Published 20-05-2021
| Article appears in June 2021 Issue

Thermal flow switch helps keep tunnels safely ventilated

thermal flow switch
The FLT93S from Fluid Components International.

Powerful fans are used to ventilate rail and road tunnels, but which direction should they be blowing in and how strong should the airflow be? That’s where a thermal flow switch such as the FLT93S from Fluid Components International (FCI) comes into the picture.

Supplied by AMS Instrumentation & Calibration, it helps manufacturers of powerful industrial fans and blowers used in the transportation industry to obtain accurate airflow control data under variable vehicle traffic conditions.

Proper tunnel ventilation is critical to maintain tunnel temperatures within acceptable limits and to help control the flow of fresh air to prevent unhealthy toxic carbon monoxide (CO) exposure, as well as smoke or gas in the case of fire or other emergencies.

Depending upon a tunnel ventilation system’s design, the airflow fans must operate in either a forward or reverse airflow configuration.  For example, during emergencies requiring air, gas or smoke evacuation, a flow switch such as the FCI Model FLT93S needs to detect forward or reverse fan airflow to prove air is blown into or extracted from a tunnel where vehicle drivers and emergency responders can be either trapped or at work.

The versatile FLT93S Thermal Flow Switch can be configured with a uni-directional shroud to help verify the airflow direction of each tunnel fan. FCI developed this special shroud for the FLT93S Switch, which allows the detection of airflow from a single direction. By installing two flow switches per fan, engineers and operating technicians are able to verify with confidence the direction of airflow for each fan in a tunnel.

The FLT93S Thermal Flow Switch is suitable for placement in tunnel ducts or fan casings up to 2.4 metres in diameter. It can detect airflow ranges from 0.08 to 37 NMPS with flow accuracy as precise as ±2% of the setpoint velocity over a ±28°C temperature range; repeatability is ±0.5% reading.  Flow elements are available for operation and survival (in the case of adverse conditions) for process temperatures up to either 260°C or 454°C.

Based on FCI’s thermal dispersion expertise, the unique sensor technology of the FLT93S Switches, combined with FlexSwitch temperature-compensation circuitry, introduces unparalleled performance capabilities for a wide range of heavy-duty industrial applications.

Featuring an all-welded, no-moving parts design, the FLT93S Switches require virtually no maintenance. There is nothing to break, clog or foul that requires continuous maintenance technician attention.  They are available with a choice of materials: 316 L stainless steel, Hastelloy or exotic materials. In addition, these switches offer an exceptionally long life and a low lifecycle cost.

AMS Instrumentation & Calibration -



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