Mass Transit Railway is the backbone of transportation systems for large cities. Many Mass Transit Railways are built underground. The air conditioner (A/C) units onboard the trains intake fresh air from the tunnel for in-cabin ventilation. The in-cabin returned air is discharged back into the tunnel environment. The quantity of "fresh" air intake from the tunnel is controlled in order to reduce the air conditioning load in the train. However, the tunnel air quality will depend on the rate of the train A/C returned air into the tunnel, and the fresh air brought into the tunnel through ventilation shafts, stations, or portals. The air quality in the tunnel will have a direct impact on the in-cabin environment. This may lead to inadequate ventilation in the train and cause discomfort to the weak and elderly people. Because the air intake rate of the train borne A/C units is taken from the tunnel air, the tunnel air quality is of great importance. The air quality also has energy implications. The lower the carbon dioxide (C02) ppm requirement, the higher the fresh air rate required. This will increase the train borne A/C unit cooling capacity.
Passenger ridership, train throughput, and the effectiveness of the train-piston action have an impact on the air quality in the tunnel as well. If the tunnel air quality is not enough by means of piston action, fan plants will need to be operated in order to provide the required tunnel air quality for the trains traveling through the tunnel. This brings energy consumption issues in the design of a mass transit system and could represent a big operating factor.
This paper identifies problems, recommends a guideline criterion and proposes a solution to deal with them two environmental control systems for the Hong Kong underground railways. A tunnel ventilation numerical analysis was performed to qualify the recommendations.