vol 8, issue 11

Puffing Devil rebuilt

11 November 2013
By Roger Dettmer
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Trevithick’s Puffing Devil

Richard Trevithick’s Puffing Devil represents a giant surge forward in transport - but comparatively few recognise it

An engineer works on the Puffing Devil replica

Unpaid enthusiasts worked on a replica of the Puffing Devil

Engineers work on the Puffing Devil replica

A total lack of working drawings was one of the main challenges for 21st-century engineers

The modern Puffing Devil

The modern Puffing Devil made its maiden outing at the Trevithick Day parade in 2001

The rebuilding of Richard Trevithick's Puffing Devil celebrated a seminal event in the history of transport.

Every year, in April, the people of the Cornish town of Camborne celebrate Trevithick Day. Named in honour of local engineering hero Richard Trevithick, it's a bit of a party. There are fairground rides, entertainers, male-voice choirs, brass bands, troupes of dancers threading their way through the local streets and, best of all, a parade of historic steam engines.

Pride of place in this parade goes to a replica of Trevithick's revolutionary 1801 steam engine, the Puffing Devil. It may not look much, just 12ft long, mounted on a wooden chassis and running on wooden wheels with steel tyres, nevertheless the Puffing Devil represents a giant step forward in the history of transport. The Wright brothers and Kitty Hawk, the site of their first manned flight, are justly famed in the history of aviation; Richard Trevithick and Camborne deserve equal recognition for their role in the development of land-based transport.

Growing up

Richard Trevithick was born on 13 April 1771. At this time Cornwall was the world's most important centre for the deep mining of metals, mostly copper and tin, and Trevithick's father was a leading mine captain or manager. A far-from-model schoolchild, but with a considerable talent for mathematics, Trevithick's real education came from living in the midst of one of the most heavily industrialised areas in Britain. At the age of 15 he was working with his father, and at 19 was appointed engineer at a local mine.

Cornish mines were notoriously wet, and the unending task of pumping the deep workings made Cornwall home to the world's greatest concentration of steam engines. The engines were absolutely vital to the operation of the mines, but the owners and operators were far from happy with their lot. Trevithick grew up surrounded by steam engines, and almost inevitably became embroiled in the fraught and contentious issues of engine operation and development.

Newcomen and Watt

The steam engines operating in Cornwall at this time were of two basic types: the Newcomen engine, introduced in Cornwall in 1720, and the Watt engine, introduced in 1777. Strictly speaking, the Newcomen engine is an atmospheric engine as it's the pressure of the atmosphere that drives the vertically mounted piston downwards. In the 'rest position' the piston is at the top of the cylinder and the working cycle begins with steam, at more or less atmospheric pressure, being introduced into the space below the piston. A jet of cold water is then directed into the cylinder, condensing some of the steam and creating a partial vacuum. The piston is then driven downwards by the net pressure acting on the top face of the piston. The Newcomen engine was mechanically crude, and with a thermal efficiency of around 1 per cent consumed prodigious amounts of coal – a key issue for Cornish operators who were wholly reliant on imported coal.

In 1763, James Watt, at the time working as an instrument maker at Glasgow University, was asked to repair a model Newcomen engine. Even when repaired the model engine barely worked, inspiring Watt to introduce a number of changes that, together, led to a radically improved engine.

The experience of working with a small model engine with a low thermal capacity made Watt very conscious of the problem of heat loss. This was a key issue as the cylinder had to be hot in order to fill the it with steam at the start of the working cycle. Heat was continuously leaking from the walls and top of the cylinder and, crucially, it was being repeatedly cooled by injections of cold water.

Watt's most famous modification to the Newcomen engine was to condense the steam in a water-cooled condenser, connected to the cylinder by a length of pipe and a control valve. He also closed off the top of the cylinder (further reducing heat loss) using steam, at around atmospheric pressure, to provide the downward force on the piston.

Watt's approach certainly led to a dramatic improvement in thermal efficiency – early Watt engines operated at around 2.7 per cent, and subsequent developments increased this to as much as 4.5 per cent. However, to fully understand the source of these efficiency gains you need to focus on what's going on in the cylinder, as opposed to the more obvious issue of heat loss.

The basic reason why the Watt engine is around three times more efficient than the Newcomen engine is because the use of a condenser results in a much better vacuum. Newcomen engines were typically operated with a partial vacuum of around 7.5lbs per square inch. The use of a separate water-cooled condenser enables the Watt engine to achieve rapid and effective cooling of the steam water-vapour mixture from the cylinder, resulting in a partial vacuum of about 1lb per square inch. So, while a Newcomen engine operates with a pressure difference across the top and bottom of the piston of around 7lbs per square inch, in a Watt engine the pressure difference is typically twice this figure. As both engines worked at about the same speed, this means that a Watt engine can deliver twice the power of a Newcomen engine with the same size of cylinder.

Understandably, Cornish mine operators were extremely keen to adopt the more efficient Watt engine. There was, however, a problem. Watt and his business partner Matthew Boulton charged users a levy equivalent to one-third of the cost of the coal saved over a Newcomen engine of equivalent power. The levy, along with the rigorous enforcement of the Watt patent, grew to be deeply resented by Cornish mine operators and engineers, leading to repeated engineering experiments designed to circumvent the patent. Trevithick became embroiled in many of these experiments and, along with several of his fellow countrymen, experienced the litigious wrath of Boulton and Watt.

Upping the ante

Watt is famous for his condenser. Trevithick's great claim to fame is his advocacy of 'strong', (i.e. high-pressure) steam, along with the design of a boiler that made high pressures possible. Watt was implacably opposed to this development, which he considered foolhardy and dangerous, famously declaring that Trevithick should be "hanged" for his efforts.

For a steam engine with a cylinder of a given size and stroke rate, the power output is determined by the net pressure acting on the piston. Watt's condenser was a valuable innovation because it took the pressure on the underside of the piston – opposing the downward motion – close to the realisable minimum, i.e. a vacuum.

This was a very useful step, but with a near vacuum achieved no further progress was possible. Trevithick's boiler was a move in the opposite direction – increasing the pressure on the topside of the piston – with progress limited solely by the capabilities of available boiler technology.

Prior to Trevithick, steam boilers were little more than glorified kettles. They had a flat base in contact with the fire and operated at around atmospheric pressure simply because they were incapable of withstanding anything higher. Trevithick designed a cylindrical boiler for inherent strength and, lacking a flat base, placed his fire and flue directly inside the boiler. Use of a U-shaped fire tube maximised the surface area of the flue in contact with the water in the boiler. This complex shape was formed from riveted sections of wrought iron, and would have tested contemporary manufacturing techniques to the limit.

The invention of his high-pressure boiler meant Trevithick could dispense with Watt's condenser to produce an efficient, compact, relatively cheap steam engine that evolved into a key power source of 19th-century industry. In the absence of a condenser Trevithick's new engine was vented directly to the atmosphere, and soon acquired the soubriquet 'puffer' from the sound of the exhaust gases pulsating up the chimney.

The first of Trevithick's high-pressure engines, working at 25lbs per square inch was installed at Cook's Kitchen mine near Camborne in 1800. It was still running 70 years later.

Puffing Devil

The puffer was so powerful and so compact that Trevithick was convinced it had the potential to propel itself. Working with a group of skilled friends and relations in Camborne, Trevithick set out to realise his dream of motorised transport. Construction began in November 1800, and by Christmas Eve 1801 the 'Puffing Devil' – with a boiler operating at 47lbs per square inch – was ready for its first run.

Trevithick, and possibly as many as eight other passengers, clambered onboard and, with night approaching and the rain coming on it, they set off. There's some uncertainty as to their exact route, but the general consensus is that they headed towards the centre of Camborne, along the modern Tehidy Road, up the steep stretch of road that is Fore Street, before stopping to turn round when the engine began to run short of steam.

This was a seminal event – the first demonstration of self-propelled vehicle as a practical mode of transport. Earlier examples of steam-powered vehicles, notably Cugnot's three-wheeled stream dray, were hopelessly impractical and essentially technological dead ends.

Recognition

Trevithick was just 29 when the Puffing Devil made its historic first run. His later engineering innovations were many, if not always wholly successful. Among his many subsequent achievements were an 1803 steam carriage that ran on the streets of London – sadly a commercial failure – and a locomotive designed for the Penydarren Ironworks in South Wales. In 1804, this successfully hauled a 10-tonne load along the nearby Merthyr Tydfil Tramroad – forming the world's first steam-hauled train, 25 years ahead of Stephenson's Rocket.

In 1816, Trevithick left Falmouth to repair his engines that had previously been sent to the silver mines in Peru. After many adventures and misfortunes he returned penniless in 1828. By a remarkable twist of fate, he was helped on his way home by a gift of £50 from Robert Stephenson of Rocket fame, whom he'd met quite by chance in Cartagena. The last years of his life were spent at J&E Hall Ltd in Deptford, where he designed a 150lb per square inch boiler.

Trevithick died a poor man on 22 April 1833 and was carried to an unmarked grave by his Hall's workmates. The contrast to James Watt, who died a wealthy man and now lies comfortably within the walls of St Mary's Church Handsworth is almost painful.

As an engineering innovator, Trevithick was every bit the equal of Watt. His personal misfortunes were, it could be said, largely self-inflicted, but his relative obscurity compared with Watt is a historic injustice. Thankfully, Trevithick's revolutionary role as the father of the steam train and portable power is becoming ever more widely recognised. In Camborne, it was never forgotten.

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The replica project

The Puffing Devil replica project was conceived as a fitting way to commemorate Richard Trevithick's 1801 achievement. It was organised under the auspices of the Trevithick Society, a registered charity focusing on the study of industrial archaeology in Cornwall.

It is unlikely that there were ever any working drawings of the original Puffing Devil, and the replica design was carefully developed from the 1804 Hazeldine Puffer, a stationary Trevithick engine, in the Science Museum, London.

The bulk of the funding came from an allocation of £40,000 from a Regional Development Fund, with contributions from the local council and private donations.

Work began in earnest in November 1999. After a huge collective effort, the Puffing Devil was ready to lead the Trevithick Day parade for the first time on 28 April 2001, where it successfully chugged and puffed its way up Fore Street.

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