Energy Balances

Abstract

Global conservation law of energy states that the total energy of a system always remains constant. Mass and energy are always conserved in a process, while entropy and exergy are not conserved. Balance equations are for both the conserved as well as for non-conserved properties, and therefore may contain a source term. The source term mainly refers to internal production or consumption of a property such as mass, energy, entropy, and exergy in a system. For example, in an internal chemical reaction in a system, a substance may be consumed and heat of reaction may be produced. Consequently, in many engineering problems energy and mass conservation are solved together. This chapter discusses the formulation of balance equations for mass, energy, entropy, and exergy for analyzing the processes. In the chapter, mass and energy balances are the same as mass and energy conservations. 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. The analyses of energy interactions in closed and open steady-state systems are discussed with ten example problems. For practice, 76 problems are added at the end of the chapter.