Fresh air flow through tunnels is essential to ensure visibility is maintained and toxic gases levels are kept within acceptable limits. Tunnel ventilation systems will usually include wall or ceiling mounted jet fans. It is necessary to measure the airflow within a tunnel to minimise the use and power consumption of the Jet Fans while maintaining optimum airflow.
Ultrasonic airflow measurement is now the predominant technology used in tunnels. With no moving parts minimal servicing is required and they can be fully sealed from the tunnel’s harsh environment. Other than a regular clean and check they 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 this it is often assumed the 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, can mean the tunnel itself is unusable for cross bore measurements.
In tunnels where cross bore instruments cannot be used, single point ultrasonic systems are a viable alternative. Provided they are installed correctly with adequate clearance around the monitor, readings will accurately track the tunnel’s air velocity. Their ease of installation, and lower purchase price means ventilation contractors can install reliable, fit and forget air flow monitoring solutions.