RACKS, CABINETS & ENCLOSURES FEATURE


COOLING TECHNIQUES FOR SPECIALISED ELECTRONICS


Dr. Adam Pawlowski, principal engineer for cooling technologies at Pentair Technical Solutionsexplores the various techniques available for effective cooling, the opportunities they offer as well as their challenges


CREATING A CONTROLLED ENVIRONMENT If a number of electronic systems, subracks or cases are installed within one electronic cabinet, it may become necessary to increase system cooling by controlling the environment within the cabinet. Door fans or a top cover may be implemented to create additional airflow from the cabinet interior to the outside environment. Cabinets may also be connected to existing air conditioning systems to further increase the effectiveness of these methods. In extreme cases, air-to-air or air-to-water heat exchangers may be used in a sealed cabinet to create a micro environment within the enclosure. Critical factors for selecting the correct


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ncreasing electronic power levels and component density generate more and more heat, elevating the thermal stress on components along with risk of failure. When selecting the cooling method for


specific application, there are two key performance factors to consider: the maximum allowable temperature for the components in the system, and the environmental conditions. Natural cooling convection is the simplest and lowest cost method. However, this is only effective if a significant amount of heat can be released from the enclosure to the environment. With this set-up, there must be free flow of


air inside and outside the enclosure. The ambient temperature also needs to be significantly lower than the operating temperature. Because the released heat impacts on the enclosure surface, when large amounts of heat is generated, natural convection may however not be the most effective solution. When air resistance of internal components becomes too significant for natural convection on its own, forced air cooling can be used as an alternative with the use of fans or blowers. Forced air techniques can be divided into two different approaches: push or pull cooling. With push designs, fans are positioned at the cool air inlet, creating positive pressure inside the enclosure and pushing air across the components towards the exit. With a pull design, fans are located at the air exit, creating low pressure within the enclosure, and pulling air across components and out the exit. Combining both methods remains common, but it is the application and environmental conditions that determine which solution.


COOLING WITHOUT ACTIVE AIRFLOW Conduction cooling methods refer to heat transfer within the enclosure, using direct material contact between hot components and a cool area, without the use of air as a medium. Each component is subject to a specific thermal resistance, impacting heat transfer through the conduction cooling chain. There are several forms of a conduction cooling solution: Clamshell conductors are equipped with


Wedge-Loks/Card-Loks on the boards as well as an inserter/extractor handle. This ensures a completely assembled configuration will secure Printed Circuit Board (PCB’s) in the system, as well as enabling heat conduction through direct contact. A clamshell design typically encloses the entire PCB including any components on both sides. Two methods that are used are a flat inner contour, or a profiled inner contour matching the PCB component landscape. The profiled contour is designed to create direct contact to hot components, allowing heat to transfer through the clamshell to the enclosure. Special elastic, such as heat conductive pads are also used to attach the clamshell to the components on the PCB. With this design, the limiting factors are the ability of the enclosure to release heat into its environment. In specific situations where a small form factor embedded system is deployed, enclosures may be fully enclosed, requiring heat from processors be conduction cooled. Typical heat conductor solutions can be solid metal blocks, with thermal paste or pads to attach to hot processors. They take up variations in height tolerance while conducting heat to the surface of the enclosure.


/ ELECTRONICS Figure 1:


Cooling solutions from Pentair


cabinet level cooling solution are based on the installation site, noise level requirements and IP protection rating requirements. Installation site design also has an impact on cold water connections and air conditioning systems. Other important environmental factors include ambient temperature, presence of other heat producing equipment, and air conditions (pollution or humidity). All of these factors need considering in the design so that cooling solutions match the system as well as the operating conditions. Each element also needs to fit with the existing equipment in place. Thermal simulation remains a necessary tool to create efficient design. Principles behind thermal simulation are fairly straightforward. The user generates a 3-D model of the main features and components, which can be imported from a CAD design system. If the simulation is being run for a specific product design, models can be included to create a more accurate model of system performance. Physical parameters such as material properties and power loss of electronic components are assigned. The software then calculates the configuration state, based on the presence of convection, forced air flow, radiation and thermal conduction using a 3-D mesh of the design. The resulting flow field is analysed and graphically represented showing temperature, pressure and air movement. Based on these results, engineers can determine any changes needed to optimise air flow, heat transfer and ultimately, system performance. Different concepts can be tested using simulation to understand their impact before the manufacturing process, saving time and costs.


Pentair Technical Solutions www.pentairprotect.com T: 01442 240471


ELECTRONICS | JULY-AUGUST 2016 27


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