Hyperloop - The Train-In-A-Vacuum-Tube Fantasy (4)

The Interior Requirements

In the interior of every public transport vehicle it is necessary to provide passengers with an environment with standardised air characteristics:
- air temperature 21 to 23 °C,
- air pressure roughly 100 kPa,
- changes of air pressure below 1 kPa/10 s,
- relative humidity 40 to 60 %,
- oxygen concentration 21 %,
- carbon dioxide concentration of 0.1 to 0.2 %.

However, on account of their individual corporal metabolisms, passengers will degrade these characteristics by:
- producing 90 W of dry heat,
- producing 30 W of moist heat (evaporating 0.05 litres of water/h in the form of sweat and exhaled water vapor),
- consumption of 15 litres of oxygen per hour,
- producing 20 litres of carbon dioxide per hour.

Air conditioning and air exchange systems must provide good air quality in the vehicle’s passenger accommodation in the vacuum environment of the tube. Not only for the duration of the journey, but also for the time needed for evacuation.

If current comfort standards for public transport vehicles are to be met within Hyperloop vehicles, it would be necessary to provide about 20 m³ of fresh air per hour for each passenger. The air humidity can be removed by cooling it below the dew point, but the limiting issue is carbon dioxide removal. The requirement of 20 m³ of fresh air per hour per person is motivated by the assumption that a person expelling 20 dm³/h of CO₂ would not increase the CO₂ concentration in the interior by more than 0.1 %.

It would be very problematic to design a vacuum tube vehicle capable of carrying such large quantities of air to meet these requirements. Even supposing the air were compressed to 1 MPa, this would still result in a requirement of 2 m³ per hour per person. By way of comparison, the main air reservoirs of locomotives have a total air volume of about 1 m³. 

However, it is not only a question of supplying sufficient fresh air. It is also necessary to store the air exhaled by passengers somewhere - in a similar manner to the waste retention tanks for train WCs. The theoretical possibility of storing exhaled air in special tanks again runs against the difficulty of providing such tanks on relatively small capsule on account of their dimensions and weight. It also does not seem rational to expel exhaled air from the vehicle into the tube, and then have to use energy to extract it from the vacuum to maintain the latter. Thus air management will be a very difficult theme to solve to enable the transport of passengers in vacuum tubes.

In addition to the air supply, each vehicle must have in its batteries an adequate store of electricity to power the air conditioning, lighting and other on-board services. Air conditioning is necessary not only for removing body-generated heat from the interior, but also for removing heat generated by technical equipment or heat penetrating through the walls of the vehicle. Since Hyperloop technology makes use of magnetic levitation (maglev) and linear electric traction drives, the issue of the effect of very powerful magnetic fields (EMC) on passengers must also be considered. This all involves much demanding, safety-relevant technical research.

Emergency Evacuation

Following a number of serious fires taking place in tunnels, both rail and road, towards the end of the 20th century, involving great losses of life, a series of rigorous and consistent safety measures have been introduced, affecting both new and existing tunnels, to ensure that emergency evacuations can take place as safely as possible and that fire protection systems are as efficient as possible. 

The measures taken are summarised under the EU technical standard EN 45 545, and cover issues such as fire resistant materials, fire detection and extinguishing systems, areas for rescue services, ventilation, information systems to assist evacuees, distances to emergency routes in vehicles and tunnels, provision of cross-galleries between bores, and much more besides.

Under the TSI SRT (Safety in Railway Tunnels) standard tunnels are classified according to length - Group A being under 5 km and Group B under 20 km, and rail vehicles are graded according to duration of fire resistance - Group A being four minutes, Group B, 15 minutes. A number of other design parameters are also taken into account. These influence design complexity and also the final cost of both the tunnel infrastructure and the vehicles which will use the tunnel. It is therefore logical and prudent that vacuum tube transport systems should also be subjected to a set of equivalent safety legislation measures.

Unless the design of a vacuum tube transport system, such as the Hyperloop, incorporates measures enabling occupants to evacuate a vehicle and the vacuum tube itself in the event of an emergency (such as a fire or a technical failure), it would be unrealistic to attempt to go ahead with a safety analysis of this transport system and ask state authorities to approve of the scheme, or to offer technical supervision. But creating practical escape routes from a vacuum tube would not be an easy matter.

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