• Since contributing this article, author Calvin Barrows has submitted further thoughts on the topic and in response to the article about expansion of metro networks in major cities of Russia that reports one of the main problems of the Moscow Metro is related with its ventilation system which cannot cope with the increase in temperatures during the summer season, and that, due to this, Moscow authorities consider building new stations at depth as currently unreasonable.

    See the full Feedback contribution on the Feedback page.

DISCUSSION FORUM

Cooling the Tube still on ice 24 Jan 2019

Calvin R Barrows, BSc (Hons), CEng, MICE*
Overheating in underground metro systems of the world is a serious and concerning issue, particularly as the numbers of passengers using older and deeper systems increase. UK Chartered Engineer Calvin Barrows, takes a slightly light-hearted approach to make some serious points about the issue and present his theories for possible solutions.

Hang onto your handrail fellow London Underground tube users, if it’s not too hot, as I am about to share my controversial theory about Cooling the Tube, which has resulted in some folk getting rather hot under the collar.

Overcrowding on a London Underground platform
Overcrowding on a London Underground platform

Please stay aboard, because for the Cooling the Tube Project ever to be successful, there are fundamental points at issue that still need to be considered and resolved. I am hoping that summarising my findings and research to date along with my interpretation of these, will initiate some serious and constructive debate among engineers, scientists and anyone with an interest in this matter.

My research:

  • considers the extent of the problem, and how it affects passengers;
  • sets out the circumstances that led me down this very different route;
  • identifies the probable causes of the overheating, based on my direct experiences, observations and temperature measurements;
  • discovers how another underground network, conceptually similar, albeit with some subtle differences, fares with overheating; and
  • suggests possible solutions specifically for the configuration of the London Underground infrastructure.

To start, the very title of the project, Cooling the Tube, is a misnomer. Consequently, the search for a solution to the problem has been misled. The challenge is not simply Cooling the Tube or even Cooling the Underground Network. To be more precise I would define it as Cooling the Whole LU Network in the Summer, since both the over-ground sections of the network as well as the underground infrastructure overheats in the summer.

Secondly, having spent years commuting on the Central Line of the London Underground and monitoring the seasonal temperature differences at various times of day, I came to the irrefutable conclusion that the trains were gaining more heat overground than underground - in the summer. Let us dispel the myth circulating that the Tube is hot all the year round, because in the winter passengers wear their thick coats and scarves while standing or sitting comfortably inside an electrically heated carriage. Conversely, in the summer, passengers wear light clothing yet are still frequently getting close to heat exhaustion during heat waves and in the hotter weather generally.

The stated objective of the Cooling the Tube project is two-fold – comfort and safety. For comfort cool trains are the priority since passengers may spend 5-10 minutes on a platform but up to 45 minutes to an hour on a train. For safety we need cool trains and cool tunnels – the cool tunnels being key to maintaining cool trains when underground.

Listed sources of heat generation in the London Underground
Listed sources of heat generation in the London Underground

If passengers were trapped for some considerable time in an overheated, stalled train, within an overheated tunnel, the outcomes for them could be very grave. So, the decision to expend effort and money cooling underground stations was erroneous, and indeed has been a wasteful diversion from addressing the real problem. Passengers are free to leave a station for a place of safety if they are overheating there!

Many alternative reasons for the overheating in the summer have been proffered, including the combined effect of the tunnel, train and station systems; traction; braking; mechanical losses; passengers; the heat accumulating in the London clay; and so on. All these are misleading either because they do not change significantly throughout the year, so they do not explain the seasonal variations, or, in the case of the London clay temperature, it is not a cause but rather an effect.

The key issue is that many of the tube trains run both over- and underground. These are inevitably going to be affected by the overground ambient temperature and solar heat gain. So rather than the primary heating of the carriages happening in the tunnels, my assertion is that the primary heating of the carriages does not happen in the tunnels.

My initial, subjective observation struck me as odd in that, travelling overground towards Epping during a late summer afternoon with progressively less passengers on board, the carriage seemed to become markedly hotter. My subsequent temperature monitoring clearly confirmed that up to a 6°C increase in temperature was not uncommon in the summer between underground network transition at Leyton and Epping Station, a journey of about 25 minutes. The only logical conclusion was that the train was being heated by the surrounding hot ambient air and super-heated by solar irradiation from the sun.

The rising temperatures and the increasing depth of the London Underground system over the years
The rising temperatures and the increasing depth of the London Underground system over the years

However, what, if anything, could that have to do with the overheating in the tunnels? My theory is that train bodies absorb heat from the 
sun on their external surfaces to become overheated storage radiators and then emit some of that absorbed heat until they reach a state of equilibrium including as they pass into the tunnel. Clearly then, overheating is a network problem – the laws of thermodynamics cannot have it any other way.

Although the level of heat emission is unharmful on the surface, when the 200 tonne trains enter the tunnel, with a thermal capacity estimated at 100-200 MJoules/°C, they continue to emit heat to detrimental effect. If all that were not enough, every time these trains enter the portals in the summer, they are also pushing/dragging in a plug of hot, ambient air, adding to the heat load in the underground network. The piston and drag effects of the 40°C temperature differences in yearly cycles of ambient overground temperatures from -5°C to +35°C, undeniably makes a significant difference to the tunnel temperatures between summer and winter.

When comparing the London Underground network with the Glasgow metro in Scotland, similar in concept but all underground and with shallower tunnels at about 9m (30ft) deep, the ambient temperature of the Glasgow system hovers around 16°C year-round, even during the city’s 31°C Summer 2018 heatwave. Because it is wholly underground, it suffers neither from solar irradiation nor the portal piston/drag effect. Additionally, unlike most of the London Underground rolling stock, the spare trains in Glasgow are stabled appropriately during the heat of the day, in roofed train sheds with the doors open, which provides a parasol effect.

Mitigating the factors causing this specific overheating problem on the London Underground network really does not need to be difficult, but does require an uncommon solution. Whilst many national over ground trains are already being successfully air-conditioned, with trains underground, the waste heat would be expelled into the overheated tunnels, leading to probable equipment failure. Moreover, any cooled air in the trains would soon be overwhelmed by this overheated tunnel air. Therefore, installing or retrofitting air-conditioning to the entire London Underground train stock, even if feasible, would be wholly unrealistic and ineffective.

In the early days, underground was advertised as the cool spot on hot days
In the early days, underground was advertised as the cool spot on hot days

However, to deal with solar irradiation there is no sensible reason why trains that run both over- and underground should not be coated with a high-performance, solar-reflective paint and fitted with high-performance, solar-reflective glass windows. Equally, trains should be appropriately stabled undercover in the non-peak times if they are likely to be affected by solar heat gain. Some depot buildings may not be very suitable, and it might be better to provide well ventilated shading, built with a finned roof and walls, similar to a Venetian blind design and with the openings facing north to create the parasol effect.

Additionally, limiting the transfer of ambient summer air into the underground network should be addressed. Indeed The Rail Safety and Standards Board (RSSB) recommends that this be considered. On sub-surface lines the piston/drag effect is less marked because of the configuration of the portal openings, the more frequent cross-passages between the two tracks and the less restricted, over-line ventilation shafts and openings. For the deep metro lines it is a different story. Their portals will probably need some re-modelling, perhaps incorporating some form of cooling or diversion or dispersal of the ambient summer air, or a combination of all three.

Clearly, there is a need for appropriate and detailed train temperature monitoring throughout an entire year to dispel the myths and agree upon the real causes of summer overheat. In summary, whilst not confusing causes with effects, we need to be very clear about both the problem and the root causes, the removal of which will resolve or dramatically reduce that problem.

My theory is that, with an over- and underground network, such as the London Underground system, the key factors causing overheating are trains travelling on the overground sections of the network combined with the summer weather. This is supported in a quote from managers of the Vienna Metro in Austria in an e-mail to me: “We appreciate your interest. As a matter of fact, the sun is the main cause for the overheating problem for the U6 [be]cause of the long overground route”.

References

* Since writing this article, author Calvin Barrows has contributed further thoughts on the topic in response to the TunnelTalk article about expansion of metro networks in major cities of Russia that reports one of the main problems of the Moscow Metro is related with its ventilation system which cannot cope with the increase in temperatures during the summer season, and that, due to this, Moscow authorities consider building new stations at depth as currently unreasonable. See the full contribution on the TunnelTalk Feedback page.

           

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