London Underground
Ventilation and cooling

When the Bakerloo line opened in 1906, it was advertised with a maximum temperature of 60 °F (16 °C), but over time the tube tunnels have warmed up. In 1938 approval was given for a ventilation improvement programme, and a refrigeration unit was installed in a lift shaft at Tottenham Court Road. Temperatures of 117 °F (47 °C) were reported in the 2006 European heat wave. It was claimed in 2002 that, if animals were being transported, temperatures on the Tube would break European Commission animal welfare laws. A 2000 study reported that air quality was 73 times worse than at street level, with a passenger inhaling the same mass of particulates during a twenty-minute journey on the Northern line as when smoking a cigarette. The main purpose of the London Underground's ventilation fans is to extract hot air from the tunnels, and fans across the network are being refurbished, although complaints of noise from local residents preclude their use at full power at night.

In June 2006 a groundwater cooling system was installed at Victoria station. In 2012, air-cooling units were installed on platforms at Green Park station using cool deep groundwater and at Oxford Circus using chiller units at the top of an adjacent building. New air-conditioned trains have been introduced on the sub-surface lines, but were initially ruled out for the tube trains due to space being considered limited on tube trains for air-conditioning units and that these would heat the tunnels even more. The New Tube for London, which will replace the trains for the Bakerloo, Central, Waterloo and City and Piccadilly lines, is planned to have air conditioning for the new trains along with better energy conservation and regenerative braking.

In the original Tube design, trains passing through close fitting tunnels act as pistons to create air pressure gradients between stations. This pressure difference drives ventilation between platforms and the surface exits through the passenger foot network. This system depends on adequate cross-sectional area of the airspace above the passengers' heads in the foot tunnels and escalators, where laminar airflow is proportional to the fourth power of the radius, the Hagen–Poiseuille equation. It also depends on an absence of turbulence in the tunnel headspace. In many stations the ventilation system is now ineffective because of alterations that reduce tunnel diameters and increase turbulence. An example is Green Park tube station, where false ceiling panels attached to metal frames have been installed that reduce the above-head airspace diameter by more than half in many parts. This has the effect of reducing laminar airflow by 94%.

Originally, air turbulence was kept to a minimum by keeping all signage flat to the tunnel walls. Now, the ventilation space above head height is crowded with ducting, conduits, cameras, speakers and equipment acting as a baffle plates with predictable reductions in flow. Often, electronic signs have their flat surface at right angles to the main air flow, causing choked flow. Temporary sign boards that stand at the top of escalators also maximise turbulence. The alterations to the ventilation system are important, not only to heat exchange.
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