Question

iii) wet steam of 95% steam quality and 65.0 bar is expanded to wet steam of 80% steam quality at 1.0 bar using a turbine in a steady-state flow process. Determine: a) the shaft work (kJ/kg) produced by the turbine, and; b) the efficiency of the expander. You may assume the process is adiabatic and potential/kinetic energy changes are negligible.

Answer 1

To solve the above question, thermodynamic tables must be looked at.

The equation to be used is: $w = h_i - h_e + \frac{V_i^2 - V_e^2}{2} + g(Z_i - Z_e)$ .

However, there is no change in kinetic or potential energy in this case. Therefore, the work can be calculated simply by looking at the change in enthalpy of the 2 states.

State 1

x₁ = 0.95

P = 65 bar = 6500000 Pa

From the tables, h_i = 2701.23 kJ/kg

State 2

x₂ = 0.8

P = 1 bar

From the tables, h_e = 2223.58 kJ/kg

a) The shaft work is 477.648 kJ/kg

b) Since the process is adiabatic, the removal of heat is zero. This makes the efficiency of the expander infinity.