D&RGW K-28 COAL FIRED MANUAL
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pump is sized to pump water in excess to the needs of the engine, and given time, will fill the boiler to its maxi
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mum capacity. During those times the locomotive is stopped and the blower is on, makeup water needs to come
via the tender pump.
Maximum water level is not critical, but should not exceed the bottom edge of the top nut of the water gauge. Over
filling the boiler results in the engine priming, or passing water to the cylinders, and can be noticed by a spray or
mist of water coming from the cylinder exhaust or blower stream exiting the smokestack. A more detailed discus-
sion of maintaining water level will be presented later in these instructions.
Coal Burning Processes
The combustion of coal in a firebox, and using the derived energy for steam production, is not a simple process.
Coal is a complex material and its combustion is intricate. Coal, a mineral compound, contains, depending upon
its specific type and source, various quantities of moisture, hydrocarbon volatile, ash, sulphur and fixed carbon .
Coal needs a significant amount of heat to initiate and maintain combustion as well as a continuous supply of oxy
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gen. Coal combustion takes two avenues. The first avenue takes place at a relatively low temperature and results
in the combustion of the volatile hydrocarbon components. As fresh coal is added to the top of the burning bed of
coal on the grates, the volatile hydrocarbons are gasified and driven off, burning in the space above the coal bed
and producing hot flue gases that transfer their heat to the firebox, tubes and flues by convection, with minimal
radiant energy. The second stage involves the fixed carbon portions of the coal bed situated on the grates.
The fixed carbon in the presence of a significant supply of oxygen, burns at a high temperature, becoming incan
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descent in the process. This second process produces considerable amounts of radiant energy as well as ad-
ditional hot flue gases. The sulphur component, which is relatively small, also bums producing a corrosive in the
form of sulphur dioxide which, in the presence of moisture, becomes sulphuric acid. The ash, or non-combustible
portion, composed mostly of clay, remains behind to form clinker, clog grates, drop into the ash pan and be de-
posited in the smoke box.
In small scale locomotive boilers the heating surface apportionment between the fire tubes and the firebox,
coupled with the small diameter of the tubes and their relative short length, generally favors the firebox for absorb
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ing the energy of combustion . Most of the heat value in coal is contained in the fixed carbon portion , and since
the fixed carbon portion combust with a high level of incandescence, this energy is readily transferred to the boiler
radiantly. This results in the firebox being the major contributor to the steaming capacity of the boiler.
The combustion of both the volatile and the fixed carbon portion takes considerable amounts of oxygen. This
necessitates that the bed of burning coal have a sizeable amount of air passing through it to maintain combustion.
This air is provided by the exhaust nozzle and a separate blower, both located in the smoke box. The nozzle, as
the engine runs , and the blower when it is stationary, reduce the pressure in the smoke box which induces air to
be drawn up through the grates and burning bed of coal, through the tubes and flue, and out the stack.
The main requirement for this to take place, is for the air to have a free and unobstructed passage through the
bed of coal, and that the coal be evenly spread across the grates. Any obstruction to the flow of air or lack of
forced draft will cause the fire to go out . As well as obstructions, if there are large holes in the coal bed that allow
air to circumvent passage through the burning portions of coal, combustion will be reduced . This is true also with
leaving the firebox door open.
How you fire and run your locomotive needs to take into account how coal burns. The frequency of stoking , the
quantity of coal stoked , how it is spread on the grates, as well as the quality of the coal , will all have considerable
impact on how your locomotive performs . The specifics of coal firing will be covered in detail further on in these
instructions.
