scope:

INTRODUCTION

In subtropical area such as Taiwan, the operating power cost of air conditioning systems for a typical commercial building accounts for more than 50% of the total power bill. Therefore, a significant energy savings can be achieved if heat recovery technologies such as run-around coils, plate-to-plate heat exchangers, heat recovery wheels, and heat pipe heat exchangers (HPHXs) are incorporated. There are several types of heat exchangers available for heat recovery applications. Run-around coils are relatively cheap, but a pump pack and an expansion tank are needed to run the system. Plate-to-plate heat exchangers are fairly efficient, but are bulky, costly, and extremely difficult to maintain. Also, condensate can be trapped on the plates with a resultant growth of molds. Heat recovery wheels are difficult to clean and cross-contamination is always a concern. Apart from these drawbacks, heat recovery wheels do not efficiently drain condensation. For many years, heat pipe heat exchangers (HPHXs) with two-phase closed thermosyphons have been widely applied as dehumidification enhancement and energy savings devices in HVAC systems in Western countries (Yau 2007), and the application of a double heat pipe heat exchanger system in the conventional air handler unit operating in a tropical climate is strongly recommended as an efficient method for humidity control and energy savings in order to maintain acceptable room conditions (Yau 2008). However, in a subtropical area such as Taiwan, the temperature difference between the evaporation side and condensation side is not so large. Therefore, the application of a heat pipe as a heat exchanger in the heat recovery system is not very popular. In semiconductor cleanrooms, therefore, potential energy recovery from the EA is enormous. Furthermore, in the hot and humid season, the moisture removal capability of chilled water coil in the heating, ventilating, and air conditioning (HVAC) systems can be enhanced if the supply air is pre-cooled before reaching the chilled water coil.

For the optimal design of a gas-to-gas heat exchanger under the limitations of no difference of pressure drop of the cold side and hot side, Bajan (1977) conducted an analysis of system entropy generation that taking into account the difference of flow rate but still ignoring the pressure drop, an optimal model for the heat exchanger design. However, the model is only applied in trend analysis of the model but not in real operating conditions (Sarangi and Chowdhury 1982). Additionally, a model proposed for minimization of the pressure drop between the cold /hot sides under a fixed dimension (Ooba 1959).