Cooling the Tube

London Underground (LU) now carries more than a billion passengers a year, and the number is rising. But despite conveying more people than ever in its 153-year history, it has only recently begun to make great strides in addressing an age-old problem - uncomfortably high summer temperatures in the portions that lie deep beneath central London. 

It might seem academic for much of the year (particularly in early February), but the punishing heat and misery that comes with using the Tube in high summer has become an unavoidable annual ritual for its users. 

Vivid memories of being hot, sweaty and crowded on heavily loaded Tube trains during heatwaves are indelibly printed on the minds of many of the capital’s commuters, driving hordes of them above ground when temperatures peak between June and August.

On July 1 2015 the mercury rose to 35°C on the Northern Line, 34°C on the Central Line, and 32°C on both the Piccadilly and Victoria Lines. Unfortunately this is not an isolated example, and for several years LU has been in the habit of offering advice for passengers on how to keep cool.

Cases of heat exhaustion are regularly recorded - notably in July 2001, when more than 600 people required treatment after three trains were halted for 90 minutes on the notoriously hot Victoria Line.  

Yet the effect of hot weather is by no means evenly distributed on the network. The Victorians built the first sections of the Tube just below street level, with ample ventilation and large tunnels to accommodate steam traction. 

As one of the original stations constructed in 1863, Baker Street is an obvious example, dating from a time when sub-surface lines were advertised as a cool refuge to escape the hot summer sun. In 2016, users of these cut-and-cover lines still benefit from airy platforms and the tunnels’ shallow proximity to the outside world, as well as from the 191-strong air-conditioned S-Stock fleet rolled out since 2010. 

The origins of today’s deeper-seated problems date to 1890, when the first deep tunnels were built under the Thames from Stockwell to the City, on what is now known as the Northern Line. More tunnels bored at depths below 20 metres followed over the next century - on the Bakerloo, Central, Victoria, Waterloo & City, Jubilee and Piccadilly lines - to form today’s Deep Tube network.

Temperatures were initially cool, and matched the ambient heat of the earth surrounding the tunnels (around 14°C in 1900). But that was before millions of passengers and a service frequency unimaginable to Victorian and Edwardian planners were added to the mix. 

Unbeknown to 19th century engineers, up to 79% of energy dissipated by trains, people and infrastructure is transferred to London’s native clay around the tunnel bores - thus the temperature of this giant heat sink has slowly climbed to today’s balmy average of 20-25°C. 

Without the valuable gift of hindsight, LU’s early engineers failed to provide adequate ventilation to dissipate this heat, and mid-tunnel shafts were either too few in number or non-existent.

Modern metro systems and tunnelling projects have learned from London’s mistakes, building higher ceilings and more ventilation shafts, and thus making LU’s oldest parts a curious international anomaly. Crossrail’s tunnels have been built at depths of up to 40 metres, and will use under-platform exhausting whereby cool air is blown across traction and braking equipment of trains when they arrive at stations, then drawn out along the underside of the platform. 

Implementing LU’s efforts to overcome the unique challenge of cooling the oldest underground railway system in the world is Programme Director for Infrastructure George McInulty and his 35-strong cooling project team based on London’s Buckingham Palace Road. It is budgeted to spend over £200 million removing heat from the network over the next ten years, to protect customers and sensitive electronic equipment from any further rise in temperature.

According to McInulty, increasing the capacity of existing ventilation tunnels is by far the cheapest and the easiest solution available to LU. But constructing shafts where they currently do not exist is not as easy as it sounds, in either financial or engineering terms. Some of the deepest tunnels lie 60m below the surface, and the supply of land is a finite resource in central London.

“Our tunnels will always be too deep and too small,” he explains. “We can increase the capacity of shafts, and we did this during the upgrade of the Victoria Line in 2011/12. But we were lucky that it was only built in the 1960s, with half an eye to improving ventilation at a later date and introducing a service of over 30 trains per hour. Elsewhere we’ve been less lucky, and have had to look at retrofitting stuff where we can.”

A further problem has arisen from building the earliest tunnels with only enough room for trains. Decades later, air-conditioning could neither be retrofitted on existing stock (due to lack of space within or on the outside of carriages), nor could the new S-Stock models fit in the tight tunnels.

LU’s New Tube for London (NTfL) programme promises to partially alleviate this, with air-cooling a key requirement of the 250 new trains that will be ordered for the Piccadilly, Waterloo & City, Bakerloo and Central lines in autumn 2017 (see Bombardier special, page 41). LU issued its Invitation To Tender on January 18, and this next generation of trains will enter service in the mid-2020s. 

However, McInulty says this can only ever be a partial remedy, as the basic mechanics of air-conditioning mean that heat energy will simply be removed from the carriages and displaced to the narrow tunnels, warming them even further. Ventilation efforts must therefore continue if this heat energy is to be dissipated. 

Conventional air-conditioning will also struggle to cope should a train come to rest inside a tunnel, making things worse by consuming more energy in the absence of fast-moving air. 

Despite the constraints, McInulty’s team has already scored some success by retrofitting ventilation and other cooling devices. Temperatures have thus been reduced in some problem areas, although these projects are bespoke in nature and will only ever cool small parts of the network. 

For example, a disused lift shaft at St Paul’s presented a perfect opportunity to install a new fan in September 2015. The system pulls air from the street and cools it using a high-capacity chiller system that circulates 16 litres of cold water every second around pipes in the ventilation shaft. This cools the air entering St Paul’s eastbound platform on the Central Line by up to 7°C. 

An even larger fan chiller, using the same technology, will be installed this summer at a mid-tunnel ventilation shaft on the Victoria Line between Walthamstow Central and Blackhorse Road stations.  

A more innovative project took place at Victoria station in 2006, using ground water from the nearby River Tyburn. Pumped through pipes in the station, the water is warmed up and carried away. However, the availability of sufficient ground water is only present at a handful of sites, restricting its scalability.