Boiler and Firebox

The source of power in the steam locomotive is the fire. There is nothing else. No electricity is collected from outside - only air, coal for the fire and water to boil are needed. Good steam generation needed a fire running at 2,500°F, and good combustion required:

• Coal of adequate quality
• A well-managed fire - in general, an even depth of coal with no thin patches, fed "little and often"
• Good ventilation through the grate with just enough 'top' or secondary air through the fire hole - too little would create black smoke
• A strong blast - by a correctly sized blast-pipe directing used (exhaust) steam up the chimney. Any driver worth his salt knew that a 'jemmy' (iron bar) placed over the blast-pipe reduced its cross-sectional area, so intensifying the blast.

A boiler converts the heat of the fire into steam via the heating surface: as the blast at the front end of the engine pulls air and gases from the firebox through the tubes, the fresh air pulled through the grate makes the fire burn more fiercely. This heats the exterior of the firebox and the tubes running through the boiler, all surrounded by water. Some heat is lost to the outside of the boiler and at the front and back but only the barrel can be lagged. A large 'heating surface' was desirable. Increasing the number of tubes, increasing the diameter of the boiler, or its length, all made this figure greater.

Compounding

In the steam locomotive, steam produced by the boiler produces power in the cylinders, forcing the pistons repeatedly fore-and-aft but then it is normally released to atmosphere via the chimney. It is common sense that if the steam could be used more than once, the engine would be more efficient. Using less fuel - coal and water - was always an important objective. But the steam locomotive is restricted for space. Compounding - where steam is used two, three or even four times - was tried very early on for stationary engines, but on moving locomotives, which shake and vibrate and have to fit through tunnels and bridges - it became a Holy Grail which was rarely achieved with success.
Engineering can never be the sole consideration - management priorities have to be taken into account too. While Mr. Richard Moon was chairman of the LNWR, dividends for shareholders were paramount. Maximum economy in running the company was the watchword controlling progress in development. Passenger trains of the time averaged only 40 mph and for goods engines, the greatest fuel economy was sought.
Having built passenger engines on the compound principle since 1880, in 1893 Mr. F.W. Webb experimented with a 'simple' (single expansion) heavy goods engine and a 3-cylinder compound He considered the trials to be a success and proceeded to build the 'A' Class, totalling 111 locomotives.