1、NA-04-7-2 Quantitative Multistage Pressurizations in Con t ro I I ed and C rit i ca I E nvi ron ment s Wei Sun, P.E. Member ASHRAE ABSTRACT The space pressurization technique is one of the key elements in design of controlled or critical environments (CCE). This technique is much more complicated fo
2、r multiple rooms under various and stugedpressurization levels, and the design criteria of this “multistage” or “cascade ” pressuriza- tion technique has traditionally been applied based on oflanded or intuitive guesswork. A recent ASHRAE research paper established a quantita- tive relationship betw
3、een a room Spressurization ratio and its air leakage flows. As a further development, the focus ofthis paper is to explore this quantitative approach in theJielci of multistage pressurizations. A simplijed case study demon- strates the detailed procedures for arranging supply, return, and exhaust ai
4、rstreams for rooms under various requiredpres- surization levels. INTRODUCTION To design a controlled or critical environment (CCE) such as a cleanroom, biosafety lab, hospital isolation room, or even a smoke control space, design engineers may realize that the pressurization (or depressurization) i
5、s always one of the most important considerations of the entire HVAC designs. Design- ing pressurization for a single room could be simple; however, the task of designing a space consisting of multiple rooms under staged pressurization levels is much more complicated. Design engineers often assume t
6、hat if a room is to have more entering air (supply) than leaving air (return and exhaust), the room will be pressurized. This is true, but further questions of how much more supply air and what the resulting pressure differential across the room will be remain uncertain. Due to lack of engineering-b
7、ased criteria, engineers often have no choice but to intuitively pick some numbers to complete a project. As a result, many CCE spaces do not function well as intended. Room airtightness, a key element in the relationship between the airstreams enteringlleaving the space and the resulting pressure d
8、ifferential, is often ignored. Poorly designed pressurization could become unpredictable, unsta- ble, or even fail to perform. For applications where product quality or personal safety is critical, unreliable or malfunctio- ing pressurization is unacceptable. A recent ASHRAE study (Sun 2002) has pro
9、posed a quantitative approach to estimate the room pressurization ratio based on a rooms leakage rate and a required pressure differ- ence across the room shell. That paper provided simplified engineering methods to substitute current offhanded approaches. As a further development, the goal of this
10、article is to establish a quantitative approach for multiple rooms under staged pressurizations. A controlled or critical environment (CCE) is an indoor space where precision controls of indoor temperature, humid- ity, and airflow direction can be achieved. Pressurization is normally either to direc
11、t desired flow patterns or to isolate air cross-contamination. It is defined as a technique that air pres- sure differences are created mechanically between rooms to introduce intentional air movement paths through room leak- age openings. This is achieved by arranging controlled volumes of supply,
12、return, and exhaust airstreams to each room within the space. These openings could be either desig- nated, such as doorways, or undesignated, such as air gaps around door frames or other cracks. Wei Sun is a principal and chief mechanical engineer with Engsysco, Inc., Ann Arbor, Mich. 02004 ASHRAE.
13、759 BACKGROUND The interaction between the airtightness condition and pressurization level in a room is a complex phenomenon. A methodology to describe pressurization quantitatively should be based on the most critical variables; other less critical or minor influences can be included in the forms o
14、f correction and/or safety factors. We start with a simple model, where a multiple-room space is at the same floor level so that the influence of stack effect can be ignored. If all rooms are located in an interior zone, the only driving force causing air movement and pres- sure difference is a mech
15、anical device, if there is no tempera- ture or humidity difference across each room shell. However, if a subject space is a high space, located at an exterior zone, or has large temperaturehumidity differences with respect to adjacent rooms, then a correction factor needs to be included for each oft
16、hese influences (ASHRAE 1999). Some key equa- tions in Suns article (2002) will be summarized and listed below as the basis for exploring the multistage pressurizations. Airflow Through Leakage Openings The power law equation is commonly used to describe air leakage through irregular cracks, such as
17、 gaps around door- frames (ASHRAE 200 I). With numerical approximation ofits flow exponent (Sherman and Dickerhoff 1998; Sun ZOOZ), it can be simplified as Q = C (AP) O.” = 51.8 ELA (AP) Oxis (I-P) (14 or Q = C (hp) O.” = I O50 ELA (AP) (SI) (1b) where Q = volumetric flow rate, cfm (L/s); AP = press
18、ure drop across opening, in. of water (Pa); C = flow coefficient, cfni/(in. of water)” (UsPa”); O. 65 = numerically averaged flow exponent, dimensionless; ELA The leakage rate of each crack is therefore characterized by its Q-AP relationship with various C or ELA values. The ELA data tables for buil
19、ding components can be found in ASHRAE Handbook-Fundamentals (2001) and ASHRAE Handbook-HVAC Applications (1 999). These data were obtained as statistical averages among many tests, although the airtightness condition of each room heavily depends on its components manufacturing quality and installat
20、ion work- manship. If a wall has several openings, then each opening area can be consolidated to have a combined ELA value (ASHRAE 1999). Another way to estimate existing room leakage is to use a “blower test” as defined by ASTM standard 1988. Cor ELA can be calculated fi-om the test result. Procedu
21、res to obtain ELA values are not included in this paper. For a space consist- = effective leakage area, in2 (m2). FAN EXHAUST FW r - CEILING 1 I G. .na V/)LVE utin VOLUME TRK!SUG . mjo BU IL D I N P,=O.Ol (2.5 Pa) Figure 2 Case study-space conjguration. each of these influences (stack effect, infilt
22、ratiodexfiltration, temperaturehumidity differences) could be included in the variables in the related equations. Furthermore, various safety factors could be applied to account for additional minor leaks, such as duct, AHU, and room background leaks. Inclusion for the correction and safety factors
23、through manual calculation is time-consuming, and it should be used cautiously. However, these factors can be easily embedded into computer programs. Detailed selection of these factors is not included in the paper. APPLICATION OF QUANTITATIVE MULTISTAGE PRESSURIZATIONS Case Description Detailed des
24、ign procedures are demonstrated through this simplified case. Figure 2 shows that small cleanroom suite with various cleanliness classifications located in an interior zone of a pharmaceutical manufacturing facility. Rooms 1 through 4 are to be served by AHU-1 and Room 5 by AHU- 2. All airflows are
25、assumed with constant volumes. Based on load calculation and equipment selection, each room requires supply air as: SA, = 800 cfm (378 L/s), SA, = 1500 cfm (708 L/s), SA, = 400 cfm (1 89 Lis), SA, cfm to be determined, and SA, = 1 O000 cfm (472 L/s). Required process exhaust air EA, (fume foods and
26、others) = 1500 cfm (708 Lh). General build- ing exhaust EA, = 2000 cfm (944 Lis). Outside makeup air OA, = 1800 cfm (850 Lis), and OA,z= 2500 cfm (1 180 Ls). Assume ail rooms are under the same or similar design temperature and humidity. The upper rows of Table 3 show the known information. Each doo
27、rway between rooms is considered as the main air connection, while the air gap around each doorframe will be used as the main leakage opening. The estimated gap area for each leakage opening is ELA,-, = 18 in? (1 1610 mm2), ELA,=27 in.2 (17415 mm2), ELA3-,=27 in.2 (17415 mm), ELA, = 18 in. (1 1610 m
28、m2), and the estimated combined 764 ASHRAE Transactions: Symposia Table 3. Space Pressurization Calculations Room 1 Room 2 Room 3 Room 4 Room 5 Room t Room function 1 Cleanroom Room cleanliness class 10 (M2.5) /Leakage area ELA between rooms lELAI.,=18 in2 (i 1610 mm2), ELA,=27 in2 (17415 mm2), ELAi
29、4=27 in2 (17415 mm2), Cleanroom Cleanroom Cleanroom General spaces Outdoor 1,000 (M4.5) 10,000 (M5.5) 100,000 (M6.5) None None Room supply air SA, SA1=800cfm SA,= 1500 (378 Lis) cfm (708 L/s) Room return air RA, TBD TBD Room exhaust air EA, None None Y 0 I Resulting pressure differenc. SA3=400 cfm T
30、BD .SA,=10000 cfm (189 L/s) (472 L/s) TBD None TBD None EA4=l 500 cfm EA,=2000 (708 L/s) cfm (944 L/s) E Leakage airflow Q between room Y- Zone to be served by AHU unit AHU outside air setting OA AHU- 1 OAAHU-F 1800 cfn (850 L/s) 1 Room return air RA, AHU-2 OAAHU-2 2500 cfm (118OL/s) Room exhaust ai
31、r EA, Zone supply air SA P1=0.08 in. P2=0.06 in. P3=0.03 in. P4-0.02 in. (20 Pa) (15 Pa) (7.5 Pa) (-5 Pa) Zone return air RA e al P5=O.01 in. P,=O (Ground- (2.5 Pa) ing) None RA,=727 RA2=1430 cfm (343 L/s) cfm (675 LIS) Known Known O O R,=1.10 R,=l.05 None None RA3=270cfm O RA,=7554 cfm NIA (1 27 L/
32、s) (3565 L/s) Known SA4=1300 cfm Known (614 L/s) O KnOWn Known R,=l.48 R4=0. 8 7 R,=l .O5 SA,.l=4000 cfm (1888 L/s) Zone/AHU relief air FA SA2,.=10000 N/A cfm (4720 L/s N/A al Zone exhaust air EA EA,e.l=1500 cfm (798 L/s) o N . 0 Zone pressurization ratio R Rzone.i=l .O2 a Zone leakage airflow Q Qzo
33、ne.,=73 cfm (34 L/s) RAzone.l= 2427 cfm (1456 LIS) EAz,.2=2000 cfm (944 L/s) Rzone.2=l .O5 Qzone-2446 cfm (21 1 L/sl RA2,-2=7554 cfm (3565 Lis) FA2,.,=227 cfm (107 L/s) Based Equatior Room air balance Room air balance 6 RAzone =ERA: EA, =CEA 7 3 4 ASHRAE Transactions: Symposia 765 leakage areas betw
34、een building shell and outdoor ELA5-, = 200 in.2 (0.129 m2). (Note: If these rooms exist, ELAS can be obtained more precisely by a blower door test.) Emergency exit doors, process pass-throughs, and room pressure relief dampers in the cleanrooms are not included for simplification. Cleanrooms usuall
35、y require tight ceiling and wall panel instal- lations; leaks through ceiling panels are ignored, as are other minor crack leaks through sealed duct/pipe penetrations. If designers choose to count other minor leaks through sealed cracks, a safety factor could be added onto a respective ELA value. To
36、 simpliQ, these factors are not included in the calcu- lations. Goals 1. 2. 3. 4. 5. 6. 7. 8. 9. 1 o. Determine required flow paths based on room cleanliness levels Determine desired static pressure (P) setting for each room and the resulting pressure differential (M) between rooms Determine leakage
37、 airflow (Q) between rooms Calculate other unknown airflows for each room Calculate relief airflows for AHU-I and AHU-2 List all airstreams for both AHU-1 and AHU-2 systems Pressurization ratio for each room Utilize rooms pressurization ratio to calculate zones pres- surization ratio Draw pressuriza
38、tion airflow diagram and illustrate all calculated results Draw airflow balance diagrams to illustrate all calculated results Results Air must flow from the cleanest to the most contaminated room. In this case, whether these doors are open or closed, airflow through a door or a door crack will be fr
39、om Room 1 , to Room 2, to Room 3, then to both Room 5 and Room 6 as shown in Figure 2. Set the rest of building close to neutral but slightly pres- surized; the desired pressure setting for each room is: Pl = 0.08 in. (20 Pa), P,= 0.06 in. (15 Pa), P,= 0.03 in. (7.5 Pa), P,= -0.02 in. (-5 Pa), andPI
40、,=O.O1 in. (2.5 Pa). Outdoor is consid- ered at zero gage pressure; therefore, = 0.02 in. (5 Pa), Oz-, = 0.03 in. (7.5 Pa), U,_,= 0.05 in. (12.5 Pa), Al,-= 0.02 in. (5 Pa), and M,-,= 0.01 in. (2.5 Pa). Solutions, along with known information, are tabulated in Table 3. Discussion Figure 3 illustrates
41、 the pressurization diagram, and Figure 4 shows the air balance diagrams. It should be noted that pres- surization (R i), neutral (R = i), or depressurization (R r I I I I SAcl500 CFM I (708 LIS) I l I LCLASS 100,0001 + P *= -0 .o 2 “ (-5 Pa) Q3-4=200 CFM - (94 L/S) P ,=O, 08 (20 Pa) _- RA,=; (3 l I
42、 (17415 MM (12.5 Pa) APr0.05 ELAP-G In (11610 Mfd 11 7 27 CFM 3 L/S) EA2000 CFM (2.5 Pa P 6= o .o o (0 Pa) Figure 3 Case studyspace pressurization airflow diagram. - SAa=1300 CFM (614 L/S) -IEA4=1500 CFM (708 L/S) !-SA3=400 CFM I (189 L/S) I ZONE 1 SERVED IPY AHU-1 -rRA3=270 CFM I (127 LIS) 1 + SAs=
43、10000 CFM (4720 LIS) ZONE 2 SERVED BY AHU-2 I r I +RA5=7554 CFM -1 I (3565 L/SS 768 ASHRAE Transactions: Symposia RF RELIEF AIR FA=227 CFM I - A I (107 LIS), OUTSIDE AIR RECIRCULATEE& RA-FA=2200 CFM (1038 L/S SA=4000 CFM RA=2427 SA=100OO CFM RA=7554 FM (472 LIS) (3565 SUPPLY AIR RETURN A 1 (211 L/S EXHAUST AIR ZONE SERVED (944 LIS) BY AHU-2 AHU-2/ZONE 2 AIR BALANCE TITAGRAM - - - - - . . - . . . . . Figure 4 Case study-AHU units/spaces air balance diagrams. i6 M ASHRAE Transactions: Symposia 769
