This research studied the heat transfer of the thermosyphon air-preheater. An empirical model was developed to predict heat transfer and applied to compute the thermosyphon air-preheater in a hot brass forging process. The thermosyphon air-preheater was designed, constructed and tested under medium temperature operating conditions with inlet hot gases, ranging between $390-440^{circ}C$ in terms of using water as the working fluid with a 50% fill by volume of the evaporator section. The experimental setup was comprised of a circular fin with 0.013 and 0.020 m internal diameter bare stainless steel tubes, with 0.0l5 m long evaporator and condenser sections. The thermosyphon air-preheater model had 6 rows, each composed of 4 columns. Experimental results found that the hot gas temperature was increased from 390 to $440^{circ}C$, the heat transfer rate increased. If the internal diameter changed from 0.013 to 0.020 m, the heat transfer rate slightly increased. The predicted results agreed well with the experimental data. This thermosyphon air-preheater has been designed, manufactured and tested for heat recovery in industry using medium temperatures ranging between $300-500^{circ}C$ to recover flue gas energy from the furnace in a hot brass forging process.