the ahu from hell

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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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