In recent years, heat-pipe heat exchangers have played a significant role in waste heat recovery. In some cases, the heat-pipe heat exchangers are required to be inclined to gravity as several experimental investigations have reported. Similarly Chato studied laminar condensation inside inclined tubes. However, no uniform conclusion on optimum inclination angle can be made due to the differences among the operating conditions. In this paper, both theoretical and experimental studies on vertical and inclined thermosyphons have been carried out. A semi-empirical correlation is presented for the purpose of designing heat pipe heat exchangers.