Why Military and Commercial Designers Use Heat Pipes, Especially Copper Heat Pipes

Heat pipes are extremely effective in high thermal conductivity. Heat pipes transfer heat from its source to heat sinks over long distances through latent vaporized working fluid heat. Typically heat pipes have three sections. They are a sealed containment vessel or shell that is vacuum tight, a working fluid, and a capillary wick structure. They work together to transfer heat evenly and efficiently. 

Heat is absorbed by vaporizing the working fluid. Vapor transports the heat to condenser regions. As vapor condenses, it releases heat to cooling mediums. The heat pipes’ wicks structure or gravity returns the condensed working fluid to the evaporators. This process creates capillary action. Planar and cylindrical heat pipes have an inner capillary lined surface of wicking material. 

The working fluid saturates the inner surface of heat pipe shell. The wicks provide the structure needed for capillary action that returns liquid from the condensers to the evaporators. Because of the vacuum seal, the working fluid boils and takes up latent heat below its atmospheric pressure boiling point. Water boils at 0ᵒ C and transfers latent heat effectively at that temperature. 

Military and commercial designers use high-performance copper heat pipes for power density capacity regardless of orientation or gravity. They are specifically designed for applications having gravity present thermal challenges or high heat loads. Long life and reliability are critical. Copper heat pipes have sintered copper powder wick structures that operate precisely against gravity. The heat pipes are rugged enough to withstand temperatures that range from -55ᵒ C to 180ᵒ C and numerous freeze-thaw cycles. 

Water is the working fluid used in copper heat pipes. It moves heat from the source to where it can be managed through dissipation liquid, air, or radiated to space. The heat pipes are incorporated into custom thermal solutions designed by engineering teams. They are integrated into metallic cold plates or heat sinks to improve efficiency and conductivity. The heat pipes are incorporated into extended surfaces, cold plates, and heat sinks with mechanical interference, solder, or epoxy.

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How Improved Extruded Aluminum Heatsinks Are Used

Due to the malleability of an aluminum alloy, extruded aluminum heatsinks can be created in a variety of unique designs. Aluminum conducts and reflects heat well. Those qualities make it useful in applications of heat transfer and reflective heat shields. An aluminum alloy is low in cost and can be tempered in various ways. Through smelting, scraping, and refining the aluminum alloy can be pounded into sheets, fins, and foils.

Aluminum provides less thermal conductivity than other metals like copper. However, it is easier by far to make heat sink extrusions from aluminum that those other metals. Creating custom aluminum heat sink variation profiles is also considerably easier. Aluminum fin profiles can be attached easily to a copper base that conducts more thermal energy to the less expensive and lighter cooling fins made of an aluminum alloy.

Heat sink extrusions serve electronic, medical, military, automotive, electrical, and telecommunication industries. A liquid cooling solution that combines extrusions made of aluminum with friction stir welding makes air-cooled heat sinks higher quality and more thermally efficient that pressed or bonded fins.

Complex fin structures are created by forcing raw aluminum through extrusion dies. The complex fins allow more heat dissipation and increased surface area. Time and cost associated with machining a shape that is equivalent from block aluminum are eliminated.

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Heat Sinks Used in LED Lighting Designs

The correct LED heat sinks must be determined for new LED lighting designs. The approach of integral models and verification test discussed here give an insight into the functional integrity and operational reliability of design in meeting market expectations.

Each LED has its set of parameters. Ambient temperature is one of the parameters. Different lights require different temperatures. Mounted, open air spotlights need 30ᵒ C, recessed ceiling lights 50 to 55ᵒ C, and automotive lighting 45ᵒ C.

The parameters must be defined. The LED Chips on Board module manufacturers provide lifetime expectations under conditions that are ideal. They calculate 90 percent reliability for the maximum junction temperature.

In calculating the required LED heatsinks, it must be understood that each part of a design adds heat due to the material’s thermal resistance. The total design should be below the maximum junction temperature required.

A mathematical calculation is made to define the maximum thermal resistance heat sinks should have or the maximum rise in temperature LED heatsinks create when dissipating power. Thermal resistance is expressed as a Rth value. Some manufacturers give thermal resistance value for heat sinks that are independent of dissipation power and ambient temperature. The Rth value of LED heatsinks is not the same under all conditions.

After applying thermal pads and heat sinks, verify the design. Manufacturers include safety margins in their designs. A maximum of 97.4ᵒ C thermal measurement point should not be subjected to temperatures more than the range of 87 to 92ᵒ C.

All high power LEDs need to dissipate the heat produced to keep it below the maximum operating temperature. Overheating can cause a shortened life, a reduction in light output, an output color change, or complete LED failure.

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All About Heat Sink Extrusions and Extruded Aluminum Heatsinks

Heat sink extrusions provide more natural convection solutions for high-powered systems and components. Complex fin structures are made by forcing raw aluminum through extrusion dies. The process of manufacturing aluminum heatsinks is somewhat like a Play-Doh factory. Pick the desired shape and squeeze Play-Doh through the shape.

The difference is extruded aluminum heatsinks exit the press opening at approximately 1000ᵒ F. In addition to the high temperature, the extrusion press pushes with a force in excess of 1500 tons. The pressure and temperature are controlled to obtain the profile required.

The complex fin profile allows greater heat dissipation by increasing the surface area while eliminating the time and cost associated with the machination of an equivalent shape made from block aluminum. The benefits of heat sink extrusion are:

• Greater efficiency than stamped heatsinks

• Cost less than assemblies that are fully machined

• Availability of many standard sizes and shapes

• Any application is easily customized

• A weight advantage over copper that is significant

• Tool and hardware mounting are eliminated by a clip system

When manufacturing extruded aluminum heatsinks, the press speed needed to maintain a steady extrusion pull-force, the weight, and the profile complexity are taken into consideration. Benefits of the system include:

• Reduced billet to billet dead cycle time

• Improved straightness

• Minimized scrap

• Improved yield throughout the process of manufacturing

The need for customized extruded aluminum heatsinks continues to grow. The growth reduces product cycle time and overall costs. Customized extruded aluminum heatsinks include:

• Shapes that require stretching and bending

• Detailed precision computerized numerical control used to make customized components that meet product application needs

• Aluminum fabrication from simple to complex precision machination

• Finishes such as heat treating, plating, painting, anodizing, or mechanical finishes

• Heat dissipation requirements.

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