Editing Heat pipe (section)

==Heat transfer==
Heat pipes rely on phase change to transfer thermal energy. They cannot lower temperatures at either end below the ambient temperature (hence they work to equalize the temperature within the pipe).

When one end of the heat pipe is heated, the working fluid inside the pipe at that end vaporizes and increases the vapor pressure inside the cavity of the heat pipe. The [[latent heat]] of vaporization absorbed by the working fluid reduces the temperature at the hot end of the pipe.

The vapor pressure over the working fluid at the hot end is higher than at the cooler end, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapor condenses, releases its latent heat, and warms the cool end. Non-condensing gases (caused by e.g., contamination) in the vapor impede the gas flow and reduce effectiveness, particularly at low temperatures, where vapor pressures are low. The speed of molecules in a gas is approximately the speed of sound, and in the absence of noncondensing gases (i.e., if there is only a gas phase present) this is the upper limit to the velocity with which they can travel. In practice, the speed of the vapor is limited by the rate of condensation at the cold end and far lower than the molecular speed.{{Citation needed|date=March 2011}} The condensation rate is close to the sticking coefficient times the molecular speed times the gas density, if the condensing surface is very cold. However, if the surface is close to the temperature of the gas, the evaporation caused by the finite temperature of the surface largely cancels this heat flux. If the temperature difference is more than some tens of degrees, the vaporization from the surface is typically negligible, as can be assessed from the vapor pressure curves. In most cases, with efficient heat transport through the gas, it is challenging to maintain significant temperature differences between the gas and the condensing surface. Moreover, this temperature differences corresponds to a large effective thermal resistance by itself. The bottleneck is often less severe at the heat source, as the gas densities are higher there, corresponding to higher maximum heat fluxes.