333-Development_and_state_of_the_art_in_road_tunnel_ventilation_technology[1].pdf

1 Development and state of the art in road tunnel ventilation technology

Dipl.-Ing. Wolfgang Mller,ZweibrckenDipl.-Phys. Jrgen Steltmann,Zweibrcken

Development and stateof the art in road tunnelventilation technologyUntil the 1950s, traffic densitieswere very low compared to todayslevels and environmental aspectswere a minor issue at best.

Subsequent economic growthbrought a progressive year-by-year increase in individual trans-port volumes (transit, vacation tra-vel, commuting). The resultingtraffic congestion in conurbatedareas and Alpine regions gave riseto pollutant emission levels thatsoon exceeded the tolerancethresholds of motorists and localresidents.

This situation forced public autho-rities to take a hard look at trafficdensities and to identify solutionsthat would ensure the environ-mental sustainability of anticipa-ted future concentrations.

Tunnel systems emerged as thesolution of choice. Europe, with itshigh population densities, soonacquired cutting-edge expertise intunnel technology.

The length of vehicular tunnels va-ried between 3 and 16 kilometers atthat time. To ventilate such longtunnel sections, planners - and, ul-timately, building contractors - hadto undertake pioneering work sin-ce experience with older tunnel sy-stems in other parts of the worldwas scarce and not easily adapta-ble to the challenges encountered.

In those days, the development of in-dividual components for tunnel venti-lation equipment proceeded concur-rently with the development of fansfor thermal power plants, mine venti-lation systems, and wind tunnels foraerodynamic testing applications.

With many components, develop-ment projects and trial results could

be directly transferred to tunnel venti-lation applications. The high level oftunnel ventilation technology attainedtoday is ultimately a direct outcome ofthis synergy effect.

A major unknown quantity was thedevelopment of future traffic densitiesand, in the final analysis, the growthin vehicle exhaust emissions in theperiod leading up to 2000.

At that time, CO emissions and opa-city had already been identified askey parameters for determining thefresh-air demand of tunnel systems.In addition, traffic volumes by the year

2000 had been extrapolated from therapid growth of the 60s and 70s.These calculations yielded highfresh-air demand levels which, as weknow today, were far overestimatedsince progress in automotive techno-logy could not be forecast with anydegree of accuracy.

The first tunnel systems were typical-ly projected with semi-transverseventilation (Diagram 1) for mediumtunnel lengths and full transverseventilation (Diagram 2) for long tun-nels. A combination of both systemswas likewise employed later.

Fig. 1: Tunnel entry / exit portal

Fig. 2: Axial-flow fan with hydraulic impeller blade pitch adjustment

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Development and state of the art in road tunnel ventilation technology 2

The broad fan control ranges requiredcould only be implemented in an eco-nomically efficient manner by combi-ning variable-flow fan designs usingon-line blade pitch adjustment withmultiple fixed rpm stages.

Semi-transverse ventilationusing axial-flow fansAs a general rule, semi-transverseventilation systems introduce justenough fresh air into the tunnel to di-lute the emitted pollutant loads. Thusdiluted, the tunnel air exits from bothportals in equal proportions.

A controlled flow of fresh air is intro-duced continuously into a separateair duct from which the requisite parti-al volumes are forced into the road-way tunnel via so-called secondaryducts.

To ensure that the direction of flowcan be controlled in the case of a fire,reversible axial-flow fan systems areemployed. Under normal conditionsthese units operate as supply fans. Ifnecessary they can be switched toexhaust mode, extracting air via thesecondary ducts.

Compared with longitudinal ventingusing jet fans, such systems offer theadvantage of producing lower longitu-dinal airflow velocities since the aircan exit through both portals. Semi-transverse ventilation is therefore via-ble for longer road tunnels as well. Onthe other hand, the cross-section ofthe tube may be reduced by the inevi-table supply air ducting. As a result,the longitudinal air velocity may be re-duced compared to longitudinal ven-ting.

Fully transverse ventilationusing centrifugal fansWith fully transverse ventilation, freshair is supplied to each point of theroadway in exactly the extractedquantity.

Fresh air is introduced into distributi-on ducts extending along each tubeslength and blown into the tunnelthrough air-supply openings (so-cal-led secondary ducts).

Diagram 1: Semi-transverse ventilation

Diagram 2: Fully-transverse ventilation

Diagram 3:Combined semi-/fully-transverse ventilation

Counter-direc-tion trafficSingle-directiontraffic




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The exhaust air carrying pollutant lo-ads is continuously extracted over thelength of the tunnel via so-called ex-haust vents. The individual exhaustair flows are combined in plenumsrunning parallel to the tunnel, then di-scharged via exhaust stacks.

In one variant of the fully transverseventilation method, referred to as re-duced transverse ventilation, the ra-ted exhaust air flows are smaller thanthe supply-air volume. The excess ofair leaves the tunnel via the portals(Diagram 3).Forced ventilationIn recent years, a clear trend towardslongitudinal ventilation has emergedfor medium tunnel lengths of up to3 kilometers.

The success of this ventilation me-thod is partly attributable to the lowercapital outlay, operating overheadand maintenance cost involved.

Jet fans arranged individually or ingroups (Diagram 4) at specified di-stances along the tunnels roof impartenergy pulses to the air column, thusinducing airflow movement.. With thisdesign, care must be taken in deter-mining fan spacings along the tunnelaxis to ensure good intermixing of thejet discharge and the remaining tun-nel airflow, as well as an optimumflow distribution across the tube.

The length of jet fan ventilated tun-nels is limited by the maximum longi-tudinal air velocity, since the speed ofthe airflow must not exceed acceptedsafety and comfort levels. For the sa-ke of completeness, the longitudinalair management method based onSaccardo nozzles should likewise bementioned here.

Saccardo nozzles (Diagram 5) arefed by axial-flow fans mounted at thetunnel portal. The nozzle injects thefan intake into the roadway tube at a15 to 20-degree angle to the tunnelaxis and air speeds between 25 and30 m/sec via a circular gap in the up-per portal area.

However, this ventilation method hasbeen found fairly sensitive to changesin airflow resistance, regardless of

Diagram 4: Longitudinal ventilation using jet fans

Diagram 5: Longitudinal ventilation using Saccardo nozzles

Diagram 6: Longitudinal ventilation using exhaust air fans




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cause (e.g., wind). It is therefore nolonger considered in newer installati-ons. Another interesting recent deve-lopment in longitudinal road ventilati-on is the use of exhaust air fans mo-unted in caverns and operating viacentral ducting (Diagram 6).In this system, fresh air is drawn infrom both portals by large axial-flowfans arranged in caverns near themiddle of the tunnel. Exhaust air isextracted and discharged via an out-let shaft. A benefit of this system liesin the fact that no exhaust air exitsfrom the portals and that the length ofthe ventilation air-way is virtually re-duced by half, with attendant advan-tageous effects on tunnel CO andopacity levels. It is often combinedwith additional jet fans inside the tun-nel which are activated in the case ofa fire to keep escape routes clear ofsmoke.

Irrespective of the foregoing, theother ventilation techniques, viz.

- reduced semi-transverse ventilation

- semi-transverse ventilation

- fully transverse ventilation

each have their own specific advanta-ges over all-longitudinal ventilation interms of fire safety, ride comfort andenvironmental compatibility and willtherefore be adopted or specified ona case-by-case basis. Needless tosay, combinations between longitudi-nal and semi-transverse ventilationmay be found effective both undernormal operating conditions and inthe event of a fire.

Modern selection criteria forventilation systemsThe choice of a tunnel ventilation sy-stem is essentially determined by thefollowing factors:

- fresh-air demand as a function ofCO, NOx and opacity-inducingemissions, taking into account legalrequirements and the anticipatedtrend in emission levels and controlthresholds over the next few years(current data are prescribed inRABT and PIARC guidelines);

- reliability of the system;

Fig. 3: Jet fan during manufacture, powder coated and with closed impeller(impeller hub sealed off against dust and corrosion)

Fig. 4: Axial-flow exhaust fan

Fig. 5: Operating building with outlet shaft


- safety in the case of a fire;

- energy consumption;

- investment cost;

- maintenance;

- service friendliness.

Control systemsEach fan control concept must be ali-gned with the demands of the tunnelventilation system.

To meet specified immission thres-holds inside the tunnel and with re-gard to the overall environment,fresh-air and exhaust flow rates haveto be continuously adapted to the cur-rent traffic load. The following flowcontrol methods are used:

Longitudinal ventilation using jet fans:activation and de-activation of indivi-dual fans or fan groups over thelength of the tunnel.

Longitudinal ventilation using exhauststacks in tunnel caverns / semi-trans-verse or transverse ventilation of lon-ger tunnels using axial-flow fresh-airand exhaust fans at the tunnel portalsor in tunnel caverns:

Here the following control options areemployed:

(Chart 1): Impeller blade pitch angleadjustment during operation, combi-ned with one or more fixed motor rpmstages (via pole-changing motors)(Chart 2): Speed-controlled fans withindividual impeller blade pitch angleadjustment on the stationary fan andvariable-speed inverter control.

Both control methods have proventheir merits in practice. The use ofone or the other method will dependon the intended operating conditions,which are often associated with diffe-rent system airflow resistance cha-racteristics.

In a ventilation system with a square-law airflow resistance graph, bothon-line blade pitch adjustment andrpm control are viable control strate-gies.

Problems will often arise if differentairflow resistance characteristics are

Fig. 6: Axial-flow fan during installation in the fan house

Fig. 7: Upright reversible axial-flow fan

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encountered in different ventilationmodes (e.g., during reverse operationof the ventilating system in the caseof a fire).The duct resistances in normal andreverse operation change with the in-let and outlet conditions at individualcomponents. In addition, deflectioneffects caused by the damper systemmay result in highly inhomogeneouslosses and hence, different tunnel re-sistance characteristics.

Both operating extremes must beknown as early as in the project pha-se (i.e., when the control concept isdetermined) and must be taken intoaccount in the fan design and ratingprocess.

In a speed-controlled system, the stalllimit for a given fixed blade positiondescribes a parabolic curve throughthe origin.

In any case, care must be taken tokeep all operating points at a safe di-stance from the fans speed-relatedstall line. To meet this requirement,compromises will often have to bemade in selecting the position of the-se points relative to the optimum effi-ciency level between the normaloperation and fire operating ran-ges.

On fans with on-line impeller pitchcontrol, such restrictions will not nor-mally apply due to the favourablecourse of the fans stall limit line rela-tive to the zero flow level.

A fans so-called holding pressure atzero output flow is usually equal tobetween 50 and 60% of the maximumpressure. From this we may generali-ze that all operating points down topart-load mode (usually 10% ofV.max) can be run safely without in-stability.

Another important aspect in selectingthe control concept is the so-calledtunnel bias pressure to be createdat the appropriate inlet and exhaustvents under the various supply andextraction conditions so as to avoiduncontrolled flow reversals over thetunnels length.

This so-called tunnel bias pressureshifts the tunnels airflow resistancegraph from zero to a negative or posi-tive pressure. Positive bias is particu-larly critical.

With speed-controlled fans, frequentintersection with the stall line (para-bolic curve through zero) in the part-load range imposes limits on theachievable minimum fan load.

The frequently voiced demand for100 to 10% variability of the volumeflow may not be attainable due to thisintersection effect (black area inChart 3). Here, too, variable-pitchfans offer performance benefits sincethese intersection problems do notoccur.

For tunnel systems with fans opera-ting at high rates of capacity utilizati-on, a mixture of the two control sy-stems may be advantageous. A com-bination of rpm control and hydraulicblade pitch adjustment can yield opti-

Chart 1: Characteristic curves of an axial-flow fan with hydraulic impeller blade pitch adjustment

Chart 2: Characteristic curves of an axial-flow fan with rpm control based on a frequency conver-ter

Installationcharacteristic curve

Stall limit

Switch over point

Stall limit

Installationcharacteristic curve


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mum energy efficiencies in day-to-day operation. The disadvantage is,that higher equipment cost will be in-curred. However, the extra capital ou-tlay may soon be reuperated by ope-rating cost savings.

Fire safety

Exhaust air fans are generally requi-red to remain operable for 1 hourmax. at temperatures up to 250C inthe event of a fire. All components, in-cluding the fan motor unit, must con-form to this specification.

In the case of exhaust air fans, theabove imposes specific design requi-rements aimed at ensuring systemsafety, e.g.,

- high temperature resistant impellerblade materials;

- sophisticated, proven pitch adjust-ment systems which allow the im-peller blade angles to be changedon the rotating fan and will not requi-re increased actuating forces or be-come blocked altogether at elevatedtemperatures;

- drive motors with special cooling sy-stems, with or without external coo-ling fans (depending on the selectedmotor protection class, e.g., IP 23 orIP 44 for open-circuit ventilated ma-

chines or IP 54 for totally enclosedfan cooled units).

Summary and outlookBecause of continuously improvingpolluant emission values for new cargenerations, fire safety objectives willincreasingly become the first and fo-remost consideration in tunnel venti-lation design.

Exhaust fans removing smoke direct-ly from the tunnel tube must be ratedto withstand temperatures of at least400C for 90 minutes or more (RABT94).

Literature:

RABT: Road Tunnel Equipment and OperatingGuidelines

PIARC (Permanent International Associationof Road Congresses) / AIPCR: Road Tunnels:Emission, Ventilation, Environment, Montreal1995

1995 Zagreb Tunnel Convention, Paper deli-vered by Mr. W. Mller in Zagreb

Chart 3: Characteristic curves of an axial-flow fan with hydraulic impeller blade pitch adjustment,superimposed by rpm control curves

rpm-cont

rol

blocked

area

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