On a hot summers day, a trip on the tube to get home can be a dreaded experience, with already hot trains overflowing with sweaty people. But how did we get to a situation where tube trains are stiflingly hot, and what’s being done about it?
A talk given by London Underground’s Head of Station Systems Engineering, Sharon Duffy, looked into the challenges the tube faces in keeping cool during the summer.
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 (pedants get very upset when sub-surface lines are called tube trains).
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 (S Stock), and as anyone who uses them appreciates, it’s a boon on hot days to get onto a cool train.
The heat from the air conditioning units is easily vented away when the trains are underground thanks to the pre-existing steam train ventilation.
The deep tube tunnels
However, it’s the deep level tube tunnels that cause the biggest problems for both passengers suffering the heat, and the London Underground in getting rid of it.
The deep level tunnels suffer a number of problems that 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. Tunnels that were a mere 14 degrees Celsius in the 1900s can now have air temperatures as high as 30 degrees Celsius on parts of the tube network.
Where does the heat come from?
Well, the passengers aren’t the problem. All those hot sweaty bodies represent roughly 2 percent of the heat in the tunnels.
Climate change is also not much of a problem. It’s an impact, but the tunnel temperatures are not much affected by what’s happening up on the surface. During a heatwave in 2006, as the surface temperatures jumped around, the tunnels were pretty much constant.
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.
Removing the heat
Anyone who has stood on the platforms will know that as the trains approach, there’s a blast of wind. This is intentional, as the trains act as air pistons in the small tunnels, and the effect of pushing air ahead and sucking air from behind soaks away about 11 per cent of the heat in the tunnels.
So when you curse as your paper flaps in the wind, think about the cooling benefits as well.
Mechanical ventilation removes about 10 per cent of the heat — that’s the big ventilation shafts that line the tunnels.
The older tunnels weren’t built with a lot of ventilation, as it wasn’t thought to be necessary — after all it’s difficult to argue that tunnels will get hot when standing in a tunnel that’s cool enough that you need to wear a jumper.
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.
In two locations, they’ve added water chillers to the intake to further cool the air down. Some of you might have noticed the shockingly cold airflow on the eastbound platform at St Paul’s tube station.
Air is taken in through an old lift shaft, cooled down, then pumped down to the platforms. A similar design was recently installed between Blackhorse Road and Walthamstow Central on the Victoria line.
Ventilation isn’t just about cooling though. On newer networks, such as Crossrail, they also act as smoke control systems should the worst happen in a tunnel. During a fire, the shafts can in places reverse flow, to blow smoke in a preferred direction. The aim being that people walking down a tunnel walk towards a fan blowing fresh air down the tunnel, while the smoke is sucked away behind them.
Over the past few years, 14 of the Victoria line shafts have been upgraded, and 50 fans across the network have had their airflow doubled, with 10 out-of-action fans brought back into use.
Despite that, fully 79 per cent of the heat in the tunnels is left to soak into the surrounding clay, which is already at or near its limit thanks to decades of absorbing heat.
The difficulty of adding more ventilation is the lack of space above ground to put new ventilation shafts. This is always going to be a problem for the older tube tunnels except on rare occasions when a surface development takes place at just the right location and agreements can be made to include a shaft down to the Underground.
It’s not just the cost of adding the new shafts and the running costs of all the electricity, but ventilation shafts also need sound attenuators to reduce the noise levels, both at the surface, but also in the tunnels so that people aren’t deafened by the noise.
People tend to be wary of having a new ventilation shaft in their neighbourhood, even though the aim is to keep the volume level to that equivalent to background city noise.
Even if new shafts are installed, at the moment they represent just 10% of the heat removed from the tunnels, so you can imagine how many extra shafts would be needed to remove a meaningful amount. So much land above the tunnels would be needed that you might as well just have a surface railway.
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