The Tube Oven

In our previous exploration, we discovered London’s subway system – the Underground, lovingly called the Tube – features some strange mosquitoes. The scientific oddities of the Underground don’t stop there, however.

The British constructed the earliest tunnels near the surface, but they quickly realized they could produce conduits deeper in the earth. One reason for this shift belongs to the geology of the region. Under the land north of the Thames lies a slew of clay, known as the London Clay Formation. Clay is fantastic for tunnel boring and engineers took advantage of the situation. Builders easily formed the circular chutes that led to the nickname “the Tube.”

In the early days of the system, officials touted the Underground as a place to go on hot days to cool down. Anyone who has a basement knows going below the surface during the summer can offer some relief. As we learned recently, caves offer the same attribute. Many caves remain at constant, cool temperatures throughout the year. Digging an artificial tunnel deep beneath the hot city, coolness could be had in London with a simple subway ride.

Geology of the London Basin; London Clay denoted by dark brown - graphic by Horace B. Woodward

Though the boring through London Clay was relatively painless, engineers still worked for economy. They crafted the tunnels to be just large enough for lines and cars to pass. The hindsight of centuries of civil engineering might make some of the nascent decisions look silly, but the tubes were crafted without extensive ventilation. The creators did not anticipate much of an issue. Trains pushing through the air in the tunnels would account for some atmospheric movement; further, caves are chilly, so who needs to worry about enthalpy?

Well, the engineers forgot about one characteristic of clay. Since time immemorial, humans have employed clay to make ovens. Clay makes tremendous ovens because clay is a tremendous insulator.

You can see where this story goes.

London Clay on the coast - photo by Simon Hugeet

While the subterranean temperature typically dips as one sinks into the crust of our planet, the Underground slowly added heat to the system during the past century and a half. And, contrary to what we might believe about our fellow sweaty humans, the heat isn’t coming from riders. Only 3% of the hotness added to the system comes from people. Trains, on the other hand, produce significantly more. Specifically, trains that rely on friction brakes. 38% of the heat added to the system comes from braking. Mechanical losses add 22% and drive losses produce 16%.

When the British dug the tunnels through the clay, the ambient temperature of the surrounding earth stood at approximately 14 degrees Celsius (57 degrees Fahrenheit). The heat has nowhere to go in the tunnels with poor ventilation but into the clay. The amount of mass around the tunnel can handle a lot of heat input because the amount of mass around the tunnel is rather…massive. That’s why the next statistic is so shudderingly incredible: today, the clay in some of the Underground tunnels now comes in at 26 degrees Celsius (79 degrees)! The amount of heat input to make a change in a mass that large is mind-bending-ly high.

Ride by ride, the London Clay has become a giant heat sink that reached its thermal capacity, which has turned the Tube into a giant oven. Today, many of the routes are not known as places to go to cool off, they are renowned as saunas. The air temperature often hits 30 degrees Celsius (85 degrees Fahrenheit) well below the surface. During the 2006 European heat wave, the air temps reached a balmy 47 °C (116 °F)!

Air coolers in an Underground station - photo by Oxyman

To the credit of the architects, they realized the issue relatively quickly and started to craft tunnels with proper ventilation systems. Plus, many modern remedies have fixed problems in some of the stations and accessible tunnels, including heat pumps, air conditioning, and groundwater cooling.  However, the initial mistake in the deep conduits has proved a conundrum.

Officials deemed digging vents to these tunnels in the 21st century untenable. Real estate is too expensive in London; the construction costs and inconveniences are non-starters. Adding units to the trains to condition air during transport is not feasible, either. They created the tunnels to closely match the size of the trains, so adding any external part cannot occur. In 2003, the mayor of London offered 100,000 pounds to the person who could solve the problems. Despite some fantastic proposals, such as turning the Tube into Venice-style canals with gondolas and dragging gargantuan bags of frozen peas through the tunnels, no viable submission could claim the reward.

Modern engineers believe they have one piece to the puzzle: regenerative braking. Instead of creating heat through friction, regenerative braking transfers energy from slowing down back into the power supply. Hybrid and electric automobiles employ this type of braking. These systems will reduce the amount of heat that enters the tunnels. However, they will not quickly change the temperatures of the clay. Without adding as much heat, the clay can transfer some of the hotness to surrounding soils and the atmosphere, but that dissipation will not happen overnight.

That's hot - image from MyLondon

Whether it’s mosquitoes or unbearable heat, the London Underground can be quite the endurance test. Scientists continue to work on potential solutions.

In the meantime, people still need to transport themselves about the metropolis, so, the next time you’re in London, mind the gap, bring insect repellent, and pack an icy drink.

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