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Case study: Air against water

overview / solution / result

Case studie: Air against water Two different elements used to protect nature, or in other words, where there is one thing there can be no other. These are the principles for the construction of an engineering structure, built by the joint venture New Railway Line Section NurembergLot North, Bilfinger Berger and Max Bцgl, parallel to the A9 motorway and level with the small town of Offenbau. This construction method can be visualised as an "oversized" 1200 m long and 14 m wide U-shaped iron, the open side of which is introduced into the soil. The legs of the "U" that is open at the bottom are made from bored piles, spaced that closely that they create an impermeable wall. The concrete lid is then cast onto these bored piles. After that, the soil enclosed by the "U" is excavated. As the ground water level in the region around Offenbau is very high, working without compressed air would not be possible. The ground water has to be pressed out of the tunnel cross-section with the aid of compressed air.

To prevent the compressed air from escaping, the Southern end of the "U-iron" is to be provided with a pressure wall. Sluices are integrated in this pressure wall. The excavated material is transported through the material sluice with lorries, pulled by electric locomotives. The material sluice has a diameter of 2.8 m and a length of 45 m. The miners, equipment drivers and skilled workers reach the work chamber through a man sluice. The work chamber is the room between pressure wall and working face. Compressed air is to be blown into this work chamber to prevent ground water from penetrating. Mr Markus Rechner, manager of the Mechanical Engineering Department at Bilfinger Berger AG recognised already at an early stage that the generally valid rules of the Compressed Air Ordinance would not be sufficient for this sensitive engineering structure. In particular the pressure fluctuations, caused by the lorries drivin in and out of the sluice, had to be kept as small as possible. In addition, the energy consumption of the air compressors has to be restricted. The calculated compressed air requirement of 400 mі/min adds up to an energy demand of approx. 2000 kW and results in costs amounting to approx. 1.0 million Euro. The temperatures inside the work chamber may reach a maximum of +25° and a minimum of +15°C. Another requirement specified by Markus Rechner was to push up the tunnel temperatures after sluicing out the personnel to allow for quicker curing of the concrete. The temperature gradient could be accurately monitored with the aid of remote data monitoring. Later on, the temperature was lowered again.

Case studie: Air against waterThe pressure fluctuations and the required energy savings are achieved by using speed-controlled electric motors. An intelligent, pressure-dependent control system, responsible for three 250 kW motors, each driving an Aerzen screw-type compressor (type VM 45), specifies the rotational speeds that have been determined by a CPU in relation to the tunnel volume. Every screw-type compressor only supplies that much volume that the specified tunnel pressure is just reached. Peter Engelke comments this as follows: Running up the pressure from below and maintaining it, that is the goal. The sluice systems (material, personnel and large equipment) were designed according to Bilfinger Berger specifications.

The tunnel pressure remained constantly at 700 mbar between 0:00 o'clock and 24:00 o'clock. Between 0:00 o'clock and 11:30 o'clock, compressor C1 was running at a constant speed of 2040 rpm and supplied a constant volume of 27.6 mі/min during this Compressor C2 took on the compensation of disruptive factors, caused by sluice work. The large equipment sluice was used between 10:06 o'clock and 14:27 o'clock. This is a sluice with pressure gates measuring 4 x 4 m. On completion of the large equipment sluice-out, the control system calculated that one compressor system was sufficient to produce the compressed air required to keep the tunnel pressure constantly at 700 mbar. Pressure fluctuations deed not exceed ± 10 mbar. The daily energy requirement was limited to 4,000 kWh.

The air compressors and sluice systems were constructed by Pressluft Frantz during the winter of 2002/2003 and were then delivered to Offenbau in April.

Case studie: Air against waterAfter only 10 months, the excavation work had been successfully completed and the pressure could be released. The compressors were switched off at the beginning of March.

The final temperatures in the screw-type compressors at +40°C climbed to +170°C. The air/air cooling units with a cooling air flow of 2 x 25.000 mі/h initially lowered the compressed air temperature to 40-60°C. The air/water cooling units, fed by a cold water kit, then achieved a further lowering to 22-23°C.

A very felicitous combination, commented Hermann Safran, the responsible foreman.

That this cooling system also worked well on cold days is demonstrated by diagram 4 of 23rd January 2004, probably the coldest day in Offenbau. Immediately at the compressor stations, the outside temperature was -5°C, while the weather station in Erding reported -12°C. It can be clearly seen that the air/air cooling unit can only cool compressed air to 37°C or 35°C. To keep the work chamber temperature between +15°C and +25°C, an air/water cooling unit is also required in winter time. Richard Loibl and his team are happy and satisfied that they have accomplished the excavations without any personal injuries and within the scheduled time frame.

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