Global conservation law of energy states that the total energy of a system always remains constant. Mass and energy are conserved in a process, while exergy is not conserved. In energy production systems, energy and mass conservations are solved together. By accounting the balance equations, energy interactions have been analyzed in turbines, compressors, nozzles, pumps, valves, mixers, and heat exchangers. Beside these, the Carnot cyclic process is also discussed briefly.