Handout 1
Space Pressurization:Concept and Practice ASHRAE Distinguished Lecture Series
Jim CooganSiemens Building Technologies
YEA Conference – Specialty Environmental DesignMarch 25, 2014
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Agenda
Introduction (concept, purpose, uses, scope)Physics: Infiltration and ContainmentPressurization MethodsDesign StepsContaminant Control PerspectiveSummary
Handout 2
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Room Pressurization
A ventilation technology that controls migration
of air contaminants by inducing drafts between spaces.
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Room Pressurization
Exhaust system removes air
Supply system delivers less
Room pressureis negative
Infiltration makes up the difference
Inward air flow contains pollutants
Handout 3
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Introduction: Who uses it? Why?
Biological and Chemical Laboratoriesprevent spread of airborne hazards
Hospital Isolation Roomsprotect patients and staff from germs
Hospital Pharmaciesfacilitate sterile compounding
Clean Manufacturingmaintain product quality
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Introduction: Who else uses it?
Office towerscontrol smoke in a fire; maintain exit path
Any Buildingseparate rest rooms from other spaces
Restaurantskeep kitchen smells out of the dining room
Any Buildingkeep unconditioned OA out of occupied spaces
These uses are out of today’s scope
Handout 4
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How is success defined?
Success is control of contaminants, not flows and pressure values
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Infiltration and Containment
Infiltration: mechanical processVelocity, Area, PressureInfiltration CurvesImportance of the EnvelopeSelect Pressurization LevelSpecifying the Envelope
Handout 5
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Infiltration Process:Pressure, Velocity, Area, Flow
Infiltration is a physical processPressurization is an engineered resultASHRAE Handbook and Ventilation Manual from ACGIH model the process
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Pressure vs. Velocity
Simple approach is to model the velocity with a discharge coefficient
ACGIH Industrial Ventilation: 7-3
ASHRAE Fundamentals Handbook presents more complex model, but the result is nearly the same
Pv )4000(6.0
Handout 6
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Infiltration Model for Pressurization
Air velocity through gaps in envelope controls contaminants
Velocity related to pressure by orifice flow
Transfer flow and HVAC flow difference is leak area times velocity
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Reality of Room Air Motion
Photograph of flow field (2D) in cross section of a room“Particle Image Velocimetry”Zhao L., ASHRAE Transactions, DA-07-044
Handout 7
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Velocity and Leakage Area
Flow is velocity times area2011 ASHRAE Handbook HVAC Applications,
puts it together: 53-9
Q = infiltration flow, cfmA = leakage area, sqft
P = pressure across envelope, inwc
PAQ 2610
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Infiltration Curve –Pressure Difference vs. Flow
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0 50 100 150 200 250 300 350 400 450 500
Infiltrating Air Flow
Pres
sure
Diff
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Handout 8
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Infiltration Curves for Several Values of Leakage Area
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0.005
0.01
0.015
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0.025
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0.045
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0 50 100 150 200 250 300 350 400 450 500
Infiltrating Air Flow
Pres
sure
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Importance of the Envelope
Leakage area is the main mechanical parameter in the pressurization system
Like knowing the hx characteristics to apply a heating coil
Like knowing the pipe diameter in a hydronic system
Handout 9
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Select Pressurization LevelChoose the flow offsetLet it determine the pressure
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0.005
0.01
0.015
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0.025
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0.035
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Infiltrating Air Flow
Pres
sure
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Select Pressurization LevelChoose the pressureLet it determine the flow offset
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0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 50 100 150 200 250
Infiltrating Air Flow
Pres
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Handout 10
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Select Pressurization Level
Different ways to express the level of pressurization in terms of the pressure differencein terms of the infiltration flow
“Specify either the pressure or the flow offset, not both.”
Unless you are trying to specify the envelope
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Pressurization and Migration
Positive room pressure drives air and contaminants out
Negative room pressure draws air and contaminants in
Neutral room pressure exchanges air and contaminants in both directions
Handout 11
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Pressurization via HVAC
Control Methods Explained and ComparedDifferential Flow ControlPressure FeedbackCascade Control
Selecting a Pressurization Control MethodHow Tight is Tight?Required Pressure Relationships
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Control Methods Compared
Three widely published methodsSpace pressure feedbackDifferential flow controlCascade control
References: 2011 ASHRAE Handbook, HVAC Applications.Chapter 16 Laboratory Systems Siemens Building Technologies: Doc #125-2412. Room Pressurization Control
Handout 12
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Control Methods Compared
Some other waysAdaptive leakage modelTrim valve
References: W Sun, ASHRAE Transactions, NA-04-7-2. Quantitative Multistage Pressurizations in Controlled and Critical EnvironmentsL. Gartner and C. Kiley, Anthology of Biosafety 2005.Animal Room Design Issues in High Containment
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Pressure Feedback
Handout 13
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Pressure Feedback
Measure pressure differenceacross room boundary
Compare to selected setpointAdjust supply flow or exhaust
to maintain pressure difference
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Differential Flow Control
Handout 14
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Differential Flow Control
Carefully control air supply to roomCarefully control all exhaust from roomEnforce a difference between themSelect the size of difference
to reliably contain pollutants
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Cascade Control
Handout 15
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Cascade Control
Has other names:“adaptive offset” “DP reset”
Measure pressure differenceacross room boundary
Compare to selected setpointControl supply and exhaust flowEnforce a difference between themDynamically adjust flow difference
to maintain the pressure setpoint
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Selecting a Control Method
Factors affecting selectionTightness of envelopeNumber of pressure levels neededSpeed of disturbances and responseDuct conditions for flow measurement
Reference: 2011 ASHRAE Handbook – HVAC Applications, Chapter 16 - Laboratory Systems, page 16.12
Handout 16
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Tightness of Envelope
ox
x
x x
o
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0 50 100 150 200 250 300 350 400 450 500
Infiltrating Air Flow
Pres
sure
Diff
eren
ce
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Number of Pressure Levels
Relatively simple requirement2-levels, OK for Differential Flow Tracking
Handout 17
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Number of Pressure Levels
Indicate intended relative pressure levels
+
++
-
--
-
----
--
------
--
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Effect of Air Flow Errors, In and Out
Nominal value ErrorExhaust flow 1000 +/- 100Supply flow 850 +/- 85Transfer flow 150 +/- 185
Numerical illustration
Handout 18
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Select Pressurization Level
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 50 100 150 200 250
Infiltrating Air Flow
Pres
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O
Based on leakage areaExample: 150 cfm for ½ square foot
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Select Accuracy Target
0
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0.015
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0.035
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Infiltrating Air Flow
Pres
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ce x
xO
Based on need to control contaminantsNot product spec’s
Handout 19
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Derive Flow Control Accuracy
Nominal value ErrorExhaust flow 1000 +/- 30Supply flow 850 +/- 30Transfer flow 150 +/- 45
Numerical illustrationBase flow control accuracy on desired infiltrationANSI Z9.5, Laboratory Ventilation
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Derive Flow Control Accuracy
Nominal value ErrorExhaust flow 1000
200+/- 30+/- 30
Supply flow 85050
+/- 30+/- 30
Transfer flow 150 +/- 45
Numerical illustrationConsider accuracy across range of flow valuesPressurization specs easier to meet at low flow
Handout 20
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Agenda
Introduction (concept, purpose, uses)Physics: Infiltration and ContainmentPressurization MethodsDesign Steps
Contaminant Control PerspectiveSummary
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Pressurization and Contaminant Control
Theory: net inward flow blocks contaminantsRecent research relates pressurization
to contaminant controlASHRAE research relates pressure to clean room contamination: RP 1344 and RP 1399Bio lab experiments: Bennet, Applied Biosafety, 2005Isolation room research, Tang, et al.
Success is control of contaminants, not flows and pressure values
Fact: contaminants cross boundaries for many reasons
Handout 21
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It’s like baseballEven if the wind’s blowing in, sometimes stuff gets out.
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Pressurization and Contaminant Control
Contaminant control can be very important or only slightly important
Biosafety standards recognize range of hazards and range of responses
Engineering and commissioning should match effort and solutions to needs
Handout 22
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Levels of Contaminant Control
Pressurization is one toolPhysical barrier is also
BSL 1 – Laboratories should have doorsBSL 2 – Doors should be self-closingBSL 3 – Series of two self-closing doorsBSL 4 – Airlock with air tight doors
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Summary
Space pressurization: tool for contamination control,not a ‘magic shield’Envelope leakage is main mechanical parameterSeveral HVAC control methods
Differential flow control is used most oftenChoice usually driven by envelope
Derive air flow accuracy spec from pressurizationAlign engineering effort with the hazard
Handout 23
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Thank you!Questions?
Jim Coogan, [email protected]