Transcript Improving Cleanroom Air Distribution Systems Using

Improving and Trouble Shooting Cleanroom
HVAC System Designs
By
George Ting-Kwo Lei, Ph.D.
Fluid Dynamics Solutions, Inc.
Clackamas, Oregon
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
Introduction to cleanroom HVAC design
• Primary functions of cleanroom HVAC systems
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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.
• Types of cleanroom flow
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Conventional type of cleanroom flow
Unidirectional flow
Mixed type of cleanroom flow
Minienvironment
• Types of Cleanroom layout
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Ballroom type
Service chase type
Minienvironment type
Conventional type of cleanroom flow
Air Supply
Critical zone
Air Exhaust
Unidirectional flow
Air Supply
Critical zone
Air Exhaust
Mixed type of cleanroom flow
Air Supply
Critical zone
Air Exhaust
Minienvironment
Air Supply
Critical zone
Air Exhaust
Ballroom type
Service area
Office and
Support area
Cleanroom
Service chase type
Service area
Office and
Support area
Cleanroom
Minienvironment type
Minienvironment
Service area
Office and
Support area
Cleanroom
• Primary cleanroom HVAC system design parameters
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Energy efficiency
Cleanliness
Cost
Temperature uniformity
Humidity control
Chemical exhaust efficiency
Noise control
Make up air supply
• Methods to improving cleanroom HVAC system design
Combinations of the following approaches
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Analysis of experimental data
Rules of thumbs and Experiences
Empirical equations
Computational Fluid Dynamics or so called Air Flow Modeling
• Common problems of a wrongly designed cleanroom HVAC system
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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
A case study: Examination of flow laminarity of a cleanroom with a
subfab underneath
Floor
Ceiling
FAB
CHASE
SUBFAB
Slab
CFD model geometry
• An example of a wrong design and method of trouble shooting
8’
24’
24’
16’
10’
Notes:
1. Flow rate of each RAU, 21,312 cfm
2. 100 % coverage
Initial design
8’
22’
16’
18’
18’
Notes:
1. Flow rate of each RAU, 21,312 cfm
2. 100 % coverage
Improved design
Comparison of two designs
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Pressure drop across the plenum excluding HEPA filter
(1)
(2)
Initial Design: 0.6 inches of water
Improved Design: 0.3 inches of water
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Energy savings for a 2 system running 2 inches of water
(0.6-0.3)/2.0 = 15%
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Avoid Failure of system air performance
Introduction to Computational Fluid Dynamics (CFD)
and CFD Applications
•
Navier-Stokes Equations


   (  v)  0
t




(  v)    (  v v )  -  p       F
t
W here
is t hedensit y

v is t he velocit yvect or
p is t hepressure
 is t hest ress t ensor

F is t hebody force
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.
CFD Assumptions
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Assumptions are often necessary when formulating CFD equations.
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Examples of assumptions
Flow entrances
Flow exits
Filters
Perforated plates
Turbulent models
Computer model geometry
Comparison among Various Cleanroom HVAC System
Design Methods
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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.
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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.
A case study: Examination of flow laminarity of a cleanroom with a
subfab underneath
Floor
Ceiling
FAB
CHASE
SUBFAB
Slab
CFD model geometry
ft/min.
Flow pathlines for the case with 35% floor peroration
degree
Flow angles for the case with 35% floor peroration
ft/min.
Flow pathlines for the case with 20% floor peroration
degree
Flow angles for the case with 20% floor peroration
ft/min.
Flow pathlines for the case with 10% floor peroration
degree
Flow angles for the case with 10% floor peroration
ft/min.
Flow pathlines for a narrower cleanroom for the case with 35% floor perforation
degree
Flow angles for a narrower cleanroom for the case with 35% floor perforation
A case study: Computer aided design of chemical exhaust systems for
vicinity near I/O of an implanter.
Exhaust system setup, Case 1
Exhaust system setup, Case 2
Flow pathlines, air originated from the ceiling at x = 8.75 inches
Chemical concentration at x = 8.75 inches
Flow pathlines, air originated from the ceiling at y = -4 inches
Chemical concentration at y = -4 inches
A case study: Computer aided design improvement of a duct transition
CFD model geometry, Case 1
CFD model geometry, Case 2
CFD model geometry, Case 3
CFD model geometry, Case 4
Velocity distribution at x = 42”, Case 1
Velocity distribution at x = 42”, Case 2
Velocity distribution at x = 42”, Case 3
Velocity distribution at x = 42”, Case 4
Pressure contour at x = 42”, Case 1
Pressure contour at x = 42”, Case 2
Pressure contour at x = 42”, Case 3
Pressure contour at x = 42”, Case 4
A case study: Size reduction of the vortexes behind equipment.
CFD model geometry, Case 1
Velocity distribution at z = 5 feet, Case 1
CFD model geometry, Case 2
Velocity distribution at z = 5 feet, Case 2
Conclusions
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CFD can be used effectively in many applications in cleanroom HVAC design.
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Case 1: Cleanroom laminarity study
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Case 2: Implanter I/O
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Estimate pressure loss of each design and assist selection of final design.
Case 4: Reduction of Vortexes
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Effectively determine exhaust duct geometry and location.
Case 3: Duct Transition
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Effectively investigate parameters affecting flow angles.
Investigate and evaluate effective method in reducing the sizes of vortexes.
CFD can help designer make decisions with more confidence.