As well as convection, conduction and radiation there is another method of transferring heat energy. This is a man made method and is an increasingly important technical solution to heat transfer. It is called the ‘heat pipe’.
Typically a heat pipe looks like a metal tube. Inside the tube is a small amount of a liquid with a suitable boiling point (when working at ‘shirtsleeves’ temperatures the liquid can be water). Other than the vapour from the liquid there is no gas in the tube. The inner surface of the tube is coated with a porous ‘wick’ (usually made of sintered metal powder). The heat pipe works like this:
The liquid at the hot end of the tube absorbs enough heat energy to provide the latent heat of vaporisation needed to boil some of the liquid.
The gas spreads rapidly down the tube, driven by the higher pressure of gas at the hot end.
As soon as it touches the cool end it condenses back to a liquid, giving up the heat energy it absorbed at the hot end.
The liquid at the cool end is absorbed into the wick.
Capillary action draws the condensed liquid back to the hot end, where the process repeats
There are three huge advantages of a heat pipe:
Because it uses the latent heat of vaporisation of the liquid a small heat pipe can transport very large amounts of heat energy in a short time.
Because there is no temperature change involved in either the boiling or condensing stages the heat pipe can continue to work with a very small temperature difference between the hot and cold ends.
As the temperature is raised more liquid boils off to gas. This raises the pressure inside the heat pipe which raises the boiling point of the liquid. Conversely, when the temperature is low the gas pressure is also low which lowers the boiling point. This means that a single liquid can work over a wide range of temperatures, Water for example can be used between around 30 and 200 °C. Other ‘working fluids’ can be used for different temperature ranges.