The ventilation must meet the following requirements:
- In controlled operation
• Provision of the vehicle drivers (and the staff during maintenance work in the tunnel) with sufficient clean breathing air
• Ensuring of sufficient visibility in the tunnel air contaminated with exhaust fumes and dust
• Prevention of impermissible pollutant immissions due to tunnel exhaust air in the area of the tunnel
- In case of a burning vehicle
• Reduction of smoke and heat effects on the escape routes in the driving area and on the rescue routes. The primary goal is to enable the tunnel users to rescue themselves on the provided escape routes.
• De-smoking of a tunnel following the rescue phase.
The appropriate ventilation system and its dimensions must be determined on the basis of optimisation calculations.
The following sections contain specifications for this.
In Appendix B, further details are listed.
4.2 Determination of the fresh air need during standard operation
4.2.1 Criteria
The fresh air supply for the tunnel must be calculated in such a way that no health-endangering effects arise for the road user in every traffic condition from smooth flow to a traffic jam at the greatest traffic volume possible derived from the forecast values, and certain demands of comfort regarding clear views and little odour are met. For rare traffic conditions, the requirements can be lower than for the traffic situations that arise regularly. In case of road works during traffic, the maximum work place concentrations (short-term and long-term limit values) apply in with respect to the duration of stay [13].
4.2.2 Determining pollutants from exhaust fumes and tyre abrasion
As criteria for the quality of air in the tunnel, the impairment of visibility due to particles (mainly diesel
soot and tyre abrasion) as well as the carbon monoxide concentration (CO) is used.
The CO emission of the vehicles with catalytic converter generally only becomes relevant from heights of more than 800 m above sea level. For tunnels that lie lower, the tunnels are primarily ventilated according to the impairment of visibility.
Tunnels that tend to suffer from traffic jams can be an exception.
The immission loads outside, which can be influenced by the tunnel air, must be assessed according to the local concentrations of nitrogen dioxide (NO2), nitrogen oxide NOx), benzene and particles.
The basis for the emission calculation of the vehicles and HGVs and the calculation method for determining the demand in fresh air are explained in Appendix B.
4.2.3 Determining traffic conditions
In general, a traffic forecast is used for the determination. If there is no such forecast, Table 6 can be used as a means of orientation. It contains empirical values of maximum traffic volumes in interurban or urban tunnels.
For the conversion of the traffic volume onto car units, the ration 1 HGV = 2 cars.
Table 6: Empirical values for maximum traffic volumes in interurban and urban tunnels in car units per km 4.2.4 Nominal values of the CO concentration and
visibility reduction
In order to assess the amount of additional fresh air that must be provided with the fan, the values contained in Table 7 for CO concentration and visibility reduction must be used for the different traffic and operating conditions to be examined.
For the calculation of the emissions (CO and visibility impairment) of the vehicles at different traffic conditions, particularly the traffic composition must
be considered (proportion of HGVs and dieses cars;
see Appendix B).
Table 7: Nominal values of the CO concentration and the visibility impairment for determining the demand in
It is necessary to close the tunnel if a CO concentration of 200 ppm or an extinction coefficient k of 12 * 10-3 m-1 is exceeded or a transmission S of 30% is not achieved.
4.2.5 Amount of fresh air and adjustability of the ventilation
The calculation of the amount of fresh air is based on average values of the traffic composition and the emissions per vehicle category. Since the emission values of the vehicles are continuing to decrease, continuously sinking fresh air amounts result for diluting the exhaust fumes in the tunnel. As a consequence, the air exchange times are getting longer so that it is no longer possible to effectively react to short-term increased emissions with a ventilation system set up for small amounts of fresh air. This mainly affects tunnels from 500 m to 1500 m in length, which are equipped with a longitudinal ventilation system for standard operation. In case of longitudinal ventilation systems, small amounts of air mean small speeds of the longitudinal flow, typically
< 1 m/s. Air speeds in the traffic area, which are so small, can only be regulated with difficulties or not at all. The consequence is so-called vision impairment cones, which can affect the driving safety. This problem can also occur for systems with semi-transverse ventilation in the zones with too little longitudinal flow.
In order to be able to react quickly in standard operation, the longitudinal ventilation must ensure a minimum speed of the longitudinal flow in the traffic area of 1 m/s and the semi-transverse and transverse ventilation must at least provide four air exchanges per hour.
4.3 Fire in the tunnel
Fires in tunnels significantly influence the design of the ventilation system. Here the safety of persons is more important for the design of the ventilation system than economical criteria for standard operation.
Note: References [14] to [19] provide detailed information on the basics, such as frequency of fires in road tunnels, fire behaviour, in particular, the expansion of smoke and the danger resulting from fires, as well as results of fire behaviour tests.
Recommendations concerning fire protection measures in other European countries can be found in [14; 19; 20 and 21].
4.3.1 Requirements
With respect to the requirements of the ventilation system in case of a burning vehicle, two phases must be differentiated:
In phase 1, which covers approximately the first 15 minutes after the fire started, self-rescue is most important. In tunnels from a certain length, see Tables 9a and 9b, escaping persons must be protected in the tunnel from impacts of smoke (loss of vision, poisonous gases and temperature) by ventilation-related measures. The ventilation system must work automatically.
In phase 2, the ventilation system is used to support fire fighting, either by means of an efficient extraction of smoke from the traffic area or by means of unidirectional smoke expulsion from the location of fire. The ventilation system is switched on and over in consultation with the fire brigade.
4.3.2 Determining fire dimension
In general, an HGV fire must be used as a basis for dimensioning the ventilation in case of fire. It would only possible to design the ventilation system to be able to deal with a burning petrol lorry with exceptional effort; however this could not provide 100% safety.
When dimensioning the ventilation for cases of fire, a nominal thermal power according to Table 8 is used as a basis. The nominal thermal power is that power that is only reached or exceeded for a short time of a few minutes of the entire duration of the fire. It is at least 30 MW. In tunnels with a higher HGV frequency, however, the possibility that a fire could affect several vehicles must be considered, which can lead to higher thermal powers.
The dimensioning according to a thermal power of 100 MW can lead to requirements of the ventilation system that cannot be provided sensibly from the constructional and system-technological view.
Therefore, cost-risk assessments must be made in the individual case and special regulations must be
made if necessary in order to achieve a technically possible solution whose costs can also be justified.
Table 8: Nominal thermal power HGV km/day
and tube
Thermal power Amount of smoke Gas at 300° C
Up to 4000 30 MW 80 m3/s
Above 4000 50 MW 120 m3/s
Above 6000 Risk analysis and, if necessary, increasing of thermal power to 100 MW and amount of smoke gas to 200 m3/s
4.3.3 Ventilation concepts in the event of fire In the event of fire, the ventilation concept significantly depends on the tunnel length. In short tunnels, an interaction by means of ventilators makes little sense due to the speed at which the smoke spreads. Thus, tunnels shorter than 400 m or 600 m in length remain without fire ventilation – see Tables 9a and 9b.
Table 9a: Types of ventilation in the event of fire for two-way or one-way traffic with daily stagnant traffic Tunnel length Type of ventilation in case of fire Up to 400 m Natural longitudinal ventilation 400 to 600 m Mechanical longitudinal ventilation 600 to 1200 m After risk analysis:
a) Mechanical longitudinal ventilation b) Smoke extraction via a large suction opening
c) Smoke extraction via intermediate ceiling with controllable suction openings
From 1200 m Smoke extraction via intermediate ceiling with controllable suction openings
Table 9b: Types of ventilation in the event of fire for one-way traffic with stagnant traffic as an exception Tunnel length Type of ventilation in case of fire Up to 600 m Natural ventilation
600 to 3000 m Mechanical longitudinal ventilation From 3000 m Longitudinal ventilation with spot
suction ≤ 2000 m or extraction via intermedi ceiling with controllable suction openings In longer tunnels, the smoke gases are either extracted in a limited section via ceiling openings or driven from the site of fire unidirectionally. For unidirectional smoke expulsion in longer tunnels, the spreading of smoke in the traffic area must be restricted by means of spot suction. With respect to longitudinal ventilation, the traffic situation, the site of fire and the speed of the tunnel air flow are decisive for the operation of the ventilation system.
In case of two-way traffic or stagnant one-way traffic or traffic jams, it is only possible to use longitudinal ventilation in a limited way due to the risk of swirling the smoke. Therefore, a risk analysis must be performed for tunnels between 600 m and 1200 m in length according to Section 0.5.
In case of one-way tunnels, a differentiation is made between tunnels with stagnant traffic as an exception and thus lower risk of traffic jams and tunnels with daily stagnant traffic and a corresponding tendency to produce traffic jams. In these tunnels, it is possible that road users are affected by the spreading smoke gases on both sides of the site of fire like in two-way tunnels.
Therefore it is always necessary for one-way tunnels with daily stagnant traffic to check whether a traffic jam in the tunnel can be avoided with traffic-controlling measures.
Tables 9a and 9b are used to select the suitable type of ventilation for the event of fire. They do not apply for choosing the type of ventilation in standard operation.
For tunnels close to the surface, it is also possible to provide individual de-smoking stations at regular intervals analogous to the suction openings instead of the smoke extraction duct.
4.3.4 Dimensioning of the ventilation for the event of fire
a) Smoke extraction
The smoke should be extracted in the area of the ceiling. Two basic solutions should be considered:
- Spot suction, i.e. extracting an entire ventilation section at one location
- Extraction via an intermediate ceiling (ceiling duct) with individually controllable suction flaps at intervals of 50 to 100 m. Smoke extraction near the portals generally is not effective. Thus, the distance between portal and the next suction opening should at least be 200 m. The suction zone above the fire usually has a length of 200 m to 300 m, depending on the flow conditions in the tunnel.
Requirements of the duct and suction flaps:
- The duct must be walkable. For this, an unobstructed height of 1.9 m is necessary.
- The flow speed in the suction opening must not exceed 20 m/s.
- It must be possible to control the flaps individually.
- Sufficient impermeability must be achieved.
- The suction flaps must have an effective flow cross section of between 2 m2 and 5 m2, depending on the suction volume flow and the distances between the flaps.
Dimensioning of the suction volume flow Spot suction
For the locally concentrated suction, the required suction amount equals the sum of the longitudinal flows in front and behind the suction location; the
following minimum values for the flow speed in the direction of the suction location apply with regards to the limitation of the spreading of smoke in the traffic area:
- before the suction location (at the site of fire) u = ucrit according to Table 10
- behind the suction location u = 1.5 m/s.
Intermediate ceiling with suction flaps
As a result of the extraction from the traffic area, due to the generally adversarial smoke arrangement in layers, a smoke/air mixture is captured particularly in case of a longitudinal flow. The necessary suction capacity thus generally is significantly greater than the produced smoke gas amount according to Table 8.
Table 10: Critical longitudinal speed
Thermal power The required suction amount Qsuction generally is calculated according to formulation (6).
Qsuction ≥ 1.5 x Qsmoke (6)
If a longitudinal flow with a speed u, which is greater than the critical speed according to [14], must be expected at the point in time when the smoke suction is switched on, it must be checked whether
u x Atunnel > 1.5 x Qsmoke
If yes, Qsuction = u x Atunnel
otherwise Qsuction = 1.5 x Qsmoke. Atunnel = traffic area cross section [m2] u= speed of the longitudinal flow [m/s]
Qsmoke = smoke gas volume [m3/s] acc. to Table 8 Qsuction = suction volume [m3/s].
In order to dimension the exhaust air ventilators, the leakage of the exhaust duct and the closed flaps must also be taken into consideration.
Qventilator = Qventilator + Qleakage (7) The volume flows Q are related to the surrounding temperature.
In the case of transverse or semi-transverse ventilation, a limited amount of fresh air must be introduced into the ventilation section affected by the fire for the purpose of providing breathing air, which must also be considered.
Influencing the longitudinal flow
An effective extraction via ceiling openings additionally requires an adjustment to the longitudinal flow in the traffic area. Therefore, it is recommended to check measures for controlling the longitudinal flow or influencing its speed. Spot ventilators or, in case of a transverse ventilation system, the specification of separate ventilation sections, in which the fresh and exhaust air volumes can be adjusted in a targeted way, can be possible.
It must be possible to set the following minimum values for the flow speed u before and behind the suction zone in the direction of the fire with the aim of limiting the spreading of the smoke to the suction zone.
- for one-way traffic that flows freely behind the site of the fire:
• before the suction zone u = ucrit acc. to table 10,
• behind the suction zone u = 0 m/s,
- for two-way traffic and stagnant one-way traffic:
• before/behind the suction zone u = 1.5 m/s.
b) Unidirectional smoke expulsion
If the smoke gases must be driven from the site of fire unidirectionally, a minimum speed for the longitudinal flow is necessary. This results from the requirement that a spreading of smoke against the expulsion direction (backlayering) must be avoided.
This “critical speed” can be calculated with the help of an internationally accepted empirical formulation [14]. For two-lane tunnel tubes reference values depending on the thermal power, the gradient and the tunnel profile result according to Table 10.
It should be possible to maintain the specified speed values for one-way traffic (phase 1 and phase 2) in a tunnel occupied to ¾ by vehicles and for two-way traffic (phase 2) in a tunnel half occupied by vehicles against a meteo-related pressure and in inclined tunnel tubes against thermal lift (chimney effect).
c) Control of the ventilation in the event of fire
The ventilation in the event of fire must at least be controlled automatically for the self-rescue phase (phase 1). Essential preconditions for this are secure fire detection and short reaction times for switching on and starting the fire ventilation system, i.e.(fire detection until reaching of the required ventilating power) < 1 minute. One minute is also planned for fire detection (see Section 6.3).
For unidirectional smoke expulsion in situations with two-way traffic or traffic jams before and behind the site of fire, it is important not to disturb a possibly existing smoke layering. The following requirements result for the ventilation control I phase 1:
- longitudinal speed ≤ 1.5 m/s
- if possible, no spot ventilator operation in the area of the smoke layer.
In case of one-way traffic and free traffic flow behind the site of fire, the smoke must be driven from the tunnel in the direction of travel at the speed that can be reached according to the construction.
In phase 2 (fire fighting) it should be possible to generate and maintain a higher speed (for minimum values see Table 10) in order to prevent the smoke from flowing back.
In case of two parallel tunnel tubes, the control of the ventilation system should be included in the non-affected tube (avoid short-circuits and build up overpressure if necessary).
4.3.5 Temperature stability of the ventilation system
Suction ventilators (rotor, flaps, guide wheel, housing and non-forced air cooled motors) and suction flaps, with which the smoke is extracted directly from the traffic area, must be designed to have a temperature stability of at least 400° C for 90 minutes. Suction ventilators that are connected to a suction duct with concrete walls are generally not used for over 250° C for 90 minutes (strong cooling effect of the duct walls). Other constructions require a separate checking of the temperatures.
Spot ventilators including the electrical connections and cables in the traffic area must provide a temperature stability of 250° C for 90 minutes. Short distances between the ventilator locations or nominal thermal powers > 30 MW can require higher temperature stability (max. 400° C for 90 minutes).
Spot ventilators in the proximity of the site of fire can fail. The number of ventilators must be determined considering these aspects among others.
4.4 Immissions due to tunnel exhaust air
4.4.1 Requirements
Legal regulations for assessing the immission situation are provided by EU directives and regulations for implementing the German immission protection act.
In general, the pollutants particles, nitrogen oxides, benzene and sulphur dioxide are decisive for an immission assessment. The according limit values are specified in statutory regulations. The overall concentration consists of the basic load and the additional immission due to the tunnel exhaust air.
4.4.2 Immission examinations
When planning a tunnel, it generally is necessary to perform examinations regarding the effects of a tunnel ventilation system. Thus it is recommendable to already write an immission report (e.g. in the
context of an environmental compatibility study) at an early stage. This can lead to consequences for selecting the ventilation system.
Due to the numerous influencing factors a calculatory immission forecast is uncertain. In critical situations, it thus can be appropriate to perform model tests and measurements to assess the actual situation.
4.4.3 Tunnel ventilation for immission protection Immission tests can provide the result that an escaping of the tunnel exhaust air from the portal must be reduced or totally prevented; this can significantly influence the selection of a ventilation system and its operation.
4.5 Ventilation systems
4.5.1 Longitudinal ventilation 4.5.1.1 Natural ventilation
Natural ventilation does not require any ventilation-related technical devices. The air exchange occurs by means of the meteorologically-related pressure differences between the portals and the air exchange generated by the vehicles.
4.5.1.2 Mechanical longitudinal ventilation
A longitudinal ventilation results from the generation of an air flow along the tunnel tube due to the
A longitudinal ventilation results from the generation of an air flow along the tunnel tube due to the