One of the challenges of designing and constructing an underground metro system is finding space at the ground level to house the vent shafts for the station environmental control system (ECS) and tunnel ventilation system (TVS), cooling towers and water tank for the station cooling system, escape staircases, and station entrances. The need for cooling equipment is primarily of concern in hot climates such as Singapore, Hong Kong, Shenzhen, Guangzhou, etc.; but it will become increasingly relevant as more subway systems consider adaptation measures for future rising temperatures. In addition to the spatial requirement for above ground equipment, there is an expectation from the public to reduce massive structures at the ground level so as to create a pleasant aesthetic environment for the area surrounding the station entrance.
Station Cooling Equipment at Ground Level
When a chilled water system is used to provide cooling for the underground station, the following equipment is typically designed for placement at ground level:
- cooling towers;
- a cooling tower make-up water tank; and
- associated pipes and valves.
For a four-car underground railway station, it is assumed that the cooling tower makeup water tank is about 60 cubic metres (m3) so that it can provide 24-hour operation for the station in case of incoming water supply failure. It is located about 3 metres (m) above the finished floor level of the cooling tower plant to facilitate the gravity feed of makeup water to the cooling towers. It occupies a footprint of 35 square metres (m2) with an effective water level1 at a height of about 1.7m. Together with the 1m maintenance and inspection space around the tank, the total space required for the cooling tower makeup water tank is approximately 70m2. The cooling tower plant creates a bulky structure near the station entrance that has a substantial visual impact to the surrounding area.
Station Sprinkler System at Basement Level
For the above mentioned four-car underground railway station, the sprinkler pump room in the concourse/platform level contains a sprinkler water tank (assume 90m3), pumps, associated pipes and valves. It is an independent system and normally does not share equipment with other systems (for example, the station environmental control system). The typical space required for the sprinkler pump room (including the sprinkler tank) is about 90m2.
Combined Cooling Tower Makeup and Sprinkler Water Tank
An innovative idea was developed to reduce the ground-level space requirements by relocating the cooling tower makeup water tank from the cooling tower enclosure at ground level to the underground plant room by using makeup water pumps.
The cooling tower makeup water tank would be combined with the sprinkler water tank so that the overall capacity of the single combined tank is the sum of the volume of the cooling tower makeup water capacity and the sprinkler water capacity. There would be two outlet pipes from the combined tank - the top one for the cooling tower makeup water system and the bottom one for the sprinkler water system. The technical and the spatial considerations for this innovative idea are shown in Figure 1.