The main reasons of heat pipes exceptional efficiency are high speed of vapor in vacuum (up to 150 mph) and highest heat transfer coefficients attainable at phase transition. Boiling curve on Figure 1 explains this phenomenon. Vacuum reduces water saturation temperature Tsat, so the temperature difference ΔT between wall temperature Tw and Tsat increases and heat transfer regime shifts into nucleate region. Heat transfer coefficients in nuclear region are two orders of magnitude higher than in convection area where hydronic boilers operates.
