OpenProcesssteady state systems play an important role in technology, for example pumps, compressors, combustion engines and much more. These systems have already been considered in the first part of the book. The necessary balancing equations, for example the first law of thermodynamics for steady state and transient systems, see Sect. 11.3 , have been applied. Figure 21.1 shows an example of an open system, i.e. a system with a mass-permeable system boundary, with a single inlet and a single outlet. Open systems in a steady state are characterised by the mass in the system remaining constant over time, so that the mass flux into the system must be balanced by the mass flux out of the system, i.e. \(\boxed {\dot{m}_{1}=\dot{m}_{2}}\) Furthermore, the state inside the system must not vary with respect to time, so that the first law of thermodynamics obeys.