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Liquid cooling halves component space, improves reliability

04 April 2013

Many renewable energy and marine resistor applications are moving towards liquid cooling because it offers superior environmental protection for drives, as well as improved reliability and smaller component sizes. Steve Hughes reports.

The advantages of liquid cooling include high efficiency, low noise and miniaturised resistor construction – a reduction by as much as 80 percent is possible. It also provides effective cooling in high ambient temperatures, very low excess surface temperature and increased component life.

In some cases, the surface temperature may fall below ambient, making water cooling appropriate for those industrial applications that demand low surface temperatures for one reason or another. These include the wood and textile processing industries as well as explosion-protected environments and wind turbines.

With the help of simulation techniques, cooling and coolant flow can be tested and optimised early on during product development, and later during manufacture. The author’s company, for example, performs simulation of gas and fluid flows, calculation of pressure, heat radiation, solid state temperature, fluid temperature, velocity and density as well as thermal effect on the environment. Products resulting from this work can utilise most liquid coolants, including some unusual and even corrosive liquids such as seawater, provided appropriate design considerations are taken into account.

By using water cooling, component temperature can be greatly reduced, which means less stress on the insulation materials and a longer component life. For example, an air-cooled braking resistor, dissipating power at 3kW will have a surface temperature of 387oC, whereas the surface temperature for an equivalent water-cooled resistor would be 35oC.

REO demonstrates the benefits
REO’s low-weight BW D330 braking resistor and its novel cooling system allows a large space saving, high ingress protection to IP69 and higher power levels up to 60kW, which is not possible with conventional air cooling. The BW D330 is also available in compact form with an integrated braking chopper. The units can also be connected in parallel to provide much larger power capacities if required.

REO has also gained a strong reputation for its encapsulated, water-cooled chokes. The single coil encapsulation, providing IP65 protection, ensures a smaller size component. REO’s Series CNW MD water-cooled chokes, which can handle currents of up to 2kA, are suitable for industrial water and standard cooling liquids, and provide easy connection for liquid cooling.

Cold plate mounting on variable speed drive frequency inverters makes use of a large metal surface instead of a traditional heat sink and can be used to cool a variety of components such as IGBTs and other power electronics devices. REO-ColdPlate is specially developed for cold plate inverter mounting, and features cooling conduits that can be connected to an existing water cooling system. Just the one cooling plate is required, saving installation space, product volume, and materials of construction.

The REOHM series BW D 158 CP, for example, provides a substructure of braking and load resistors in a cold plate mounting format for a variety of liquid cooled industrial and transportation applications. Other components can also be directly attached to the resistor and be liquid cooled.

It is also possible to build multiple components into one compact unit so cooling can be directly integrated, thus improving cooling performance and reducing component footprint. The customer only has to make two water connections, simplifying panel design.

Coolants
Whilst the technology is, broadly speaking, referred to as water cooling, the kinds of liquids that can be used as coolants are actually quite diverse and include non-conductive liquids like deionised water. Typical coolant and material suitability for water depends on percentages of calcium, magnesium, chlorides and sulphates, which can lead to deposits and corrosion.

Deionised water, or demineralised water, must have all calcium, sodium, iron, copper, chlorides, bromides, salts and impurities removed as the higher the insulation effect, the higher the corrosiveness. Glycol and aqueous solutions like ethyl glycol water have very good physical properties but are toxic, while propylene glycol water has slightly inferior properties but is more suitable for use in the food industry.

As mentioned earlier, salt water can also be used but subsequent deposits, severe corrosion and contamination must be taken into consideration. In contrast, dielectric liquids have properties similar to deionised water and are not very corrosive.

The most frequent kinds of contamination in industrial cooling systems are of a mechanical nature – deposition caused by excessive water hardness or chemical contamination and biological contaminants like algae and bacteria.

Pressure loss
Every component has a pressure loss that must be taken into consideration when designing the cooling system and the coolant pump. The coolant loss is a pressure difference created by wall friction and internal fluid friction in pipelines, fittings and valves.

Resistance is dependent on volume flow, geometry and Reynolds number. Calculation of the total pressure drop when connecting multiple cooling components will include a determination of the individual pressure drops, which can be ascertained in advance by using simulation procedures and calculation programs.

It is also possible that cooled components could overheat and be destroyed if the water cooling system fails. It is therefore important that the temperatures of cooled components are monitored. Should the nominal temperature be exceeded, a temperature activated switch can be configured to trigger an alarm. As an additional safety measure all REO water cooled resistors are designed to run at full power without the cooling medium for a minimum of five minutes.

Steve Hughes is with REO UK


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