Ventilation control and monitoring

There are two principal aims of a well-designed ventilation control system:

• in routine operations, to provide fresh air at a rate that is consistent both with the comfort of the tunnel users, and with economic operations i.e. at the minimum rate for an acceptable level of air quality.
• in exceptional circumstances or emergency cases (equipment breakdowns, accidents or fire in the tunnel), the ventilation system must be capable of responding quickly and reliably to each specific ventilation demand.

It is therefore important to bear in mind that the objectives of the ventilation system and the associated control system are different depending on the situation of the tunnel.

Normal operation

During a normal operating situation, the levels of pollutants in the tunnel may increase (depending on the traffic conditions and the natural ventilation of the tunnel) until it becomes necessary to activate the mechanical ventilation, in which case it can be done either automatically or manually.

In order to obtain information on the levels of pollutants, carbon monoxide and visibility sensors are usually available in the tunnel, although the installation of other types of sensors such as nitrogen oxides is becoming increasingly common.

Originally, the measurements of contaminants in the tunnel were used to perform the actions on ventilation manually. In some tunnels with a low level of supervision, these actions are implemented in the tunnel monitoring and control system that carries them out automatically through a series of predefined algorithms or sequences.

However, nowadays, most tunnels have automatic ventilation control systems whose objective is not only to guarantee the required air quality levels, but also to achieve this efficiently by minimising energy consumption and equipment maintenance needs (see section 4.3 "Actions to reduce tunnel operating costs" of the PIARC report 2017 R02 "Road Tunnel Operations: First Steps towards a sustainable approach").

Where routine ventilation is concerned, an optimal air flow rate is one that satisfies two conflicting requirements; the rate of ventilation must be sufficient to dilute the pollutants generated by the vehicles, while at the same time the air flow quantities should be as small as possible in order to reduce the energy consumption at the fans and therefore reduce running costs.

The optimisation of ventilation control for air quality considerations during normal operation is crucial to reduce energy consumption. This is an important issue since this consumption represents a significant part of the operational cost of a tunnel.

The constant adjustment of air flow to cope with the needs is a difficult problem especially in the case of long and complex tunnels where the control of the ventilating airflows can be difficult to maintain.

Chapter IV "Ventilation control" of the 2000 05.09.B. PIARC report “Pollution by nitrogen dioxide in road tunnels” report describes some of the criteria usually considered and the most common criteria.

Emergency ventilation

Emergency ventilation, on the other hand, needs fast and well-targeted interventions, short response times, and a well-defined sequence of all the operations. The objectives of incident ventilation are therefore quite different from those of normal ventilation, and economic considerations are no longer the principal concern.

In a fire situation, the actions on the ventilation are normally associated with automatic detection systems or with supervision from the control centre, which allow the triggering of the action sequences for the pre-defined ventilation. Thus, ventilation control systems are one more tool in the set of tools available to mitigate the consequences of a fire situation. 

For this reason, ventilation control must be considered with a global approach that takes into account the strong link between the emergency services' action procedures, the actions of the control centre operator and the tunnel results in terms of smoke behaviour. A general discussion of the importance of taking ventilation into account in the definition of fire response plans can be found in section VIII.4.1 "Fire response planning" of the 1999 PIARC report 05.05 "Fire and Smoke Control in Road Tunnels".

The design of appropriate ventilation control scenarios for each possible fire situation is a very important part of the process: see PIARC technical report 2011 R02 "Road tunnels: Operational strategies for emergency ventilation". These scenarios can be simple, especially when the purely longitudinal strategy is applied, or involve a large number of measurement and ventilation devices in complex, transverse-ventilated tunnels or in tunnels with massive point extraction for which the control of the longitudinal airflow can be necessary.

There are numerous ways and strategies to approach the design of a fire ventilation control system which depend on multiple factors such as the degree of supervision, means of detection, ventilation strategies and the type of system.

Firstly, its design must take into account the expected evolution of the incident, the different stages that occur throughout the emergency and the influence of ventilation on the behaviour of the fumes. Although in some cases the actions on the ventilation equipment are simple and do not require complex control systems, in many cases it is necessary to take into account sophisticated criteria.

Today, the development and implementation of ventilation control systems in case of fire represents an important part of ventilation design, in which the definition of the necessary control algorithms is particularly important and it is crucial to be able to guarantee the quality and reliability of field measurements (speed and smoke detection sensors).

The chapter "Response of ventilation control systems to fire" of the PIARC technical report 2011 R02 "Road tunnels: Operational strategies for emergency ventilation" provides detailed information about the challenges and needs of control systems for ventilation management and current experiences in this field.