Selecting Heat Pipe

Heat Pipe Basics

What is a heat pipe?

A heat pipe is a very efficient heat conductor. A typical heat pipe consists of a vessel in which its inner walls are lined with a wick structure. The vessel is first vacuumed, then charged with a working fluid and hermetically sealed. When the heat pipe is heated at one end, the working fluid evaporates from liquid to vapor (phase change). The vapor travels through the hollow core to the other end of the heat pipe at near sonic speed where a heat sink or other means removes the heat energy. Here, the vapor condenses back to liquid form and releases heat at the same time. The liquid then travels back to the original end via the wick by capillary action. The energy required to change phase from liquid to gas is called the latent heat of evaporation. For example, the latent heat of evaporation for water is 539 cal/g. The sensible heat of water is 1cal/g˚C. Therefore, the working fluid in a heat pipe can transport a very large amount of heat and make heat pipes 30 to 100 times better than a solid copper rod. Fig. 1 is a schematic of 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.

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

The four heat transport limitations can be simplified as follows:

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

What materials can be used to construct a heat pipe?

A particular working fluid can only be functional at certain temperature ranges. Also, the particular working fluid needs a compatible vessel material to prevent corrosion or chemical reaction between the fluid and the vessel. Corrosion will damage the vessel and chemical reaction can produce a non-condensable gas.

Table 1 illustrates the typical operating characteristics of heat pipe; from past researches, experiments and from commercial production.  For example, the liquid ammonia heat pipe has a temperature range from -70 to +60˚C and is compatible with aluminum, nickel, and stainless steel.

The liquid ammonia heat pipe has been widely used in space and only aluminum vessels are used due to lightweight. Water heat pipes, with a temperature range from 5 to 230˚C, are most effective for electronics cooling applications and copper vessels are compatible with water.

Heat pipes are not functional when the temperature of the pipe is lower than the freezing point of the working fluid. Freezing and thawing is a design issue, which may destroy the sealed joint of a heat pipe when placed vertically. Proper engineering and design can overcome this limitation.

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.

What is a wick structure and how does it affect the performance of the heat pipe?

A heat pipe is a vessel whose inner walls are lined up with the wick structure. There are four common wick structures:

  • Groove
  • Wire mesh
  • Sintered powder metal
  • Fiber/spring

The wick structure allows the liquid to travel from one end of the heat pipe to the other via capillary action. Each wick structure has its advantages and disadvantages. Every wick structure has its own capillary limit.

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