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.
The new suggested scheme would have the following configuration:
Makeup Water Transfer Tank at Ground Level
A cooling tower makeup water transfer tank with a capacity of 2m3 will be provided at the cooling tower enclosure at the ground level instead of the 60m3 tank.
Since the 60m3 cooling tower makeup tank is to be relocated underground and replaced by a small 2m³ makeup water transfer tank at ground level, the footprint of the cooling tower enclosure at ground level will be reduced. The tabulation shown in Figure 2 is the configuration of the cooling tower enclosure for the two schemes.
Makeup Water Pumps
Incoming water will feed directly to the 2m3 makeup water transfer tank for normal usage and to the underground cooling tower makeup water tank for 24-hour storage. An extra set of water makeup pumps is required to pump the makeup water from the underground cooling tower makeup water tank to the 2m3 makeup water transfer tank at the ground level cooling tower enclosure in case of incoming water supply failure. It is anticipated that the pumps will not be operated under normal usage. From a routine maintenance standpoint, the pump set needs to operate occasionally to avoid stagnant water inside the underground cooling tower makeup water tank. The pumps will be controlled by the level sensors in the at-ground level makeup water transfer tank. More control points will be required for the additional tank and pumps. Also, more pipe works and control devices would be needed to transfer the makeup water to the makeup water transfer tank and then to the underground cooling tower.
Combined Tank at Basement Level
When the cooling tower makeup water tank is relocated underground, the floor area of the combined tank will be smaller than that of the two individual tanks by taking advantage of the available spare headroom at the concourse/platform level. In general, most of the underground levels have a clear headroom of 5 metres and so it is envisaged that the effective water level could be increased. When the cooling tower makeup water tank is combined with the sprinkler water tank, the overall area of the fire pump room is increased to approximately 126m2.
Also, the extra set of makeup water pumps required to transfer the makeup water to the makeup water transfer tank at the ground floor occupy an area of 3m2 and are usually located next to the underground combined cooling tower makeup and sprinkler water tank or within the chiller plant next to the combined tank (see Figure 3).
Impact on Fire Sprinkler System
In general, a fire services system is an independent system and is not affected by the operation and maintenance of other systems (for example, the environmental control system). The cooling tower makeup water in the upper portion of the combined water tank will only be used when a water supply is not available, which is a very remote condition. The outlet for the cooling tower makeup water is above the sprinkler water level, and the sprinkler tank storage water will not be used for cooling tower makeup water. However, as an added benefit, it is expected that the suggested scheme can provide extra capacity for the fire sprinkler system if needed and, as a result, the local fire services authority has no objection to the implementation of this suggested scheme.
The cost of underground structure construction is higher than above ground construction. However, reduced use of space at the ground level also results in less ‘land take’. Land take, or the area of land that is ‘taken’ by infrastructure itself and the facilities that go along with the infrastructure, is very important in places like Singapore, Hong Kong, etc., where land resources are limited. A reduction of space needed at ground level results in the equivalent area being saved in land take, while additional space underground does not require the equivalent area of land take. As a rough example, 100m2 of land saved at ground level equals 100m2 of saved land take; while 100m2 of saved space underground equals approximately 50m2 of saved land take.
Based on the discussion above, the advantages and disadvantages of the proposal are summarized in Figure 4.
With proper station planning, the cooling tower water makeup tank can be combined with the sprinkler water tank in the underground concourse/platform level. This arrangement can reduce the overall at-grade footprint of the cooling tower plant as well as provide more design flexibility for the architect to improve the overall aesthetic ambience of the station entrances.
(The authors gratefully acknowledge Steven Lai, Director, Infrastructure, China Region, and Senior Professional Associate in the Hong Kong office of WSP, for his advice, support and assistance with this article.)
1Effective water level is the height between the water outlet and the water level. There is some “dead water” below the water outlet level that cannot be used. The height of the tank is 2m in this case.