The two types of
coal in use today are Anthracite (Welsh Coal) and Bituminous Coal. Bituminous
Coal is the more common although its composition varies from pit to
pit and seam to seam. The crew of the locomotive must adapt to the type
of coal being used to achieve the best results.
Combustion requires
Heat, Air, Space and Time. In the 1860‘s the Midland Railway introduced
several features which resulted in improved combustion within the boiler.
These included the brick arch (which increased the space available for
combustion by extending the path of the volatile gases), the rocking
grate and the fire hole door (improved air flow into the firebox).
Air is admitted
into the firebox from two main points. Primary Air (Bottom
Air) is admitted through the grate and Secondary AIr (Top
Air) enters through the firehole door. There is also a third source
of air known as Supplementary Air which travels through
the gaps around the fire doors.
An
even fire built up and well burned through (left) would give good results,
maintaining steam at high pressure whilst the engine is being heavily
worked. Air is being admitted through the front damper and firehole
door. The smoke at the chimney should be a light grey showing that the
correct amount of air is being mixed with the volatile gases resulting
in complete combustion. No smoke at the chimney indicates too much air
and the coal is burnt through. It is time to fire again. However, too
much coal will result in smoke.
Smoke does not make Steam!
Black
smoke indicates a lack of air and unburnt volatiles escaping to atmosphere.
A fire of the kind shown (right) gives very bad results in maintaining
steam, causes a great loss of heat and much smoke! The large mound of
coal is cooling the bed of the fire and prevents an even flow of air
through the fire grate.
The
size of coal also effects combustion. If coal is broken into pieces
no larger than a man‘s fist, then a greater surface area is exposed
and the fire burns better.
Here, an example
of bad firing is shown (left). The fire is built up under the brick
arch and the area which is normally the hottest part of the fire is
the coolest. Air cannot flow easily, the brick arch is cooler than it
should be and its effectiveness as a radiant is impaired. Good steaming
results cannot be obtained.
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An
even fire built up and well burned through (left) would give good results,
maintaining steam at high pressure whilst the engine is being heavily
worked. Air is being admitted through the front damper and firehole
door. The smoke at the chimney should be a light grey showing that the
correct amount of air is being mixed with the volatile gases resulting
in complete combustion. No smoke at the chimney indicates too much air
and the coal is burnt through. It is time to fire again. However, too
much coal will result in smoke.
Black
smoke indicates a lack of air and unburnt volatiles escaping to atmosphere.
A fire of the kind shown (right) gives very bad results in maintaining
steam, causes a great loss of heat and much smoke! The large mound of
coal is cooling the bed of the fire and prevents an even flow of air
through the fire grate.
The
size of coal also effects combustion. If coal is broken into pieces
no larger than a man‘s fist, then a greater surface area is exposed
and the fire burns better.