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Thursday, April 11, 2013

Steam Engines for transportation

The steam engine that James Watt designed significantly improved the efficiency of stationary engines used for pumping water out of mines.  However, since it was an "atmospheric" engine (meaning the steam supplied to the cylinder was at normal air pressure), the cylinder had to be quite large to accomplish meaningful work.

Below is an example of a steam-powered blowing engine, used to provide low-pressure compressed air for use in steel furnaces.  The cylinder on the left is the steam engine, and the one on the right is the air compressor.

In order for a steam engine to be practical for powering a vehicle (ship, tractor, or train), cylinder size would need to be shrunk, and so steam pressure and temperatures would need to increase.

By 1800 metallurgy had come far enough along that steam could be contained under significant pressure.  In 1804 a steam engine using 145 psi steam pressure hauled a load of 10 tons for 10 miles in England.  This was proof that steam power could be used to move significant amounts of goods.

Here is a photo of the first practical steam locomotive. 

This design is quite complicated-looking, but not too difficult to understand.  The driver stands on a separate railcar (not shown) to the right.  Fire passes through tubes and out the stack at the left.  The fire and hot gas passing through the tube bundle boils water that surrounds the tubes and makes steam.  The steam pushes a piston - which cannot be seen because it is inside the body of the boiler. 

The piston goes in and out of the front of the front of this engine like a giant slide trombone.  Connected to the piston rod is the diagonal connecting rod, which is hooked to a crank at the rear (upper right) of the engine.  A huge flywheel on the opposite side of the engine smoothes the pulses (and wheelspin on the steel tracks) caused by the pulsed power delivery.  A pair of the wheels are then driven by using a series of gears, again to reduce wheelspin. 

Here is a video of the original locomotive in action.  Looks a bit dangerous to be the engineer ;)

Steam Safety:

Early problems (fatalities) with using higher steam pressures usually involved loss of water level and overpressure conditions. 

Loss of water level would leave the boiler tubes uncovered by water, allowing them to melt and release massive volumes of steam into the firebox, blowing ash and steam everywhere.  It may not be obvious, but water can absorb enormous amounts of heat, but steam cannot.

Steam overpressure is bad for equally obvious reasons - boiler rupture and explosion. 

The low water level condition was solved by adding a lead plug in the boiler at the lowest allowable water level.  When the water dropped below that level, the lead plug would be uncovered, it would melt, and release steam pressure (and excess energy) in a safe direction.  The lead plug would be installed at a slightly higher level than the bundle of boiler tubes, to prevent uncovering a tube and rupturing it.

The steam overpressure condition was corrected by fitting safety valves to the boiler, allowing excess steam to be safely vented off, in the event that there was too much fire for the amount of steam needed for the engine.

With these problems more or less resolved, the tranportation side of steam technology could march on.

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