of power may require forced air cooling, especially once an enclosure becomes involved. Therefore, selecting the right fan for the job is crucial as its efficiency and effectiveness can greatly impact the lifespan of a system and prevent component failure.

System Profiling

System profiling is an important first step to understanding where and how much heat is generated in an application. This data can be gathered by placing temperature sensors around a PCB and within an enclosure. System impedance, which looks at a drop in air pressure between inlet and outlet also plays a major part in calculating airflow required from a fan, and thus, the size and type of fan required. This too can be achieved by using sensors and measuring the pressure drop or by placing the system in an air chamber.

For larger systems, such as data centers, modeling the system using computational fluid dynamics (CFD) provides an even more accurate profile of a system's cooling requirements.

Determining Cooling Requirements

The next question to answer is, how much the internal temperature can change without increasing the risk of failure? The operating temperature of the "most critical" component will give a maximum ambient temperature, while the cumulative power dissipation for all relevant components, such as power transistors, microprocessors, amplifiers and communication interfaces, will provide a figure for the amount of power dissipated by the overall design.

Power dissipated (W), converts linearly to energy (Joules/second), which in turn is exhibited as heat. During operation the temperature of the air around components will continue to rise until it reaches a level that will inhibit further heat from being removed. At this time replacing the heated air with ambient air via forced air cooling will become necessary and is where selecting the appropriate level of airflow comes in.

Airflow Calculations

Equation 1 shows the relationship between temperature rise and airflow, where q is the amount of heat absorbed by the air (W), w is the mass flow of air (kg/s), Cp is the specific heat of air (J/kg x K) and ΔT is the temperature rise of the air (°C).

Once the maximum permissible temperature within the enclosure is known and the amount of heat generated is derived (based on the cumulative power/heat dissipated by the components) it is possible to calculate the amount of airflow required. Since mass flow (w) = air flow (Q) x density (ρ), substituting and solving for Q, Equation 1 can be rewritten to get Equation 2 (where Q is the airflow in CMM (m³/min), q is the amount of heat to be dissipated (W) and ρ is the density of air (kg/m³)).

Equation 1: Calculating Heat Absorption

q = w x C_Ρ x ΔT

Equation 2: Calculating the Amount of Airflow Required

Q = [q/(ρ x C_Ρ x ΔT)] x 60

Equation 3: Calculating Heat Absorption

Q = 0.05 x q/ΔT; for Q in CMM
Q = 1.76 x q/ΔT; for Q in CFM

Fan Performance Curves

The figure above shows the performance curve of the CFM-120 series from CUI Devices, a 120 mm by 120 mm frame axial fan with dual ball bearing construction. As shown, manufacturers characterize fans by plotting airflow (measured in either Cubic Feet per Minute, CFM, or Cubic Meters per Minute, CMM) against static pressure (measured in either inches or millimeters of water, often written as Inch H2O or mm H2O). Unfortunately, the result calculated in Equation 3 on the previous slide is only accurate for conditions with no back pressure or system impedance, but in reality there will always be some system impedance that needs to be calculated or estimated. Once system impedance is accounted for it can then be plotted on the fan's performance curve shown on the figure below and the point at which they cross can be taken as the operating point for the fan.

However, if the airflow through an enclosure cannot be measured, then an alternative would be to specify the operating point above the number gathered from Equation 3. For example, if the airflow calculated is 50 CFM with zero back pressure, over-specifying the fan at 100 CFM with the intention of operating it at 75 CFM provides a good margin of error, as well as the ability to increase airflow during operation. Furthermore, taking steps at the design stage to minimize system impedance can positively impact the size and power of fan needed. Best practices include keeping areas around air inlets and outlets clear, considering the system impedance a filter will introduce, and ensuring component placement directs airflow to and around critical components.

Fan Types and Considerations

Fans are generally categorized by the way air enters and leaves the fan, which comes in two common forms: the axial fan where air is drawn in from one side and expelled from the other on the same plane or the centrifugal fan design where air is drawn in and expelled in a different direction. This style of fan, most commonly found as a blower, effectively compresses the air, allowing for the delivery of airflow at 90° to the intake under different pressures. The application requirements will determine which design is more suitable as axial fans deliver greater airflow in systems with low static pressure, while centrifugal fans offer lower airflow, but can deliver it against higher static pressure.

Audible and electrical noise are also important considerations when selecting a fan. In general, the greater the airflow required, the greater the audible noise which means that careful design to optimize airflow and reduce system impedance will reduce the required CFM and minimize noise. Axial fans will also tend to be quieter than their centrifugal fan counterparts. In addition to noise, electromagnetic interference (EMI) generated by the dc motor in a fan should also be considered. This unwanted system effect is usually limited to conducted EMI in the power leads and can be effectively suppressed with ferrite beads, shielding, or filtering.

Fan Type	Airflow	Application Type	Noise
Axial Fans	Higher	Low Static Pressure	Quieter
Centrifugal Blowers	Lower	High Static Pressure	Louder

Fan Bearing Types

Depending on the application, fan bearing type should also be a consideration with sleeve and ball bearings being the most common. Sleeve bearings, which are the simpler and lower cost option, have proven to operate as well as ball bearing fans at consistently lower temperatures. However, at variable or high temperatures sleeve bearings tend to degrade more quickly and can begin to experience wobble, noise, and friction issues commonly found in the sleeve bearing design. On the other hand, ball bearings address many of the uneven wear and friction problems found in sleeve bearings resulting in a significantly higher operating life. They can also be operated at any angle for use in portable applications, but at the same time are less impact resistant, more complex, and costlier than sleeve bearings.

As a third option, CUI Devices has developed the omniCOOL™ system, an advanced sleeve bearing design incorporating a magnetic structure that enables rotor-balancing to minimize tilt, wobble, and friction, allowing for operation at any angle. This magnetic structure, along with a specially hardened sleeve that provides additional heat resistance as well as reduced friction, combine to make the omniCOOL system a more reliable and cost-effective design compared to traditional bearing technologies.

Sleeve Bearing

Ball Bearing

omniCOOL™ System Bearing

Learn more about CUI Devices' omniCOOL™ system bearing design ▶

Fan Control Options

Many fans now come with additional features that provide greater control over fan speed and operation to improve system performance. Having a fan run constantly even when maximum cooling is not required does not result in an efficient system and can reduce the operating lifetime of a fan. That is why many systems now monitor the temperature within an enclosure and only operate a fan when it is required. However, this can present problems with thermal lag or a fault condition if the fan was unable to start due to an obstruction. To address this, most modern dc fans feature auto-restart protection that detects when a fan motor is prevented from rotating and automatically cuts the drive current. Additional fan control options include:

Tachometer Signal

Detects rotational speed of fan motor and provides a pulsed output
If motor stops, output stops pulsing and stays at logic high or low

Rotation Detector

Doubles as a lock sensor where output remains at logic low during normal operation, but is driven to logic high if fan motor stops

PWM Control Signal

Gives the ability to control the speed of the fan
Duty cycle of this input determines speed of fan’s rotation, relationship between duty cycle, and whether fan’s speed is linear
When used with a microcontroller one can create a sophisticated thermal management solution used to adapt to system conditions and provide more efficient operation

Available Products

CUI Devices' dc axial fans and centrifugal blowers are available with frame sizes ranging from 25 mm to 120 mm and airflows up to 200 CFM. All models come as standard with auto restart protection and feature ball bearing, sleeve bearing, or CUI Devices' advanced omniCOOL bearing system that provides a unique alternative to traditional bearing designs. Options with tachometer signal, rotation detector, and PWM control signal are also offered. CUI Devices' dc fans and blowers further feature 1700 to 13000 RPM rated speeds, noise levels from 10.7 up to 69.9 dBA, and static pressures from 0.04 up to 5.22 inch H2O, making them ideally suited for a variety of forced air cooling needs.

Dc Axial Fans

CFM & CFM-V Series

25 to 120 mm frame sizes
1.35 to 200.9 CFM airflows
1700 to 13000 RPM rated speeds
5, 12, 24, 48 Vdc rated voltages

Dc Centrifugal Blowers

CBM-B & CBM-V Series

50 to 120 mm frame sizes
2.6 to 54.7 CFM airflows
0.15 to 5.22 in H2O static pressures
Ideal for high back pressure applications

Summary

Forced air cooling is an efficient way of implementing thermal management for an enclosed PCB and with semiconductors and PCBs becoming ever more complex and dense, component failure is often linked to insufficient cooling that causes overheating. If a fan is chosen for cooling, then selection of the right fan should not be taken lightly. Determining a system's thermal profile, calculating needed airflow and performance, and understanding fan types and features are all important things to consider. Thankfully CUI Devices offers dc fans and blowers with various features and performance ratings to meet a range of thermal requirements, which can mean the difference between premature failure and an efficiently operating system.

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PRODUCT SPOTLIGHT:DC FANS