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The AHU From
A Case Study Presented by
David Sellers
Portland Energy Conservation Inc.
INLAND EMPIRE CHAPTER
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At the time this was all going on, the cleanroom conditions were stable and within
specifications; 68F+1-1/2F, 45%+3%relative humidity.
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Space condition
Outdoor conditionLeaving preheat
coil
Leaving fans
Leaving reheatcoil
Leaving cooling coil,as found condition
Leaving cooling coil,repaired condition; leaving
humidifier, as foundcondition
The process as found was using $5,000 - $7,000 ofunnecessary energy per month.
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How Could This Be?
Clean room conditions were at spec
Fast track design build project Specific clean room requirements
Non-specific HVAC process requirements
Clean room qualif ication basedcommissioning focus
The nature of the semi-conductorindustry business cycle
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Preheat Problems
Shut down steam whenits not needed
Integral face and bypasstype preheat coil
Valve in place, not controlsignal
Calibrate sensor Add Alarms
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Preheat Problems
Integrate control cycle Eliminate multimode control approach
Operating cycles based on OAT rather thanclean room
Inconsistent operation
Sequence with chilled water and reheat
Lock out when not required
Maintain an independent low limit
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Cooling Problems
Calibrate sensor A little tricky with an averaging element
Relative calibration more important thanabsolute calibration
Lock out when not required
Add alarms
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Humidification and ReheatProblems
Eliminated by the other fixes Improved space sensor calibration
routine
Add alarms Fan heat provided most of the reheat
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Educating the Operators EnsuredPersistence
Diligent, talented operators Lacking training in psychrometrics
Lacking training in clean room process
Informal training provided empowerment
Kept a psych chart by the operators console Nature of engineering support requests changed
Acted on their own initiative
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Systems Integration Problems
24 hour per day plant operating spec takenliterally
No light switches in the clean rooms No provisions for coordinated start-up and shut
down
Power failures and equipment failures
caused severe pressure fluctuations in thefab
Contamination issue
Safety issue
Restart issue15 -
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Systems Integration Problems
Multiple power sources on
the same system Controls on uninterruptible
power supply
Drives on emergency powerwith different auto-restartoptions selected
Control interlock circuit onnormal power
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Power Coordination Coupled withan Assembly Problem for aDISASTER!!!
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Fab Leakage = Energy Burden +Operational Danger
Two parallel, redundant fans designed so:
Only one fan runs Provides 27,000 cfm at 3.5 inches w.c.
Clean room leakage resulted in runningboth fans to achieve pressurization
required 14,000 extra cfm of outside air to move and
condition
Static requirement increased to 7 in.w.c.
Duct pressure class was 4 in.w.c.!!18 -
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Pressure Relief Doors ProvideProtection
Protect from air
hammer effectsdue to suddendamper positionchanges.
Protect over-
pressurizedducts to theextent possible
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Improved Damper Reinforcement
Simply matched factory
instructions Reinforcing plate between
sections
Bracing is an alternativeapproach
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Permissive Interlocks and BetterQuality Limit Switches
Start command opensdampers
Limit switches provingstarts the fan Wired to work in all
selector switch positions Local control mode on the
drive disabled
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HVAC Process/EfficiencyCommissioning Benefits
Reduction in operating cost and
improved performance and reliability inretrocommissioning arena.
Reduction in operating cost and first
cost with improved performance andreliability in new construction.
Reduction in CO2 emissions.Complements energy savings.
Potential for additional financial leverage.22 -
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Barriers to Moving fromQualification Based Approach
Rapid fluctuations in the semiconductormarket cycle.Short paybacks demanded.
Design innovations not embraced.
Drive to get on line and in production ASAP.
Production process tolerances.Run on detection limits.
Extremely costly to ship bad product.
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