One of the challenges of air conditioning an underground metro system is to find 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 appliances is primarily concern especially in hot and humid climates. Furthermore, the demand increasing exponentially as more subway systems consider adaptation measures for future rising temperatures due to global warming. In addition to the spatial requirement for the above systems, there is an expectation from the public to reduce massive structures at the ground level so as to develop pleasant aesthetic environment for the area surrounding the station entrance.
The chilled water system is mostly used to provide cooling for the underground station, the following equipment is basically designed for placement at ground level: for a four-car underground railway station, it is assumed that the cooling tower makeup water tank is about 60 cubic metres so that it can provide 24-hour operation for the station in case of incoming water supply failure. It is located about 3 metres 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 with an effective water level at a height of about 1.7-metre. Together with the 1-metre maintenance and inspection space around the tank, the total space required for the cooling tower makeup water tank is approximately 70 sq.m. The cooling tower plant creates a bulky structure near the station entrance that has a substantial visual impact to the surrounding area.
Centralised chilled water plant
For metro station of large capacity, it has been proposed a centralised chilled water plant concept in order to improve the overall energy efficiency of the air-conditioning system and reduce the station footprint. This concept can also improve the aesthetic ambience of the station entrances. Due to the large plant room size required to house them, the chiller plant and cooling tower plant are the key elements (Fig.1) of effective station planning. The extent and location of the cooling tower plant also introduce a visual and aesthetic impact to the station entrance and could present a noise nuisance to the neighbourhood. In this, the centralised chilled water plant will be located at Station B with chilled water supplied to the adjacent Station A and Station C (see Fig. 2). The chilled water supply and return pipes will run along the cut and cover tunnels and bored tunnels to serve the adjacent stations.
In consideration of system reliability and additional pressure loss along the tunnels, two pairs of chilled water pipes are proposed. Under a normal situation, each pair of chilled water pipes is designed to supply 50 per cent of station load. In case the pipe work in one tunnel cannot be used, the pipe work in the non-incident tunnel can provide a total of 75 per cent of station load. This is to ensure that there is no interruption of the air-conditioning supply to the stations’ critical plant rooms in the event of a tunnel fire emergency or potential mechanical damage to the pipes.
Chilled water system is used in for air conditioning components of which are given below.
- Water cooled chiller (Screw chillers).
- Twin cell cooling towers for Condenser water piping connected with chiller for heat rejection.
- AHUs to blow air through ducts.
- Chilled water pumps and Condenser water pumps to pump water to chiller and cooling towers.
These are the major equipment used in the air conditioning in metro stations. Apart from this, two water cooled chillers are also used for the air conditioning of various other rooms like server rooms, controlling rooms and ticket office rooms.
In some cases, geothermal heat pumps are also used as shown in Fig. 3 for thermal comfort of the metro stations. That provides an excellent opportunity to use this renewable energy source in cities with densely built underground infrastructures. Geothermal energy that uses bore holes up to 400 metres in depth.
Now coming to the ventilation part, ventilation on metro stations is being provided with the help of TVS. Here, OTE dampers, which look like square cut-outs, are provided to exhaust the heat dissipated from the OHE and friction.
Having compared the system requirement of the two schemes, the advantages and disadvantages are summarised as follows:
- By sharing equipment, a centralised chilled water plant requires less maintenance space to accommodate the chillers, cooling towers, and pump accessories, although the equipment will be larger.
- The use of larger chillers is more energy efficient.
- The station footprints at the adjacent stations (Station A and Station C) are reduced as underground space does not have to be allocated for the chiller plant.
- Station entrances at the adjacent stations are streamlined as it is not necessary to allocate at-grade space to house the cooling towers.
- The total amount of equipment is reduced and less maintenance work will be required.
- There is flexibility in selecting the stations along the metro lines which will have cooling towers installed.
- The station which houses the centralised cooling tower plant (Station B) will require a large at-grade footprint.
- It is necessary for two pairs of chilled water pipes to be run inside the tunnel to serve each of the adjacent stations. This requires close coordination with the electrical and mechanical services group to free up space for mounting of the chilled water pipes on the tunnel wall.
- Inspection and maintenance of the chilled water pipes inside the tunnel will be required, also possible replacement.
- As there is limited tunnel wall space at the cross-passage door location, tunnel services may be routed into the cross passage as a buffer zone to facilitate services crossing the cross-passage door opening. Considering the large pipe size, chilled water pipes would be located to avoid running in the inner wall to avoid such situation.
- Due to the temperature difference between the tunnel environment and the chilled. water supply/return pipe, it is necessary for the chilled water pipe to be insulated, which may impose a spatial constraint within the tunnel.
- Station chilled water pumps in the centralised chilled water plant need to be upsized to compensate for: additional pipe and fitting loss within the station with the central plant; additional pressure loss along tunnels; and pipe failure in one tunnel.
The adoption of a centralised chilled water plant concept offers benefits that outweigh the disadvantages. It achieves an overall cost savings in initial cost, operating cost, and maintenance cost and effort. The use of a larger centralised chiller will improve overall energy efficiency. And for stations with limited footprints, it eliminates the need for chiller plant and cooling tower space, thereby improving the ambience of the underground station and the station entrance. There have been concerns that future replacement of a centralised chilled water plant and the pipe works in the tunnel may affect the daily operation of the chiller plant. The implementation of a centralised chilled water plant concept has to be in the early design stages of metro station cooling projects. It will be difficult to implement the scheme once the land take is done, as large space is required for the station with the centralised chilled water plant.