4.5 Entropy Balance for Open Systems
 
As with energy balance for open systems, once can extend the equation 4.24 to a generalized entropy balance equation that may be written for the system shown in fig. 3.6. There is, however, an important point of departure from the first law. Unlike energy, entropyis not a conserved quantityfor real world processes in which both mechanical and thermal irreversibilities are inevitable. Hence for such processes are attended by a positive entropy generation rate,. This conclusion may be expressed in mathematical terms as follows:  
