A heat pipe consists of a tube sealed at both ends, with its interior maintained in a vacuum and filled with a fixed quantity of heat-transfer medium. The tube is inclined at a fixed angle and fitted with a division plate at the midpoint. Hot fluid flows through the lower side, while cold fluid flows through the higher side, thereby inducing heat transfer from the hot to the cold fluid.

Heat pipes mainly use fin tubes. As the heat transfer medium absorbs heat from the hot fluid and evaporates, the vapor pressure on the lower side rises. On the higher side, the medium releases heat to the cold fluid and condenses, thereby lowering the vapor pressure there. This pressure differential drives the evaporated medium to move rapidly through the heat pipe interior, while the condensed medium flows back down by gravity, resulting in continuous heat transfer throughout the cycle.

When heat pipes are arranged in series and applied to a heat pipe-type air preheater that recovers waste heat from the fire-tube boiler exhaust gas to preheat the combustion air, boiler energy efficiency is reported to improve by 5% or more. The installation of a Heat-pipe-type Air Preheater can be considered essential in condensing boilers designed to reduce harmful pollutant emissions in the exhaust gas.

The advantages of the Heat-pipe type Air Preheater over the Tubular Air Preheater are as follows.

No air-to-gas leakage Easy tube cleaning Minimal maintenance required, and the same performance is achievable with a compact size Easy to install and cost-effective Since each tube operates on an independent heat transfer cycle, the impact on overall system efficiency due to the failure of individual tubes is minimized