Cross bore or single point solutions for ultrasonic tunnel airflow monitoring

Road tunnels require continual fresh air flow to ensure that visibility is maintained and toxic gases levels are kept within acceptable limits. In the event that levels exceed those limits, the tunnels’ ventilation systems will begin operating wall or ceiling mounted jet fans.
Jet fans are extremely expensive to run based on their enormous power consumption.

The optimal way to maintain airflow and minimise fan usage is to implement an airflow measuring system.

Ultrasonic airflow measurement is now the most commonly used technology in tunnels. With no moving parts, minimal servicing is required and it can be fully sealed from the tunnel’s harsh environment. Other than a regular clean and check, it can be considered a fit and forget solution.
While the measurement principle is well defined and accepted, the ventilation contractor is presented with a choice of either single point or cross bore ultrasonic airflow monitors.

Typically, cross bore systems require more upfront investment and are more complex to install. Because of these factors it is often assumed that its measurement technique is superior to a single point airflow monitor.
While calculating an average airflow reading across the tunnel bore will provide more representative information, the system can only be installed if the tunnel’s design is suitable.

Limited head room above the traffic envelope, structural supports, cable trays and other obstacles may mean that a cross bore measurement system would not be viable.

In tunnels where cross bore instruments cannot be used, single point ultrasonic systems are a practical alternative. When installed correctly with adequate clearance around the sensor, single point ultrasonic systems will provide readings that accurately track the tunnel’s air velocity.

With easy installation, ventilation contractors can install single point, reliable, ‘fit and forget’ air flow monitoring solutions at a lower price.

Acoem provides solutions for both cross bore and single point ultrasonic tunnel airflow monitoring and will work with tunnel contractors to ensure optimal performance based on their individual application or project requirements.

The CROSSFLOW is an open path, cross bore air flow sensor designed to measure air speed and direction in multi-lane and bidirectional tunnels. The system uses reliable ultrasonic transit time measurements between a pair of transceivers which are mounted and aligned on either side of the tunnel, while the AIRFLOW MkII single point system provides consistent and accurate measurements using proven, ultrasound time of flight technology.

by Acoem contributor | June 13, 2022
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