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Coal Firing a Steam EngineCoal firing a steam engine is a little more complicated than it first appears.
Fundamentally we must first look at the fire, the heat source, and for a fire to be productive it needs four things:
Let us assume your coal fire is lit and there is a good supply of coal to hand; the exhaust gases then need to be removed to provide room for more air to pass either through or around the fire to keep it burning, otherwise the fire will simply be smothered and die down and eventually extinguish itself.
Obviously the fire will burn more fiercely if air passes through the hot coals and this is where a chimney comes in. By heating the chimney, fresh air is drawn over (or through) the fire to create better combustion.
It will be seen that the heat is most concentrated close to the fire itself, and as the exhaust gases move away from the fire the heat disperses, so a chimney is actually cooling the gases, which in turn reduces the draw of air through the fire.
From this it can be understood that a chimney’s height and diameter, along with its distance from the fire itself, needs to be proportional to the area of the grate beneath the fire (ie, the size of the fire) and the boiler size.
For a steam engine this sizing can be overcome somewhat by using the exhaust from the steam engine to blast the gases through the chimney.
But therein lies another little problem. The blast pipe diameter must not detract too much from the steam engines power by restricting the waste steam flow (building up too much back pressure in the exhaust line from the engine), while it must also be proportionate to the chimneys diameter and the height and position of the blast pipe up the chimney.
This works very well when a tall chimney is used, but for locomotives, where they need to pass through tunnels and under bridges of a set height, there is a need to use a blower on a regular basis. This is a separate feed from the boiler, passing steam through a fine jet in the smokebox to provide draft through the chimney, that way drawing the fire, which in effect makes short chimneyed vehicles just a little less efficient than long chimneyed for fuel consumption.
The grate the fire sits on must also be considered as this can easily become blocked up by ash from the fire if the gaps are too close, that way blocking off the air supply, meaning regular raking by the fireman is needed to keep up a good head of steam.
So... You have the fire lit and the hot gases are working their way along your boiler, transferring some heat into the water inside your boiler, cooling as they do so, making the gases denser, harder to move along the almost horizontal fire tubes, which is again trying to smother the fire. These hot gases are still way above the boiling point of water and so still hold lots of useful heat, meaning either the longer your boiler, or the longer you can keep those hot gases inside your boiler, the more efficient your engine will run.
But there is a limit...
This is where there are set ratios for boiler tube diameter and the number of boiler tubes, their length, grate size, water capacity, running pressure, blower jet sizes and chimney dimensions.
However, this is beginning to turn into a tale of epic proportions, so for the time being we will continue with firing an existing boiler.
Grate size versus boiler size.
If your firebox area (fire) is too large for your boiler you are wasting coal.
The heat generated by the fire will provide gases that are extremely hot as they vent out of the chimney. The obvious sign of this are sparks coming out of the chimney when under power; the smokebox paint peeling from being overheated with the smokebox even glowing; the safety valve lifting regularly and a frequent need to close the air damper to restrict the flow of air through the fire.
On the other hand, if it is too small, then you will find it takes a little longer to get up pressure and journeys need regular stops to allow the pressure to be built back up before you can continue again.
This can be compensated for by using a blower as mentioned above, that way drawing more air through the fire to produce a hotter fire.
More on this topic to follow:
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