How do steam engines work?

The Industrial Revolution is said to have started with the use of hydro-powered machines water-powered lathes, mills, and metalworking plants are some examples of these early industrial machines. However, the one machine that was most important to the Industrial Revolution was the steam engine.

Suddenly manufacturing machines could be set up anywhere, not just next to convenient rivers. The first practical steam engine application was to power James Watts pumps for keeping mines dry. From there steam engines were improved, adapted, and adjusted to perform all manner of jobs: powering locomotives, ships, factories, and eventually electrical generators.

A steam engine is any machine that uses steam and the properties of steam to allow for heat to be used to generate power. There are, in general, two differing kinds of steam engines: the piston-type and the turbine type. What both of these have in common is a source of steam, or boiler, where water is heated to the boiling point, in a pressure vessel. In order to make sure that only steam is passed through the steam engine, and to improve the amount of force available from the steam, this vessel will often be operating at pressures up to 1200 pounds per square inch.

From the boiler, the steam passes through piping to the engine proper. One familiar steam engine involves a piston arrangement where the steam is applied above a piston head, driving it down the cylinder, which motion is then transferred via mechanical linkages to where the power is needed. This is the manner in which steam is used in a steam locomotive to make the driving wheels turn, moving the train.

The other common steam engine is the turbine, where the steam is admitted to a series of wheels with metal fan blades, which absorb the energy of the steam by spinning, and then turning a shaft to make the power of the steam available for use. This is the steam engine that is used in electrical power plants to turn generator shafts. In both of these machines the steam, after its energy has been harnessed is expelled into a low pressure and low-temperature environment.

The ideal efficiency of a heat cycle engine is called a Carnot engine, after the Frenchman who first described the limiting factors. For the Carnot engine calculations, a number of assumptions are made: First, all processes are occurring at saturation temperatures and pressures this means that the steam is never heated above the boiling point for the pressure it is at; Second, entropy remains constant since entropy always increases, this is one reason why Carnot efficiency is ideal, not real.

The Carnot efficiency of a cycle is the maximum efficiency that can be described, not a goal that can be reached in the real world. Simply put Carnot efficiency is described as the hot temperature (in degrees Kelvin or Rankin) minus the cold temperature (again, in degrees Kelvin or Rankin) divided by the hot temperature.

It is the difference between the pressure and temperature of the boiler and the pressure and temperature of the discharge environment that provides the energy difference that allows a steam engine to work. The two ways to increase the amount of energy being transferred per unit mass of steam are to raise the steam’s temperature and pressure in the boiler or to lower the pressure and temperature that the steam is expelled to after it has completed its work.

With the steam piston arrangement familiar to steam locomotives, the discharge environment is the normal environment outside, and there is little that can be done to control that by the locomotive’s engineers. A steam turbine, however, discharges to a vessel called a condenser, where the steam vapor is condensed back to the water, so it can be pumped back to the boiler, where the whole process repeats infinitely. By keeping this condenser vessel below atmospheric pressure, it is possible to extract even more energy from the steam than simply discharging to the air would allow.

Steam engines are not as common today as they once were: diesel powers most ships, and locomotives today, most pumps are electric, and cars are either powered by gasoline engines or electric motors, or a hybrid of the two. However, steam is still used in most nuclear plants to transfer the energy from the reactor to the generators; likewise most oil power generation plants are still operating on a steam cycle.

Steam engines are one of the cycles where larger plants and facilities are more closely able to approach ideal efficiency than smaller plants, and so are going to be used for years to come.