A heat pipe transports thermal energy via the latent heat of vaporization of an internal working fluid rather than simple solid-state conduction. Liquid vaporizes in the evaporator section absorbing enormous heat (e.g. ~2,300 kJ/kg for water), travels rapidly through the vapor core, and condenses in the cooling zone, releasing its latent heat isothermally. This two-phase cycle yields an effective thermal conductivity (keff) of 10,000 to 50,000 W/m·K, compared to only 390 W/m·K for pure solid copper.

The Capillary Limit represents the maximum pumping pressure developed by the microscopic meniscus pores of the wick structure. The capillary pressure (2*sigma / rc) must overcome the frictional pressure drop of the returning liquid through the porous wick plus the vapor pressure drop and any gravitational head. If the heat load exceeds the capillary limit, liquid cannot return fast enough, causing evaporator wick dryout and sudden thermal runaway.

A heat pipe contains an internal porous wick (such as sintered copper powder, wire mesh, or axial grooves) that pumps liquid by capillary action, allowing it to operate horizontally or even against gravity. A thermosyphon is completely wickless and relies 100% on gravity to return condensate. Thermosyphons can carry significantly higher heat loads at lower cost, but must always operate with the evaporator positioned below the condenser.

Trace chemical impurities or lack of proper internal chemical passivation allow water to react with copper or iron contaminants, releasing free hydrogen gas (H2). Because hydrogen cannot condense at operating temperatures, vapor flow sweeps it into the far end of the condenser section, creating a dead gas slug that blinds condensing area and degrades heat transfer efficiency.

The Sonic Limit occurs when vapor velocity reaches Mach 1 at the evaporator exit, choking mass flow. The Entrainment Limit occurs when high-velocity counter-current vapor shears liquid droplets off the wick surface, starving the evaporator. The Boiling Limit occurs when high radial heat flux causes nucleate boiling inside the wick, forming vapor bubbles that insulate the heated wall.