**1. Introduction**

The energy consumption of many countries has been increased mostly in growing countries. A rise in energy-consuming has been recognized because of the main developments in different sectors [1]. It is expected that energy-consuming even has a quicker increase in equatorial (hot) countries in comparison with the other countries. The energy side and global warming are the principal matters now and one of the main challenges for scientists and policymakers. One of the main processes that participate in the emissions lowering is an application of a heat recovery system to improve the thermal performance in the expression of energy-consuming. Air conditioning units are one of the important energy consumers, and commonly the running electrical cost of air conditioning systems accounts for about 50% of whole energy bills [2]. The heat will recover if the energy recovery applications are combined with air conditioning units to decrease the energy required of air conditioning units [3].

**Figure 1** shows the use of a heat recovery device in the HVAC system, a high temperature (hot) and humid inlet air is required in an occupied building. The return air (cold) can be used to precool the incoming outdoor (hot) air by employing a heat recovery coil. Conventional heat recovery coils include stationary finned or plate-type air-to-air heat exchangers or rotary heat wheels [4]. Heat pipe heat exchanger (HPHX) suggested as an efficient heat exchanger is for this purpose [3].

The advantages of using the heat recovery device in HVAC systems are:

1.Energy reduction and, thus, primary energy consumption, which led to reduce emission production.

**Figure 1.**

*Air conditioning system with precool and reheat coils [4].*


The purpose of a heat pipe heat exchanger for heat recovery in hot and cold climates is widely known. The heat pipe exchanger offers a low air pressure drop, related to possible by loop configurations, and heat recovery applications can be extended to milder climates and still pay for themselves. A new possibility is 'cooling' recovery in the summertime, which is now economical enough to be considered. The application of heat pipe as a heat exchanger to recover wasted heat is participated to enhance the dehumidification capacity of a conventional cooling coil is one of the most attractive applications. The used heat pipe heat exchanger in HVAC systems as a dehumidifier device can decrease 10% the air relative humidity that leads to resulting in a remarkable enhancement of the quality of fresh air and a decrease in power demand. The heat pipe is a promising technology to enhance the quality of fresh air, and at the same time help save energy.

The objectives of this chapter are to view heat pipe technology, the mechanism of heat and mass transfer, and assess the performance of heat-pipe heat recovery units for the HVAC system. This chapter presents the updated development status of the heat pipe used as a heat exchanger used in the HVAC system to recover the wasted heat. In this chapter, the experimental and theoretical investigations of the heat pipe, Thermosyphon, and OHP are systematically summarized.
