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Tube B, but not Tube A

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technical & practice

Temperatures on London's Tube get people hot under the collar. How to cool commuters down, that is the question London mayor Ken Livingstone, in a rather exuberant flight of fancy, recently announced that he would award a prize of £100,000 to anyone who could come up with an effective way of cooling London's increasingly sweltering Underground network*.

I claim my prize. After all, providing an effective air conditioning system is relatively straightforward - air-con is not exactly a new science.

Constructing new, cooler units with exhaust pipes to dump the heat and inlet air-shafts to compensate for increased ventilation flows is simply a question of investment and greater infrastructural development.

However, as you might have suspected, it isn't as easy as this. London Underground (LUL) has cunningly inserted a clause on what it considers to be the criteria for a workable solution.

'Solutions, ' it says, 'must have economic viability in terms of whole-life costs; ie manufacture, installation, operational, maintenance and decommissioning, ' adding that it must meet all issues of environmental control, including 'all anticipated operating modes of normal, degraded and emergency'.

So the quest is on to find a solution that cools the Tube in its anticipated degraded state. Not, perhaps, the best starting point for a workable brief, but at least it will be cheap.

Winter draws on

As the cooler autumnal months loom into view, air conditioning may seem like a less pressing need, but the Underground still registers exhausting temperatures even in winter.

LUL suggests that estimated average diurnal air temperatures in the tunnels are 25°C in summer and 20°C in winter, although it is reported that the internal temperatures on the platforms can actually rise to be 10°C hotter than the external ambient temperature. Peak temperatures of 37°C were recorded on 15 July this year.

At the moment, on hot days, LUL's consideration for its customers extends to exhortations that passengers wear light clothing and carry a bottle of water. Given this self-help state of affairs, any investment, so the argument goes, can only make things better. With one Israeli firm responding to the challenge by recommending that giant snow machines be built above ground, lateral thinking is the order of the day.

Here we report two diverse submissions from the same engineering office, Hoare Lea, which, we understand, represent the views of the entrants rather than those of their company.

The ice van cometh Nick Cullen's proposal goes back to the principles of early air conditioning. It is recorded that in 1881, the ailing US President Garfield, who had just been shot by an aggrieved attorney, had his sick-room cooled by passing air over several buckets of rapidly melting ice. Sadly, the ice, replaced at a rate of 200kg of ice per hour, doesn't seem to have cooled his fever and he died in the same year.

Cullen updates the same principle and proposes that a cost-effective and simple method would be the coupling of a cooling carriage to each train, which would be used to cool the tunnels and indirectly the platforms and carriages.

Each cooling carriage would hold a block of ice that would be used to offset the heat from the trains and the passengers. The ice would be generated when the trains were running outside the tunnels, to enable heat created in the process to be vented away.

The London Underground Tube network relies on the throughput of outside air to remove the heat generated by trains and passengers. The air is driven by the piston effect of the trains as they run through the tunnels or by fans moving air through station concourses and dedicated ventilation shafts. In turn the trains rely on the air in the tunnels to provide cooling to the individual carriages. As outdoor air temperatures soar, as they have done this summer, the capacity of the outside air to cool the tunnels and platforms diminishes. Consequently temperatures rise both on platforms and within carriages.

This solution would need minimal electrical infrastructural enhancements, as the power for the cooling systems would be readily available in the third rail that provides power to the motor drives. To understand why this would not work in its basic arrangement it is necessary to understand the cooling is generated. When cooling, or 'coolth', is generated a similar amount of heat is also generated, which must be rejected from the machine. If cooling units were installed in train carriages this heat would be rejected to the air outside the carriage. If the train is within a tunnel, the discarded heat would only serve to raise the tunnel and platform temperatures.

The solution must be train-based but not reject heat into the tunnels.

Ice - used in buildings or industry as a means of storing coolth - has the capacity to store large amounts of cooling energy, about 80 times the equivalent volume of water, which it releases while melting. The idea, in this case, would be to use a refrigeration machine, mounted on an additional carriage at the back of a train to generate ice while running outside the tunnel network. Once within the tunnels, the cooling energy stored in the ice would be used to provide cooling either directly to the carriages or more cost effectively to cool the air in the tunnel, from which of course the carriages themselves could obtain their cooling.

Some early computer simulations have shown that the air would be cooled by 10°C. Passengers on a platform would experience a cooling breeze as the train departed the station - a pleasant experience on a warm day.

The cooling carriage is based on known, proven technology and could provide cooling to the tunnel network without any disruption. Initial calculations indicate that about 8 tonnes of ice would be sufficient to provide enough cooling for one 45minute train journey through the below ground portion of the system.

The store, together with a chiller and ice store, could fit on a standard carriage floor plate.

Providing cooling to the Underground uses energy, which in turn will lead to increased carbon emissions. However, by limiting the provision of cooling to times when required, perhaps as little as one month a year, energy is only used when absolutely necessary.

Push me, pull you

Meanwhile, Cullen's colleague Graham Dane argues that the carriages are relatively comfortable in winter only because platform temperatures are more moderate than the summer months. The only successful way to reduce carriage temperatures then, he suggests, is to reduce platform temperatures in a managed way.

'Cooling the platforms will cause them to become havens of coolth, ' he says.

The recognition of the nature of the problem and the understanding of the mechanics of the London Underground system are obviously shared by both Dane and Cullen. Currently - and it is too expensive to change the basic principle of the network - trains passing through the tunnels act like the piston in a bicycle pump. The stairs, escalators and concourse act as pressure release vents for air pushed out from in front of the train; and consequently air is sucked into the tunnel network behind the train.

The enormous thermal capacity of the Underground - a large surface area built of concrete and steel - can absorb a considerable amount of heat with very little rise in temperature. The downside of this phenomenon is that while old Victorian stations have reached their equilibrium temperatures, newer stations such as those built on the Jubilee Line have not. At present, they are less hot in summer than the older lines, but they will become progressively warmer over time.

Furthermore, due to thermal inertia, in winter 200kW of heating is provided by heat released from the mass of the underground structure.

In summer 200kW of cooling effect is given by the heat absorbed by the ventilation effect of the trains moving in the tunnels. The average heat gain of 300kW per station is removed year round by the ventilation effect of the train movement.

Dane's solution is that the carriage temperatures should be as close as practical to the platform temperature.As a humane gesture, he suggests that forced ventilation by means of fans should be increased to limit the internal temperature rise in each car, but the key to his proposal is to introduce chiller pipes into each station platform.

The circulation of refrigerant is the most effective means of transporting heat, but because of the distances and complex routing, refrigerant is untenable and so water, in two 250mm diameter pipes, is, for Dane, the preferred option. The circulation of water is nearly as efficient as refrigerant with the advantage of being able to be carried long distances; it is safer and can be modified/maintained without decommissioning the system.

Placing two pipes at high level along each platform would absorb the heat from the tunnels to be carried out away from the network (to avoid taking heat from one place in the system and dumping somewhere else underground). A rooftop plant room will be required for each station to house the heat rejection equipment, pumping controls and electrical equipment. For a simple station such as Goodge Street, Dane says, this would probably be about 250m 2.For a more complex one, such as Bank, this could be as large as 1,000m 2.Locating this equipment is an important issue for the development of 'air rights' above the Tube stations, and cooling plant rooms will need to be reserved in whatever developments are proposed.

These two schemes show how practically simple is the answer to the problem - requiring only a credible investment plan and a recognition that something's got to change. After all, taking warm air out of the system, or introducing cool air into it, is all that it takes. Ken, it's as simple as that.

Thanks go to Nick Cullen, associate in research and development, and Graham Dane, senior partner, at Hoare Lea Consulting Engineers. Tel 020 7890 2524.

*NB: The competition is now closed.

All ideas herein remain the copyright of the above named entrants

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