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Space Pressurization:Concept and Practice ASHRAE Distinguished Lecture Series

Jim CooganSiemens Building Technologies

ASHRAE, Oryx Qatar ChapterMarch 8, 2014

Page 5 Copyright © 2014. All rights reserved.

Agenda

Introduction (concept, purpose, uses, scope)Physics: Infiltration and ContainmentPressurization via HVACPressurization and Contaminant ControlDesigning PressurizationAir Flow Control ComponentsAir Flow Control AccuracyReview Design ProcessExamplesSummary


Room Pressurization

A ventilation technology that controls migration

of air contaminants by inducing drafts between spaces.


Room Pressurization

Exhaust system removes air

Supply system delivers less

Room pressureis negative

Infiltration makes up the difference

Inward air flow contains pollutants


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


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


How is success defined?

Success is control of contaminants, not flows and pressure values


Theory and Concepts 1: Infiltration and Containment

Infiltration: mechanical processVelocity, Area, PressureInfiltration CurvesImportance of the EnvelopeSelect Pressurization LevelSpecifying the Envelope


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


Infiltration Process:Pressure, Velocity, Area, Flow

Infiltration is a physical processPressurization is an engineered resultASHRAE Handbook and Ventilation Manual from ACGIH model the process


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


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


Infiltration Curve –Pressure Difference vs. Flow

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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


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


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


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


Select Pressurization LevelChoose the flow offsetLet it determine the pressure

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Select Pressurization LevelChoose the pressureLet it determine the flow offset

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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


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


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


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


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)


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


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


Pressure Feedback


Pressure Feedback

Measure pressure differenceacross room boundary

Compare to selected setpointAdjust supply flow or exhaust

to maintain pressure difference


Differential Flow Control


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


Cascade Control


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


Special Methods


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


Tightness of Envelope

ox

x

x x

o

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How Tight is Tight?

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0.05

0.075

0.1

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Offset Airflow (cfm)

Diff

eren

tial P

ress

ure

(in. w

g.)

Trim Valve

Flow/Pressure Cascade

Pressure Feedback

Flow Offset


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


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


Number of Pressure Levels

Relatively simple requirement2-levels, OK for Differential Flow Tracking


Pressurization and Contaminant Control

Contaminant control can be very important or only slightly important

Biosafety standards recognize range of hazards and range of responses


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


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


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


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


End of Part 1Questions?

Jim Coogan, [email protected]

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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


Designing Pressurizationand Control


Required Pressure Relationships

Indicate intended direction of air flow between all adjacent spaces


Required Pressure Relationships

Indicate intended relative pressure levels

+

++

-

--

-

----

--

------

--


Designing Pressure Feedback Systems

Design control sequenceSpecify the componentsConsider the envelope


Design the Control Sequence

Identify the air flow terminalsDecide which one controls the room pressureConsider start-up sequencesConsider response to failures


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.


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!


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


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


Select Pressurization Level

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Based on leakage areaExample: 150 cfm for ½ square foot


Select Accuracy Target

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Based on need to control contaminantsNot product spec’s


Effect of Errors in Flow In and Out

Nominal value ErrorExhaust flow 1000 +/- 100Supply flow 850 +/- 85Transfer flow 150 +/- 185

Numerical illustration


Derive Flow Accuracy Spec

Use equation that combinessupply and exhaust errors

Apply it with desired infiltration accuracy

22esd eee



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


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


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


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


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


Air flow control components

DampersFlow SensorsControllersSpecifying components or performance


Defining Air Terminal Performance

Range of air flowsControl accuracyPressure dropSound


Flow Control Dampers

Single-blade, venturi, bladder

AIRFLOW


Cut-Away View of Venturi/Cone/Spring/Shaft

Spring

Cone Cap(Slides on Shaft)

CenterShaft

ShaftBracket

ShaftBearingDashtube

Bushing(Slides in Dashtube)


Damper’s Job:Selectively restrict air path

Venturi Damper – fully open

Venturi Damper – nearly closed

Blade Damper – fully open

Blade Damper – nearly closed


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


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


Air Flow Sensors

3 Common typesVelocity pressureVortex sheddingThermal


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

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1200

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Act

ual F

low

Ran

ges

ExhaustSupplyFlow Difference


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


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


Theory and Concepts 2: Air Flow Sensing AccuracyControl Loop Accuracy, Sensing AccuracyEnd-to-end AccuracyKinds of Sensing ErrorsErrors in Flow Sensing System


Air Flow Sensors

3 Common typesVelocity pressureVortex sheddingThermaland “no sensor”


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


Output

Input

ideal curve

actual curve

Offs

etEr

ror


To talk about accuracy, think about error

Different kinds of errorsoffsetspannon-linearityhysteresiscross-sensitivity



Output

Input

ideal curve

actual curve

Non

linea

rity

Erro

r

Output

Input

ideal curve

actual curve

Span

Erro

r



Spec’s may state only overall error

Details used to optimizing sensing system

Output

Input

ideal curve

actual curve

Ove

rall

Erro

r


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


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


Effect of Flow Pick-up Error

Numerical example to illustrate the math

Span error: 5% after field calibrationOffset error: 0

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30

40

50

60

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Air FlowFl

ow E

rror

Flow Rate Accuracy

1000 cfm 50 cfm400 cfm 20 cfm200 cfm 10 cfm100 cfm 5 cfm


Errors in VP Sensing System

Characteristics of dp transmitterlinearity: not an issuespan error: various grades available,typically 1% or betteroffset: typically 1% or better


Effect of Transmitter Error


span error: 0%offset error: 1%10 inch round duct‘unity gain’ probe0.25 inwc transmitter

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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


Effect of Transmitter Error


span error: 0%offset error: 1%10 inch round duct‘unity gain’ probe0.25 inwc transmitter

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50

60

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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.


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


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


Combined Sensing Error

Numerical exampleduct: 10” roundtransmitter: 1.0 inwcprobe gain: 1.5span error: 3% in flowoffset: 0.5% of range

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or


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


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?


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


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

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Air FlowFl

ow E

rror


Design Process

Determine pressure relationshipsSelect pressurization levelCalculate required flow accuracyCheck for practicalityAdjust as neededExample


End of Part 2Questions?









Agenda

Introduction (concept, purpose, uses, scope)Physics: infiltration and containmentPressurization via HVACDesign for flow trackingAir flow control components


Design Process

Start design with pressurization needs

Then derive component spec’s

Compile box schedule

room req’sand descriptions

box schedule

Design Process


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?


Desired Pressure Relationships

Covered in Part 1


Select Pressurization Level and Accuracy Target

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Covered in Part 1Based on pressurization need (not product spec’s)



Use equation that combinessupply and exhaust errors

Apply it with desired infiltration accuracyAllocate allowable error among terminals

22esd eee


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)


Corridor

Laboratory

150

Supply Flow

Exhaust Flow

Infiltration Flow

850 / 200 700 / 50

Example 1: Ventilation Schematic


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



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%



Combine accuracy spec’s with flow rangesCompare to available products:

What equipment meets the spec?8” terminal meets spec; 10” may be acceptable

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Err

or


Example 2: Same Room, More Flow


Negative pressurization (150 cfm, 0.5 ft2)Cooling flow irrelevant,

less than the ventilation rateHigh ventilation (1250 cfm)



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



Combine accuracy spec’s with flow rangesCan’t quite meet it with these componentsMay need to adjust the design

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60

70

80

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Air Flow

Flow

Err

or


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”


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


Example 3: Same Flows, More Terminals


Negative pressurization (150 cfm, 0.5 ft2)Cooling flow irrelevant,

less than ventilation requirementHigh ventilation rate (1200 cfm)


Example 3: Ventilation Schematic

CorridorLaboratory

Supply Flow

Exhaust Flow

Infiltration Flow

550

150

550

625625



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


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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50

60

70

80

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Air Flow

Flow

Err

or


What equipment can meet spec?Try 2 pairs of 8” supply and exhaust terminalsIn this case, 2 terminals are

more accurate than 1


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


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


Designing for Pressurization

Determine pressurization relationships

Select pressurization level and accuracy

Calculate flow accuraciesCheck for practicalityAdjust as needed

22esd eee

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80

0 200 400 600 800 1000 1200 1400 1600 1800 2000

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Flow

Err

or

CorridorLaboratory

Supply Flow

Exhaust Flow

Infiltration Flow

550

150

550

625625

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Dif

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xO

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 5 0 100 15 0 200 250


Pres

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Dif

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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


Thank you!Questions?


Documents

ASHRAE WILL GG IVE YY OUOU THEOU THE WW ORLDashraeqatar.org/uploads/3/4/5/4/34547927/spacepressurizationdoha.pdf · ASHRAE WILL GG IVE YY OUOU THEOU THE WW ORLD ... pressure by orifice