As a unit of mass enters a system, the surroundings do flow work on each unit of mass, increasing the energy inside the system.
Flow work done on a unit of mass entering the system is
Similarly, as a unit of mass leaves a system, the system does flow work on each unit of mass, decreasing the energy inside the system.
Flow work done on a unit of mass leaving the system is
If mass is crossing the boundary at rate
then the rate of flow work is:
So, for every unit of mass that enters the system, the associated flow work is Pv. Each unit of mass crossing the boundary also carries internal energy u, so the total energy associated with each unit of mass in u + Pv. Similarly, the total energy associated with each unit of mass leaving the system has energy every unit of mass leaving the system also has energy u + Pv. This combination of internal properties is given the name enthalpy, h = u + Pv. For convenience in solving control volume problems, h is included in property tables.