Rail tunnels have always been able to maintain a comfortable temperature because air circulates freely from one end of the passage to the other.
But with the construction of the world's longest underwater tunnel, beneath the English Channel between Britain and France, come several unforeseen challenges in keeping things cool underground.
In fact, engineers designing the project acknowledge that one of their initial surprises was that the tunnel needed to be air-conditioned at all.
"The discovery that the tunnels needed air-conditioning was made purely by the contractors checking some of the aerodynamics of the tunnel," said Robert Vance, director of the industrial and marine refrigeration group for the British subsidiary of York International Corp.
York, based in York, Pa., about 50 miles north of Baltimore, is the world's largest maker of air-conditioning and refrigeration systems and has the contract to provide air-conditioning systems for the tunnel.
The length, depth and narrow width of the Channel tunnel, known as the Chunnel, and the heat that high-speed trains will generate as they pass through, will combine to build temperatures as high as 130 degrees Fahrenheit in the passageway.
That would not only make the trains unbearably warm for passengers, but it probably would cause equipment to malfunction and the tracks to ultimately buckle.
Therefore, the engineers involved in the project have been wrestling with ways of incorporating in the existing design of the tunnel, which is scheduled to open in mid-1993, features that will keep it cool enough for the safety of both equipment and passengers.
That meant a special cooling system had to be designed. "The challenge for us was to develop an air-conditioning system that would cause the temperature to be somewhere between 38 degrees and 50 degrees," Vance said. "The major challenge we faced was how to get the heat out of the tunnels."
Other underground tunnels are short enough to provide adequate air flow, allowing for a comfortable temperature. But the Chunnel is 32.2 miles long, nine times as long as the tunnel that connects San Francisco with the East Bay. "And that presented a number of problems for us," Vance said.
For one thing, the distance between the trains and the walls of the Chunnel is shorter than in most tunnels. That, coupled with the speed of the trains traveling at 100 miles an hour, will create pressure and air friction that will cause the temperature to rise to 130 degrees.
The solution was developed by York and Transmanche-Link, a )) consortium of British and French construction companies that is building the Chunnel.
The plan called for constructing two systems of pipes to run the length of each tunnel and act as heat exchangers to cool the air. The movement of the trains will circulate the air.
Water will leave large refrigeration units, called chillers, at 38 degrees, flow through the pipes and return 21 degrees warmer, to be chilled again.
Four chillers, each with 2,000 tons of cooling capacity -- Pennsylvania Station in New York has a total of 3,000 tons -- will pTC be installed on the English side of the channel and another four, with 1,700 tons of cooling capacity, will be constructed on the French side. The Chunnel is to run from Folkestone, England, to Sangatte, France.
The refrigeration units on the French side will be on land. But those on the British side will be constructed on an artificial island below the cliffs of Dover.
The heart of the system is the refrigeration operation, which is generated, in peak times, by 52 megawatts of electricity an hour, more than four times the amount needed to air-condition Pennsylvania Station.
The pipes will be 24 inches in diameter and will handle 18.5 million gallons of water -- nearly twice the volume of a large intercontinental supertanker -- in a 300-mile network.
The Chunnel, which is expected to cost more than $14 billion, is one of the largest projects in the history of construction.
When complete, passenger and cargo trains will travel 130 feet below the seabed of the English Channel. The air-conditioning ,, project, whose equipment, materials and labor will cost about $200 million, is already being described as the world's most expensive air-cooling system.