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Cooling the tube – Engineering heat out of the Underground

Read More: https://www.ianvisits.co.uk/blog/201...e-underground/

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The “tube” can be split into two types of service — the tube proper which runs through tube tunnels, and the older sub-surface lines which are just below street level. The older sub-surface tunnels were built for steam trains, so had loads of big holes in the ground included in the design to deal with removing smoke, and they are also much larger than tube tunnels. This has allowed the London Underground to fit air-conditioning units to its new fleet of sub-surface trains.

- However, it’s the deep level tube tunnels that cause the biggest problems for both passengers suffering the heat, and London Underground in getting rid of it. The deep level tunnels suffer a number of problems which are individually a nuisance, but collectively add up to show why there is such a huge problem cooling the Underground. One of the biggest problems is a side-effect of what made it possible to dig the deep level tunnels in the first place — namely the very solid and nice to tunnel through London Clay which sits under the city.

- In fact, when the early tube tunnels were dug, they were so cool down there that the cool tube was seen as a respite from the summer heat on the surface. Why suffer on a bus in the heat when there’s a cool tube to take instead, said the marketing men. So why is the Bakerloo line, once the coolest place to be, now a mobile sauna? While that heavy thick clay is lovely to tunnel through, it is also a heat insulator. Over the years, the heat from the trains soaked into the clay to the point where it can no longer absorb any more heat.

- About 21% of the heat in the tunnels comes from the movement of the trains themselves, from aerodynamic drag and other frictional losses. The motor engines account for 15%, the electrical and auxiliary systems are the remaining 12%. About half the heat in the tunnels though comes from just one source –from the trains slowing down — the conversion of movement into heat by applying the brakes. So it can be seen that cutting the heat from applying the brakes is where the biggest win would be, and indeed, the use of regenerative braking now converts about half the heat loss back into electricity.

- However, that can only work where trains are accelerating and braking at the same time, on the same electricity sub-station loop. Experiments have been underway to improve that by use of an inverting substation, supplied by Alstom, which can send unused power from braking trains back into the national grid. --- By the time the Victoria line came along, the engineers were very well aware of the problem and it was built with considerably more ventilation shafts than older tunnels would have been supplied with. Although it varies depending on location, in general cooler air is sucked down through the stations, and then ventilation shafts in the tube tunnels sucks out the hot air.

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