steam

Part 4 - Steam

James Watt did not invent the steam engine. The first effective steam-powered device was a water pump, developed in 1698 by Thomas Savery. It used condensing steam to create a vacuum which raised water from below and then used steam pressure to raise it higher. It was used in mines, pumping stations and to supply water to water wheels that powered textile machinery. Savery's engine was low cost but very inefficient and burned a lot of coal for a small result.

In 1705 France, Denis Papin built a boat powered by his steam engine, mechanically connected to paddles. This was the first steam-powered boat and first steam driven vehicle of any kind. Unfortunately, the boat was destroyed by a guild of boatmen who did not want competition from Papin's steamboat. 

The first commercially-successful, stationary steam engine was invented by Thomas Newcomen around 1712. It was the first machine designed to replace the water mills and horse-powered water pumps that were used to extract water from the deep tin mines of south west Britain.

It may have been based on Denis Papin's atmospheric engine but it worked by condensing steam under a piston to create a partial vacuum within a cylinder. Atmospheric pressure then pushed the piston down and this drove a piston water pump at the bottom of the mine through a long chain. It was an improvement on Savery's engine but still very inefficient.

But the steam age really began about 1772, when James Watt realized Newcomen's engine wasted energy by condensing the steam inside the cylinder. He designed a separate condenser to suck the steam from the cylinder so that the cylinder remained hot. Watt's separate condensers were initially used to retrofit the hundreds of Newcomen engines that were already in use before Watt started making complete engines.

Newcomen steam engines were powered by atmospheric air pressure (14.7 pounds/sq.inch at sea level) so the engine cylinders had to be large. Watt was well aware of the advantage of using high pressure steam but this required steam boilers and valves that did not leak or burst under pressure and more precisely machined pistons and cylinders than were available. 

But Watt was already developing a better engine with help from Matthew Bolton and some of the best iron workers in the world. John Wilkinson had developed precision boring techniques to make cannon and he solved the problem of making large cylinders with tightly fitting pistons. The new engines used half as much coal as Newcomen's engine while pumping the same amount of water.

In the process, Watt devised methods of measuring the work done by various engines compared to the amount of coal used to boil water for steam. He invented the measurement 'horse power' based on the number of pounds of water a horse could raise one foot high. He estimated one horse power was 550 foot-pounds per second (equivalent to 745.7 watts). He also invented many devices such as a pressure relief valve and the centrifugal governor used to control engine speed.

Urged by his business partner, Mathew Bolton, Watt developed high pressure engines to provide a rotary motion suitable for driving spinning and weaving machines and also a new machine tool industry required to make steam engines. The steam engines made it possible to site factories wherever convenient instead of near sources of water. Steam engines rapidly replaced wind and water mills for most industrial purposes by the end of the century. For marine and stationary boilers, tall chimneys (smokestacks) were typically used to maintain the draft of air entering the fire box.

The fireman (stoker), while shovelling coal into a high pressure boiler, had to keep the water level in the boiler between lines marked on a sight glass. If the water level was too high, less steam was produced and water might be carried into the cylinders, possibly causing damage. Conversely, a low water level might expose the top of the firebox which could fail, letting high-pressure steam into the firebox and severely injuring the fireman.

The driver or fireman monitored steam pressure gauges and could manually released steam but, if the pressure reached the boiler's maximum working pressure, a spring-loaded safety valve automatically reduced the pressure.

The wet (or saturated) steam from the boiler was often passed through a super-heater (a set of flues at the top of the boiler) where it was dried by further heating. This eliminated water droplets suspended in the saturated steam, thus preventing damage to the cylinders while also improving thermal efficiency.

The superheated steam was then directed to valves inside steam chests adjacent to each cylinder.  The valves admitted steam to each end of each piston alternately (so it delivered power with each stroke) and exhausted the used steam at the end of each stroke.

The 1849 stationary steam engine, built in the USA by George Henry Corliss, was one of the most successful. Corliss' engines were generally about 30 percent more fuel efficient than conventional steam engines with fixed cutoff valves, because Corliss used one inlet valve and one exhaust valve for each end of the double acting cylinder (4 per cylinder). This meant that the inlet valves were not cooled by exhaust steam as were engines with only 2 valves, which handled both inlet and exhaust steam at each end of the double acting cylinder. The Corliss engine also provided variable control of each valve from a centrifugal speed governor so that the inlet steam could be cut off at any point of the piston's travel. A decrease in load allowed the engine speed to increase and the speed governor would then automatically adjust the inlet steam valve cutoff point to reduce the power output and maintain the constant speed needed for cotton spinning.