Comparison of airflow in two different passively ventilated subway systems—New York City (USA) and Dortmund (Germany)

The investigations on conditions of air flow and air exchange in the Subway-system of New York City began in the summer 1998. These should help to provide sustainable health prevention regarding bioclimatic and air hygienic loads; furthermore, evacuation measures in sudden catastrophic situations such as big fires should be developed. Main characteristic of the investigation has been to determine, and characterise the air movements in between the Subway stations and the tunnel sys-tem, and to identify and quantify different influencing factors. In addition to the influence of train movements further influences on the underground system of air currents could be identified (e.g. the effects of regional and local weather conditions and especially the gradient of air temperature between the air inside and outside the stations).

Based on these experiences gained in the New Yorker Subway a comparable measurement program has been developed for the Underground in Dortmund, Germany. The latter clearly differs from the former as follows: 1. Different structure of stations and tunnels, 2. shorter lengths of trains, 3. no air conditioning of the trains, and 4. no overheating effects due to absence of air conditioning in underground areas (rooms for Subway personal) and distant heat transfer systems that heat up tunnel walls (in Dortmund).

Despite these differences, the main common characteristic is that both systems are ventilated pas-sively by entrances/ exits and tunnel openings. Thus, air currents inside the station and tunnel sys-tem of both Underground systems are mainly influenced by effects of regional and local weather conditions (in addition to train movements). The most important are: 1. Temperature/density and humidity gradients between subway and urban atmosphere (upward or downward winds). Here, especially diurnal and seasonal changes are of major importance. 2. Regional wind speed and direction. 3. Microscale/street-level weather conditions, including: wind eddies above the exit/entrance area

As both investigation areas differ greatly due to their climatic background situation (e.g. in their extreme values of climatic variables) distinct differences appear regarding air flow conditions and air exchange with the urban atmosphere, respectively.