Selecting Heat Pipe

Selecting a Heat Pipe

When selecting a heat pipe for your next thermal management project, be sure to consider all of the following variables that apply to your specific scenario.

  1. Investigate and determine the operational parameters of your specific application:
    1. Heat load and geometry of the heat source
    2. Possible heat sink location, the distance and orientation relative to the heat source
    3. Temperature profile of heat source, heat sink and ambient
    4. Environmental condition (such as existence of corrosive gas)
  1. Select appropriate pipe material, wick structure, and working fluid
    1. Determine the working fluid appropriate for your application
    2. Select pipe material compatible to the working fluid
    3. Select wick structure for the operating orientation
    4. Decide on the protective coating
  2. Determine the length, size, and shape of the heat pipe your specific application requires
    1. Determine the dimension needed for your specific application
    2. Determine the length needed for your specific application
    3. Determine whether any custom bends will be needed in your heat pipe

What is a heat pipe?

A heat pipe is an extremely efficient heat conductor consisting of a vessel, a working fluid, and a wick structure. All three of these elements work simultaneously to create an efficient heat transfer device.

How is a heat pipe made?

The process of making an Enertron heat pipe begins by thoroughly vacuuming the heat pipe vessel to clear the passage of any obstructions, charging the vessel with the working fluid desired, and sealing the vessel. Once the heat pipe has been hermetically sealed, it is ready for use.

How does a heat pipe work?

A heat pipe is activated by heating one end of the pipe to allow the working fluid inside of it to evaporate from liquid to vapor. The vapor then travels through the hollow core of the heat pipe from the evaporator end to the condenser end at near sonic speed.

During this process, the lower temperature at the condenser end induces the vapor to condense back to liquid while dissipating heat that is being removed by a heat sink or other means. The liquid then travels back to the evaporator end of the heat pipe via the wick structure using Capillary Force.

The energy required to change phase from liquid to gas is called the Latent Heat of Evaporation. As an example, the latent heat of evaporation for water is 539 cal/g. The specific heat of water is 1 cal/g°C.

Therefore, the working fluid in a heat pipe can transport a very large amount of heat and make heat pipes 100 to 1,000 times better than a solid copper rod alone.

Figure 1 is a schematic of a heat pipe.

Enertron Heat Pipe Structure Showing Evaporation & Condensation Zone as Working Fluid Passes Through Vessel

What are the four heat transport limitations of a heat pipe?

The four heat transport limitations can be simplified as follows:

    • Sonic limit– the rate that vapor travels from evaporator to condenser
    • Entrainment limit– friction between working fluid and vapor that travels in opposite directions
    • Capillary limit– the rate at which the working fluid travels from condenser to evaporator through the wick
    • Boiling limit– the rate at which the working fluid vaporizes from the added heat.

When to consider a heat pipe?

There are three thermal conditions that may lead to the use of heat pipe:

  1. To act as a primary heat conductive path
    • When a heat source and heat sink need to be placed apart, a heat pipe can be a very effective heat conduction path for heat transportation from the heat source to the heat sink.
  2. To aid heat conducting of a solid
    • Heat pipe can add the efficiency and transport capacity of a thermal shunt.
  3. To aid heat spreading across the plane
    • Heat pipes can be used to increase the heat spreading across a large heat sink base, thereby effectively increasing the base thermal conductivity. The effect of this is the decrease of the temperature gradient across the base (increase in efficiency), thereby lowering the heat source temperature.
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