One of the major parameters to consider when specifying enclosures for electronics is thermal performance. With enclosures ranging from small hand-held boxes through to desktop instrument cases, 482.6mm (19 ) racks and associated sub-assemblies to street furniture (outdoor enclosures), the thermal aspects vary almost as much as the style of housing.
Most of them have to contend with the same issues, such as providing a suitable mechanical retention for the printed circuit board(s) and power supply, allowing space for input/output connections, and consideration of environmental issues. These are best addressed during design as rectification can be a costly process.
As the volume occupied by the electronics (to perform the same function) shrinks with time, so heat becomes a greater issue. If the space is maintained, it is likely that functionality will be increased accordingly, and thus contribute towards the thermal problems. Designers need to look for innovative solutions to keep their electronics cool and maintain or improve the mean time to failure of their system.
Unfortunately, other constraints may hamper the heat removal process. Where the system is sited in a dusty or damp environment (or outdoors), it may require isolation from the offending contaminants. This is normally achieved with the enclosure by sealing to a recognised standard such as an IP rating. In turn, this prevents the use of standard air circulation for cooling, as the openings for the airflow will allow moisture and dirt access to the enclosure.
Rittal has been developing innovative solutions to the problems of heat removal from systems and enclosures for many years. Initially by offering fans and baffles to circulate and direct cooling air both within and through the enclosure, followed by the development of cabinet air conditioning systems, which may be roof, door or side panel mounted.
A recent cooling introduction removes the heat directly from source and pumps it to a remote heat exchanger. Whether this heat is generated by microprocessors, or, as in the case of AdvancedTCA, by onboard power conversion or large scale DSP, a simple liquid filled heat exchanger is fitted around the source and coupled to a manifold incorporated as part of the vertical structure of the rack. The manifold is connected in turn to pipework, which terminates in an air to liquid heat exchanger, which can be situated outside the room or even outside the building.
This solution has been developed for applications where a very large amount of computing is required, for example in scientific experiments. As processors become faster and more powerful, a general flow of cooling air is insufficient and they require individual attention to keep them cool (just look inside your PC to see the heat removal system installed for the processor!) This problem is amplified in the application above, by the number of processors housed in the same enclosure.
By using water as the main cooling transfer medium (water is 1000 times more effective than air at removing and carrying heat), the size of the local heat transfer interface at the point of generation is also significantly reduced. In fact, the heat exchanger piggy backed onto the processor is much smaller than the previous generation of air-cooled heatsink which it replaces.
For ease of use and increased effectiveness, the Rittal system uses snap-together pipework connections and also incorporates a manifold in the uprights of the server rack itself. Thus only a single flow and return are needed between the rack and the remote water to air or water/water heat exchanger.
The effectiveness of this solution is such that the CPUs in 21 x 42U racks when loaded at 4 CPUs per U of height can be maintained at a long life working temperature, while keeping the noise in the computer room to a minimum as no fans or blowers need to be present.
More recently the liquid cooling system has been integrated such that the plug-in board can be hot swapped as normal, with the liquid flow and return automatically shutting off and drip-proof connections maintaining safety during the process.Other thermal issues, which are affecting enclosure design and configuration, can be found in the latest specifications to come from PICMG, the PCI Manufacturers Group.
A major change between the PICMG2.x (CompactPCI) and PICMG3.x series of specifications introduced for the Advanced Telecom Computing Architecture (Advanced TCA) is to distribute -48V power to all plug-in boards via the backplane, and then to use on-board conversion for the operating voltages required. This obviates the need for very high currents at low voltage to be distributed around the subrack.
Additionally each plug-in board may house several mezzanine boards. This can increase the total heat generated to around 200W per board. Also, due to the density of PCB population, there is likely to be increased resistance to air flow, thus demanding air movers which operate effectively at higher static pressure than conventional axial fans.
Rittal has developed high specification RiCool blowers which can be hot-swapped and which fit in a minimal 1U of subrack height to meet these needs.
Partly because of the space required to accommodate the heatsinks for the processors and other high power silicon, and the mezzanine boards, the ATCA specification has increased the slot width from 4HP to 6HP (20.32 to 30.48mm) between the plug-in boards.
However, the Rittal fluid filled thermal cooling system could be adapted for use with microprocessors, large scale DSP and on-board power conversion within the current framework of AdvancedTCA with a likely reduction in overall system height brought about by removing over half the heat generated at plug-in board level.
The increasing need to remove heat from ever more powerful systems is creating pressure on enclosure manufacturers to create a working balance between thermal and other considerations, such as EMC, and look for new ways to overcome the problems presented.