steam

Part 6 - Locomotives

In 1784, William Murdock demonstrated his steam driven carriage to his neighbour Richard Trevithick and, almost immediately after Isaac Watt's patent expired in 1800, Trevithick and Oliver Evans independently introduced high-pressure steam engines. 

On 21 February 1804, Trevithick's steam locomotive hauled the world's first railway train along the tramway between Merthyr Tydfil and Abercynon in South Wales, Britain.

Trevithick adapted the Cornish boiler and later used the more efficient Lancashire boiler and, to increase efficiency, the hot gases leaving the central flues were routed through a brick chamber encasing the underside of the boiler then back again along the boiler sides to the tall chimney pipe. The brickwork and other insulation minimized the heat lost to the atmosphere.

George and Robert Stephenson's locomotive, the Rocket, was selected for the proposed Liverpool and Manchester Railway in Britain at the Rainhill Trials, in 1829, where it averaged 12 miles per hour (19 km/h), with a top speed of 30 miles per hour (48 km/h) while hauling 13 tons.

The Rocket's cylindrical boiler was a water filled tank that, in addition to a single large flue, contained a large numbers of small-diameter fire-tubes which greatly increased the surface area for heat transfer. The heat, thermally conducted through the walls of the tubes, produced steam at a much higher rate than Cornish boilers and the increased efficiency revolutionized land and sea transport.The hot gases, directed through the fire-tubes, generated saturated ("wet") steam that rose to the highest point of the boiler, typically a steam dome. For increased efficiency, the boiler was insulated to reduce heat loss and exhaust gases were used to pre-heat the boiler feed water. Engineers quickly found that injecting the exhaust steam from the cylinders into the smokestack, through a blast-pipe, eliminated need for tall chimneys (smokestacks), typically used on early locomotives. As the exhaust steam condensed, the partial vacuum sucked air through the firebox so the draft increased automatically as the engine used more steam.

A blower fan provided a draft when during start up, as heating cold water in a boiler could take hours. The blower was also used when the locomotive was stopped or coasting, when there was no exhaust steam to create a draft and maintain steam pressure. Steam not only drove the locomotive but it also powered air compressors for the brakes, the pump that supplied water to the boiler, the passenger car heating system and the whistle. In 1837, Robert Davidson built the first electric locomotive and in 1864 the District and Metropolitan Railways was established to complete an underground railway in London. The first part of the line used gas-lit wooden carriages hauled by steam locomotives and it was 1907 before the underground sections of their lines were electrified. 

Stoking on a steam locomotive was hard work. The run from London to Edinburgh, Britain, was about 400 miles and, at 60 mph (96 km/h), a good driver would burn about 10 tons of the best quality coal, which meant that the fireman had to shovel coal from the tender into the firebox at the rate of about 56 pounds (25 kg) each minute for more than 6 hours. 

As steam locomotives seldom carried condensers, exhaust steam was vented to the atmosphere and so they needed a constant supply of fresh water. Clean water was essential, as dissolved minerals accumulated in the boiler and, if they crystalized and were carried over with the steam they could damage the cylinders. To prevent this and reduce boiler corrosion, the hot mineral-concentrated water was periodically blown down (wasted). However boilers had to be repaired or replaced when they failed a pressure test. Otherwise, under pressure, the boiler could fail explosively.