More recent tunnel ventilation fans
Perhaps the notable feature of more recent tunnels has been the almost universal use of axial flow fans. The development of high duty aluminium alloys for the aircraft industry has meant that the tip speeds necessary for reasonable pressure development make the axial fan highly competitive. Flexible in design and much more compact, it can be installed horizontally, vertically or at any angle such that duct runs can be considerably simplified.
One of the early users of appreciable numbers of these fans was London Transport which has over 325 kilometres of tube railway. Generation of heat arises naturally from the continuous input of energy from train operation. There is a steady rise in the temperature of the air over a number of years due to the heat build up in the clay surrounding the tunnels which has to be corrected by ventilation.
Many fans for this usage were vertically mounted (Figure 1.44) and driven from a vertical motor through Vee-belts. The fans had to operate against widely fluctuating system pressures due to the piston effects of approaching or receding trains. They were designed with relatively low pitch angled blades to give a rising pressure characteristic back to zero flow, guarding against flow reversal. A number of manufacturers supplied these in sizes around 2.5 m diameter.
Probably the most recent usage of centrifugal fans for tunnel ventilation in Europe was in the late 60s by the Greater London Council. Both the Hyde Park Corner and Strand underpasses
Figure 1.46 Mersey (Kingsway) tunnel Aerex fan
Figure 1.44 Vertically mounted tunnel ventilation fan
Used backward bladed aerofoil fans. The former incorporated 8 Carter Howden 2.4 m diameter units (Figure 1.45). The Strand underpass, which was a conversion of the old Kingsway tram tunnel, used two 1.8 m double inlet double width fans, although these have subsequently been replaced by axial flow fans.
The first Mersey (Queensway) tunnel had been engineered on the grand scale and in 1925 no-one would have believed that it would ever reach vehicular saturation point. During its first year it handled over 3 million vehicles and by 1959, this had risen to
11 million vehicles. The original ventilation system could no longer cope and in 1964 additional axial flow fans were installed. Traffic continued to rise and in 1968 no less than 17 million vehicles were handled with 60,000 in one day.
Planning for a second (Kingsway)tunnel began in 1958 and this was opened in 1971. Ventilation was by the same upward semi-transverse system as used in the first tunnel with supply at the rate of about 0.3m3/s per metre run. The blowing shafts were offset from the line of the tunnel whilst the adjacent exhaust shafts were positioned directly above (Figure 1.45). Ventilating stations were over their respective shafts, behind the promenade at Seacombe, and on the inland side of Dock Road. Both stations were surmounted by evasees which whilst not so meritorious as the ventilating stations of the first tunnel, nevertheless are noteworthy landmarks. They could even be said to have a 70s-style “pipe of peace” affinity with the Roman Catholic Cathedral, scurrilously known as “Paddy’s Wigwam”.
Figure 1.45 Mersey (Kingsway) tunnel ventilation system 16 FANS & VENTILATION
Each tunnel tube is ventilated by two supply and two extract fans, one of each on either side of the river. An additional complete standby fan is linked together with the operating fan on bogies having traversing drives and carried on rails (Figures 1.46 and 1.47).
In operation, one fan is held in the surface position in line with the ventilation shaft, whilst its partner rests over a maintenance pit. In the event of failure, the fans automatically traverse to bring the standby into operation. Perhaps in imitation of the original tunnel, the order was split between Aerex and Davidson.
Each fan is driven through a 90° reduction gearbox coupled to a low speed induction motor. Fan speed is controlled by carbon monoxide monitors in the tunnels. Supply fans are 5.2 m diameter and have a duty of about 350 m3/s against 750 Pa at 129
Rev/min absorbing 306 kW. The extract fans are 6.1 m diameter and have a duty of about 387 m3/s against 200 Pa at 245 rev/min for a power of 107 kW.
The Ahmed Hamdi tunnel is a 1640 metres long, two lane, two way road tunnel beneath the Suez Canal at El Shallufa, approximately 10 miles north of Suez, in Egypt. The ventilation is a fully transverse system supplying air through ducts under the road and extracting through the false ceiling which forms the extract duct. A total of 16 two stage fans, 1.9 m diameter, were installed in two extract and two supply fan chambers. The system was designed to reduce the carbon monoxide level to 250 ppm maximum and the diesel smoke level to 20% Westing — house maximum.
Equipment had to withstand sand and dust storms and an ambient temperature of 45°C. It was also necessary for equipment to withstand a temperature of 250°C for one hour before breakdown. In the event of a fire, supply fans would be reversed and all 16 two stage fans would be extracting smoke from the tunnel.
To cater for the enormous increase in cross harbour traffic over the famous Sydney Harbour Bridge, Australia, and to relieve the subsequent heavy congestion on the bridge approach roads, it was decided that a tunnel should be constructed underneath the natural harbour. A newspaper article from the Australian Telegraph Mirror of 27th August 1992, illustrates the novel approach taken, see Figure 1.48.
The tunnel is 2.3 km in length. Two of the main requirements were that the supply fans had to be capable of running in reverse in an emergency and all fans be rated for smoke extract. Each of the fans has a duty of 53 to 103 m3/s. (Figure 1.49). The testing programme was one of the most comprehensive ever, covering flowrate and pressure, power measurements, sound levels, bearing vibration, X-raying of all impeller components, high temperature tests at 200 °C for 2 hours, impeller strain gauged for centrifugal and fluctuating stress, and 24 hour run tests with reversals.
In Hong Kong, a number of tunnels (Eastern Harbour, Junk Bay, Lion Rock, Tates Cairn, MTR Island Line, etc) have been built to link the island to the mainland for both road and rail traf-
Figure 1.49 Supply fan for the Sydney Harbour tunnel 18 FANS & VENTILATION
Fic. Some of these have been characterized by increasing fan capacity as traffic density has increased.
The Eastern Harbour crossing is but one of many and is a combined road (2.1 km) and rail (6 km) tunnel in one immersed tube which links Cha Kwo Ling near Kwun Tong on the Kowloon Peninsula with Quarry Bay on Hong Kong Island. The equipment was designed to cover normal tunnel ventilation, dilution and extraction of smoke and gases in a road tunnel through both overhead and low level side ducts. Emphasis was placed on the suitability of fans and associated acoustic treatment material being capable of working in high temperatures and in a hazardous environment. Fresh air is supplied from ventilation buildings located at each end of the tunnel, using 20 2.5 m diameter axial type fans. During emergency conditions 10,2.8 m diameter exhaust fans operate to extract smoke. The environment is maintained by an intelligent computer control system. A total of some 180 fans in varying sizes are used.
Piston effects from moving trains in the Channel Tunnel cause the fans to operate over an extensive range of the fan characteristic. This calls for aerodynamic stability from windmilling to flow reversal, with a continuously rising and power limited fan characteristic. These criteria apply for both forward and reverse modes. The axial fans selected for both normal (NVS) and supplementary (SVS) ventilation (Figure 1.50) are hydraulically actuated with controllable blade pitch in motion. There are four 2 m diameter NVS axial fans having a capacity of 89 m3/s and four 4 m SVS axial fans (Figure 1.51) each with a capacity of 300 m3/s.
All these fans are aerodynamically stabilized by means of the Axico anti-stall ring which introduces two chambers, one on either side of the impeller, providing stable flow conditions and continuously rising fan characteristics in both flow directions. When in the stall region, the separated and highly turbulent flow is removed from the main flow annulus and entered into the stabilizing peripheral ring-shaped duct just upstream of the impeller blades.
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