Emerging Technologies in (Suite) Pressure Control, Performance Modeling and Design Practices Wei Sun, P.E. ASHRAE Principal, Director of Engineering Engsysco, Inc. Ann Arbor, Michigan, USA Emerging Technologies in (Suite) Pressure Control, Performance Modeling and Design Practices Presented by Wei Sun, P.E. ASHRAE Clean Spaces Technical Committee (TC9.) Chairman Healthcare Facilities Technical Committee (TC9.6) Member Laboratory Systems Technical Committee (TC9.) Member Principal, Director of Engineering Engsysco, Inc. Ann Arbor, Michigan, USA www.engsysco.com Engsysco Introduction Pressurization Technique Purposes Direct desired flow patterns Isolate airborne cross contamination Definition A technique that air pressure differences are created mechanically between rooms to introduce intentional air movement paths through room leakage openings. These openings could be either designated, such as doorways, or undesignated, such as air gaps around doorframes or other duct/piping penetration cracks. How to achieve It can be achieved by arranging the controlled volumes of supply, return, and exhaust airstreams to each room within the space. Introduction Basic Rules connection between two adjacent rooms is through connecting opening(s). If a door between two rooms is open, the doorway will be the main designated flow path. If the door is closed, then the leakage will be through undesignated paths, such as air gaps along doorframes, joints, pipe and duct penetrations and gaps around ceiling panels etc. Most of these controllable cracks (except for operable doors) in typical controlled spaces are required to be permanently sealed. > P2 2 RA + EA RA 2 + EA 2 Leakage Flows Closed SA SA 2 P2 > P2 2 RA + EA RA 2 + EA 2 Leakage Flows Opened SA SA 2 P2
Introduction Basic Rules To Achieve P > P 2, SA > (RA +EA ), and SA 2 < (RA 2 +EA 2 ) SA = (RA +EA ) + Q SA 2 + Q = (RA 2 +EA 2 ) Q is the leakage (transfer) air from to 2, if both rooms are tightly sealed, except for the opening between rooms. 2 RA + EA RA 2 + EA 2 Q Leakage Flow Leakage SA SA 2 P2 Introduction Relationship between Leakage Flow, Leakage Area and Pressure Drop across Leakage Path The pressure drop (differential) across an opening (either a crack 2 or a doorway) is strongly related with the leakage opening size RA + EA RA 2 + EA 2 (effective leakage area) and Q Leakage Flow A leakage flow through the opening. Leakage Area To quantitatively achieve a SA SA 2 P2 desired room pressure (or, pressure differential between rooms), leakage openings and Pressure Differential respective leakage airflows need to be studied together. flow through Leakage Power Equation: (Esq.. ) n Q = C () 2 RA + EA Q Leakage Flow A Leakage Area where Pressure Differential Q = volumetric flow rate CFM (L/s) = pressure drop across opening in. of water (Pa) C = flow coefficient CFM/(in. of water n ) (L/s/Pa n ) n = flow exponent dimensionless SA RA 2 + EA 2 SA 2 P2 flow through Large Designated Orifice Equation: (Esq.. 2) Q = 26 A (I-P unit) 2 RA + EA Q Leakage Flow RA 2 + EA 2 A Leakage Area Q = 84 A SA SA 2 P2 (SI unit) where Pressure Differential Q = volumetric flow rate CFM (L/s) = pressure drop across opening in. of water (Pa) A = large designated opening area's) ft 2 (m 2 ) 26 = unit conversion factor dimensionless (I-P unit) 84 = unit conversion factor dimensionless (SI unit) Orifice Equation is more popularly used in design community
Leakage Rate vs. Pressure Difference for Various Leakage Areas (Based on Orifice Equation) Leakage Flowrate (cfm) 2,,9,8,7,6,,4,3,2,, 9 8 7 6 4 3 2 Leakage Area (Sq. in.)....2.2.3.3.4.4...6.6.7.7.8 Pressure Differential Between s (in.) 4 38 36 34 32 3 28 26 24 22 2 8 6 4 2 8 6 4 2 Leakage Area Value Determination Large designated openings such as doorway can be easily measured. However irregular opening such as a crack can not be measured physically, there is other means to estimate the equivalent size, or called Effective Leakage Area (ELA). For Existing s: Field Blower Test (ASTM 987, CGSB 986) to obtain more precious data. For Future s during design phase: Use ASHRAE ELA tables for building components (doors, walls, joints, etc.) as estimated values. ASTM Blower Test, - Traditional -tightness Test Portable Pressurization Blower Test can produce a set of data of Q - P, and a power equation curve fit with calculated constants (C, n, ELA) that defines a room s unique and dynamic leakage characteristic. Abnormal test ranges: ASTM (987): 2. - 7 Pa (. -.3 in.) CGSB (986): - Pa (.2 -.2 in.) Labor intense, time consuming Disruption to occupied spaces Blower Test - Multiple-Point Test Data for Power Equation Curve Fitting Power equation: n Q = C ( ) Once obtained Q - data set, C and n can be calculated: m m m ( lnqk ln k ) m (lnqk ln k ) n = k = k = k = m m 2 2 ( ln k ) m (ln k ) k = k = m m lnqk n ln k C = EXP k = k = m
Qi Resistance Analysis Define: Leakage Flow Resistance R Leakage flow resistances connected in parallel and series Q Q P i P P R = ELA n ELA = ELA i T i= ELAT = n 2 i= ( ELAi ) Pressurization Scenarios and Variable Relationship Scennario : Prerssurized SA - (EA+RA) = ΔV = ΣQ > Total Total Supply flow and/or (SA) Return flow (EA+RA) Positively Pressurized + Total Leakage flows ΣQ Total Supply flow (SA) Total and/or Return flow (EA+RA) Offset Flow ΔV Pressurization Scenarios and Variable Relationship Total Supply flow (SA) Scennario 2: Non-Prerssurized SA - (EA+RA) = ΔV = ΣQ = Total and/or Return flow (EA+RA) Non-Pressurized Total Leakage flows ΣQ = Total Supply flow (SA) Total and/or Return flow (EA+RA) Offset Flow ΔV = Pressurization Scenarios and Variable Relationship Scennario 3: De-prerssurized SA - (EA+RA) = ΔV = ΣQ < Total Total Supply flow and/or (SA) Return flow (EA+RA) Negatively - De-pressurized Total Leakage flows ΣQ Offset Flow ΔV Total Supply flow (SA) Total and/or Return flow (EA+RA)
Central Handling System & Pressurization SA = Volume of total supply air entering the space/zone RA = Volume of total return air leaving the space/zone EA = Volume of total exhaust air leaving the space/zone OA = Volume of outside air drawn into the AHU FA = Volume of relief air released from return air RA-FA = Volume of recalculated air Q = Volume of total leakage air through space shell/zone Central Handling Unit & Pressurization Two volumetric balance equations (Mass balance equation under assumption of same air density) SA = RA + EA + Q (Volume balance for a space) SA = OA + (RA FA) (Volume balance for a typical AHU) Space Pressurization Ratio (R) Define as the Ratio between SA and (RA+EA), as an indicator of pressurization scale: SA SA R = = RA + EA SA Q By specifying SA values, R will be a function of Q. R Value Chart is convenient for design engineers to determine SA and (RA+EA) ratio during air distribution arrangement. Chart Pressurization Ratio vs. Leakage Rate for Various Supply Rates
Space versus Pressurization Ratios The relationship between the space pressurization ratio and its individual room pressurization ratios: R = n ( SAi SA) i = R i The space pressurization ratio, an indicator of relative pressurization level, can be used to adjust air gains or losses among zones in order to arrange desired air flows within a building. Adjacent s under Various Pressures If a room has several leakage openings with adjacent rooms, the room s pressurization ratio is: R R R = n SA R SA i = Q i Pressure Differential and Crack Velocity Criterion (Pressure Differential ) For single room: :. in. of water (2. Pa) For multiple-room space with staged pressurizations: :.2 in. ~.3 in. ( Pa ~ 7. Pa) for each pressure step Criterion 2 ( Crack Velocity V) fpm (3 M/m) From comparison below, the pressure criterion of =. in. is much more conservative than the velocity criterion of V = fpm. Pressurization Criterion Pressure Differential Crack Leakage Velocity V Large Velocity V Unit Pressurization Criterion Comparison Basis In...2.3.4..6.8. fpm 9 374 87 764 92,64,98,444,67 Eq. (), when n=.6 fpm 26 369 42 22 84 639 738 82 Eq. (2a) Pressurization Variables and Control Strategies flow differential between entering airflow (supply airflow, SA) and leaving airflow (exhaust and/or return airflows, EA+RA), normally called offset value (ΔV), which equals the total leakage airflow (ΣQ) of the room. To maintain a specific room pressure value, the room s offset airflow (ΔV) must be controlled and maintained at the appropriate value. s offset airflow can be controlled directly or indirectly. The treatment of the room offset value defines a pressurization control strategy. Typical pressurization control techniques are: Direct Pressure-Differential Control, Differential Flow Tracking Control, Hybrid Control and Adaptive Control.
SUPPLY AIR Leakage SUPPLY AIR SUPPLY AIR Monitor Leakage Leakage Supply Total Supply to Switch Thermostat Supply Total Supply to Switch Supply Thermostat Total Supply to Switch Thermostat CHEMICAL LAB ROOM CONTROLLER CHEMICAL LAB CHEMICAL LAB ROOM CONTROLLER ROOM CONTROLLER Fume Fume Fume Flow Flow Velocity or Sash & Controller Velocity or Sash Velocity or Sash & Controller & Controller Total from Flow Flow Leakage Total from Total from Leakage Leakage Direct Pressure-Differential Control () Utilizes a pressure differential sensor to measure the pressure difference between a controlled room and an adjacent space such as a corridor. It basically ignores the specific offset value as required, instead, it directly controls the airflow control devices to achieve the required pressure differential. T Suitable for a tightly constructed room with limited traffics. switch is recommended to trigger a reduced pressure differential setpoint if the door opens. Differential Flow Tracking Control (DF) Intuitively assumes an offset value which is used as a flow difference between the entering and leaving airflows to control their respective airflow devices. Maintain the same offset value throughout the operation to keep pressurization constant, or maintain a constant percentage offset value which creates a weaker pressurization at lower flow. T Suitable for open-style rooms or rooms with frequent traffics Hybrid Control (+DF) Combines the pressure accuracy of the direct pressure differential control and the stability of the flow tracking control. The offset value is reset-able based on pressure differential reading. The offset value reset schedule is pre-determined and controller s parameters are fixed manually in field. This method is also called cascaded control. T Suitable for open-style rooms or rooms with frequent traffics Multiple- (Suite) Pressure Control Strategies Single room control technologies often cause problems in Suite Pressure Control during air balancing, since the following phenomena are often ignored: Adjusting one room s offset value will impact adjacent rooms air pressures if they were just balanced earlier. One room s air gain could be another room s air loss through leakages. Example - Pharmaceutical Aseptic Suite
CLEANER ROOM.6 In. AIRLOCK.3 In.. In. SUITE CONTROLLER CLEANEST ROOM.8 In. GENERAL CHEMICAL LAB -.2 In. CONTAINMENT LAB -.6 In. Manifolded or Open to Corridor Supply Designated Leakage Flow Minor Leaks Thru. Cracks Pressure Differential Switch Position Outputs Flowrate Inputs SUITE CONTROLLER Pressure Inputs Return Switch Inputs Adaptive Control (+DF+AD) The three traditional methods (, DF and +DF) are either to ignore, assume or manually fix in field the offset value respectively. The adaptive (+DF+AD) approach directly accounts for leakage flows between the rooms in a suite. It controls all rooms pressures all together as an optimized system, instead of controlling each room pressure independently. It actively adjusts the flow offset of each room according to an on-line pressurization model. The model uses flow and pressure differential measurements to estimate the leakage values between the rooms and adjust flow offset of each room automatically. Automated -tightness Test Pre-condition for Truly Adaptive Control Similarly as Blower Test, but fully automated. A room s unique dynamic leakage characterization can also be automatically achieved by digital controller, precision pressure differential sensor (±. in./.2 Pa) and airflow control devices (±%). These devices are often permanently installed in lab and clean room environments. This automated pressurization test (Q- P data set) is faster and cheaper, and can be handled remotely. Adaptive Control (Example: Control of Multiple s) Legend Flows between s flow Between s
RMX RM2 RM3 RM RM4 RM RM6 - Numbe - Wall - Flow Direction - Induced Flow (by Pressurization) - Node () - Flow Resistance @ Major - Flow Resistance @ Minor - Forced Flow (by Fan) RM2 RM3 RM RM4 RM RM6 Personnel Flows between s Personnel Flow Between s More Considerations Correction and Safety Factors Add as required Correction Factors (Refer to ASHRAE Handbooks 999 & 2, detailed procedures will be included in the next phase of the study) Stack effect Wind effect Interior zones with high temperature or humidity differences Safety Factors (Detailed procedures will be included in the next phase of the study) background leaks Duct leaks AHU unit leak Samples of Pressurization Control Devices Flow Control & Measure Pressure Measure Static Pressure Measuring Probes Control Damper Type Pressure Transmitter Type 2 Pressure Transmitter - and Monitor Type 3 Case Study - flow Resistance and Leakage Flow Simulation Major and minor leakage openings, connection in parallel and series Network Flow with Major s Only Network Flow with Major and Minor s
Pressure Drop Across Autom atic Swing Opens to 9 o in 3 Seconds; Size 4 ft. (W ) x 7 ft. (H). Width of s Across The /Wall Are Maintained with Constant Supply and Return Flows. Initial Pressure Differential Across is 68.9 Pa, it drops to Pa less than 2 seconds. Angle of Swing (Degree) 2 3 4 6 7 8 9 7 6 6 4 4 3 3 2 2.. 2 2. 3 7 6 4 3 2 Pressure Drop Across Width of Time of (Second) Automatic Sliding Opens at Speed of 6 in./sec.; Size 4 ft. (W) x 7 ft. (H); s Across The /Wall Are Maintained with Constant Supply and Return Flows, Initial Pressure Differential Across is 68.9 Pa, it drops to Pa around 2 seconds... 2 2. 3 Time of (Second) 6 4 3 2 > P2. First First Closing Both s Closed Second Second Closing.9.9.8.8.7 Cleanroom.7.6.6...4.4 lock.3.3.2.2.. Corridor.. -. -...9.9.8.8.7.7.6.6...4.4.3.3.2.2.... -. -. lock Sliding Operation Cycle > P2 First First Closing Both s Closed Second Second Closing (Cleanroom and Corridor) ( ) ( 2) lock Sliding Operation Cycle P2 P2 Modeling of Transient Pressurization. Pressurization Loss Characteristic During a Swing or Closing L ( t) A ( t) A ( t ) = = = 2 H W H where, L ( t) W θ ( t sin 2 = 2 H ) W ω t sin 2 ( (6 θ < θ o 6 ) o 9 ) L=width (gap) of door opening in. (cm) W=width of door in. (cm) θ=angle of door opening degree ω=speed of door turning degree/sec. t=time sec. H=door height in. (cm) A=effective door opening width (gap) in 2 (cm 2 ) 2 Swing Transient Flow Through Swing θ W L Wall P Pressure Differential Across (Pa) Modeling of Transient Pressurization. Pressurization Loss Characteristic During a Swing or Closing Transient Pressure Differential Across When A Swing Opens Width (Gap) of (in.) Static Pressure (in. WC) lock Pressure Profile Modeling of Transient Pressurization 2. Pressurization Loss Characteristic During a Sliding or Closing where, L=width (gap) of door opening in. (cm) W=width of door in. (cm) t=time sec. s=speed of door opening in./sec. (cm/sec.) H=door height in. (cm) A=effective door opening width (gap) in2(cm2) 2 Sliding Transient Flow Through Sliding W L Wall P Pressure Differential Across (Pa) Modeling of Transient Pressurization 2. Pressurization Loss Characteristic During a Sliding or Closing Transient Pressure Differential Across When A Sliding Opens Width (Gap) of (in.) Pressure Differential between s (in. WC) Pressure Differentials Between s
-6-3 3 6 2-6 -3 3 6 s 2 s Transient Impact on Pressurization Control Any passive motor-driven or actuator-driven HVAC system (such VAV box or valve) will not have enough time to react effectively to prevent possible cross contamination. A single barrier door could cause a short duration of backflow contamination until the motor or actuator completes the modulation cycle of re-balancing, additional means to prevent possible backflow contamination, such as double-door airlock is necessary. Dynamic Pressurization Control Strategies - lock Lock Type Cascading Bubble Sink Dual- Compartment AIRLOCK ++ AIRLOCK - - + CLEANROOM +++ CASCADING AIRLOCK + SINK AIRLOCK CLEANROOM - AIRLOCK ++ AIRLOCK ++ + BUBBLE AIRLOCK CLEANROOM - CLEANROOM - - AIRLOCK - - DUAL COMPARTMENT AIRLOCK Dynamic Pressurization Control Strategies - lock lock Physical Model Cleanroom.6 in. Corridor. in. Wait! lock (Cascading).3 in. -6-3 3 6 Network Flow Simulation Result of Network Flow Simulation Clean room.6 in. Wait! lock (Cascading).3 in. Corridor. in. -6-3 3 6-6 -3 3 6-6 -3 3 6
CFD Model to Study lock Transient Performance - Physical Conditions Case Class, Case 2 Class (2 CFM, 7 ACH) (6 CFM, ACH) Leakage 73 CFM (2 CFM, 7 ACH) 278 CFM Leakage 2 CFM 948 CFM Clean : lock:, Corridor: 6 (CFM, ACH) 84 CFM, 3 ACH) Leakage 73 CFM 8378 CFM Leakage 2 CFM (48 CFM, 3 ACH) 47948 CFM Clean : lock: Corridor: Steady State flow Distribution Case Class, Case 2 Class Steady State Cleanroom Particle Concentration Case Class, Case 2 Class Corridor Particles Enter lock Case Class, Case 2 Class
Profile of Pressure Differential Across When Is & Closing (Initial Condition: Pa = -.6 In.) 2 2 Closing - -2 2 3 4 6 7 8 9 2 3 4 6 2 2 - -2 2 2 - -2 2 2 - -2 Profile of Pressure Differential Across When Is & Closing (Initial Condition: - Pa = -.4 In.) Closing 2 3 4 6 7 8 9 2 3 4 6 Profile of Pressure Differential Across When Is & Closing (Initial Condition: Pa = -.2 In.) Closing 2 3 4 6 7 8 9 2 3 4 6 Profile of Pressure Differential Across When Is & Closing (Initial Condition: Pa = In.) Closing 2 3 4 6 7 8 9 2 3 4 6 2 2 - -2 2 2 - -2 2 2 - -2 2 2 - -2 Profile of Pressure Differential Across When Is & Closing (Initial Condition: Pa =.2 In.) O pening Closing 2 3 4 6 7 8 9 2 3 4 6 Profile of Pressure Differential Across When Is & Closing (Initial Condition: Pa =.4 In.) Closing 2 3 4 6 7 8 9 2 3 4 6 Profile of Pressure Differential Across When Is & Closing (Initial Condition: Pa =.6 In.) Closing 2 3 4 6 7 8 9 2 3 4 6 Profile of Pressure Differential Across When Is & Closing (Initial Condition: 2 Pa =.8 In.) Closing 2 3 4 6 7 8 9 2 3 4 6 lock Particles Enter Clean and Corridor Case Class, Case 2 Class lock Particles Enter Clean and Corridor Case Class, Case 2 Class Variation of Corridor Particle Concentration Case Class, Case 2 Class lock Transient Performance Pressure Differential Across Cleanroom During Walk-Through Pressure Differential Across (Pa) Pressure Differential Across (Pa) Pressure Differential Across (Pa) Pressure Differential Across (Pa) Pressure Differential Across (Pa) Pressure Differential Across (Pa) Pressure Differential Across (Pa) Pressure Differential Across (Pa)
8, 7, 6,, 4, 3, 2,, 8, 7, 6,, 4, 3, 2,, 8, 7, 6,, 4, 3, 2,, 8, 7, 6,, 4, 3, 2,, Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Depressurization @ Pa = -.6 In. ) Inside Cleanroom Clos ing = 8.9% Outsi de Cleanroom A ver age 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Depressurization @ - Pa = -.4 In. ) Inside Cleanroom Closing = 8.% Outside Cleanroom 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Depressurization @ Pa = -.2 In. ) Inside Cleanroom Closing = 6.9% Outside Cleanroom 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Neutral @ Pa = In. ) Inside Cleanroom Closing = 4.2% Outsi de Cl eanroom A ver age 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 8, 7, 6,, 4, 3, 2,, 8, 7, 6,, 4, 3, 2,, 8, 7, 6,, 4, 3, 2,, 8, 7, 6,, 4, 3, 2,, Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Pressurization @ Pa =.2 In. ) Inside Cleanroom Closing = 2.2% Outsi de Cl eanroom A verage 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Pressurization @ Pa =.4 In. ) Inside Cleanroom Cl osing =.7% Outside Cleanroom 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Pressurization @ Pa =.6 In. ) Insi de Cleanroom A ver age Clos ing =.8% Outsi de Cl eanr oom 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 Particle Concentrations Across Cleanroom When is & Closing (Initial Condition: Pressurization @ 2 Pa =.8 In. ) Ins ide Cleanroom A ver age Closing =.3% 2 3 4 6 7 8 9 2 3 4 6 7 8 9 2 2 22 Outside Cleanroom (Note: Pa =.2 In., Particle Measured @. µm) & Closing W/O People Traffic A Person Walks Through 2% 2% % % % % - 2 2% 2% % % % Initial Pressure Differential Across (Pa) (Note: Pa =.2 In., Particle Measured @. µm) R 2 =.966 (No People Traffic) & Closing W/O People Traffic A Person Walks Through Regression ( & Closing W/O People Traffic) Regression (A Person Walks Through ) R 2 =.929 (With People Traffic) % - 2 Initial Pressure Differential Across (Pa) Contamination Risk Factor () is a criterion which is to quantity the effectiveness of cleanroom particle containment in preventing the airborne particles migration into cleanroom. = P C / P O = Contamination Risk Factor P C = Number of Particles inside Protected Cleanroom Near P O = Number of Particles at Corridor Entrance Near This criterion is applied for a Barrier Device which is to minimize particle migration. This barrier could be single door, an airlock (two doors in series), mini environment, or glove box. The lower of the level, the better barrier s performance, or the better decontamination effectiveness. This expression can not only apply for airborne particle, but also for airborne microorganism egress, in which the particle counts will be replaced with Colony Forming Unit (CFU). Particle Concentrations & Across Cleanroom Under Various Pressure Differentials Particle Concentrations Across Particle Concentrations Across Particle Concentrations Across Particle Concentrations Across Particle Concentrations Across Particle Concentrations Across Contamination Risk Factor (, %) borne Particle Contamination Risk Factor () Under Various Pressure Differentials Across Cleanroom borne Particle Contamination Risk Factor () Under Various Pressure Differentials Across Cleanroom Particle Concentrations Across Particle Concentrations Across Contamination Risk Factor (, %) Regression Curve: =.332e -.8*PD Regression Curve: =.48e -.73*PD Dynamic Pressurization Control Strategies Adjustable Pressure Stabilizer A leakage regulator, controllable pressure relief damper across a wall to maintain a minimum required pressurization. When a door is normally closed, this damper should stay open and maintain normal pressure differential; when the door opens, the damper shall be automatically closed either by spring-loaded or counterweight gravity damper, and maintain a lower while acceptable pressure differential. Pressure Stabilizer Importance In addition to design engineers and research scientists, the information presented may also benefit manufacturers in the fields of: -handling unit control Lab HVAC control Prefabricated clean room Precision environmental test chamber Smoke management control distribution system
Pressurization Study Q & A