This ASHRAE Distinguished Lecturer is brought to you by the Society Chapter Technology Transfer Committee

ASHRAE WILL ILL G IVE Y OU THE W ORLD 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 Coogan Siemens Building Technologies ASHRAE, Oryx Qatar Chapter March 8, 2014

Agenda Introduction (concept, purpose, uses, scope) Physics: Infiltration and Containment Pressurization via HVAC Pressurization and Contaminant Control Designing Pressurization Air Flow Control Components Air Flow Control Accuracy Review Design Process Examples Summary Page 5

Room Pressurization A ventilation technology that controls migration of air contaminants by inducing drafts between spaces. Page 6

Room Pressurization Exhaust system removes air Supply system delivers less Room pressure is negative Infiltration makes up the difference Inward air flow contains pollutants Page 7

Introduction: Who uses it? Why? Biological and Chemical Laboratories prevent spread of airborne hazards Hospital Isolation Rooms protect patients and staff from germs Hospital Pharmacies facilitate sterile compounding Clean Manufacturing maintain product quality Page 8

Introduction: Who else uses it? Office towers control smoke in a fire; maintain exit path Any Building separate rest rooms from other spaces Restaurants keep kitchen smells out of the dining room Any Building keep unconditioned OA out of occupied spaces These uses are out of today s scope Page 9

How is success defined? Success is control of contaminants, not flows and pressure values Page 10

Theory and Concepts 1: Infiltration and Containment Infiltration: mechanical process Velocity, Area, Pressure Infiltration Curves Importance of the Envelope Select Pressurization Level Specifying the Envelope Page 11

Theory of Pressurization Theory: pressure blocks contaminants Theory: net inward flow blocks contaminants Surprisingly little work done correlating pressurization to contaminant control Current ASHRAE research correlates pressure with contamination Earlier work: Bennet, Applied Biosafety, 2005 Success is control of contaminants, not flows and pressure values Page 12

Infiltration Process: Pressure, Velocity, Area, Flow Infiltration is a physical process Pressurization is an engineered result ASHRAE Handbook and Ventilation Manual from ACGIH model the process Page 13

Pressure vs. Velocity Simple approach is to model the velocity with a discharge coefficient ACGIH Industrial Ventilation: 7-3 v 0.6(4000) P ASHRAE Fundamentals Handbook presents more complex model, but the result is nearly the same Page 14

Velocity and Leakage Area Flow is velocity times area 2011 ASHRAE Handbook HVAC Applications, puts it together: 53-9 Q 2610A P Q = infiltration flow, cfm A = leakage area, sqft P = pressure across envelope, inwc Page 15

Infiltration Curve Pressure Difference vs. Flow 0.05 0.045 0.04 0.035 Pressure Difference 0.03 0.025 0.02 0.015 0.01 0.005 0 0 50 100 150 200 250 300 350 400 450 500 Infiltrating Air Flow Page 16

Infiltration Curves for Several Values of Leakage Area 0.05 0.045 0.04 0.035 Pressure Difference 0.03 0.025 0.02 0.015 0.01 0.005 Page 17 0 0 50 100 150 200 250 300 350 400 450 500 Infiltrating Air Flow

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 Page 18

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 Page 19

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 Page 20

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 Page 21

Select Pressurization Level Choose the flow offset Let it determine the pressure 0.035 0.03 Pressure Difference 0.025 0.02 0.015 0.01 0.005 0 0 50 100 150 200 250 Infiltrating Air Flow Page 22

Select Pressurization Level Choose the pressure Let it determine the flow offset 0.035 0.03 Pressure Difference 0.025 0.02 0.015 0.01 0.005 0 0 50 100 150 200 250 Infiltrating Air Flow Page 23

Select Pressurization Level Different ways to express the level of pressurization in terms of the pressure difference in terms of the infiltration flow Specify either the pressure or the flow offset, not both. Unless you are trying to specify the envelope Page 24

Specifying the Envelope Specify a value for one variable Specify a range for the other Implies accepted range of leakage 0.035 0.03 0.025 Pressure Difference 0.02 0.015 0.01 0.005 Page 25 0 0 50 100 150 200 250 Infiltrating Air Flow

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 rate for hospital isolation rooms Page 26

Test and Adjust the Envelope If it s in the spec... Cx agent or TAB contractor tests the envelope Directs contractor to adjust leakage area to specified range Correction can include: adjustable door sweep transfer opening with restriction seal cracks Reference: A. Geeslin et al., ASHRAE Transactions, SL-08-044 Air Leakage Analysis of Special Ventilation Hospital Rooms Page 27

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 Page 28

Pressurization via HVAC Required Pressure Relationships Control Methods Explained and Compared Differential Flow Control Pressure Feedback Cascade Control Selecting a Pressurization Control Method How Tight is Tight? Required Pressure Relationships (again) Page 29

Control Methods Compared Three widely published methods Space pressure feedback Differential flow control Cascade control References: 2011 ASHRAE Handbook, HVAC Applications. Chapter 16 Laboratory Systems Siemens Building Technologies: Doc #125-2412. Room Pressurization Control Page 30

Control Methods Compared Some other ways Adaptive leakage model Trim valve References: W Sun, ASHRAE Transactions, NA-04-7-2. Quantitative Multistage Pressurizations in Controlled and Critical Environments L. Gartner and C. Kiley, Anthology of Biosafety 2005. Animal Room Design Issues in High Containment Page 31

Pressure Feedback Page 32

Pressure Feedback Measure pressure difference across room boundary Compare to selected setpoint Adjust supply flow or exhaust to maintain pressure difference Page 33

Differential Flow Control Page 34

Differential Flow Control Carefully control air supply to room Carefully control all exhaust from room Enforce a difference between them Select the size of difference to reliably contain pollutants Page 35

Cascade Control Page 36

Cascade Control Has other names: adaptive offset DP reset Measure pressure difference across room boundary Compare to selected setpoint Control supply and exhaust flow Enforce a difference between them Dynamically adjust flow difference to maintain the pressure setpoint Page 37

Special Methods Page 38

Selecting a Control Method Factors affecting selection Tightness of envelope Number of pressure levels needed Speed of disturbances and response Duct conditions for flow measurement Reference: 2011 ASHRAE Handbook HVAC Applications, Chapter 16 - Laboratory Systems, page 16.12 Page 39

Tightness of Envelope x 0.05 0.045 0.04 Pressure Difference 0.035 0.03 0.025 0.02 0.015 0.01 0.005 x o x o x 0 0 50 100 150 200 250 300 350 400 450 500 Infiltrating Air Flow Page 40

How Tight is Tight? 0.1 0.075 Trim Valve Pressure Feedback Differential Pressure (in. wg.) 0.05 0.025 Flow/Pressure Cascade Flow Offset 0 0 50 100 150 200 250 300 Offset Airflow (cfm) Page 41

How Tight is Tight? Rough guides for selecting control method Tight enough for pressure feedback? pressure difference > 0.03 inwc, 8 Pa at a practical infiltration rate Too tight for flow offset control? infiltration flow < 5 x flow control accuracy example: 1000 cfm supply +/- 3% need offset > 5 (3%) 1000 cfm = 150 cfm for effective flow offset control Page 42

Room Leakage Spec s Project spec s calling out sealing methods ASHRAE Standard 170 lists numerical flow/pressure relationship CDC suggests leakage area ~40 in 2 (~0.03m 2 ) for infectious isolation rooms NIH Design Standard D.4.5: 47 L/s per door Page 43

Number of Pressure Levels Relatively simple requirement 2-levels, OK for Differential Flow Tracking Page 44

Pressurization and Contaminant Control Contaminant control can be very important or only slightly important Biosafety standards recognize range of hazards and range of responses Page 45

Levels of Contaminant Control Pressurization is one tool Physical barrier is also BSL 1 Laboratories should have doors BSL 2 Doors should be self-closing BSL 3 Series of two self-closing doors BSL 4 Airlock with air tight doors Page 46

Pressurization and Contaminant Control Air contaminants can move against net inward flow Even with good pressurization some air escapes Lab Ventilation Standard: Z9.5 opposes migration of air contaminants; it does not eliminate it. Current research shows effects Page 47

Recent Research Projects Projects study movement of contaminants with: Open doors Moving doors Moving people ASHRAE RP 1344 and 1431 measured with particle source and counter Wei Sun, ASHRAE Research Report, RP 1344, Clean Room Pressurization Strategy Update Page 48

Recent Research Projects Projects study movement of contaminants with: Open doors Moving doors Moving 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 Page 49

End of Part 1 Questions? 0.05 0.045 0.04 0.035 Pressure Difference 0.03 0.025 0.02 0.015 0.01 0.005 0 0 50 100 150 200 250 300 350 400 450 500 Infiltrating Air Flow Jim Coogan, PE Jim.Coogan@Siemens.com Page 50

ASHRAE WILL ILL G IVE Y OU THE W ORLD This ASHRAE Distinguished Lecturer is brought to you by the Society Chapter Technology Transfer Committee

Space Pressurization: Concept and Practice ASHRAE Distinguished Lecture Series Jim Coogan Siemens Building Technologies ASHRAE, Oryx Qatar Chapter March 8, 2014

Agenda Introduction (concept, purpose, uses, scope) Physics: Infiltration and Containment Pressurization via HVAC Pressurization and Contaminant Control Designing Pressurization Air Flow Control Components Air Flow Control Accuracy Review Design Process Examples Summary Page 3

Designing Pressurization and Control Page 4

Required Pressure Relationships Indicate intended direction of air flow between all adjacent spaces Page 5

Required Pressure Relationships Indicate intended relative pressure levels - -- -- -- -- ++ - + -- -- -- -- Page 6

Designing Pressure Feedback Systems Design control sequence Specify the components Consider the envelope Page 7

Design the Control Sequence Identify the air flow terminals Decide which one controls the room pressure Consider start-up sequences Consider response to failures Page 8

Specify the Components Room pressure sensor heart of the system? if sensor measures critical dp, accuracy is less critical if sensor measures to a reference, accuracy is more critical check zero periodically +/- range often selected Air flow terminals Do not select mechanical pressure independence. Pressure control loop is not the place for it. Page 9

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 limits Room air flow out of balance Surrounding spaces affected Come back and seal the room! Page 10

Can the Room Be Too Tight? What if it s too tight? unlikely with Pressure Feedback calculate expected sensitivity: how much does the room pressure change for a small movement of the damper? Consider adjusting leakage slightly anticipate: select adjustable door sweep or other adjustable feature only feasible if planned from the start Page 11

Designing Flow Tracking Systems Design control sequence Consider the envelope Select pressurization level Select accuracy target Specify the components Calculate corresponding flow accuracies Check for practicality Adjust as needed Page 12

Select Pressurization Level 0.035 Based on leakage area Example: 150 cfm for ½ square foot 0.03 Pressure Difference 0.025 0.02 0.015 0.01 O 0.005 Page 13 0 0 50 100 150 200 250 Infiltrating Air Flow

Select Accuracy Target 0.035 Based on need to control contaminants Not product spec s 0.03 Pressure Difference 0.025 0.02 0.015 0.01 x O x 0.005 Page 14 0 0 50 100 150 200 250 Infiltrating Air Flow

Effect of Errors in Flow In and Out Numerical illustration Nominal value Error Exhaust flow 1000 +/- 100 Supply flow 850 +/- 85 Transfer flow 150 +/- 185 Page 15

Derive Flow Accuracy Spec Use equation that combines supply and exhaust errors e d e 2 s Apply it with desired infiltration accuracy e 2 e Page 16

Derive Flow Accuracy Spec Allow same error on supply and exhaust (Arbitrary allocation, others are possible.) Example: e e s d e d 2 e 2e 2 s Supply and exhaust tolerance = 30 cfm 2 s e 2 e 150(30%) 1.4 30 Page 17

Check for Practicality Can we find flow control products that meet the needs? flow range / pressure drop flow 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 Page 18

Adapting the Design If pressurization design does not work adjust flow offset choose pressure feedback instead of flow resize terminals reselect sensors reduce air flow ranges add leakage, move design point Address as soon as possible Page 19

Adapting the Design Pressure too low for pressure feedback Flow offset too small for control accuracy 0.035 0.03 Pressure Difference 0.025 0.02 0.015 0.01 O 0.005 Page 20 0 0 50 100 150 200 250 Infiltrating Air Flow

Adapting the Design Increase flow difference or Use pressure feedback 0.035 0.03 O Pressure Difference 0.025 0.02 0.015 0.01 0.005 Page 21 0 0 50 100 150 200 250 Infiltrating Air Flow

Adapting the Design Add leakage to make the system less sensitive Increase the flow offset 0.035 0.03 0.025 Pressure Difference 0.02 0.015 0.01 O 0.005 Page 22 0 0 50 100 150 200 250 Infiltrating Air Flow

Adapting the Design Improve flow control accuracy Resize terminals, reselect sensors, reduce flow range 0.035 0.03 Pressure Difference 0.025 0.02 0.015 0.01 O 0.005 Page 23 0 0 50 100 150 200 250 Infiltrating Air Flow

Specifying Flow Control Sample Calculation VAV cooling load: 700 cfm Minimum supply 50 cfm Intended pressurization: 0.015 inwc, 4 Pa, negative Anticipated leakage: 0.5 sqft Calculated infiltration: 150 cfm Desired infiltration accuracy: 30%* 150 = 50 cfm Allocated sup/exh accuracy: 50/1.4 = 30 cfm Page 24

Specifying Flow Control Sample Calculation: result Flow control performance spec calculated from pressurization requirements Max Min Supply 700 +/- 30 50 +/- 30 Exhaust 850 +/- 30 200 +/- 30 Page 25

Air flow control components Dampers Flow Sensors Controllers Specifying components or performance Page 26

Defining Air Terminal Performance Range of air flows Control accuracy Pressure drop Sound Page 27

Flow Control Dampers Single-blade, venturi, bladder AIRFLOW Page 28

Cut-Away View of Venturi/Cone/Spring/Shaft Spring Shaft Bracket Dashtube Shaft Bearing Center Shaft hing des in Dashtube) Cone Cap (Slides on Shaft) Page 29

Damper s Job: Selectively restrict air path Venturi Damper fully open Blade Damper fully open Venturi Damper nearly closed Blade Damper nearly closed Page 30

Flow Control Dampers Single-blade, venturi, bladder Which kind do you need? Do you need to choose? Consider specifying performance range of air flows control accuracy pressure drop sound Page 31

Closed loop vs. open loop If you care about airflow, MEASURE IT! Closed loop control flow rate affects controller output damper curve is not crucial uses a flow sensor delivers flow data 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 Page 32

Air Flow Sensors 3 Common types Velocity pressure Vortex shedding Thermal Page 33

Specifying Flow Sensors Specify performance: ASHRAE Guideline 13 5% of reading? 3% of max relates better to pressurization Require on-site commissioning Actual Flow Ranges 1200 1000 800 600 400 200 Exhaust Supply Flow Difference Pressurization Accuracy Fixed Accuracy Target 0 0 200 400 600 800 1000 1200 Nominal Exhaust Flow Page 34

Accurate Airflow Control, at Low & High Pressure Drop System performance: terminal sensor, actuator and controller Test covers air flow range 112 cfm to 1400 cfm 55 l/s to 700 l/s and pressure range 0.5 inwc to 5.0 inwc 125 Pa to 1250 Pa Error at low flow usually smaller than at high flow Page 35

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 Page 36

Theory and Concepts 2: Air Flow Sensing Accuracy Control Loop Accuracy, Sensing Accuracy End-to-end Accuracy Kinds of Sensing Errors Errors in Flow Sensing System Page 37

Air Flow Sensors 3 Common types Velocity pressure Vortex shedding Thermal and no sensor Page 38

Sensing Accuracy Concepts Physical Air Flow Velocity Pressure Probe Signal Pressure Differential Pressure Transmitter Instrument Current Input Electronic Circuits A/D Value DDC Calculations Sensed Air Flow Sensing system can include multiple components Each component has accuracy characteristics Combined effect is what counts Sometimes called end-to-end accuracy Page 39

Sensing Accuracy Concepts To talk about accuracy, think about error Different kinds of errors offset span non-linearity hysteresis cross-sensitivity Output actual curve ideal curve Offset Error Input Page 40

Sensing Accuracy Concepts Output Output actual curve ideal curve Span Error ideal curve actual curve Nonlinearity Error Input Input Page 41

Sensing Accuracy Concepts Output ideal curve Overall Error actual curve Spec s may state only overall error Details used to optimizing sensing system Input Page 42

Velocity Pressure Sensing Flow computed from measured pressure Q = A k (P v ) 1/2 Shape of the curve affects sensing performance A ir F lo w 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Velocity Pressure Page 43

Errors in VP Sensing System Characteristics of flow pick-up linearity: typically good, can be affected by installation span error: affected by installation, corrected by balancer offset: non-existent Page 44

Effect of Flow Pick-up Error Numerical example to illustrate the math Span error: 5% after field calibration Offset error: 0 F lo w E rro r 60 50 40 30 20 Flow Rate Accuracy 1000 cfm 50 cfm 400 cfm 20 cfm 200 cfm 10 cfm 100 cfm 5 cfm 10 0 0 200 400 600 800 1000 1200 1400 Air Flow Page 45

Errors in VP Sensing System Characteristics of dp transmitter linearity: not an issue span error: various grades available, typically 1% or better offset: typically 1% or better Page 46

Effect of Transmitter Error Numerical example to illustrate the math span error: 0% offset error: 1% 10 inch round duct unity gain probe 0.25 inwc transmitter F lo w E rro r 60 50 40 30 20 10 Flow Rate Accuracy 1000 cfm 6 cfm 400 cfm 15 cfm 200 cfm 27 cfm 100 cfm 48 cfm 0 0 200 400 600 800 1000 1200 1400 Air Flow Page 47

Effect of Transmitter Error Numerical example to illustrate the math span error: 0% offset error: 1% 10 inch round duct unity gain Looks probe bad? 0.25 inwc Same transmitter effect as 5% at 1000 cfm. F lo w E rro r 60 50 40 30 20 10 Page 48 Flow Rate Accuracy 1000 cfm 6 cfm 400 cfm 15 cfm 200 cfm 27 cfm 100 cfm 48 cfm 0 0 200 400 600 800 1000 1200 1400 Air Flow

Errors in VP System ROUGHLY span error comes from the probe offset error comes from the pressure transmitter Physical Air Flow Velocity Pressure Probe Signal Pressure Differential Pressure Transmitter Instrument Current Input Electronic Circuits A/D Value DDC Calculations Sensed Air Flow Page 49

Flow Sensing Arithmetic Flow computed from measured pressure Q = A k (P v ) 1/2 Flow error comes from pressure error Q + dq = A k (P v + dp v ) 1/2 Pressure error has 2 components dp v = e s P v + e o P Range Flow error is sensitive to turndown dq/q = (1+ e s + T 2 e o ) 1/2-1 Page 50

Combined Sensing Error Numerical example duct: 10 round transmitter: 1.0 inwc probe gain: 1.5 span error: 3% in flow offset: 0.5% of range F lo w E rro r 60 50 40 30 20 10 0 0 200 400 600 800 1000 1200 1400 Air Flow Page 51

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 negligible span error is smaller than probe error Page 52

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 control What s the solution? Page 53

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 Page 54

Combined Sensing Error After Zeroing the Transmitter Numerical example duct: 10 round transmitter: 1.0 inwc probe gain: 1.5 span error: 3% in flow offset: 0.25% of range Makes velocity pressure 0 methods viable in pressurized spaces F lo w E r r o r 60 50 40 30 20 10 0 200 400 600 800 1000 1200 1400 Air Flow Page 55

Design Process Determine pressure relationships Select pressurization level Calculate required flow accuracy Check for practicality Adjust as needed Example Page 56

End of Part 2 Questions? Jim Coogan, PE Jim.Coogan@Siemens.com Page 57

ASHRAE WILL ILL G IVE Y OU THE W ORLD This ASHRAE Distinguished Lecturer is brought to you by the Society Chapter Technology Transfer Committee

Space Pressurization: Concept and Practice ASHRAE Distinguished Lecture Series Jim Coogan Siemens Building Technologies ASHRAE, Oryx Qatar Chapter March 8, 2014

Agenda Introduction (concept, purpose, uses, scope) Physics: infiltration and containment Pressurization via HVAC Design for flow tracking Air flow control components Page 3

Design Process Start design with pressurization needs Then derive component spec s Compile box schedule room req s and descriptions Design Process box schedule Page 4

Designing Air Flow Tracking Select pressurization level and accuracy Calculate flow accuracy spec for each terminal room req s and descriptions Check practicality with available components Identify terminals and air flow ranges Determine pressurization relationships Select pressurization level and accuracy Calculate flow accuracies Check for practicality Adjust as needed Adjust design? box schedule Page 5

Desired Pressure Relationships Covered in Part 1 Page 6

Select Pressurization Level and Accuracy Target 0.035 Covered in Part 1 Based on pressurization need (not product spec s) 0.03 Pressure Difference 0.025 0.02 0.015 0.01 x O x 0.005 Page 7 0 0 50 100 150 200 250 Infiltrating Air Flow

Derive Flow Accuracy Spec Use equation that combines supply and exhaust errors e d e 2 s Apply it with desired infiltration accuracy Allocate allowable error among terminals e 2 e Page 8

Example 1: Simple Biological Lab Small room, no special exhaust equipment 1 supply, 1 exhaust Negative pressurization (150 cfm, 0.5 ft 2 ) VAV for cooling (maximum 700 cfm) Minimum flow when occupied (200 cfm) Page 9

Example 1: Ventilation Schematic Corridor 150 Laboratory 850 / 200 700 / 50 Page 10 Supply Flow Exhaust Flow Infiltration Flow

Calculate Flow Accuracies Choose to allow equal error on supply and exhaust Calculate accuracy needed e 150(30%) e d s 30 2 1.4 30 cfm allowed on supply and exhaust Max Min Supply 700 +/- 30 50 +/- 30 Exhaust 850 +/- 30 200 +/- 30 Page 11

Check for Practicality 30 cfm allowed on supply and exhaust No challenge at the low flows Exhaust is a little tight at the high end Max Supply 700 +/- 30 700 +/- 4% Exhaust 850 +/- 30 850 +/- 3% Min 50 +/- 30 50 +/- 60% 200 +/- 30 200 +/-15% Page 12

Check for Practicality Combine accuracy spec s with flow ranges Compare to available products: What equipment meets the spec? 8 terminal meets spec; 10 may be acceptable 80 70 60 Flow Error 50 40 30 20 10 Page 13 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Air Flow

Example 2: Same Room, More Flow Small room, no special exhaust equipment 1 supply, 1 exhaust Negative pressurization (150 cfm, 0.5 ft 2 ) Cooling flow irrelevant, less than the ventilation rate High ventilation (1250 cfm) Page 14

Calculate Flow Accuracies Calculate accuracy needed at supply and exhaust 30 cfm allowance on supply and exhaust Same envelope, same pressurization, same allowable error Supply 1100 +/- 30 Exhaust 1250 +/- 30 Page 15

Check for Practicality Combine accuracy spec s with flow ranges Can t quite meet it with these components May need to adjust the design 80 70 60 Flow Error 50 40 30 20 10 Page 16 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Air Flow

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 Page 17

Adjust the Design as Needed Covered in part 1 If spec s are not practical adjust flow offset choose pressure feedback instead of flow resize terminals reselect sensors reduce air flow ranges add leakage, move design point Page 18

Example 3: Same Flows, More Terminals Small room, no special exhaust equipment 2 supply, 2 exhaust Negative pressurization (150 cfm, 0.5 ft 2 ) Cooling flow irrelevant, less than ventilation requirement High ventilation rate (1200 cfm) Page 19

Example 3: Ventilation Schematic Corridor Laboratory 150 550 550 625 625 Supply Flow Exhaust Flow Infiltration Flow Page 20

Calculate Flow Accuracies Calculate accuracy needed on supply and exhaust e If terminal accuracies are equal e d s e 2 s e e e 150(30%) 1 s2 e1 e2 e 150(30%) d 1 2 2 2 22.5 Or we can adjust the allocation 2 2 45 Page 21

Allocate Error to Terminals Exhaust flow is a little higher Round exhaust error up and supply error down Terminal 1 Terminal 2 Supply 550 +/- 20 550 +/- 20 Exhaust 625 +/- 25 625 +/- 25 Page 22

Check for Practicality What equipment can meet spec? Try 2 pairs of 8 supply and exhaust terminals In this case, 2 terminals are more accurate than 1 80 70 60 Flow Error 50 40 30 20 10 Page 23 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Air Flow

How Does That Work? Math to combine errors accounts for the chance that errors cancel or add Square root equation favors more terms Is it realistic? experience says that large rooms, with many terminals are easier to commission than rooms with 1 in and 1 out Page 24

Summary of Examples Worked 3 examples with the same pressurization requirement leakage area: 0.5 ft 2 flow 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 Page 25

0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 0 50 100 150 200 250 80 70 60 50 40 30 20 10 0 0 200 400 600 8 00 100 0 1200 1 400 1 600 1 800 2 000 Supply Flow Exhaust Flow Infiltration Flow Designing for Pressurization Pressure Difference e d x O x Infiltrating Air Flow e e Flow Error 2 s 2 e Corridor Laboratory 550 550 625 625 150 Determine pressurization relationships Select pressurization level and accuracy Calculate flow accuracies Check for practicality Adjust as needed Air Flow Page 26

Summary Space pressurization: tool for contamination control, not a magic shield Envelope leakage is main mechanical parameter Several HVAC control methods Differential flow control is used most often Choice usually driven by envelope Derive air flow accuracy spec from pressurization Page 27

Thank you! Questions? Jim Coogan, PE Jim.Coogan@Siemens.com Page 28