Functionality of the heat pipe

Fig. 2 shows the functional principle of the heat pipe. A small quantity of liquid (i.e. water, for high temperature applications sodium or potassium) is placed in a tube from which the air is then evacuated and the tube is sealed. One end of the tube is heated causing the liquid to vaporise and the vapour to move to the cold end of the tube where it is condensed. The condensate is returned to the hot end by several mechanisms depending on the type of heat pipe. Normally the gravity (in this case the system is called thermosyphon instead of heat pipe) or capillary force returns the condensate. Since the latent heat of evaporation is large, consinderable quantities of heat can be transported with a very small temperature difference from end to end. Thus the structure will have a high effective thermal conductance. This is the main advantage for the use of heat pipes in the above showen allothermal gasifier.

Fig. 2: functional principle of the heat pipe