Heat Sinks with Heat Pipes

Heat sinks with heat pipes are used to remove heat from a concentrated source and transport it to a larger surface area to improve heat transfer and dissipation efficiency. it is required to remove heat from concentrated source immediately & transport to larger surface area to improve heat transfer and heat dissipation efficiency. Heat pipe are available in various diameter, length & various power handling capacities. Heat sinks with Heat pipe are used for handling power from 40W to multi kilowatt.

 

How Heat Pipes Work:

A heat pipe is a simple device that can quickly transfer heat from one point to another. It consists of a sealed copper container whose inner surfaces have a capillary wicking material. A heat pipe is similar to a thermo siphon. It differs from a thermo siphon by virtue of its ability to transport heat against gravity by an evaporation-condensation cycle with the help of porous capillaries that form the wick. The wick provides the capillary driving force to return the condensate to the evaporator. The quality and type of wick usually determines the performance of the heat pipe, for this is the heart of the product. Different types of wicks are used depending on the application for which the heat pipe is being used.

 

When heat is applied at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe.

 

Design Guideline

Heat pipes have an effective thermal conductivity many thousands of times that of copper. The heat transfer or transport capacity of a heat pipe is specified by its “Axial Power Rating”. It is the energy moving axially along the pipe. The larger the heat pipe diameter, greater is the APR. Similarly, longer the heat pipe lesser is the APR. Heat pipes can be built in almost any size and shape.

 

  1. Orientation: Heat pipe normally perform best when they are used in vertical position however it can be used in position other than gravity however performance will be degraded & depends on many factor like heat flux, working fluid, wick structure, length of heat pipe e.t.c.
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  3. Forming:Most often need arise to bend the heat pipe thus affecting the performance. Following factor affect heat pipe performance.
    • Bend radius
    • Location of bend
    • Amount of flattening

    Careful design can minimize the performance loss & better prediction of performance.

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  5. Operating temperature:Operating temperature limits for heat pipe depends on working fluid used.Refer chart below for various fluids used & useful operating temperature range.
     

    Medium Melting Point Boiling Point Operating Range
    Water 0 100 30 to 200
    Ammonia -78 -33 -60 to 100
    Methanol -98 64 10 to 130
    Ethanol -112 78 0 to 130
    Sodium -98 892 600 to 1200
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  7. Wick or Capillary Structure: The prime purpose of the wick is to generate capillary pressure to the working fluid from the condenser to the evaporator. It must also be able to distribute the liquid around the evaporator section to any area where heat is likely to be received by the heat pipe. Often these two functions require wicks of different forms. The selection of the wick for a heat pipe depends on many factors, several of which are closely linked to the properties of the working fluid. Following are most common used wick structure in heat pipes
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    • Groove tube surface
    • Screened mesh
    • Sintered powder

     

  9. Reliability of heat Pipe: Heat pipe has no moving parts & can have long reliability but largely depends upon method of manufacturing, purity of working fluid, method of sealing, cleanliness of internal surface before sealing. It is recommended to specify reliability of heat pipe required while ordering the heat pipe.
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  11. Heat removal: It is recommended to specify the area of heat removal so heat pipe can be oriented accordingly as heat can be removed from condenser area of heat-pipe.

 

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