ashrae will gg ive yy ouou theou the ww orldvisit . ashrae members who attend their monthly chapter...
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ASHRAE WASHRAE WILLILL GGIVEIVEYYOUOU THETHE WWORLDORLD
ASHRAE WASHRAE WILLILL GGIVEIVEYYOUOU THETHE WWORLDORLD
This ASHRAE Distinguished Lecturer is brought to you by the Society Chapter Technology Transfer Committee
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Complete the Distinguished Lecturer Complete the Distinguished Lecturer Event Summary CritiqueEvent Summary Critique
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Forms are available at:Forms are available at:www.ashrae.org/distinguishedlecturers
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BBECOMEECOME A FA FUTUREUTURE LLEADEREADER ININ ASHRAE ASHRAE –– WWRITERITE THETHE NNEXTEXTCCHAPTERHAPTER IINN YYOUROUR CCAREERAREERBBECOMEECOME A FA FUTUREUTURE LLEADEREADER ININ ASHRAE ASHRAE –– WWRITERITE THETHE NNEXTEXTCCHAPTERHAPTER IINN YYOUROUR CCAREERAREER
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Find your Place in ASHRAE! Visit Find your Place in ASHRAE! Visit www.ashrae.orgFind your Place in ASHRAE! Visit Find your Place in ASHRAE! Visit www.ashrae.org
ASHRAE Members who attend their monthly chapter meetings become leaders and bring information and technology back to their job.
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Space Pressurization:Concept and Practice ASHRAE Distinguished Lecture Series
Jim CooganSiemens Building Technologies
ASHRAE, Oryx Qatar ChapterMarch 8, 2014
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Agenda
Introduction (concept, purpose, uses, scope)Physics: Infiltration and ContainmentPressurization via HVACPressurization and Contaminant ControlDesigning PressurizationAir Flow Control ComponentsAir Flow Control AccuracyReview Design ProcessExamplesSummary
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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
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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
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How is success defined?
Success is control of contaminants, not flows and pressure values
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Theory and Concepts 1: Infiltration and Containment
Infiltration: mechanical processVelocity, Area, PressureInfiltration CurvesImportance of the EnvelopeSelect Pressurization LevelSpecifying the Envelope
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Theory of PressurizationTheory: pressure blocks contaminantsTheory: net inward flow blocks contaminantsSurprisingly little work done correlating pressurization
to contaminant controlCurrent ASHRAE research correlates pressure with contaminationEarlier work: Bennet, Applied Biosafety, 2005
Success is control of contaminants, not flows and pressure values
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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
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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
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Infiltrating Air Flow
Pres
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Infiltration Curves for Several Values of Leakage Area
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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
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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
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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
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Select Pressurization LevelChoose the flow offsetLet it determine the pressure
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Infiltrating Air Flow
Pres
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Select Pressurization LevelChoose the pressureLet it determine the flow offset
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Infiltrating Air Flow
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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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Specifying the EnvelopeSpecify a value for one variable Specify a range for the other Implies accepted range of leakage
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Specifying the Envelope
Set one parameter (flow or pressure) as the intended operating point
Set an allowable range for the other as a way to specify leakage area
ASHRAE Standard 170 suggests leakage ratefor hospital isolation rooms
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Test and Adjust the EnvelopeIf it’s in the spec...Cx agent or TAB contractor tests the envelopeDirects contractor to adjust leakage area
to specified rangeCorrection can include:
adjustable door sweeptransfer opening with restrictionseal cracks
Reference: A. Geeslin et al., ASHRAE Transactions, SL-08-044Air Leakage Analysis of Special Ventilation Hospital Rooms
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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 both directions
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Pressurization via HVAC
Required Pressure RelationshipsControl Methods Explained and Compared
Differential Flow ControlPressure FeedbackCascade Control
Selecting a Pressurization Control MethodHow Tight is Tight?Required Pressure Relationships (again)
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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
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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
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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
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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
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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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Special Methods
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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
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Tightness of Envelope
ox
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x x
o
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How Tight is Tight?
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Offset Airflow (cfm)
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Trim Valve
Flow/Pressure Cascade
Pressure Feedback
Flow Offset
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How Tight is Tight?
Rough guides for selecting control methodTight enough for pressure feedback?
pressure difference > 0.03 inwc, 8 Paat a practical infiltration rate
Too tight for flow offset control?infiltration flow < 5 x flow control accuracyexample: 1000 cfm supply +/- 3%need offset > 5 (3%) 1000 cfm = 150 cfmfor effective flow offset control
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Room Leakage Spec’s
Project spec’s calling out sealing methodsASHRAE Standard 170 lists numerical
flow/pressure relationshipCDC suggests leakage area ~40 in2 (~0.03m2)
for infectious isolation roomsNIH Design Standard D.4.5:
47 L/s per door
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Number of Pressure Levels
Relatively simple requirement2-levels, OK for Differential Flow Tracking
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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
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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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Pressurization and Contaminant ControlAir contaminants can move against net inward flowEven with good pressurization some air escapesLab Ventilation Standard: Z9.5
“opposes migration of air contaminants; it does not eliminate it.”
Current research shows effects
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Recent Research Projects
Projects study movement of contaminants with:
Open doorsMoving doorsMoving people
ASHRAE RP 1344 and 1431 measured with particle source and counter
Wei Sun, ASHRAE Research Report, RP 1344, Clean Room Pressurization Strategy Update
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Recent Research Projects
Projects study movement of contaminants with:
Open doorsMoving doorsMoving people
Hospital study used water tank model
Tang JW, Nicolle A, Pantelic J, Klettner CA, Su R, et al. (2013) Different Types of Door-Opening Motions as Contributing Factors to Containment Failures in Hospital Isolation Rooms. PLoS ONE 8(6): e66663. doi:10.1371/journal.pone.0066663
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End of Part 1Questions?
Jim Coogan, [email protected]
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ASHRAE WASHRAE WILLILL GGIVEIVEYYOUOU THETHE WWORLDORLD
ASHRAE WASHRAE WILLILL GGIVEIVEYYOUOU THETHE WWORLDORLD
This ASHRAE Distinguished Lecturer is brought to you by the Society Chapter Technology Transfer Committee
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Space Pressurization:Concept and Practice ASHRAE Distinguished Lecture Series
Jim CooganSiemens Building Technologies
ASHRAE, Oryx Qatar ChapterMarch 8, 2014
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Agenda
Introduction (concept, purpose, uses, scope)Physics: Infiltration and ContainmentPressurization via HVACPressurization and Contaminant Control
Designing PressurizationAir Flow Control ComponentsAir Flow Control AccuracyReview Design ProcessExamplesSummary
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Designing Pressurizationand Control
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Required Pressure Relationships
Indicate intended direction of air flow between all adjacent spaces
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Required Pressure Relationships
Indicate intended relative pressure levels
+
++
-
--
-
----
--
------
--
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Designing Pressure Feedback Systems
Design control sequenceSpecify the componentsConsider the envelope
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Design the Control Sequence
Identify the air flow terminalsDecide which one controls the room pressureConsider start-up sequencesConsider response to failures
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Specify the Components
Room pressure sensor – heart of the system?if sensor measures critical dp, accuracy is less criticalif sensor measures to a reference, accuracy is more criticalcheck zero periodically+/- range often selected
Air flow terminalsDo not select mechanical pressure independence. Pressure control loop is not the place for it.
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Consider the Envelope
At pressure setpoint, at nominal air flow, indicate anticipated air flow offset
If offset exceeds spec, or if pressure setpoint is unattainable, envelope leaks too much
Controlling air terminal hits flow limitsRoom air flow out of balanceSurrounding spaces affected
Come back and seal the room!
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Can the Room Be Too Tight?
What if it’s too tight?unlikely with Pressure Feedbackcalculate expected sensitivity: how much does the room pressure change for a small movement of the damper?
Consider adjusting leakage slightlyanticipate: select adjustable door sweep or other adjustable featureonly feasible if planned from the start
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Designing Flow Tracking Systems
Design control sequenceConsider the envelope
Select pressurization levelSelect accuracy target
Specify the componentsCalculate corresponding flow accuraciesCheck for practicality
Adjust as needed
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Select Pressurization Level
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Based on leakage areaExample: 150 cfm for ½ square foot
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Select Accuracy Target
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Based on need to control contaminantsNot product spec’s
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Effect of Errors in Flow In and Out
Nominal value ErrorExhaust flow 1000 +/- 100Supply flow 850 +/- 85Transfer flow 150 +/- 185
Numerical illustration
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Derive Flow Accuracy Spec
Use equation that combinessupply and exhaust errors
Apply it with desired infiltration accuracy
22esd eee
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Derive Flow Accuracy Spec
Allow same error on supply and exhaust (Arbitrary allocation, others are possible.)
Example:
Supply and exhaust tolerance = 30 cfm
222 2 sesd eeee
304.1
%)30(1502d
see
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Check for Practicality
Can we find flow control products that meet the needs?
flow range / pressure dropflow accuracy
Is the envelope too loose?flow needed to pressurize is excessive
Is the envelope too tight?infiltrating flow is small compared to controlled flow
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Adapting the Design
If pressurization design does not workadjust flow offsetchoose pressure feedback instead of flowresize terminalsreselect sensorsreduce air flow rangesadd leakage, move design point
Address as soon as possible
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Adapting the Design
Pressure too low for pressure feedbackFlow offset too small for control accuracy
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Adapting the Design
Increase flow difference or Use pressure feedback
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Adapting the Design
Add leakage to make the system less sensitiveIncrease the flow offset
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Adapting the Design
Improve flow control accuracyResize terminals, reselect sensors, reduce flow range
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Specifying Flow Control
Sample CalculationVAV cooling load: 700 cfmMinimum supply 50 cfmIntended pressurization: 0.015 inwc, 4 Pa, negativeAnticipated leakage: 0.5 sqftCalculated infiltration: 150 cfmDesired infiltration accuracy: 30%* 150 = 50 cfmAllocated sup/exh accuracy: 50/1.4 = 30 cfm
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Specifying Flow Control
Sample Calculation: result Flow control performance speccalculated from pressurization requirements
Max Min
Supply 700 +/- 30 50 +/- 30
Exhaust 850 +/- 30 200 +/- 30
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Air flow control components
DampersFlow SensorsControllersSpecifying components or performance
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Defining Air Terminal Performance
Range of air flowsControl accuracyPressure dropSound
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Flow Control Dampers
Single-blade, venturi, bladder
AIRFLOW
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Cut-Away View of Venturi/Cone/Spring/Shaft
Spring
Cone Cap(Slides on Shaft)
CenterShaft
ShaftBracket
ShaftBearingDashtube
Bushing(Slides in Dashtube)
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Damper’s Job:Selectively restrict air path
Venturi Damper – fully open
Venturi Damper – nearly closed
Blade Damper – fully open
Blade Damper – nearly closed
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Flow Control Dampers
Single-blade, venturi, bladderWhich kind do you need?Do you need to choose?Consider specifying performance
range of air flowscontrol accuracypressure dropsound
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Closed loop vs. open loopIf you care about airflow, MEASURE IT!
Open loop control• flow rate does not affect
controller output• depends on calibration of
damper and actuator• doesn’t need a sensor• delivers no data
Closed loop control• flow rate affects
controller output• damper curve is not
crucial• uses a flow sensor• delivers flow data
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Air Flow Sensors
3 Common typesVelocity pressureVortex sheddingThermal
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Specifying Flow Sensors
Specify performance: ASHRAE Guideline 13
5% of reading?3% of max relates better to pressurization
Require on-site commissioning
Pressurization AccuracyFixed Accuracy Target
0
200
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600
800
1000
1200
0 200 400 600 800 1000 1200Nominal Exhaust Flow
Act
ual F
low
Ran
ges
ExhaustSupplyFlow Difference
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Accurate Airflow Control, at Low & High Pressure Drop
System performance: terminal sensor, actuator and controller
Test covers air flow range112 cfm to 1400 cfm55 l/s to 700 l/s
and pressure range0.5 inwc to 5.0 inwc125 Pa to 1250 Pa
Error at low flow usually smaller than at high flow
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Rating Standards for Air Flow Controls
ASHRAE 195P:Method of Test for Rating Air Terminal Unit Controls
AMCA 610: Laboratory Method of Testing Airflow Measurement Stations for Performance Rating
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Theory and Concepts 2: Air Flow Sensing AccuracyControl Loop Accuracy, Sensing AccuracyEnd-to-end AccuracyKinds of Sensing ErrorsErrors in Flow Sensing System
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Air Flow Sensors
3 Common typesVelocity pressureVortex sheddingThermaland “no sensor”
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Sensing Accuracy Concepts
Sensing system can include multiple components
Each component has accuracy characteristicsCombined effect is what countsSometimes called ‘end-to-end accuracy’
Velocity Pressure
Probe
Differential Pressure
Transmitter
Input Electronic Circuits
DDC Calculations
SignalPressure
InstrumentCurrent
A/DValue
PhysicalAir Flow
SensedAir Flow
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Output
Input
ideal curve
actual curve
Offs
etEr
ror
Sensing Accuracy Concepts
To talk about accuracy, think about error
Different kinds of errorsoffsetspannon-linearityhysteresiscross-sensitivity
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Sensing Accuracy Concepts
Output
Input
ideal curve
actual curve
Non
linea
rity
Erro
r
Output
Input
ideal curve
actual curve
Span
Erro
r
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Sensing Accuracy Concepts
Spec’s may state only overall error
Details used to optimizing sensing system
Output
Input
ideal curve
actual curve
Ove
rall
Erro
r
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Velocity Pressure Sensing
Flow computed from measured pressure
Q = A k (Pv)1/2
Shape of the curve affects sensing performance
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Velocity Pressure
Air
Flow
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Errors in VP Sensing System
Characteristics of flow pick-uplinearity: typically good, can be affected by installationspan error: affected by installation, corrected by balanceroffset: non-existent
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Effect of Flow Pick-up Error
Numerical example to illustrate the math
Span error: 5% after field calibrationOffset error: 0
0
10
20
30
40
50
60
0 200 400 600 800 1000 1200 1400
Air FlowFl
ow E
rror
Flow Rate Accuracy
1000 cfm 50 cfm400 cfm 20 cfm200 cfm 10 cfm100 cfm 5 cfm
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Errors in VP Sensing System
Characteristics of dp transmitterlinearity: not an issuespan error: various grades available,typically 1% or betteroffset: typically 1% or better
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Effect of Transmitter Error
Numerical example to illustrate the math
span error: 0%offset error: 1%10 inch round duct‘unity gain’ probe0.25 inwc transmitter
0
10
20
30
40
50
60
0 200 400 600 800 1000 1200 1400
Air Flow
Flow
Err
orFlow Rate Accuracy1000 cfm 6 cfm
400 cfm 15 cfm200 cfm 27 cfm100 cfm 48 cfm
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Effect of Transmitter Error
Numerical example to illustrate the math
span error: 0%offset error: 1%10 inch round duct‘unity gain’ probe0.25 inwc transmitter
0
10
20
30
40
50
60
0 200 400 600 800 1000 1200 1400
Air Flow
Flow
Err
orFlow Rate Accuracy1000 cfm 6 cfm
400 cfm 15 cfm200 cfm 27 cfm100 cfm 48 cfm
Looks bad?Same effect as
5% at 1000 cfm.
Looks bad?Same effect as
5% at 1000 cfm.
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Errors in VP System
ROUGHLYspan error comes from the probeoffset error comes from the pressure transmitter
Velocity Pressure
Probe
Differential Pressure
Transmitter
Input Electronic Circuits
DDC Calculations
SignalPressure
InstrumentCurrent
A/DValue
PhysicalAir Flow
SensedAir Flow
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Flow Sensing Arithmetic
Flow computed from measured pressureQ = A k (Pv)1/2
Flow error comes from pressure errorQ + dQ = A k (Pv + dPv)1/2
Pressure error has 2 componentsdPv = esPv + eoPRange
Flow error is sensitive to turndowndQ/Q = (1+ es+ T2eo)1/2 - 1
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Combined Sensing Error
Numerical exampleduct: 10” roundtransmitter: 1.0 inwcprobe gain: 1.5span error: 3% in flowoffset: 0.5% of range
0
10
20
30
40
50
60
0 200 400 600 800 1000 1200 1400
Air Flow
Flow
Err
or
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Behavior at High Flow
Error is almost entirely due to the probe and air flow issues in the duct
Transmitter errors are much less significant at high flow
offset is completely negligiblespan error is smaller than probe error
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Behavior at Low Flow
Error is almost completely due to offset in the transmitter
Span errors in transmitter and probe are much smaller
Offset can disrupt effective controlWhat’s the solution?
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Zero the DP Transmitter
Offset can be almost completely eliminated by zeroing in the field
Highly reliable process, much easier than other field calibration tasks
Manual or automatic
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Combined Sensing ErrorAfter Zeroing the Transmitter
Numerical exampleduct: 10” roundtransmitter: 1.0 inwcprobe gain: 1.5span error: 3% in flowoffset: 0.25% of range
Makes velocity pressure methods viable in pressurized spaces
0
10
20
30
40
50
60
0 200 400 600 800 1000 1200 1400
Air FlowFl
ow E
rror
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Design Process
Determine pressure relationshipsSelect pressurization levelCalculate required flow accuracyCheck for practicalityAdjust as neededExample
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End of Part 2Questions?
Jim Coogan, [email protected]
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ASHRAE WASHRAE WILLILL GGIVEIVEYYOUOU THETHE WWORLDORLD
ASHRAE WASHRAE WILLILL GGIVEIVEYYOUOU THETHE WWORLDORLD
This ASHRAE Distinguished Lecturer is brought to you by the Society Chapter Technology Transfer Committee
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Space Pressurization:Concept and Practice ASHRAE Distinguished Lecture Series
Jim CooganSiemens Building Technologies
ASHRAE, Oryx Qatar ChapterMarch 8, 2014
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Agenda
Introduction (concept, purpose, uses, scope)Physics: infiltration and containmentPressurization via HVACDesign for flow trackingAir flow control components
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Design Process
Start design with pressurization needs
Then derive component spec’s
Compile box schedule
room req’sand descriptions
box schedule
Design Process
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Designing Air Flow Tracking
Determine pressurization relationships
Select pressurization level and accuracy
Calculate flow accuraciesCheck for practicalityAdjust as needed
room req’sand descriptions
box schedule
Select pressurization
level and accuracy
Identify terminals and air
flow ranges
Calculate flow accuracy spec
for each terminal
Check practicality with available components
Adjust design?
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Desired Pressure Relationships
Covered in Part 1
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Select Pressurization Level and Accuracy Target
x
xO
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
sure
Diff
eren
ce
Covered in Part 1Based on pressurization need (not product spec’s)
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Derive Flow Accuracy Spec
Use equation that combinessupply and exhaust errors
Apply it with desired infiltration accuracyAllocate allowable error among terminals
22esd eee
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Example 1: Simple Biological Lab
Small room, no special exhaust equipment1 supply, 1 exhaust
Negative pressurization (150 cfm, 0.5 ft2)VAV for cooling (maximum 700 cfm)Minimum flow when occupied (200 cfm)
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Corridor
Laboratory
150
Supply Flow
Exhaust Flow
Infiltration Flow
850 / 200 700 / 50
Example 1: Ventilation Schematic
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Calculate Flow Accuracies
Choose to allow equal error on supply and exhaust
Calculate accuracy needed
30 cfm allowed on supply and exhaust
304.1
%)30(1502d
see
Max Min
Supply 700 +/- 30 50 +/- 30
Exhaust 850 +/- 30 200 +/- 30
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Check for Practicality
30 cfm allowed on supply and exhaustNo challenge at the low flowsExhaust is a little tight at the high end
Max Min
Supply 700 +/- 30700 +/- 4%
50 +/- 3050 +/- 60%
Exhaust 850 +/- 30850 +/- 3%
200 +/- 30200 +/-15%
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Check for Practicality
Combine accuracy spec’s with flow rangesCompare to available products:
What equipment meets the spec?8” terminal meets spec; 10” may be acceptable
0
10
20
30
40
50
60
70
80
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Air Flow
Flow
Err
or
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Example 2: Same Room, More Flow
Small room, no special exhaust equipment1 supply, 1 exhaust
Negative pressurization (150 cfm, 0.5 ft2)Cooling flow irrelevant,
less than the ventilation rateHigh ventilation (1250 cfm)
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Calculate Flow Accuracies
Calculate accuracy needed at supply and exhaust
30 cfm allowance on supply and exhaustSame envelope, same pressurization,
same allowable error
Supply 1100 +/- 30
Exhaust 1250 +/- 30
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Check for Practicality
Combine accuracy spec’s with flow rangesCan’t quite meet it with these componentsMay need to adjust the design
0
10
20
30
40
50
60
70
80
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Air Flow
Flow
Err
or
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What’s the Problem?
Lots of ways to look at it“Air change rate is too high”“Room is too tight for offset control”“Flow offset is too small”“Flow control not accurate enough”
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Covered in part 1If spec’s are not practical
adjust flow offsetchoose pressure feedback instead of flowresize terminalsreselect sensorsreduce air flow rangesadd leakage, move design point
Adjust the Design as Needed
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Example 3: Same Flows, More Terminals
Small room, no special exhaust equipment2 supply, 2 exhaust
Negative pressurization (150 cfm, 0.5 ft2)Cooling flow irrelevant,
less than ventilation requirementHigh ventilation rate (1200 cfm)
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Example 3: Ventilation Schematic
CorridorLaboratory
Supply Flow
Exhaust Flow
Infiltration Flow
550
150
550
625625
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Calculate Flow Accuracies
Calculate accuracy needed on supply and exhaust
If terminal accuracies are equal
Or we can adjust the allocation
5.222
%)30(15021d
see
45%)30(15022
21
22
21 eessd eeeee
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Allocate Error to Terminals
Exhaust flow is a little higherRound exhaust error up
and supply error down
Terminal 1 Terminal 2
Supply 550 +/- 20 550 +/- 20
Exhaust 625 +/- 25 625 +/- 25
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0
10
20
30
40
50
60
70
80
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Air Flow
Flow
Err
or
Check for Practicality
What equipment can meet spec?Try 2 pairs of 8” supply and exhaust terminalsIn this case, 2 terminals are
more accurate than 1
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How Does That Work?
Math to combine errors accounts for the chance that errors cancel or add
Square root equation ‘favors’ more termsIs it realistic?
experience says that large rooms, with many terminals are easier to commission than rooms with 1 in and 1 out
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Summary of Examples
Worked 3 examples with the same pressurization requirementleakage area: 0.5 ft2flow offset: 150 cfm +/- 30%
Increased air flow challenged the design
Supply Flows Exhaust Flows Design
1 700/50 +/- 30 850/200 +/- 30 8” or 10” supply and exhaust
2 1100 +/- 30 1250 +/- 30 Adjust design
3 2x 550 +/- 20 2x 625 +/- 25 2 pairs of 8” terminals
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Designing for Pressurization
Determine pressurization relationships
Select pressurization level and accuracy
Calculate flow accuraciesCheck for practicalityAdjust as needed
22esd eee
0
10
20
30
40
50
60
70
80
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Air Flow
Flow
Err
or
CorridorLaboratory
Supply Flow
Exhaust Flow
Infiltration Flow
550
150
550
625625
x
xO
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 5 0 100 15 0 200 250
Infiltrating Air Flow
Pres
sure
Dif
fere
nce x
xO
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 5 0 100 15 0 200 250
Infiltrating Air Flow
Pres
sure
Dif
fere
nce
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Summary
Space pressurization: tool for contamination control,not a ‘magic shield’Envelope leakage is main mechanical parameterSeveral HVAC control methodsDifferential flow control is used most oftenChoice usually driven by envelopeDerive air flow accuracy spec from pressurization