Why is the Tube so hot? London’s underground heat explained

There is a direct and indisputable relationship (and subconscious awareness) between the chronic overheating of the London Underground and heat waves; however, I appreciate that the direction of my analysis may seem counter-intuitive to many, and this is clearly causing some “cognitive dissonance” in our engineering community.

Absorb the Sun: Does solar gain from the Overground carry heat to the Underground? (Photo by mammuth/iStock)

The first point to consider is that the Tube is a “mixed” over/underground network, which overheats, whereas fully or mostly underground networks like Glasgow, Warsaw or Prague generally do not overheat. Even taking into account their year-round operational heat sources, their networks maintain a constant temperature of around 15⁰C.

Whether it is only aerial rail networks or mixed networks, all rolling stock is at the mercy of solar gain during its movement on the surface. Traveling above ground in the summer, cars are relatively easy to cool internally with air conditioning (AC), the hot exhaust air being released into the atmosphere.

However, when comparing tunnels that serve a purely underground network with tunnels that serve a mixed network whose trains run both above and below ground, the thermal behavior results will be significantly different.

It is important to note that the total heat absorbed from these latter trains equals the sum of all the heat sources in an underground-only situation, as well as the significant solar gain effects – both direct and indirect. At David Attenborough perfect planet series, episode 2 at minute 38, he reminds us: The solar energy that hits our planet in just one hour contains more energy than that used by all of humanity in an entire year.

What does Transport for London say about the Tube overheating?

Transport for London (TfL) and London Underground (LU) blamed “brakes” and other sources of operational (year-round) heat, including passengers’ body heat. If the sources were only the effects of operational heat, these would cause overheating in the underground-only networks, which is not the case. However, although TfL/LU have taken isolated, unrelated snapshots of the overheated trains using thermal imaging, they have not determined whether the increase in heat is truly gradual.

With a more robust assessment, they may have discovered the primary role of direct and indirect irradiation effects of solar gain, both on the track and on the trains as they travel above ground.

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The resulting problem is that not only the internal surfaces overheat, but also the external surfaces, some by direct irradiation and others, such as the running gear, by indirect irradiation from the irradiated rails and track ballast.

How Underground Tunnels Affect Tube Temperatures

The use of alternating current in the interior space of the wagons does not deal with the heat reirradiated from the external surfaces of the train in the tunnel, but adds to the overall heat of the tunnel due to the heated air that the alternating current exhausts. The best way to deal with this problem is to first keep the trains from getting hot, both inside and out.

Obviously, traction motors and brakes get considerably hotter in seasonal hot weather, not only because of their operation, but also because they have been overheated in the environment and location they are in – while they are on the surface.

TfL/LU’s own data confirms the seasonality of overheating, as does Cockram and Birnie’s clay temperature chart London Underground ventilation. Additionally, while TfL/LU constantly proclaim that climate change is sure to make matters worse, they strongly resist the assumption that the current climate might be involved.

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As a result, and despite numerous past and planned, untargeted and misinterpreted (in my opinion) multi-million pound mitigation measures, realistic proposals for the prevention of these huge solar effects are sorely lacking.

That’s why the focus on “the brakes” got so little improvement, and why regenerative braking also wasn’t effective in reducing overheating. Additionally, all of these thermal conductivity processes described above, including the intensifying effect of irradiated ballast, have been largely ignored or neglected with respect to the management of overheated rails and the resulting rail buckling.

Minimal attention has been paid to the benefits of solar reflectance measurements. For example, the beneficial use of standard white paint on non-wearing rail surfaces is around 5⁰C, but using a solar reflective white paint could reduce excess heat from the rails. ‘about 15 to 20⁰C, and even more if the ballast was also protected. of irradiation.

Without sufficient understanding and inclusion of the role of irradiation at the surface, TfL/LU and their academic advisors and consultants may have misinterpreted the evidence and their modeling will always be wrong, massively underestimating the amount of heat transported in the tunnels.

Despite WSP/Parsons Brinckerhoff’s (PB) modern involvement in TfL/LU’s Cooling the Tube (CtTP) project, their engineers and others working on this project ignored or (more likely) completely ignored historical findings. of PB in the mid-1970s. in collaboration with the leaders of the American rail networks.

PB’s findings in these early iterations of their seminal work on tunnel design refer to the study of numerous rail/subway systems in the United States, including the Pennsylvania Transportation Authority of southeastern Pennsylvania. [SEPTA]who pointed out, very explicitly, that their Market Street Line cars were absorbing solar heat when they were above ground and bringing that heat into the subway.

Unfortunately, these discoveries seem to have been forgotten over time. Yet this research and its findings highlight a message that must be taken very seriously.

How is TfL coping with the heat on the London Underground?

Much of the funds TfL/LU has spent have been on station cooling programs. When such high temperatures occur in the station environment, passengers have a choice – stay or exit to the relative safety of street level. However, this focus completely misses the point, as it is the overheated tunnels that will harm passengers if their train stalls between stations when the saloon temperature is above 40°C – and similar temperatures have indeed been reported anecdotally on the Central Tube. Line.

In a broken down train, everyone is totally dependent on the train staff to isolate the current, open the doors and get off or evacuate the passengers. Historically, such events have often taken hours, not minutes, and for many such a delay could prove fatal.

In its early days, the London Underground network was advertised as “it’s cooler down below” because at the time it was an underground-only network. The overheating consequences of the decision to extend it overhead to the suburbs could not have been foreseen or assessed at that time.

Meanwhile, Warsaw’s underground network engineers are planning the construction of one or more other lines.

If any of this expansion is in the open, over time it will almost certainly create the same problems that Tube faces today. Moreover, the Piccadilly Line’s new air-conditioned trains will not solve the problem, they will only exacerbate it. Crossrail will also eventually overheat because it’s modeled after the New York City subway, which has been air-conditioned for 30 years – and it’s still overheating.

In many ways London’s Tube was and is a brilliant piece of engineering. However, it would be so much nicer, and safer, if it didn’t overheat. Based on the above analysis, if the mid-1970s observations of PB and confirmed by my own research are taken into account with an open mind, I do not see this overheating as an intractable problem.

Data visualization by Katharine Swindells.

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Bonny J. Streater