Space to breathe

At 200m long, weighing 319t, fitted with air conditioning, carrying up to 1500 passengers, and designed to sprint between closely spaced stations, the Elizabeth line trains produce a lot of heat.

When the line is running at full capacity there will be up to 24 trains per hour. The braking and traction energy required for the trains to stop at and depart from the stations is released as heat into the tunnel.

Maintaining a comfortable climate for passengers requires constant air movement. Designing and delivering it was not straightforward.

Most stations have two ticket halls, one at either end of the platforms, and many connect with the existing rail or London Underground network. This adds to the complexity of air flow underground. Our aerodynamic and thermodynamic studies considered normal and abnormal operating conditions to understand the airflow in every space – the running tunnels, cross-passages, shafts, platforms and ticket halls.

Circulation is driven by a system of fans in ventilation shafts to supply cool fresh air and extract warm air. Full height platform screens provide a safety barrier between passengers and platform edges, but also serve to control air flow, minimising the scale of the challenge of providing comfort cooling. Passengers transition between well-ventilated platforms and the interiors of air-conditioned trains, while air in the running tunnels – and on the track side of the platform screens – is comparatively warm. Without the screens, that warm air would be drawn by the piston action of the trains through the stations.

Under-platform extract ventilation systems extract warm air from the tunnels around trains stopped at the stations and draw cooler fresh air down through draught relief ducts and through the stations.

There are ventilation shafts where air exchange takes place. But their number and locations were carefully examined as part of a value engineering process, resulting in the removal of eight proposed shafts – a third of the total number – including several which would have caused significant disruption during construction. Their removal was balanced by the inclusion of cross passages to maintain sufficient routes for evacuation from the tunnels. “Reducing the number of shafts reduced project costs without compromising safety. Our client, London Fire Brigade and other design disciplines were all involved in developing and approving the final scheme,” David says.

We used our specialist ventilation analysis software, Mott Tunnels, to determine the optimum arrangement of fans, finalise the 18 vent shaft designs, and ensure exhaust air would not affect people living and working in existing or proposed buildings above.

We also produced temporary ventilation designs for construction so that workers in the tunnels and shafts could safely install track, overhead power lines, communications and signalling, lighting and the permanent ventilation equipment.

The inclusion of full height platform screens improves control of airflows driven by the fans, and by the piston effect of the trains.

David Eckford

Mott MacDonald principal mechanical engineer