Problem # p003

Open System, steady state , steady flow

In certain situations, when only superheated vapor is available, a need for saturated steam might arise. This can be produced in what is called a steady-flow desuperheater, in which liquid water is mixed with superheated steam at such a rate that the resulting mixture is exactly saturated vapor.

Liquid water : p1=4 MPa , T1 = 40oC
Superheated Steam : p2=4 MPa , T2 = 360oC
Saturated Steam : p3=4 MPa

Excerpts from a table of properties of water at 4 MPa

 T[oC]v[m3/kg] u[kJ/kg]h[kJ/kg]
compressed liquid400.001167171
saturated vapor2500.05026022801
super-heated vapor   3600.06828463117

Task :

  1. Write out the law of conservation of mass for the desuperheater using the symbols for the mass flow rates given in the figure above.

  2. Write out the law of conservation of energy for the desuperheater. Assume that the desuperheater is well-insulated and that kinetic and potential energies can be neglected.

  3. Determine the mass flow rates, and , for the incoming liquid water and super-heated vapor if we wish to generate a mass flow rate = 1.2 [kg/s] for the saturated steam exiting the desuperheater.