A concept is analysed for metro projects in congested urban areas - a centralised chilled water plant, located at one underground metro station, supplies two adjacent underground stations through a series of supply and return pipes that run along the railway tunnels.
For some metro projects, such as the Tsuen Wan Line in Hong Kong, WSP | Parsons Brinckerhoff has proposed a centralized 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.
This article compares and analyses the use of stand-alone chilled water plants (original scheme) versus a single centralized chilled water plant concept (suggested scheme) to serve three typical underground railway stations.
Original Scheme (Self-Contained Stand-Alone Chilled Water Plant)
For each station, a self-contained, stand-alone chilled water plant comprised of an underground chiller plant and an at-grade cooling tower plant has been adopted. Details of the chilled water plant are summarized in Table 1.
Due to the large plant room size required to house them, the chiller plant and cooling tower plant are the key elements 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 neighborhood.
Each station has a stand-alone chilled water plant with three chillers (two duty and one standby) in the underground chiller plant, and three at-grade cooling towers (two duty and one standby) attached to the station entrance/vent shaft structure. For this study, it is assumed that the cooling load of a station is 1600kW; hence, 3 x 800kW chillers are selected as the typical chiller configuration. Table 2 shows the system requirement of the stand-alone plant.
Suggested Scheme (Centralised Chilled Water Plant)
In this scheme, the centralised chilled water plant will be located at Station B with chilled water supplied to the adjacent Station A and Station C (see Table 3). 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 DN200 chilled water pipes are proposed. Under a normal situation, each pair of DN200 chilled water pipes is designed to supply 50 percent of station load. In case the pipework in one tunnel cannot be used, the pipework in the non-incident tunnel can provide a total of 75 percent 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.
For this scheme, a chiller plant and cooling tower plant are not required at the adjacent Station A and Station C. The system requirement is summarized in Table 4.
Design Considerations
Having compared the system requirement of the two schemes, the advantages and disadvantages are summarized as follows:
Advantages:
1. 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.
2. The use of larger chillers is more energy efficient.
3. 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.
4. Station entrances at the adjacent stations are streamlined as it is not necessary to allocate at-grade space to house the cooling towers.
5. The total amount of equipment is reduced and less maintenance work will be required.
6. There is flexibility in selecting the stations along the metro lines which will have cooling towers installed.
Disadvantages:
1. The station which houses the centralised cooling tower plant (Station B) will require a large at-grade footprint.
2. 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.
3. Inspection and maintenance of the chilled water pipes inside the tunnel will be required, also possible replacement.
4. 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.
5. 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.
6. 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.
Conclusion
The adoption of a centralized 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 centralized 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. Recently, the centralized chilled water plant and the pipe works in the tunnel for the Tsuen Wan Line in Hong Kong have been replaced after about 30 years of operation. During the replacement, all major equipment and materials were delivered by goods wagons1 through the railway tunnel during non-traffic hours from 01:30am to 04:00am daily. Because of the height limitation in the railway tunnel, the chiller had to be disassembled before loading to the goods wagon. With proper design and planning, the replacement of the centralised chilled water plant and the pipe works in the tunnel can be managed efficiently.
The implementation of a centralised chilled water plant concept has to be in the early design stages of the project. It will be difficult to implement the scheme once the land take2 is done, as large space is required for the station with the centralised chilled water plant.
(The authors gratefully acknowledge Steven Lai, Director, Infrastructure, China Region, and Senior Professional Associate in the Hong Kong office of WSP | Parsons Brinckerhoff, for his advice, support and assistance with this article.)
1Goods wagons or freight wagons (North America: goods cars or freight cars) are unpowered railway vehicles that are used for the transportation of cargo.
2Land take - the area of land that is 'taken' by infrastructure itself and other facilities that necessarily go along with the infrastructure.
