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 coal for the fire, air, and water to boil is needed. Good steam generation needed a fire running at 2,500F, and good combustion required:-

• Coal of adequate quality
• A well-managed fire — in general, an even depth of coal with no thin patches, fed “a 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 (i.e. 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 fiercer still. This heats the exterior of the firebox plus 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 up 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 very constricted for space. Compounding  — where steam is used two-, three- or even four times — was tried very early in 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.

But engineering can never be the sole consideration: Management priorities have to be considered. While Mr. Richard Moon  was chairman of the LNWR, shareholder dividends 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 a success and proceeded to build the ‘A’ class , totalling 111 locomotives.