Burgeoning urbanisation leading to rise in traffic woes has generated an impetus for Mass Rapid Transit System in India. The metro rail has been witnessing exponential growth since its first introduction in Kolkata and later in Delhi. The segment has gained momentum with successful installation of metro projects in Kochi, Mumbai, Chennai, Jaipur, Lucknow, Bengaluru, Hyderabad, and Gurugram. As a result, HVAC sector has envisaged the growth potential due to spike in metro projects that are crisscrossing length and breadth of India.
Underground metro projects have major requirements for cooling system, ventilation system, pressurisation system, fire smoke extraction system etc. Elevated metro projects usually do not require major HVAC system except for booking counters and mechanical ventilation of a few utility areas and axial fans at the platform area. In underground confined area metro stations, it is necessary to supply mechanical ventilation in order to extract the heat dissipated by metro rails, passengers’ movement, lights and fans, escalators, air conditioning units etc.
The cooling load at metro stations is primarily the heat generated by arriving, stationary and departing metro rails. The skin loads are a much smaller percentage as the stations being underground where the temperatures range from 22 to 27-degree Celsius. The sudden rush of passengers alighting a train adds a short burst of load. The cooling system, thus, has to cater to sharp load fluctuations and adjust cooling medium flows of chilled water and recirculating air, explains Nimesh Mehta, Chairman & Managing Director, Perfect Infra Limited.
While shading the light on working of cooling systems at metro stations, Stefano Ruzzon, HVAC Projects and Dealers, CAREL Group Sales Manager briefs, for metro systems, solutions can be exploited that lower the ambient temperature by using water as the source of cooling. When one kg of water evaporates, it absorbs around 700 W of energy from the surrounding air. If the evaporation process is triggered by a device with lower power consumption, such a device will be greener than equivalent traditional cooling equipment, such as a chiller. Having significantly lower energy consumption, evaporative cooling systems are, therefore, particularly cost-effective in summer.
In the Madrid Metro, for example, CAREL has installed highpressure spray units with stainless steel racks housing the nozzles used to atomise and cool the air. The humiFog solution proposed cools the environment to within the limits established by law. A special pump delivers high-pressure water that is sprayed through stainless steel nozzles, producing an imperceptible and uniform mist. The droplets evaporate spontaneously and almost instantly, humidifying and cooling the air. Consuming around 5 W of electricity per l/h of atomised water, the system provides about 700 W of cooling: the ratio between cooling capacity and power consumption is 140:1, elaborates Ruzzon.
The CAREL system ensures an inside temperature of 28°C when the outside temperature is 34-degree C. The experience in Spain has allowed CAREL to monitor and compare real data, highlighting how evaporative cooling has proven to be more advantageous than traditional systems, due to lower energy consumption and less maintenance.
Akhilesh Shriwas, Senior HVAC Engineer, Egis India, explains, in cooling system for underground metro stations – chilled water is provided from a chiller plant room and the same is supplied to the Air Handling Units (AHUs) and Fan Coil Units (FCUs) at various levels of the stations to serve the public areas, offices and ancillary rooms etc. Water-cooled chillers operate during revenue hours and air-cooled chillers operate during non-revenue night hours. For ancillary or 24X7 rooms, standby FCUs provide for uninterrupted air conditioning of those rooms.
Cooling system for above-ground metro stations is done by Variable Refrigerant Flow (VRF) air conditioning system with standby system arrangement (complete VRF system for 24×7 operating rooms) and public area remain unconditioned, adds Shriwas.
According to Detailed Project Report of Pune Metro, the metro station will be cooled by using AHUs located in environmental control plant rooms throughout the station. Each platform will be served by at least two separate AHUs with the distribution systems combined along each platform to ensure coverage of all areas.
Detailing the nuances of working of these air conditioning systems, the report further explains, these air conditioning systems mix return air with a desired quantity of outside air. The outside air requirement depends on occupancy with a minimum of five liters per second per person or 10 per cent of circulated air volume, whichever is the greater. The provision of free cooling by a simple two-position economiser control system will be included with the use of enthalpy sensors to determine the benefits of using return air or outside air. This will signal the control system to operate dampers between minimum and full fresh air so as to minimise the enthalpy reduction needed to be achieved by the cooling coil. The report recommends to use energy-efficient water-cooled chiller units with screw compressors at each station.
Mukesh Sehgal, Managing Director, Contec Airflow Projects, suggests, the underground stations will need to be designed around Variable Refrigerant Volume (VRV) based system. The tunnel ventilation, fire smoke exhaust fans will require major design, feasibility study before these systems can be proposed for cooling with VRV systems. For pressurisation and ventilation, if required temperature control can be integrated through VRV system.
Chennai Metro is exploring gas-based cooling technology to keep the stations cool in the underground stretch. According to Chennai Metro Rail Limited, the new VRF technology will reduce the consumption of water as cooling medium that is necessary for operating water-based cooling system. Refrigerants are used as cooling medium in VRF units. The flow of refrigerant is controlled by the inverter compressor of the VRF outdoor unit. Then, it is circulated within the station to multiple units.
Dubai Metro has installed the world’s first district-cooled mass transit system. In District cooling, the chilled water is distributed through an underground insulated pipeline in order to cool the indoor air of all stations. Being economical, it is energyefficient helping in reduction of CO2.
Like all other infrastructure projects, HVAC metro projects have its own challenges. To achieve operating performance requirements of HVAC system i.e. to maintain the temperature and relative humidity in the metro station at desired level especially during peak hours is the biggest challenge.
Dealing with large size equipment in underground metro stations is also a challenge i.e. lifting, shifting and installation of heavy HVAC equipment like chillers, pumps, and fans with proper implementation of safety. Usually, metro projects pass through major commercial and residential areas. The heat and noise dissipating through the outdoor units is also one the major problems in its placement.
For installations of above ground metro outdoor units can be aesthetically installed with proper consideration in the orientation of heat and sound dissipation in the nearby residential or commercial establishments. These smaller units do not pose any major challenge. Placement of outdoor units in underground metro will definitely require careful planning for heat and sound dissipation and space requirement of these units, states Sehgal.
He further suggests that VRV based cooling systems will be an ideal choice. This will not only save on space but also in other utilities like make up water (for water cooled condensers), space, sound and aesthetics.
Optimum design calculation for capacity sizing of equipment, appropriate selection and installation of equipment for 24×7 rooms in metro stations are necessary for easy service and maintenance of the equipment in 24×7 rooms. Selection of the equipment and materials is in full compliance with the local climate and operating conditions keeping in mind performance requirement and lifecycle of selected equipment, concludes Shriwas from Egis India.