Control Surface and Flow Work - thermodynamics:

Control Surface: A control system has control volume that is separated from its surrounding by real or imaginary control surface that is fixed in shape, position and orientation. Matter can flow continually in and out of control Volume and heat and work can cross control surface. This is an example of open system.

Steam Generator: The volume of generator is constant. Water is supplied, and then Heat is supplied and Steam comes out. It is control system as well as an open system.

The flow process can be analyzed as a closed system by applying the concept of control volume. The control surface can be selected and all the energies of system including flow energies can be considered inside system. The changes of state of working substance (mass) is required not be considered during its passage through system.

PE = force × Distance = (p₁A₁).x.

= p₁V₁(J)

Now the specific volume of working substance is p₁

Fe = p₁v₁(J/kg).