1、2010 ASHRAE 3ABSTRACTThis paper provides an overview of the design of naturalventilation systems to control smoke movement in tunnels. Thepaper discusses the current standards and requirements forthese types of systems. It then discusses the various elementsthat are used to design such systems. Thes
2、e include both itemsin direct control of the designer and those that are uncontrol-lable in the design. The paper also presents a case study wherethe various design elements are utilized to create a workingnatural ventilation smoke control system in a short rail tunnel.INTRODUCTIONThe control of smo
3、ke movement in tunnels is of criticalimportance to the safety of people in a tunnel. Two generalapproaches are available to designers of these systems. Thefirst is through passive means and the second is through the useof mechanical equipment. The passive systems are oftenreferred to as natural vent
4、ilation (NV) systems. The activesystems are referred to as mechanical ventilation (MV)systems. NV systems are only used in existing systems and inshort tunnels, while MV systems are implemented in theremaining tunnels. There are many practical advantages tousing natural ventilation systems and this
5、paper discussesthem along with important design elements that help insure asuccessful design.What Is a Natural Ventilation SystemNV systems are designed to allow smoke to move natu-rally away from people who may be located within the tunnel.This movement is accomplished generally through the buoy-an
6、cy forces that hot smoke generates. This force naturallymoves the hot smoke upward toward the ceiling of the tunnel.Once the smoke reaches the ceiling the smoke spreads later-ally along the ceiling. In a NV system the smoke is accumu-lated in the ceiling area or channeled away from any people inthe
7、tunnel. Another characteristic of NV systems is that thereare no mechanical devices that need to activate for the systemto operate (although certain systems utilize dampers that mustopen or close in a fire emergency).Types of NV Systems. The first type is one in whichsmoke is carried away from the f
8、ire through ventilation open-ings in the ceilings that lead to the surface. By removing thesmoke through these openings, a tenable environment can besustained in the tunnel so that evacuation can occur. Thesecond type of NV system utilizes a smoke capture system inthe crown of the tunnel. Sufficient
9、 space is provided such thata layer of smoke forms high above any patrons and spreadslaterally as the fire progresses.What Is a Mechanical Ventilation SystemMV systems are characterized by the use of fans to forcesmoke to move in the proper direction. Dampers are alsocommonly used. The fans generall
10、y overpower any buoyancyforces that may be present. In these types of systems moreprecise control over where smoke travels can be accom-plished.Why Choose a Natural Ventilation System?The use of NV systems in tunnels has many advantagesover their MV counterparts. The first being that the addedcosts
11、of MV systems is often quite large compared to that ofNV systems. Some of the costs are associated with the actualpurchase of equipment such as fans, dampers, sensors, motorcontrollers, and wiring. Other costs come from the actualThe Design of Natural Ventilation Systems to Control Smoke Movement in
12、 TunnelsThomas P. ODwyer, PEMember ASHRAEThomas ODwyer is a supervising mechanical engineer in Mechanical and Electrical Technical Excellence Center of Parsons BrinckerhoffAmericas Inc. New York.OR-10-001 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashr
13、ae.org). Published in ASHRAE Transactions 2010, Vol. 116, 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. 4 ASHRAE Transactionsconstruction costs for the physical spaces n
14、ecessary to housethe equipment. Still other costs stem from providing adequatepower for this equipment. Finally costs stem from the main-tenance associated with the equipment. Natural ventilationsystems have little to none of these costs, with perhaps anexception being in the physical space requirem
15、ents.NV systems are also generally less complex than MVsystems. Because NV systems are generally passive systems,they must generally perform without any action by operators.These systems perform based upon mostly geometric condi-tions within the tunnel such as ceiling height and openings tothe surfa
16、ce.REQUIREMENTSNFPA 130 and NFPA 502The two main sources for fire life safety standards intunnels are the National Fire Protection Association Standards130 and 502. NFPA 130 covers rail tunnels while NFPA 502covers road tunnels. Both standards have similar goals withrespect to smoke control. Primari
17、ly, the goal is to provide atenable path of evacuation from a fire incident. To accomplishthis smoke must be kept away from evacuating passengersthrough either active or passive means.Tunnel Length. Both of these standards use tunnel lengthas one means of determining whether MV or NV is applicable.N
18、FPA 130 requires an MV system for rail tunnels greater than300 m (984 ft). For rail tunnels between 60 m (197 ft) and 300m (984 ft), this standard allows for engineering analysis todetermine whether MV or NV systems are required. For railtunnels less than 60 m (197 ft), MV systems are not required.F
19、or road tunnels, NFPA 502 does not set a definitive lowerlimit on tunnel length for which MV systems are required, butallows for NV systems where engineering analysis showsacceptable performance.Tenable Environment. Both standards have slightlydifferent definitions of what a tenable environment is.
20、In eithercase, the standards cover exposure to high temperature, radi-ation, toxic gases, high noise levels, high air velocities andlimited visibility.Zone of UntenabilityOnly NFPA 130 mentions this, but because of impracti-cality of providing a tenable environment at points very closeto a tunnel fi
21、re, there is a zone around a fire where the smokecontrol requirements are relaxed. According to NFPA 130, thiszone of untenability can include areas up to 30 m (98 ft) awayfrom the fire location. The size of this zone is most directlyrelated to the severity of the design fire incident. This zoneallo
22、ws designers to provide a reasonable amount of smokecontrol away from a fire without over-designing for thosecases, close to the fire, which are difficult to impossible tocontrol without excessive cost.Path of EgressThe standards require a path of egress from the fire. Thestandards are vague on whet
23、her all patrons need to have a pathof evacuation provided or only a single path is required forsome of the patrons. Because of this vagueness, longitudinalsystems, which push smoke and hot gasses over a portion ofthe tunnel which may contain patrons are allowable. In thesecases only one path of egre
24、ss is provided for a portion of thepatrons.Smoke Control in Chosen Evacuation Path. One of thecharacteristics of a controlled system is that it respond as thedesigner or operator intended under a variety of conditions. Ifthe designer expected the ventilation direction to be in acertain direction the
25、n the system should operate in that direc-tion when implemented. For NV systems this intended direc-tion is always the same for any given fire location or size. InNV systems the smoke movement must be away from the pathof egress for all conditions that may be reasonably encoun-tered. If the results
26、are different for different conditions the NVsystem does not really provide control of the path of egress.Point of Safety. A point of safety is a location that affordsadequate protection to patrons during a fire emergency. Byadequate, it is meant that the area will remain tenable for theduration of
27、the incident. In deep tunnels, far from the surface,points of safety are often employed so that patrons can evac-uate there instead of traveling long distances to the surface.Control of Smoke MovementIn order to successfully control smoke movement thesystem must definitively move smoke in the intend
28、ed direc-tion. The intended direction is that which the designer choseas a response to a particular fire incident. In the MV case thisgenerally means using fans to move smoke away from the pathof egress. In the NV case, control is based upon the geometricconfiguration of the tunnel and how buoyant h
29、ot smoke movesup and away from a fire incident. The geometric configurationis designed to facilitate this smoke movement in an intendeddirection, therefore maintaining control of the situation.DESIGN ELEMENTSThe designer of NV systems for tunnels has to balancemany items; those both under his/her co
30、ntrol and those that arenot.External InfluencesThe impact of those items beyond the designers controlneeds to be mitigated such that they do not have great influenceon the control of smoke movement. There are several itemsthat the designer has no control over in designing a naturalventilation system
31、 for a tunnel. These items must be taken intoaccount and measures must be taken to mitigate against theireffects.Wind. Wind is perhaps the single most difficult influenceon a natural ventilation system. When present, wind forces can 2010, American Society of Heating, Refrigerating and Air-Conditioni
32、ng Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, 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. ASHRAE Transactions 5be larger than t
33、he buoyancy forces generated by a fire. Sincewind direction and magnitude are variable, even for locationswith a prevailing wind, the wind force can not be relied uponto provide natural smoke control or neglected in its adverseimpact on a NV system.The only design choice for NV systems is to shield
34、theexposed external connections to a tunnel (portals and ventshafts) from the wind. This generally involves shielding theseexposed elements from the wind. When the elements are notexposed directly to oncoming winds, the impact of the windcan often be minimized.Ambient Temperature. The ambient temper
35、ature bothexternal to and inside the tunnel can have an impact on thebuoyant forces generated by a tunnel fire. In warm conditions,the temperature difference between the smoke and ambient airis less than in colder conditions. The size of these temperaturedifferences are directly related to the size
36、of the buoyant forcesgenerated by a tunnel fire.Generally, NV systems perform better when buoyantforces are stronger, therefore to be conservative, the designshould definitely examine situations where ambient tempera-tures are high and therefore buoyant forces are low. Thisshould not preclude also l
37、ooking at how the NV systemperforms when low ambient temperatures are encountered. An important ancillary item related to ambient tempera-ture is the wall temperatures that exist in the tunnel. The walltemperature impacts the amount of fire heat absorbed by thewalls and also impacts the buoyancy for
38、ces generated. Heat Release Rate and Smoke Release Rate. Althoughtrains and road vehicles can be designed to be fire hardened,generally the tunnel designer has little control over the vehi-cles traversing the tunnels they are designing. Therefore thefires that can be expected within a tunnel are bey
39、ond thedesigners control. Most projects have a set fire heat releaserate (FHRR) and fire smoke release rate (FSRR) profile basedupon a reasonable worst expected case scenario.The FHRR and FSRR have two effects on the design ofNV systems. The first is the impact that the FHRR has on thestrength of th
40、e buoyant forces. The larger the FHRR the largerthe buoyant forces due to a greater mass of hot fire gases. Thisis generally a beneficial effect. The second is that larger theFHRR the larger the resulting hot smoke cloud will be. This isgenerally a detrimental effect as the size of the resulting unt
41、en-able zone is larger. Generally the larger untenable region hasa greater detrimental effect than the beneficial effect of theadded buoyant forces for larger fires.Tunnel GeometryThe choice of tunnel geometry is the most importantconsideration when developing a working NV system. Designdecisions ar
42、e made to facilitate one of the two types of NVsystems described earlier. These geometric choices are oftencostly to implement and care needs to be taken to minimizetheir costs while still providing a working NV system.Length. There are no particular reasons that NV systemscan not work in longer tun
43、nels. The reason they are not oftenemployed is that the costs of ventilation openings to thesurface and the increased underground space requirements ofceiling capture type systems is prohibitive. NFPA 130 has anupper limit of 300 m for rail tunnels before MV systems arerequired, while NFPA 502 has n
44、o upper limit at which MVsystems are required.Cross Sectional Area. Increasing the tunnels crosssectional area can be beneficial to a NV system in that a largercross section can hold more smoke and hot gasses near theceiling. This makes the smoke layer thickness shallower andthus generally higher aw
45、ay from patrons. Unfortunately,tunnels are often built with the smallest cross section possibleto reduce costs.Ceiling Height. The ceiling height has a profound impacton the performance of a NV system. The higher the ceilingheight the higher any smoke layer will be with respect topatrons. Unfortunat
46、ely, tunnels are often built with the lowestpossible ceiling. In certain cut-and-cover tunnels a higher ceil-ing height can be achieved at relatively low cost if less over-fill is placed on top of the tunnel.Grade. The vertical curvature or grade of a tunnel isanother determining factor in the desig
47、n of NV systems. Thegrade of the ceiling gives a buoyant plume of smoke a naturaldirection to move laterally. In combination with a tailored ceil-ing height and ventilation openings the clever use of the tunnelgrade can make a NV system work by funneling smoke and hotgases to the high point of the t
48、unnel.Ventilation Openings. Sufficient openings need to beprovided and located throughout the tunnel such that during afire emergency the majority of the smoke will exit throughthese openings. These openings should be located in or nearthe tunnel ceiling. This is an important consideration becauseso
49、metimes these openings are located slightly below the ceil-ing level where their effect is reduced. In this situation, smokemust fill the area above the opening soffit before a significantamount of smoke exits the openings. The design of these open-ings should be such that there are no long horizontal runswhich tend to hamper the buoyant flow of smoke.Special care should be taken at the surface where venti-lation openings are exposed to winds. In certain situationswhere the openings are flush with the surface the buoyant flowcan be hampered by the flow of wind. Care should be taken t
