For a flow process, let s 1 denote the specific entropy (entropy per unit mass) at the inlet, where mass flows in, and s 2 the specific entropy at the outlet, where mass flows out. Similarly, denote the specific enthalpies by h 1 and h 2 . The inlet and outlet, in this case, function as initial and final states a process: mass enters the system at an initial state (the inlet, indexed "1"), undergoes some process, and then leaves at a final state (the outlet, indexed "2").