steam

Part 5 - Boilers

Water retains more heat energy per pound than most other materials and when the temperature increases to 100 degrees Celsius (at sea level) water absorbs more energy (latent heat) as it is evaporated into steam. As more heat is added the temperature remains constant until all of the water is converted into steam. In a closed container (boiler) more heat increases the pressure and the temperature of the steam.

Early boilers, built for the first steam engines, were simple 'kettles', often a dome shaped tank with a flat bottom and a hemispherical top supported on a brick fire-box. They were partially filled with water and heated by a fire underneath. They were not sealed so the steam pressure was no greater than atmospheric pressure and they were not insulated so most of the heat was wasted.

Richard Trevithick adapted one of the first fire tube boilers (the Cornish Boiler) for a pumping engine a the Dolcoath mine Cornwall, England, in 1812, and doubled the efficiency of the Bolton and Watt steam engine. In the same year he installed another for a a threshing machine on a farm at Probus, Cornwall, powered by a steam engine. It was less expensive to operate than the horses it replaced.

The Cornish boiler was a horizontal cylindrical tank of water (the strongest practical shape for a pressurized container) constructed from curved steel sheets rivetted together to form cylinder 4 ft. (1.25 m) to 6 ft. (1.75 m) in diameter and 13 ft (4 m) to 21ft. (7 m) long. Inside and running the length was a cylinder (or flue), typically 0.6 times the diameter of the cylindrical shell, forming the furnace tube or firebox. It could produce steam at the rate of 6500 kg/hr and could withstand a maximum steam pressure of about 150 psi (1000 kPa).

The Lancashire Boiler was a big improvement, capable of producing steam at the rate of 9000kg/hr at a similar steam pressure. The design was attributed to William Fairbairn in 1844, although Stephenson's locomotive "Lancashire Witch" had used a similar boiler in 1829. 

 It was similar to the Cornish boiler but instead had two furnace tubes (flues) thereby increasing the area of the flues heating the water. A spring-loaded safety valve was mounted on top of the boiler to release steam if the pressure rose above a safe working pressure. A stop valve, regulated the steam between the steam pipe and boiler and a low water safety valve released steam if the water level fell below the top of the furnace tubes.

A "Scotch" marine boiler was a fire-tube boiler, developed from the Lancashire boiler, and it used several furnace tubes to give greater heating area for a given furnace capacity but it was also fitted with many small-diameter fire-tubes (each typically 3 or 4 inches [76 or 102 mm] diameter ) further increasing the heating area. 

The Scotch marine boiler was used with most steam engines until the 20th century, particularly for the most advanced piston engines like the triple-expansion engines used on the Titanic. It was finally replaced by high-pressure water-tube boilers such as the Yarrow used, with steam turbines, especially on warships.

The Titanic had 29 Scotch-type boilers, with a total of 159 coal burning furnace tubes suppling steam at a pressure of 215 pounds per square inch (1,480 kPa) to the engine room. The larger boilers were 15 feet 9 inches (4.80 m) diameter and 20 feet (6.1 m) long.

Clean water was essential to reduce the rate of boiler corrosion. Any dissolved minerals accumulated in the boiler and not only increased the rate of corrosion but, if they crystalized and were carried over with the steam, they could damage the cylinders. 

Stationary boilers were usually supplied from with water from air cooled towers where the exhaust steam was condensed. Some of the water was lost to evaporation but it was purer than the local water supply. 

Mobile steam engines did not typically carry condensers, except in areas where water was not readily available. Exhaust steam was simply released to the atmosphere and wasted. This meant that the train had to refill its water tank periodically from rail-side water tanks supplied with municipally treated water or the cleanest water from rain, rivers or streams. 

On ships, the exhaust steam was passed through sea water cooled, condensers (heat exchangers). Periodically, to reduce corrosion, the hot mineral-concentrated water was deliberately blown down (wasted). However, over time, corrosion eventually degraded the margin of safety and the boiler failed a pressure test and was repaired or replaced. Otherwise, under pressure, the boiler could fail explosively. And, in the early days of the steam age, many did.

 1948 explosion