A Short History of Steam Engines
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The Cornish Boiler William Wycherley said it, "Necessity, the mother of invention." The invention of a workable steam boiler and engine made mass transportation possible. However hazardous it might have been. Economics and the need to get the cargo safely to a destination was the driving force behind new designs for boilers. The passengers also enjoyed arriving, in one piece. Development of the Cornish Boiler was a step in that direction. Until that time designers had always placed the furnace beneath the water cylinder. Attempts to make use of the heat that was going up the chimney were limited to simply rapping the hot gasses around the boiler several times. Them some genius had the idea of putting the fire where it would do the most good, in with the water. Not actually "IN" the water but literally inside the cylinder containing the water. The Cornish Boiler made several design changes. First, as I already said, the furnace was placed inside a metal tube measuring three(3) feet or more in diameter (from side to side). And this tube was placed inside the boiler. Having all that hot metal inside the water greatly increased the amount of heat transferred to the water. Like the Plain Cylinder Boiler, the fire and hot gasses were still routed through three flue's which run along both sides and beneath the cylinder. But unlike the Plain Cylinder Boiler, the order in which the gasses moved was changed. After leaving flue #1 (the metal tube running through the water) the hot gasses were divided at the aft end and moved forward along flue #2 which runs along both sides of the cylinder at the same time. At the front of the boiler the hot gasses were directed downward into flue #3 and traveled aft beneath the boiler to the chimney. This helped reduce the amount of mud that accumulated in the bottom of the boiler and that increased the boilers efficiency even more. The flat ends of the cylinder are another obvious design change made necessary by the internal furnace. What is not so obvious from in the illustration is the size of the boiler. Unlike the Plain Cylinder Boiler, the Cornish Boiler is quite a bit larger. It holds less water than the Plain Cylinder Boiler but transfers heat to that water at a much greater rate producing more steam, at higher pressures, in a shorter time. Fuel efficiency was just as important in the 1800's as it is today. All these improvements came at a high price for the designers did not take into account the effects of heat expansion on the metal flue. The internal tube with its furnace and fierce heat was constantly changing length. When heated the metal tube would expand several inches causing the ends of the boiler to bulge outward. As the furnace used up the fuel in the fire box, the temperature would drop slightly. Not a big drop in temperature but enough to allow the metal tube to contract. After more fuel was added the tube would heat up again and cause another bulge in the ends of the boiler. This constant back and forth movement placed a great amount of stress on both ends of the boiler. The result of all that repeated back and forth stress was something called a, "collapsed flue." Which is just another type of boiler explosion. The results are the same. If you twist a wire coat hanger back and forth many times you will see just what happens to the metal, without the big explosion. Boiler explosions were common. Way to common in those early days of steamboats. But progress was being made in boiler design both in efficiency and safety. - K. Spitzner, Georgia 
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