Cooling methods for electronic equipment

Cutting-edge electronic systems and related challenges

As a result of technology development, computers must process more and more data. They are required to perform complex tasks, while at the same time manufacturers strive to miniaturise electronic equipment, ideally to a pocket size. Computers that once took up entire rooms were only tasked with simple calculations. Modern computers fit in the palm of your hand and their capabilities seem almost infinite. However, the issues connected with heat generation by electronic circuitry remain unchanged. Moreover, the increasing amount of computing power brings with it a proportional increase in heat to be dissipated. It won’t come as a surprise to anybody that excess heat is one of the greatest enemies of electronic systems. It greatly accelerates the wearing of such electronic components as transistors, resistors or relays. In special cases, high temperatures can even damage control systems irreparably. A device repair cost may then exceed the profitability threshold. While comparing computers designed a few decades ago to those of today, we need to remember that the 20th-century devices equipped with extensive cooling systems in fact operated under much more favourable conditions.

Nowadays, electronic systems set the pace for human life, help provide safety and streamline further technology advancements. To ensure their faultless operation at the maximum efficiency, adequate cooling and protection against overheating must be provided. The challenges faced by consumer mobile devices or advanced industrial systems force manufacturers to go to great lengths to find the best way to maintain the most sensitive components at safe temperatures.

Tailor-made cooling systems

When it comes to selecting a cooling system, the following are the most popular solutions:

Fans

The first and most common application of fans in electronic devices is to dissipate heat. They are installed both in desktop computers and in laptops.

Fans are equipped with motors driving the impeller blades. Convection air movements forced by impeller rotations dissipate and remove the heat generated in an electronic device. The heat energy is extracted through the grilles located in the computer enclosure. The fan movement is triggered by a temperature sensor. After a certain setpoint is exceeded, the fan impeller starts rotating to facilitate device circuitry cooling. The mass of air flowing through a fan is specified in cubic metres per hour (m³/h).

A fan also plays another important role, ie, it acts as a ‘dust cleaner’ inside a computer unit. To ensure its trouble-free operation, dust must be removed from the impeller blades on a regular basis.

It is also worth noting that fans currently offered by the leading manufacturers undergo rigorous noise level tests. Therefore, their operation does not cause computer users’ fatigue. However, the noise level increases in proportion to the size of a fan and the amount of air it has to push.

Using the active cooling feature causes a risk of damaging certain components, eg, the drive system or the driven impeller itself. This risk is eliminated when using the passive cooling feature mentioned below.

Heat sinks

These are finned metal elements most often used in conjunction with the fans described above to increase heat dissipation efficiency.

As per the heat exchange process principles, the greater the surface area absorbing thermal radiation, the higher the cooling capacity. In a heat sink design, properly shaped fins maximise the heat exchange surface area. It is obvious that the larger the heat sink, the more intensive the cooling process is. Still, the size of the device for which the component is to be used is the biggest limitation for its performance. The distance between the heat sink and the heated component is another very important aspect. The smaller the distance, the more efficient the heat dissipation is. In order to maximally intensify the temperature transfer process, thermally conductive tapes are placed in the contact area between both surfaces.

In a fan/heat sink assembly, the latter accepts hot air masses pushed by fan impeller blades, and dissipates the heat into the surroundings.

Peltier cells

Peltier modules are made of parallel ceramic plates, between which type N and type P semiconductors are alternately placed. Direct contact between these plates is ensured by using copper sheets to transfer electrons. A current flow forces temperature changes in the places of contact between dissimilar semiconductors. Fault-free operation, a relatively small and compact design and no need to use a coolant are the main advantages of this cooling system. Another important aspect is the possibility of expanding and increasing the capacity of a Peltier cell by adding additional modules. Then, the ‘hot’ side of the first module is connected with the ‘cold’ side of another one. Therefore, the heat collection capability depends on the available space and current intensity.

These cells are a perfect solution for harsh environmental conditions, eg, locations where large amounts of dust are present.

Liquid cooling

Effective heat dissipation in state-of-the-art high-power electronic systems requires high cooling capacity. Note that the thermal capacity of air, ie, the amount of energy that air is capable of storing, is approx 1 kJ/m³, whereas the thermal capacity of water is 4000 kJ/m³. Therefore, demanding electronic systems benefit from liquid cooling systems. ‘Water blocks’ are based on water (or another cooling liquid) flowing around the heat sink in a sealed enclosure. A pump system ensures liquid circulation. Liquid cooling systems require a high degree of manufacturing precision. This results from the necessity to avoid contact of electrically powered systems with water. Even if a system is filled with a non-conductive substance and simple contact with this substance will not destroy the device, the coolant loss will disrupt the cooling process. Such solutions are implemented, for example, in computers designed to perform complex calculations or simulations for the research and engineering industry.

Heat tubes

A heat tube is a simple device transferring heat in line with the convection principle. They are used in numerous fields of application, from cooling and heating solutions, through cutting-edge computers, to the space industry.

A heat tube is divided into three zones:

• Evaporation zone (evaporator);
• Intermediate zone — heat transfer without exchange with the surroundings (adiabatic);
• Condensation zone (condenser).
   

Heat is absorbed in the evaporator in which the liquid medium is evaporated. The pressure in the evaporator space is higher than the pressure in the condenser space. This pressure difference forces the movement of vapour to the condensing space where it is condensed and transfers heat to the upper source.

The heat tube technology is applied in space vessels and the chemical or power engineering industry.

The importance of cooling for electronic system operation

Excessive heat in equipment reduces its service life, adversely impacts its performance and often results in inevitable electronic system failures. Proper selection of cooling elements helps avoid all of these negative effects. It is a considerable challenge for designers, as numerous factors, such as key technical parameters and dimensions of devices, have to be taken into account. Despite installing high-end heat dissipation systems, it is still the users who have great impact on equipment lifetime by providing adequate space for air circulation and using it only in line with its intended purpose.

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Image credit: iStock.com/Marina N. Mak