Today, the visibility of the air in road tunnels is the main criterion for the design of ventilation systems. In normal operation, i.e. with free-flowing traffic, slow-moving traffic or traffic congestion, it serves as the main control variable for the operation of the tunnel ventilation. How does visual clouding occur and what can be done about it?
Visibility is impaired by particles resulting from combustion and by other particles. These other non-combustion particles originate from abrasion (brakes, tires) and resuspension (swirling fine dust particles). During winter application of road salts also adds to the problem.
Road tunnel ventilation is required to ensure that good visibility is maintained allowing motorists to travel safely at any time of the day or the night and irrespective of the traff ic conditions. Furthermore, pollutants (toxic emission, particles) have to be continually removed from the tunnel during such ventilation. The Sigrist visibility monitor VisGuard has been designed in accordance with the requirements in road tunnels in such a manner that all requirements as regards kind of installation, corrosion resistance and maintenance intervals are fulfilled. The instrument is available for installation directly in the tunnel tubes (insitu), as well as for mounting in separate rooms (extractive).
The presence of fog influences visibility measurements and needs to be eliminated. Optional heaters successfully suppress these influences and are mostly used in the tunnel portal area.
The cost of energy for ventilation control is by far the largest proportion in the operating costs of a road tunnel. For the 17 kilometres of the Gott hard tunnel, the total yearly energy consumption amounts to approx. 14.5 Mio. Kwh. About 86% of the energy is consumed by the ventilation, while only 4% of the energy is used for the illumination. With the installation of the VisGuard, the ventilation can be controlled in such a manner that the ventilation can be substantially reduced or even completely turned off at times of litt le traff ic while at the same time the contaminants are suff iciently eliminated and the visibility is guaranteed in accordance with the existing directives.
The ventilation systems installed in the road tunnels are equipped with variable volume flow. This allows the ventilation to be adapted to the constantly changing traffic conditions. The control system is based on visibility measurement, usually in combination with other measurements (CO, NO, wind speed) and helps to minimize the massive electricity costs for ventilation.
The above diagram shows the typical pattern of visibility during 24 hours. During those hours of the night with little traffic, the values are around or even < 1mE/m. During this time, the ventilation can be reduced to a minimum. At around 7 o’clock in the morning, the values increase continually and reach a maximum around midday. In the afternoon, the volume of traffic varies and reaches another peak late in the evening. Afterwards the volume of traffic and also the visibility values drop during the night. The red line marks the threshold value for closing the tunnel.
Daily pattern of the visibility in a road tunnel