Pneumatic tube transport may seem to have gone the way of steam engines, but its unique characteristics mean not only does it live on, but so does the dream of pneumatic transit.
Picture the scene: a wealthy, high-profile inventor makes headlines in cities across the US, promising to revolutionise transit by using air pressure to convey passenger carriages down a tightly fitting tube at unheard-of speeds.
We’re not talking about a 2013-era Elon Musk touting the hyperloop, but the 1870 creation of New York’s first subway – Alfred Ely Beach’s Pneumatic Transit.
Built in secret, the short demonstration line ran beneath Broadway for just 90 m. A single, richly upholstered carriage seated 22 and fit snugly inside the 2.4 m wide tunnel, propelled along at 16 km/h by a 44 t steam-powered Roots blower. When it reached the end of the line, it tripped a switch, reversing the blower’s baffles and pulling the carriage back. In its first year, it carried more than 400,000 people.
While this pneumatic transit line never took off, it came at the beginning of a golden age for pneumatic transport.
Quick dispatch
The first known pneumatic tube transport system, built in 1853, connected the London Stock Exchange to a telegraph company 200 m away, allowing for timely financial news to be shared and replacing runners who had carried messages between the buildings.
Within decades, pneumatic systems had been installed across the UK, Europe and North America. They used compressed air or vacuums to propel cylindrical carriers through networks of tubes. Their speed and low operating costs made them immensely popular for moving small items – mail, documents, cash, even merchandise – within and between businesses. In Scotland’s Aberdeen, a pneumatic line connected the fish market to the post office to speed the sale of the perishable commodity.
Some at the time were thinking bigger than a few letters. In 1860s London, for example, a few kilometres of tunnels 150 cm wide carried carriages bearing up to 6 t of parcels (and even the occasional passenger) at 50km/h.
The fullest expression of the potential of pneumatic dispatch systems might have been the Poste Pneumatique of Paris.
Construction began in 1866. By 1934, it had expanded to 427 km of route, connecting 130 post and telegraph offices. In 1945, it was handling an estimated 30 million shipments annually. It finally closed to the public in 1984, while a government system stayed in use until 2004.
Route blockers
These systems posed numerous engineering challenges beyond planning and building the system. They included maintaining the system’s airtightness, preventing and locating the leaks that could sap the efficiency and performance of the system.
The most critical challenge, however, was dealing with the blockages that could halt operation entirely. When carriers got stuck, operators would first try reversing airflow to dislodge them. If that failed, many deployed specialised, markedly heavier carriers to shunt them free.
For more stubborn obstructions, one solution was to fire a pistol near the tube opening. A recording cylinder and chronograph were used to precisely time the returning echo, allowing operators to pinpoint the obstruction’s location before physically accessing the tube.
Hospital efficiency
While pneumatic transport systems have fallen from public view, overtaken by newer technologies, they remain popular in a surprising number of niches. The most widespread is probably health care. Most major hospitals still use pneumatic transport systems, facilitating rapid and hygienic movement of pathology samples, medication, documents and blood products.
In 2016, the newly built Royal Adelaide Hospital unveiled the southern hemisphere’s largest pneumatic tube hospital system. The network serviced by its 72 stations, features many advancements missing from pneumatic transport’s golden age, including real-time carrier tracking, powered by radio-frequency identification.
Pneumatic tube systems are also still often found in banks and casinos, where the technology allows for highly secure movement of cash and other valuables.
The broadest application of pneumatic transport systems do away with canisters altogether. Pneumatic conveying systems are found throughout the materials handling world, popular for moving dry-bulk materials and powders, from sugar to mineral ore to cement. Some of these systems can easily transport upwards of 60 t an hour while minimising dust and spillages.
There are even a few examples of pneumatic garbage collection, including on New York’s Roosevelt Island.
Pipe dreams
Almost exactly 150 years after New York’s Pneumatic Transit line carried its last passenger, the concept got a high-pressure boost when Elon Musk published a white paper proposing a so-called hyperloop.
The paper described an ultra high-speed passenger and freight transportation system, composed of capsules in low-pressure tunnels, kept at 1 hectopascal – less than 1/1000th of sea-level air pressure. Friction would be minimised by using air bearings. Later iterations of the concept proposed electromagnetic propulsion.
In the following decade, proposals to plan and build hyperloop routes emerged everywhere from the US to South Korea to India to Norway. Backers argued the technology offered numerous advantages over high-speed rail, including speeds potentially near-sonic, reduced energy demands and simpler construction. However, despite countless press releases and promotional mock-ups, not even a full-scale prototype has yet been demonstrated.
The challenges that have dogged these projects range from regulatory – getting approval to build dead-straight tunnels between major population centres is no small task – to technical.
“A steel tube on pylons,” wrote Canadian Professor Emeritus Vaclav Smil, “would have to be engineered to maintain the thousandfold pressure difference between its inside and outside walls that threatens to crush it, and it would have to do so reliably along hundreds of kilometres of the track while also supporting the pressure generated by the rapidly moving pods and coping not just with overall thermal expansion along its course, but with the differential thermal expansion between the tube’s top and bottom.”
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