Subway Ventilation

Ventilation Performance in a Subway Station

The Monitoring Need

The air quality in a subway station with many diesel trains sets high demands on the ability of the ventilation system to remove obnoxious gases and particles. In order to evaluate the ventilation system and air quality it was necessary to use two tracer-gas monitoring systems: one stationary system for dosing the tracer-gas in the ventilation system and one mobile system for measuring at the same time. Freon 134a was used as the tracer-gas because SF6 is forbidden in Denmark due to its high Global Warming Potential, GWP-index and will most likely be forbidden in other countries soon. The GWP-index for Freon 134a is 1300 where as for SF6 it is 23900.

The Solution

The Photoacoustic Multi-gas Monitor 1312 (now 1412) is well suited for these types of measurements. The monitor is easily operated and can measure the gases of interest within one minute. Due to the distance between the dosing point at the air intake and the various sampling points around the station, two independent systems were used.

For measuring the CO, CO2 and Freon 134a concentrations in the various sections of the station, one Photoacoustic Multi-gas Monitor 1312 was used. When connected to an inverter and car battery this system is fully mobile.

For dosing of the tracer-gas a Multi-point Sampler and Doser 1303 is used with a 1312 and the Application Software 7620. The system is set up with a constant dosage rate of Freon 134a. This occurs in the ventilation inlet just before the fan and Insures a good mixing before the air is distributed to the platform. Using the tracer-gas technique, where the ventilation air is "marked" with a detectable tracer, which is not present in the atmosphere, it is possible to detect how well different sections of the platform are ventilated. Furthermore, it is possible to measure the spreading of pollutants to other parts of the subway system, by measuring the tracer-gas concentrations at these locations. By measuring the tracer-gas concentration both upstream and downstream of the dosing point, it is possible to calculate the volume flow rate in the duct.