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
- 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
- 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.
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.