Numerical Simulation of Flow Circulation inside a Clean Room of an Electronic Manufacturing Plant

Transcription

1 The 9 th Asian Symposium on Visualization Hong Kong, 4-8 June, 2007 Numerical Simulation of Flow Circulation inside a Clean Room of an Electronic Manufacturing Plant Abstract S. Chutima, T. Chanthasopeephan and W. Wechsatol Center of Operation for Computer Aided Research Engineering (COCARE), Department of Mechanical Engineering, King Mongkut s University of Technology Thonburi, 126 Pracha-utid, Bangmod, Thungkru, Bangkok, Thailand, Corresponding author W. Wechsatol In this work, the finite element technique of computational fluid dynamics is used to visualize the flow circulation inside a clean room of an electronic component manufacturing plant. The numerical results show the flow circulation inside the clean room caused by the high velocity of air flow from the fan-ionizer. The study shows that the flow circulation can be prevented by reducing the speed of fan-ionizer and/or by increasing the space between each work-station inside the room. 1. Introduction Clean rooms are widely used in many electronic industries in order to prevent contamination during the manufacturing process. Clean rooms are categorized by the allowed number of particulates with size of smaller than 0.5 micron inside the rooms. In this work, a clean room of class 100, in which the allowed number of particulates is less than 100 particles per cubic feet is examined. In order to achieve class 100 of clean room by preventing flow circulation, the room is designed to have downward flow in laminar regime [1, 2]. Air is filtered by a high efficiency particulate air filter (HEPA filter) before entering the room through the ceiling and leaves the room through the perforated floor, bringing particulates along with it. The air flowing through the room is expected to be in laminar regime without any changes in direction and create no circulation of flow. However the installation of work-stations inside the room such as in Fig. 1 disturbs flow direction and velocity flow field as well as creates flow circulation. Chen et al. [3] used the concept of Figure 1 Installation of work-stations inside a clean room. ASV n-1

2 S. Chutima, T. Chanthasopeephan and W. Wechsatol Integrated Accessibility of Contaminant Source (IACS) to consider the contaminant removal effectiveness of many ventilation styles. They reported that the optimal ventilation strategy depends on each situation and location of contaminant sources. Marshall et al. [4] suggested that contamination in a clean room is rather caused by the air flow pattern than the air flow rate. Therefore, in this work the focus is on the air flow pattern inside the clean room after the installation of the work-stations with fan-ionizers. Furthermore, the effects of the air speed from the ionizer and the distance between each work-station on the flow pattern of air inside the clean room are considered. 2. Modelling Instead of modelling the whole room with all the work-stations, modelling of just one workstation and using the symmetry condition would give a reasonable prediction of flow field inside the room with feasible accuracy, thus saving computational time. The model of each work-station is assumed to have simple geometrical shapes for the sake of simplicity in computation. The workstation shown in Fig. 2 has the dimension of 60 cm wide 80 cm long 60 cm high with three-side dust-partition of 80 cm high and 10 cm gap between the desk and the partition. The height of the room is 2.4 m. The work-station is installed with three fan-ionizers to release the required ion over the working area. The space surrounded the work-station model in Fig. 2 is half of the adjacent distance between each work-station in the clean room due to the geometric symmetry. The temperature inside the room is assumed unchanged at 20 C, so that the properties of air such as density and viscosity remain constant throughout the flow filed. The flow field at steady state inside the room must satisfy the continuity and Navier-Stokes equations, v = 0 and ( v ) v v v v v v P = + ν 2r (1) ρ where v v is velocity vector of air flow, P is pressure distribution, ρ is density and ν is kinematic viscosity. Air enters the room though the HEPA filter installed above the ceiling with uniform velocity at 0.32 m/s and leaves the room though perforated floor at atmospheric pressure. The air is accelerated and leaves the fan-ionizers with downward velocity of 4 m/s. The symmetrical condition was applied throughout the side wall of the computational model in Fig. 2; and the no-slip condition was applied throughout the solid surface. Figure 2 model of a work- station with three fan-ionizers. 9 th Asian Symposium on Visualization, Hong Kong SAR, China, n-2

3 Numerical Simulation of Flow Circulation inside a Clean Room of an Electronic Manufacturing Plant In order to simulate the flow pattern, a commercial finite element program FEMLAB [5] was used to solve the set of Eq. (1). The unstructured grids with triangular shaped elements were used to create the mesh. The numerical grid was refined by reducing the size of the triangular shaped elements by half until the step change in the norm of velocity throughout the flow field is less than 2%. 3. Results and Discussion This study emphasises on the flow pattern of air inside the clean room. Figure 3 and 4 show the air flow pattern inside the room when the distance between each adjacent work-station is increased. The high relative velocity of air created by the fan-ionizer compared to the inlet air velocity through the ceiling causes the flow circulation inside the room. When the distance between each adjacent work-station is narrow, the recirculation of flow between the gap and the fan-ionizer may carry particulates along with it and may cause the accumulation of particulates as well damage to the electronic parts on top of the work-station. Figure 3 and 4 show that the recirculation of flow can be diminished by increasing the space between each adjacent work-station. (a) (b) (c) Figure 3 Front view: the velocity flow field when the frontal distance between each work-station is ( a) 40 cm, (b) 60 cm and (c) 80 cm. (a) (b) (c) Figure 4 Side view: the velocity flow field when the side distance behind each work-station is (a) 27 cm, (b) 40 cm and (c) 60 cm. 9th Asian Symposium on Visualization, Hong Kong SAR, China, n-3

4 S. Chutima, T. Chanthasopeephan and W. Wechsatol The effects of three different positions of fan-ionizers on the air flow pattern were investigated. Figure 5 shows that the size of recirculation on top of the work-station was reduced by moving the fan-ionizers closer to the partition. For the operating speed of fan-ionizers, Fig. 6 shows that the circulation of flow inside the clean room is diminished when the speed of ionized air is less than 2 m/s. Those flow circulations could be prevented by reducing ionizer speed; however the risk of insufficient ion must be considered. 4. Conclusion and Remarks This work is an example of how to use the visualization technique to improve the particulate removal effectiveness of a clean room. Without such technique, it is almost impossible to understand the invisible flow field and may not be able to effectively remove and prevent particulates from damaging electronic equipments. The numerical results illustrated that the high air velocity from the ionizers causes the circulation of flow inside the room as shown in Figs The size of circulation can be reduced by increasing the distance between each desk and repositioning the fan-ionizers as well as reducing the speed of the ionized air. The suitable distance between each desk preventing the circulation of flow that can cause the accumulation of particulates above the working area on the desk was presented. Figure 5 The effect of ionizer positions on the velocity flow field. Figure 6 The velocity flow field when the air velocity from the fan-ionizer is 2 m/s, the frontal distance is 40 cm and the side distance is 45 cm. 9th Asian Symposium on Visualization, Hong Kong SAR, China, n-4

5 Numerical Simulation of Flow Circulation inside a Clean Room of an Electronic Manufacturing Plant 6. Acknowledgement Authors would like to show sincere gratitude to Prof. J. C. Ordonez and Center of Advanced Power Systems, Florida State University, USA for allowing access to FEMLAB. 7. References 1. W. Whyte, Clean Room Technology: Fundamental of Design, Testing and Operation, Wiley, England, W. Whyte, Clean room Design, 2 nd ed. Wiley, Chichester, X. Chen, B. Zhao and X. Li, Numerical investigation on the influence of contaminant source location, occupant distribution and air distribution on emergency ventilation strategy, Indoor a and Built Environment 14, 6, pp , J.W. Marshall, J.H. Vincent, T.H. Kuehn and L.M. Brosseau, Studies of ventilation efficiency in a protective isolation room by the use of a scale mode, infection control hospital epidemiology, 17 (1), pp. 5-10, Comsol: Chemical Engineering Module Library, Version 3.2, Comsol AB, Stockholm, th Asian Symposium on Visualization, Hong Kong SAR, China, n-5