ASHRAE OR-10-031

Abstract

<strong>INTRODUCTION</strong> <br>In the traditional arrangement of the airflow path in a cleanroom, shown in Figure 1, supply air (SA) is introduced from the ceiling and return air (RA) is located on the wall at a low level (IEST, 1995 and Whyte 2001). This system is hereafter referred to as a wall return system cleanroom. In this system, the direction of airflow is maintained consistent with the movement of gravitationally settling particles (or bioaerosols in a bio-cleanroom such as a pharmaceutical cleanroom or hospital operating room). In this type of cleanroom, the return air shaft (RAS) occupies space in the cleanroom and increases the power input of fan-filter units (FFU) due to a long flow path to/from the FFU. Moreover, the return airflow is significantly influenced by production tools and devices, as well as the movements of operators. In summary, existing problems in the wall return system are as follows: (1) the external static pressure of FFU must be high (to overcome pressure resistance of the return air grill (RAG), RAS, and dry cooling coils (DCC)), (2) the downward cold supply air from the FFU encounters upward air currents due to the heat-load from process tools, and (3) fixed positions of the RAS and DCC. Installation of supply inlets and exhaust outlets on the ceiling conflicts with the traditional concept of cleanroom airflow path arrangement. Murakami et al. (1992) indicated that changes in arrangement or in the number of exhaust openings do not have a significant effect on the entire flow field; however, such changes often have a large influence on the particle diffusion field (for 0.3 &#956;m particles) since the particle transportation path is changed by the position of the exhaust outlets. Murakami et al. (1989) also indicated that a locally balanced supply-exhaust airflow system (the supply and exhaust airflow rates balanced locally within a flow unit) exhibits better exhaust particle performance than that of a wall return type cleanroom. For industrial cleanrooms with a high cooling load, such as semiconductor cleanrooms for semiconductor testing and for thin filming (with furnace equipment), the cleanliness level must be maintained and temperature controlled within a strict interval, and currently, the size of the particles of concern fall into the sub-micron region due to advances in semiconductor manufacturing technology. To solve the problems mentioned above, this study proposes a unique local air distribution scheme that can maintain the cleanliness level within the requirements while removing cooling load efficiently. The new proposed system (see Figure 2) is referred as a Fan Dry Coil Unit system, in which the supply inlet and exhaust outlet are installed in the ceiling and heat is removed by a dry coil located just above the process tools. This study aims to evaluate the performance of the proposed system and to compare the system with the traditional wall return system by experimental measurements in a full-scale cleanroom with a typical cooling load using a semiconductor testing machine.