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TBM Big Dig Oct '01

    

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History shows that if something has been used successfully in the past, it most likely will work in the future. A method of tunnel ventilation by Jack Barry & Associates Inc. is being used for several contracts on the Big Dig showing that the contractors have confidence in what this system can do.

In terms of numbers, Barry, president of the Irvine, Calif., company that also does noise control, said that with this system, an additional 325,000 cfm of air is being produced at no additional cost. This is being done with a 25 hp fan.

On the Big Dig, the fan system starts out with 50,000 cfm. This volume of air induces an additional 325,000 dm for a total of 375,000 cdm. The contracts in which Barry uses the induced draft system all use a 54-in. fan.

Other than the cost-savings that stem from the more energy-efficient system, ventilation designed by Jack Barry & Associates also brings with it the knowledge that safety is the primary concern. 

"In any design of an underground tunnel, cut-and-cover or mine, the primary purpose of ventilation is to supply the proper quantity and quality of oxygen to the people working underground," Barry said. "There have been studies performed that say that when people are oxygenated properly, they perform better, productivity increases, there are fewer accidents and more work. "Plus it's a much safer environment," he said.

The induced draft system is being used for ventilation during the construction phase of the contracts. Barry said a significant feature of this system is that there are no bulkheads, which also saves on cost because they do not have to be constructed.

"There are no doors that have to be opened and closed and no doors that need to be maintained," he said. "What normally happens with a bulkheaded door system is that the doors never seem to stay closed because people are driving back and forth through the doors. By using this system, we ensure the workers
have a clean, safe environment to work in at all times, not just when the doors are closed."

Dan Hudd, electrical mechanical coordinator for Modern Continental Construction Co., one of the Big Dig contractors using the induced draft system, said the ventilation is cleaning the area.

"We're doing a lot of sandblasting to clean off the girders. It was getting dusty and dirty down there," Hudd said. "From Cashman's job, their system blows and ties into our system. Once we hooked up this fan, all that clears out pretty fast."

Joe Folco, project engineer for Perini/Kiewit/Cashman, another Big Dig contractor, said Barry's company did the mainline tunnel ventilation. Although the joint venture finished up last year, the system still is in use for other contractors.

How does the system work if Barry isn't using a bulkheaded door system? It starts with the fan being a vaneaxial fan. Barry uses this type of fan for its high efficiency resulting from adjustable pitch blades.

"We mount a fan in the right of way, and we design a system that will allow us to have a high enough duct air velocity at the discharge point," he said.

After calculations, Barry said that they can then determine what the air velocity will be when the air intersects with the circumference of the tunnel. Based on science and practical experience, he said, they may install a nozzle at the end of the discharge system that will allow the proper speed of air. At the contact point, an air bulkhead is established, with "one side with positive air and one side with negative air."

Barry's development of this type of ventilation goes back to when he owned Bonanza Fans Inc., also of Irvine, a company that designed and built fans for the mining and tunneling industries. At Bonanza Fans, Barry learned about vaneaxial fans from the pioneers of the original fans' and other key players. It was at Bonanza where discharge cones became more important.

"I got involved in a study of discharge cones, and I found out that by controlling the speed of the air, we could optimize the regenerative regain," he said. "When you discharge air say you have a 36-in. diameter pipe and you're discharging air, there will be eddies that form all around the circumference of this duct so that the actual area open for discharge may be as small as 30in. It takes more energy to discharge the air."

Barry said that with a discharge cone, it takes less energy to get the air moving.

"When we put a discharge cone on a system, we take and we slow the air down, holding it and controlling the air flow, hence the speed," he said. "We create a large area that slows that air down, which allows it to go out without these eddies.

"Further investigation showed us that speeding up the air at the discharge point would accomplish something else. If I could control the exit velocity, I then could control the speed and pressure of the air when it came in contact with the C circumference of the tunnel. I discovered that I could create an air bulkhead which: allowed for a sealed tunnel, hence total control of air throughout the tunnel." !

For the design of the system, Barry also C credits Dr. Jan Wolski, a retired professor: from New Mexico Tech. Barry discovered Wolski was using the same technique, and the two collaborated.

Other projects of note that Barry used the system on include the City of Niagara Falls (N.Y.) Raw Water Intake Tunnel for contractor Herbert E Darling Inc., Railroad Tunnel No. 26 in Simi Valley, Calif., for Herzog Contracting Corp. and a portion of the TARP project in Chicago, which was 9 miles long and 30 ft in diameter
for Perini/ICNO&C.

The system works only when the tunnel is open on both ends. "If you're using ducting, there's resistance in the ducting," Barry said. "As soon as you hole through, you can use the induced draft system, and you use a fraction of the horsepower to move the air."

Anne Sedjo is assistant editor for TBM: Tunnel Business Magazine.

 

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