Steam Engine - Steam Cycle

Steam Cycle

Main article: Rankine cycle See also: Thermodynamics See also: Heat transfer

The Rankine cycle is the fundamental thermodynamic underpinning of the steam engine. It is a way to illustrate the flow of the working fluid steam and the heat added, converted to work and rejected as waste heat in the course of engine operation. The heat is supplied externally to a closed loop, which in steam engines contains water and steam. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is named after William John Macquorn Rankine, a Scottish polymath.

The Rankine cycle is sometimes referred to as a practical Carnot cycle because, when an efficient turbine is used, the TS diagram begins to resemble the Carnot cycle. The main difference is that heat addition (in the boiler) and rejection (in the condenser) are isobaric (constant pressure) in the Rankine cycle and isothermal (constant temperature) in the theoretical Carnot cycle. In this cycle a pump is also used to pressurize the working fluid received from the condenser as a liquid instead of as a gas. Pumping the working fluid through the cycle as a liquid requires a very small fraction of the energy needed to transport it as compared to compressing the working fluid as a gas in a compressor (as in the Carnot cycle).

The working fluid in a Rankine cycle follows a closed loop and is reused constantly. While many substances could be used in the Rankine cycle, water is usually the fluid of choice due to its favourable properties, such as non-toxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties.

The steam engine contributed much to theory in the development of thermodynamics; however, the only applications of scientific theory that influenced the steam engine were the original concepts of harnessing the power of steam and atmospheric pressure and Watt's development of the separate condenser. Otherwise, the improvements to the engine itself were more mechanical in nature. The thermodynamic concepts of the Rankine cycle did give engineers the understanding to calculate efficiency and to save time and effort in developing modern high temperature and pressure boilers and steam turbines.