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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 heat pipe 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 of the heat pipe to the other end of the heat pipe at near sonic speed, where heat energy is being removed by a heat sink or other means. Here, the vapor condenses back to liquid and releases heat at the same time. The liquid then travels back to the original end of the heat pipe via the wick by capillary action. The energy required to change phase from liquid to gas is called the latent heat of evaporation. Figure 1 is a schematic of a heat pipe.

When to Consider a Heat Pipe?
There are three thermal conditions that may lead to the use of heat pipe:

  1. Heat pipe 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. Heat pipe to aid heat conducting of a solid. Heat pipe can add the efficiency and transport capacity of a thermal shunt.
  3. Heat pipe 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 heat sink base thermal conductivity. The effect of this is the decrease of the temperature gradient across the heat sink base (increase in efficiency), thereby lowering the heat source temperature.

Things to Consider When Designing With Heat Pipes
As an effective heat conductor, heat pipe can be used in situations when a heat source and a heat sink need to be placed apart, to aid heat conduction of a solid, or to aid heat spreading of a plane. However, not every heat pipe is suitable for all applications. For that reason, the following need to be considered when designing with heat pipes:

  1. Heat transport limitation of the heat pipe
  2. Wick structure of the heat pipe
  3. Length and diameter of the heat pipe
  4. Heat pipe orientation
  5. Effect of bending and flattening of the heat pipe
  6. Heat pipe reliability

Heat Transport Limitations of a Heat Pipe
There are four heat transport limitations of a heat pipe:

  1. Sonic limit - the rate that vapor travels from evaporator to condenser
  2. Entrainment limit - Friction between working fluid and vapor which travel 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

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

  1. Groove
  2. Wire mesh
  3. Powder metal
  4. 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.

Heat Pipes Reliability
Heat pipes have no moving parts and you are looking at 20 years MTBF operational. However, care must be given when designing and manufacturing the heat pipe. Two manufacturing factors can reduce the reliability of the heat pipe: the seal of the pipe and the cleanness of pipe internal chamber. Any leakage in the heat pipe will eventually fail the pipe. If the internal chamber is not thoroughly clean, when the heat pipe subjected to heat, the residual may generate non-condensable gas and degrade the pipe performance. Improper bending and flattening of the pipe may also cause the leakage on the pipe seal. There are some external factors that may also shorten the life of a heat pipe such as shock, vibration, force impact, thermal shock and corrosive environment.

Based on all of the designing criteria and limitations of the heat pipes, designing with heat pipes might not be an easy task. You can consult Enertron engineer for assist you in clarifying any design dilemmas, or other questions regarding heat pipes that you might have.

Thermoguide Ltd. Advanced Thermal Solutions 972-54-7251937 zamish1@netvision.net