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Improving and Trouble Shooting Cleanroom HVAC System Designs By George Ting-Kwo Lei, Ph.D. Fluid Dynamics Solutions, Inc. Clackamas, Oregon

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Outline Introduction to cleanroom HVAC design Introduction to Computational Fluid Dynamics (CFD) and its applications A case study: Examination of flow laminarity of a cleanroom with a subfab underneath A case study: Computer aided design of chemical exhaust systems for vicinity near I/O of an implanter. A case study: Computer aided design improvement of a duct transition A case study: Size reduction of the vortexes behind equipment Conclusions

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Primary functions of cleanroom HVAC systems Provide filtered supply air at sufficient flow rate and with effective flow patterns to reach a specified class of cleanliness. Provide filtered outdoor air for occupants and equipment. Exhaust effectively unwanted chemicals. Maintain specified cleanroom pressure. Add or remove moisture to regulate cleanroom humidity. Add or remove thermal energy to regulate cleanroom temperature. Introduction to cleanroom HVAC design

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Types of cleanroom flow Conventional type of cleanroom flow Unidirectional flow Mixed type of cleanroom flow Minienvironment Types of Cleanroom layout Ballroom type Service chase type Minienvironment type

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Conventional type of cleanroom flow Air Supply Air Exhaust Critical zone

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Unidirectional flow Air Supply Air Exhaust Critical zone

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Mixed type of cleanroom flow Air Supply Air Exhaust Critical zone

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Minienvironment Air Supply Air Exhaust Critical zone

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Ballroom type Office and Support area Cleanroom Service area

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Service chase type Office and Support area Cleanroom Service area

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Minienvironment type Office and Support area Cleanroom Service area Minienvironment

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Primary cleanroom HVAC system design parameters Energy efficiency Cleanliness Cost Temperature uniformity Humidity control Chemical exhaust efficiency Noise control Make up air supply

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Methods to improving cleanroom HVAC system design Combinations of the following approaches Analysis of experimental data Rules of thumbs and Experiences Empirical equations Computational Fluid Dynamics or so called Air Flow Modeling

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Common problems of a wrongly designed cleanroom HVAC system Insufficient air flow Inadequate laminarity Fail to pressurize to specified pressure level Local stagnition near point of service Big stagnition zones Ineffective chemical vapor exhaust Too high noise Temperature variation above specifications Humidity variation above specifications

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A case study: Examination of flow laminarity of a cleanroom with a subfab underneath CFD model geometry Floor Ceiling Slab FAB SUBFAB CHASE

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An example of a wrong design and method of trouble shooting 24’ 16’ 10’ 8’ Notes:1. Flow rate of each RAU, 21,312 cfm 2. 100 % coverage Initial design

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22’ 16’ 18’ 8’ Notes:1. Flow rate of each RAU, 21,312 cfm 2. 100 % coverage Improved design

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Comparison of two designs Pressure drop across the plenum excluding HEPA filter (1)Initial Design: 0.6 inches of water (2)Improved Design: 0.3 inches of water Energy savings for a 2 system running 2 inches of water (0.6-0.3)/2.0 = 15% Avoid Failure of system air performance

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Navier-Stokes Equations Introduction to Computational Fluid Dynamics (CFD) and CFD Applications

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1.Divide solution domain into finite cells. 2.Formulate CFD equations by Finite Volume or Finite Element method. 3.Solve CFD equations by a digital computer.

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CFD Assumptions Assumptions are often necessary when formulating CFD equations. Examples of assumptions Flow entrances Flow exits Filters Perforated plates Turbulent models Computer model geometry

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Comparison among Various Cleanroom HVAC System Design Methods Rules of thumb Advantages: Designs are done very quickly and inexpensively. Disadvantage: Rules are very general and may require large safety margins to ensure that the design is successful. Empirical equations Advantages: The equations can be used to quickly predict conventional usage of the design. Disadvantages: When the parameters of the design vary, the uncertainties of solutions can often be significant.

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Physical Modeling Advantages: Designer can see and feel the environment governed by this design. Disadvantages: Expensive. Computational Fluid Dynamics (CFD) Advantages: (1) Less expensive compared to physical modeling. (2) May sometimes predict some potential design flaws so that they can be remedied before the facility is constructed. (3) May quickly explore possible opportunity for improved performance. (4) Can model a variety of options for both planned and operating designs so that the most economical solutions can be pursued with a high degree of confidence in their validity. Note: In some applications, physical modeling is still required after flow modeling. However, flow modeling can reduce the number of prototypes.

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A case study: Examination of flow laminarity of a cleanroom with a subfab underneath CFD model geometry Floor Ceiling Slab FAB SUBFAB CHASE

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Flow pathlines for the case with 35% floor peroration ft/min.

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Flow angles for the case with 35% floor peroration degree

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Flow pathlines for the case with 20% floor peroration ft/min.

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Flow angles for the case with 20% floor peroration degree

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Flow pathlines for the case with 10% floor peroration ft/min.

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Flow angles for the case with 10% floor peroration degree

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Flow pathlines for a narrower cleanroom for the case with 35% floor perforation ft/min.

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degree Flow angles for a narrower cleanroom for the case with 35% floor perforation

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A case study: Computer aided design of chemical exhaust systems for vicinity near I/O of an implanter. Exhaust system setup, Case 1

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Exhaust system setup, Case 2

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Flow pathlines, air originated from the ceiling at x = 8.75 inches

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Chemical concentration at x = 8.75 inches

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Flow pathlines, air originated from the ceiling at y = -4 inches

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Chemical concentration at y = -4 inches

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A case study: Computer aided design improvement of a duct transition CFD model geometry, Case 1

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CFD model geometry, Case 2

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CFD model geometry, Case 3

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CFD model geometry, Case 4

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Velocity distribution at x = 42”, Case 1

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Velocity distribution at x = 42”, Case 2

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Velocity distribution at x = 42”, Case 3

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Velocity distribution at x = 42”, Case 4

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Pressure contour at x = 42”, Case 1

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Pressure contour at x = 42”, Case 2

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Pressure contour at x = 42”, Case 3

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Pressure contour at x = 42”, Case 4

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A case study: Size reduction of the vortexes behind equipment. CFD model geometry, Case 1

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Velocity distribution at z = 5 feet, Case 1

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CFD model geometry, Case 2

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Velocity distribution at z = 5 feet, Case 2

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Conclusions CFD can be used effectively in many applications in cleanroom HVAC design. Case 1: Cleanroom laminarity study –Effectively investigate parameters affecting flow angles. Case 2: Implanter I/O –Effectively determine exhaust duct geometry and location. Case 3: Duct Transition –Estimate pressure loss of each design and assist selection of final design. Case 4: Reduction of Vortexes –Investigate and evaluate effective method in reducing the sizes of vortexes. CFD can help designer make decisions with more confidence.

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