Control Volume

While the system notion focuses attention of the procedures undergone by a fixed body of material, in most of the engineering problems, the center of attention is a piece of equipment via which material flows continuously. Some illustrations are a hydro-electric plant, steam or gas turbine, heat exchanger, gasoline or diesel engine, nozzle, diffuser, and so on.  

In such a situation it is helpful to define the control volume. The control volume is any volume of fixed shape, and of fixed place and orientation associative to the observer.