In thermodynamics we explore an array of cycles that are used to explain a process. Most notably, we use cycles to look at steam engines, combustion engines, and refrigeration systems. By examining idealized cycles, we can determine a fairly accurate approximation for the work and heat involved in an actual process. Understanding these ideas are paramount to maintaining everything from huge refineries to the refrigerator in your house. I’ll first briefly explain the concept of an engine and then look into the Rankine cycle, which is an idealized cycle of a steam engine.
The image above is the classical Carnot engine, which operates at the highest possible efficiency of engine. The Carnot cycle is used to analyze other cycles and determine their efficiency. Efficiency is just the relationship between the amount of useful work obtained and the heat put into the system. The higher the efficiency, the better because less energy is lost. In essence, the Carnot cycle shows that energy (Qh) is taken from a hot reservoir at temperature (Th). The circle represents the engine, which is an assortment of turbines, compressors, condensers, etc. The engine produces usable work (such as turning a turbine) as the fluid moves into the cold reservoir. The flow of heat from the hot reservoir to the cold reservoir/sink produces work. The Carnot engine is adiabatic, reversible, and the expansion of gas is isentropic (no change in entropy).
The Rankine cycle is used to model steam-operated heat engines which are commonly found in power generation plants.
The images above a great representations of what is occurring in a Rankine cycle. In steps 1-2 the fluid is pumped from a low pressure to high pressure, because it requires less energy and is easier to heat/boil a liquid when it is at a higher pressure. From 2-3 the liquid passes through a boiler where it is heated at constant pressure and eventually saturated vapor leaves the boiler at point 3. The saturated vapor then passes through a turbine which is how power is generated. High pressure vapor enters a turbine which decreases the temperature and pressure, while producing work. The exiting vapor-liquid passes through a condenser at point 4 which acts as the cold reservoir and cools the fluid back into a saturated liquid. The fluid constantly moves through the cycle and produces work. Diagrams and tables are essential because they are used to determine the different parameters of the fluid at different points in the cycle. It’s possible to determine enthalpy, entropy, temperature, pressure, and superheat among many other variables. If just two of the variables are known/fixed, it’s possible to find all the other values. With certain known information (isoentropic expansion, constant pressure across boiler/condenser) it is simple to use the cycle to model steam engine systems.