The heat pipes are found as one of the promising technologies, which provide massive improvement of the energy performance in the energy systems. However, there are still some challenges in the employment of this technology in various applications, such as the ones with moderate-temperature operating conditions. The purpose of this study is the development of a moderate-temperature heat pipe on heat recovery, especially on replacing Ljungstrom in steam power plants with heat pipe heat exchangers. The temperature of the smoke passing through the Ljungstrom in the steam power plant is around 320 °C. For this purpose, 304 stainless steel vertical wickless heat pipe filled with special oil as working fluid is analyzed theoretically and experimentally. An experimental setup is fabricated, and the effects of aspect ratio, filling ratio, and exhaust temperature on the heat transfer rate and temperature distribution along the moderate-temperature wickless heat pipe are studied. Moreover, the operating limitations of the wickless heat pipe are investigated. A simple but accurate mathematical model is developed based on the equivalent thermal resistances and various physical limitations. By increasing the filling ratio of the heat pipe, the thermal performance of the system in all aspect ratios is reduced. The amount of heat transfer increased by increment of exhaust flow temperature up to 375 °C for aspect ratios of 11 and 25; however, for aspect ratio of 18, it is changed minimally. Based on the obtained results, the maximum heat transfer to cooling water, which is equal to 262.13 W, is achieved when FR and AR equal 30% and 18, consecutively. Furthermore, the average error for the equivalent thermal model is approximately 21% for a moderate-temperature wickless heat pipe, based on the assessment performed in this study.