**Abstract**

With increasing global demands for energy (especially in developing countries), energy production will increase, the wasted energy will increase, and the emission and pollution will increase also. That makes the researchers focus on recovering the wasted heat and enhancing the recovery devices to improve the energy-saving amount. Heat pipe technology is one of the promising methods of transfer heat efficiently between two species. There are three common types of heat pipe; conventional heat pipe, thermosyphon, and oscillating heat pipe. Each type contains three sections: evaporator, adiabatic, and condenser section. The heat pipe as a heat exchanger was investigated and experimentally used by many authors to recover the wasted energy in many engineering applications.

**Keywords:** heat transfer, heat exchanger, heat recovery, heat pipe, pulsating heat pipe
