VENTILATION | INSIGHT
Left: Ventilation Schematic Project 2025
construction shall be avoided or kept at a minimum. Also, it must be evaluated if urgent measures must be fast-tracked as deviations from the safety targets may require immediate action. Some system components, such as the variable frequency drives for the exhaust axial fans, the airflow monitors as well as the visibility monitors in the tunnel have already been replaced as part of regular maintenance. It is expected that the tunnel sensors can be used until 2040. The exhaust fans are in good condition, but their capacity does not meet the requirement of the new design code; it requires an increase of 33% to a flow rate of 192m³/s. This equates to a flow velocity in the tunnel of 2m/s from both sides towards the extraction section. The total thrust of the tunnel jet fans does not comply with the current design code. For new tunnels, the code requires more conservative assumptions for barometric and wind pressure. Also, the design should consider increased traffic volume since 2004. New jet fans would have to have the thrust increased by 28%. The ventilation control system has already exceeded
the nominal design life. While the current operation is regularly tested, support by the original supplier is no longer available. The algorithm for airflow control in emergency ventilation is outdated. New systems are equipped with standard PI-controllers to allow faster and more reliable airflow control. The design of the egress pressurisation system
includes safety margins as it had to be based on assumptions regarding leakage. This causes higher power consumption. Since 2004, the requirements for egress pressurisation systems have been updated based on the experience of new egress tunnels in Switzerland.
DESIGN DECISIONS One significant advantage of a refurbishment design for a tunnel ventilation system is that the design can be based on measurements in lieu of assumptions regarding friction losses and leakage. As part of a research project in 2011, measurements of pressure losses and flow rates have been conducted for Tunnel Giswil. These have now been used as a reference for the design. Three options have been investigated for the extraction capacity: ● 1:1 replacement of the fans to achieve the current exhaust flow rate 144m³/s;
● Design for an exhaust flow rate of 175m³/s, optimised according to the admissible pressure loads of the false ceiling and the smoke dampers; and,
● Design according to the current standard 192m³/s.
It was decided to deviate from the design code and
implement an optimised extraction capacity of 175m³/s. With the reduced flow rate, exhaust fans can be selected with the same size as the current fans. Changes to the fan room layout can be avoided. Also, the static pressure in the smoke duct does not
exceed the design limits of the smoke dampers and of the false ceiling. The reduced flow rate achieves a tunnel airflow of 1.8m/s from both sides towards the extraction section, which is sufficient to limit smoke propagation. For the smoke dampers, three options have been investigated: ● 1:1 replacement; ● Replacement by dampers with increased cross-section – to match the increased extraction flow rate; and,
● Redistribution of the dampers in equal distances along the tunnel.
A redistribution of the dampers and an increase of
the damper size would require a significant change to the civil construction and prolonged tunnel closures. While the current dampers may cause an increased pressure drop with the higher flow rate, the effect is acceptable. A close inspection of the smoke dampers and their maintenance records led to the expectation that the dampers can be operated safety until 2040 without major intervention beyond regular scheduled maintenance. The damper actuators must be replaced as spare parts are no longer available. Also for the jet fans, three options have been investigated: ● 1:1 replacement; ● Replacement of four jet fans with a total thrust to meet the new design code; and,
● Replacement of six jet fans with a total thrust to meet the new design code, including redundancy for one group of jet fans being unavailable.
Today’s four jet fans will be replaced by six jet fans
to meet the requirements of the current design code regarding thrust and redundancy. This is possible due to the 400m-long section between the end of the smoke duct and the southern tunnel portal. While in a new tunnel only 300m would be acceptable, this section can now be used to accommodate three pairs of jet fans. Two of the six fans will be controlled by variable speed drives to allow improved airflow control in emergency ventilation. This partly compensates the exhaust capacity not meeting the requirement of 192m³/s.
January 2026 | 19
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