Tunnel ventilation has to perform several important and varying functions, and of course the combination of functions varies widely, depending on the application. There may be little in common between the performance requirements of road traffic tunnel, rail tunnel and underground metro stations due to operational factors such as the piston effect of traffic (usually trains), the risk of fire or collision emergencies and any significant natural ventilation effects due to location and climatic conditions.

These increasing demands on ventilation are not only affecting the design of new tunnels but also creating a major market in refurbishment. Combining the two are the tunnel fires of recent years that, if the authorities deem the traffic flow to be high enough, have necessitated the construction of a parallel, second, bore to separate traffic flows and create a safe refuge/escape route for tunnel users.

One such case is the Tauern Tunnel on the A10 route in Austria where, following the collision and fire of 1999, the national highway operator Asfina, decided to construct a second bore with improved operational systems that was opened in April this year. This allowed the original bore’s systems to be upgraded, with a date next year for both to be fully operational. Civils refurbishment on the original bore has been carried out by Ostu-Stettin with Zitron Netherlands supplying and installing the ventilation equipment for a cross (traverse) flow system in both tubes.

The equipment supplied by Zitron included eight EB (extraction) fans with hydraulic vane control suitable for operation at up to 400 degrees C for 120 minutes, eight fresh air fans, 52 pressure jet fans for facilitating escape in an emergency and 18 louvre valves for ventilation control. As well as installation, Zitron was also responsible for surveying the flow and pressure losses, test assemblies in the factory, leak tests, ventilation fire tests and volumes delivered, plus maintenance planning.

The wide range of axial and jet fans available from Howden Ventilatoren features corrosion protection and flameproof motors, with auto-variable pitch or variable speed drives, and potential for up to 100 per cent reverse flow. Tunnel fans supplied include those for the renewal and upgrade of the Mon Blanc Tunnel and the Gotthard Tunnel.

Higher rating
More accurate and demanding test fire profiles have led to requirements for fans, particularly in exhaust situations, that will perform longer in the event of a fire. In addition flow capacity requirements have often increased for rapid clearance of contaminants from road tunnels, but also from metro systems.

Flakt Woods, for one, has anticipated the need for larger fans, achieving EN12101-3 certification by successfully testing a 1MW axial fan of 2.24m diameter and running at 1500 rev/min. The fan range, of up to 2.8m diameter, can operate at 400 degrees C for two hours. The fan, with steel blades and steel hub, is the most highly stressed in the range with the greatest power requirement, allowing the test to cover the whole range. The manufacturer says, however, that though there is little actual requirements for a 1MW unit, with ever increasing performance boundaries, this is looking to the future in range terms.

Jet fans
A major development in road tunnel ventilation over recent years, at least for longitudinal movement, has been jet fans. These work mainly by inducing airflow around them, multiplying the air actually passed through each fan. With a dedicated control system they can be effective in aiding smoke or fume control during emergency situations, especially when reversible drives are used, to limit the spread of the contaminants and improve the chances of a safe escape route. Usually used mainly in shorter tunnels (less than 3km long) where they can reduce the overall costs of a ventilation system, but can also be used in combination with more conventional major fan with shafts and or ducts. They also hold the possibility of easier flow control for varying traffic levels throughout the day.

Even now the actual mechanism of flow inducement does not appear to be completely understood, and the calculation and testing of fan capacity required has been called into question resulting in excessive installations.

There have been a number of design improvements since the first introduction of jet fans, in order to reduce some of their disadvantages. As stated by Fathi Tarada, of Mosen and the developer of the recently introduced MoJet fan, these include:
• Loss of thrust due to ‘friction’ (Coanda effect),
• Large number of units required to generate sufficient thrust,
• Fans unit are large and heavy, ‘leading to installation and maintenance challenges’.

The alternatives for longitudinal ventilation are Saccardo (impulse) nozzles (patented in 1898), and fans in ventilation shafts working on a push-pull effect, both of which can require substantial extra civils works for new or refurbished tunnels.

The Banana Jet, available from Witt & Sohn of Germany, improves on installation efficiency by having a housing that directs the jet towards the tunnel centreline at an angle of seven degrees instead of a straight tube. It can therefore have a major improvement when installed in a niche in the tunnel wall or crown. Up to 30 per cent more effective thrust is claimed compared to traditional jet fans of the same size.

Tarada claims that directing the jet away from the axis generates a ‘wall jet’ on the road surface, above which any smoke can move upstream, although the Coanda effect is reduced. In some cases the nozzles can encroach on possible traffic space he claims. The alternative of deflection louvers on the fan duct generates a large pressure drop, he points out, at both ends of the fan duct.

The Mosen MoJet, however, combines the advantages of jet fans and the induced flow of Saccardo nozzle installations by having convergent nozzles on one or both ends of the fan. This gives accelerated flow and hence increased thrust (typically 7 to 20 per cent), and directs the flow to the tunnel centre-line to increase the installation efficiency (giving a further 18 to 37 per cent thrust increase). An overall thrust increase of up to 64 per cent may be available, Tarada claims. The nozzle reduces mass flow through the fan, but if the fan characteristic is ‘steep’ enough, this is compensated for by the increase in air velocity.

Metro ventilation
With the London Underground metro system, the fire at King’s Cross-St Pancras station, and the multiple bombings of 2005 have affected the thinking on the necessary system ventilation in such a large network. In parallel with this scenario there is an underlying need to cool the system, particularly in summer and in overnight maintenance periods when ventilation by train movements have proved to be inadequate to prevent heat build-up and thermal radiation from the surrounding ground.

The major refurbishment scheme currently underway on the London Underground network includes ventilation renewal in various projects across most network routes. This includes a recent award to Mansell (Balfour Beatty Group) of a GBP 6.3M (USD 9.8M) second contract to upgrade four mid-tunnel ventilation shafts with new fans on the Victoria Line. The ventilation refurbishment, as well as providing emergency ventilation, is often, as with this concrete, also part of a scheme to reduce heat build-up in the underground rail network. The new contract is due for completion in November next year.

Dampers or louver valves are the less glamorous but nevertheless vital component of tunnel ventilation systems where some degree of sealing off of passages is required, such as for emergency control of smoke extraction and fresh air redirection, or for maintenance.

As part of the Europe wide move to comply with EU Directive 2004/54EG, the ventilation of existing Trans European Network (TEN) road tunnels is being upgraded, and some other operators non-TEN routes.

The major highway crossing of the River Elbe in Hamburg is now comprised of four tunnels. Comprehensive refit of the first three bores was started last year including the retrofitting of 410 Trox dampers for smoke extraction for fire protection. They are made of stainless steel and can withstand 400 degrees C for 90 minutes.

Trox dampers are already fitted to the Mrazovka Tunnel in the Czech Republic and the Vielha Tunnel in Spain—the ventilation of both having been rated as ‘very good’ by ADAC, the German automotive society.

Another manufacturer, Connols-Air of Singapore supplies the OLS SD60 smoke dampers particularly for metro systems. It points out that dampers should have a failsafe position during any power interruption and so must be tested within a ventilation system to prove reliability.

The damper is appropriate for automatic ventilation control from a central control using electric or pneumatic actuators. Stainless steel moving parts are employed to prevent seizure, within a galvanised steel structure. The louver blades are designed for high-pressure fluctuations and are tested to 6kPa.

Howden Ventilatoren fans in the north fan station of the Plabutschtunnel A large Flakt Woods axial fan during certification tests Testing a MoJet fan