The Classification of Mass Balances

It will have been noted that a number of different types of mass balances exist, and their use is dictated by the type of process under consideration. Thus, in Illustration 2.2, the mass balance was taken over a finite entity, the water basin, and the balance space in Illustration 2.3 was an incremental quantity Az, which ultimately shrank to a point in space. A further distinction is the appearance of time in Illustration 2.2 and an absence of spatial coordinates. The reverse was the case in Illustration 2.3. Clearly, both time and spatial geometry determine the type of mass balance that has to be made. 

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