1、2008 ASHRAE 409ABSTRACTSince the terrorist attacks on 911 the safety and securityof occupants in at-risk buildings has been elevated in impor-tance for the design and operations of buildings. With a highlevel of occupant safety as a goal, architects, and structuraland mechanical engineers must work
2、collaboratively. This isparticularly true for decisions related to the design and oper-ation of heating, ventilating and air-conditioning (HVAC)systems. Generally terrorist attacks on buildings will be theresult of either detonation of an explosive devise or release ofa chemical or biological agent.
3、 It is this latter approach thatis of most concern for the design and operations of HVACsystems as they may reduce or increase risk depending on theiroperation. This paper presents the beginning of a decision-support framework for the design of buildings and HVACsystems to reduce the risk to occupan
4、ts in the event of a releaseof a chemical or biological agent. The paper attempts to iden-tify issues that should be considered by architects and HVACengineers during the design process. The paper proposes thatdesign and operations decisions are based on risk assessment,owner/design goals, constrain
5、ts, performance satisfaction,and cost assessment. An overview of these decision-makingfactors is discussed as they relate to HVAC systems.INTRODUCTIONThe design of buildings and HVAC systems in response tothe release of a chemical or biological agent (CBA) is a multi-dimensional problem. Arguably fe
6、w architects or HVACsystem designers have much experience designing buildingsfor the protection of occupants from the release of a CBA.Consequently, if the design decision-making process can becaptured and mapped with inputs, constraints, assessmentprocedures and outputs identified then designers co
7、uld betterunderstand the relevant issues associated with this problem.Decision-making relative to this topic must involve the assess-ment of risk, owner and designer goals and intentions, consid-eration of constraints, satisfaction of performance mandatesand cost. This paper is an initial proposal f
8、or developing sucha decision-support framework for design of buildings andHVAC systems in response to Homeland Security issues.Decision-making relative to the design of buildings andHVAC systems in response to a CBA threat must include: 1)identifying the decision situation and understanding ownerobj
9、ectives, 2) identify alternatives, 3) decompose the problemand process, 4) choose the best alternative, and 5) performsensitivity analysis. The process is shown in Figure 1.DECISION SUPPORTDecision support systems are not new, their history beganaround 1965 as part of a movement toward distributedco
10、mputing. Decision support systems today provide inte-grated support for managers working alone, in teams and inorganizational hierarchies to manage organizations and makebetter-informed decisions. While traditionally applied to thebusiness and manufacturing sectors, decision support systemsare now b
11、eing adapted to other disciplines such as engineeringand architecture. The decision-making process shown in Figure 1 beginswith identifying the decision situation and understandingdesign objectives. This may be further broken down into 1)risk assessment, 2) consideration of owner and designer goals,
12、3) consideration of constraints, 4) satisfaction of performancemandates and 5) cost assessment, as shown in Figure 2.HVAC Design and Operations inResponse to Homeland Security IssuesA Decision-Making ProcessJames Jones, PhD Harmohindar Singh, PhD, PEFellow ASHRAEJames Jones is an associate professor
13、 at the College of Architecture and Urban Studies, Virginia Tech, Blacksburg, VA. Harmohindar Singhis a professor at the College of Engineering, McNair Hall, North Carolina A&T State University, Greensboro, NC.NY-08-0482008, American Society of Heating, Refrigerating and Air-Conditioning Engineers,
14、Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 114, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.410 ASHRAE TransactionsIDENTIFYING THE DECISION SITUATI
15、ONRisk AssessmentSome HVAC system design and operating decisions mayimpose increased cost and spatial needs for the building. Forexample, the decision to include whole-building medium orhigh arrestance filtration may increase first costs as well asoperating and maintenance costs when compared to the
16、 alter-native of not implementing such an approach. The desire toisolate zones of the building with pressure differences maysuggest the need for low porocity barriers, carefully specifiedrelief and outside air imbalance and distributed HVAC systemlayout with more systems taking up more space. These
17、cost-impacting decisions must be weighed against the relative riskof attack and harm to the occupants. Therefore the decision-making process begins with determining the degree of risk andvulnerability of the proposed building. The basic elements ofrisk analysis are shown in Figure 3. The main steps
18、of riskanalysis include:1. Identify suitable risk indices.2. Develop a model of the activity or system being analyzed,linking more detail elements of the system and the overallrisk indices.3. Estimate unknown parameters of the model.4. Use the model to generate an estimate of the risk indices.Releva
19、nt questions associated with risk assessment mightinclude:1. What are the types of threat and is each significant?2. How long would it take to recover from each type ofevent?3. Would there be warning prior to each type of event?4. How many people might be affected from each event?5. Would the event
20、affect the infrastructure?6. What are the mitigation strategies?7. What is the prioritized action sequence?For buildings, risk of a CBA attack is hypothesized to bea function of:1. Building function (A)2. Visibility (Military, federal, multi-national business,etc.)(B)3. Location (large urban, urban,
21、 sub-urban, rural) (C)Figure 1 Overview of decision-making process (Clemen1996).Figure 2 Decision issues for identifying the decisionsolution.ASHRAE Transactions 4114. “Defensibility” (setback from adjacent parking, streets,etc.)(D)A simple representation of the risk assessment procedureis shown in
22、Figure 4. The diagram suggests a compoundingrisk assessment model.Consideration of Owner and Designer GoalsThe goals and intentions of the owner/client and thedesigner (architect and HVAC consultant) potentially createopportunities and constraints on the design process, andconsequently must be expli
23、citly incorporated into the deci-sion-making process. Opportunities are presented when the owner/clientacknowledge the importance of providing a high level ofprotection to the building occupants. This may be mandated aswith some government and military facilities or may be volun-tary as with some mu
24、lti-national corporations. In either caseby the owner accepting building safety and security as a prior-ity both the design process and the outcome may be positivelyimpacted.Owner/client goals can also impose constraints on theproject. For example, an owners goal of protecting occupantsmay translate
25、 into a building massing scheme that separatesfunctional zones into wings or provide near-impermeablebarriers between zones. This, in turn, may require a distributedHVAC system solution with proper air distribution and spec-ified relief/OA imbalance with more units and greater floorarea. The owner/c
26、lient goals and intentions must be translatedinto opportunities and constraints.The system designer may also impose opportunities andconstraints on the design process. For example, in response toan intention to minimize risk the HVAC engineer may be will-ing to try new and emerging HVAC systems such
27、 as displace-ment ventilation which limit the distribution of a contaminantto a much smaller area when compared to mixed ventilationsolutions. If the owner can be convinced to agree to this designdirection then the understanding and performance of thesenew systems may be improved both within the des
28、ign firm andto the profession at large. On the other hand, familiarity withcertain solutions and systems and a lack of willingness to takeon the risk of specifying an unfamiliar system can be aconstraint. This represents a second level of risk into theprocess, namely the risk of attack on the buildi
29、ng must also beweighed with the personal and financial risk (potential litiga-tion) to the owner and designer for specifying untried anduntested system solutions. The owner and system designermust work within their person range of risk tolerance. ConstraintsAs suggested the owner and designer goals
30、can imposeconstraints on the project. Therefore their goals and intensionsmust be translated into opportunities and constraints. Otherpotential constraints relative to the release of a CBA must alsobe identified and incorporated into the decision-makingFigure 4 Schematic risk assessment structure. W
31、here: Hrepresents high risk and L low risk for the riskassessment criteria.Figure 3 Overview of the risk analysis process.412 ASHRAE Transactionsprocess. Relevant constraints may be categorized as thoseassociated with 1) the site conditions, 2) building spatiallayout, 3) building and zonal enclosure
32、s, and 4) HVAC system.Site constraints may be regulatory such as set-backrequirements from the street, adjacent buildings or vehicles.They may also be non-regulatory as design objectives toinclude buffers, barriers, terracing and other elements toprevent the movement of vehicles or large containers
33、near thebuilding. The incorporation of a surveillance system may alsoimpose constraints on the design in terms of placement forconcealment, viewing angle and accessibility for mainte-nance.The building spatial layout may also be constrained by thedesire to have a high level of protection. For exampl
34、e, differentfunctions of the building may be identified as having high riskrelative to other areas of the building. The main lobby, circu-lation areas, mail rooms and loading docks may be consideredhigh risk for agent release. These areas should be isolated fromother operational areas of the buildin
35、g. This isolation can beachieved by physical separation or potentially by HVACsystem operation. A risk-based hierarchical space planningscheme may impose constraints on the overall massing of thebuilding.Another design strategy that is growing in popularity andthat has significant consequences for s
36、pace planning andHVAC system layout and operation is the concept of “shelter-in-place”. This strategy incorporates a room within the build-ing that is a refuge from the attack due to controlled space pres-surization and air cleaning. Decisions related to shelter-in-place begins by asking whether thi
37、s strategy is appropriate forthe given inputs (owner/client goals, level of risk, costconstraints, etc.). If a shelter-in-place strategy is undertakenthen the decision-making process must include: 1) what willbe the SIP level of protection (normal, enhanced, expedient,specialized or pressurization),
38、 and 2) addressing the consid-erations (location, enclosure air tightness, size and capacity,and accessories). These questions should be translated intoprocedural steps in the decision-making structure.HVAC ConstraintsThe desire to protect building occupants from a CBA hasdirect consequence for the
39、design and operation of the HVACsystem. This goal can impose certain constraints on thedesigners of these systems. For example, in a general sense theASHRAE Presidential Ad Hoc Committee on HomelandSecurity has issued numerous recommendations on how toprotect buildings against CBA (ASHRAE 2003). The
40、 recom-mendations may be summarized as follows:1. Make sure that the ventilation system is working asdesigned by evaluating its operation relative to its designintent i.e. it should operate in accordance with ASHRAEStandard 62. This may impose a need to include commis-sioning as a step in the buildi
41、ng hand-off process.Although not suggested by ASHRAE for at-risk build-ings continuous system monitoring and commissioningshould be considered. This may impose a cost that shouldbe included in a life-cycle-cost analysis.2. Secure mechanical rooms and outdoor air intakes toprevent tampering. Air inta
42、ke should be located as high aspossible above ground. Where locating the air intakesabove ground is not possible, monitoring systems such assurveillance cameras or alarms should be used.3. Understand the consequences of any HVAC changes thatare considered in response to CBA incidents. Changesmade to
43、 building operation with the intention of reducingbuilding vulnerability should not degrade indoor air qual-ity or comfort under normal operation.More specifically, some design and operationalconstraints on the HVAC system are identifiable and quanti-fiable as part of decision-making. For example, o
44、utdoor airintakes are potential targets for a CBA, therefore when design-ing the HVAC system several guidelines can constrain thelocation of these intakes. FEMA 426 Guidelines specify thatair intakes should be located at least 12 ft above ground. Simi-lar guidelines apply from government and US Depa
45、rtment ofDefense sources that require OA intakes to be located at least10 feet above ground. When this is not possible security andsurveillance measures may be needed. When a hierarchy of risk is applied to functional zoningand HVAC system layout, such as shelter-in-place, then pres-sure differentia
46、ls may be used to insure directional flow andisolation of high risk areas of the building. For this both thesystems selection and operations are intended to maintain adesired pressure relationship between zones. These pressuredifferentials may be viewed as operational constraints on thesystems. For
47、example, the Center for Disease Control (CDC)guidelines for preventing the transmission of mycobacteriumtuberculosis in health-care facilities stipulates that a minimumdifferential pressure of 0.249 Pa (0.001 in w.c.) is needed toachieve differential airflow into or out of a room (Wiseman2003). Wise
48、man states that “This value is deemed too smallsince it can be adversely affected by thermal stratification in aroom, room supply air diffusion and door swings (Wiseman2003). The American Institute of Architects “Guidelines forthe design and construction of hospitals and health care facil-ities” als
49、o recommends a minimum differential pressure of2.49 Pa (0.01 in w.c.). The American Conference on Govern-mental Industrial Hygienists (ACGIH) “Industrial ventilation,a manual of recommended practice” notes that the proper flowdifferential will depend on the physical conditions of the area,but a general guideline would be to set a 5% flow differencebut no less that 24 L/s (50 cfm) (Wiseman 2003). It goes on torecommend that pressure differences should be greater than0.05 in w.c. (12.45 Pa). Depending on the guideline beingdesigned for these all impose potential constraints on thedesign and
