Question

Refrigerant R-12 at 30°C, 0.75 MPa enters a steady flow device and exits at 30°C, 100 kPa. Assume the process is isothermal and reversible. Find the change in availability of the refrigerant. 

Answer 1

h_i = 64.59 kJ/kg, s_i = 0.2399 kJ/kg K, compr. liquid.

h_e = 210.02 kJ/kg, s_e = 0.8488 kJ/kg K, sup. vapor 

From Eq. 

∆ψ = h_e - h_i - T₀(s_e - s_i ) = 210.02 - 64.59 - 298.15(0.8488 - 0.2399)

= -36.1 kJ/kg

Remark: Why did the availability drop? The exit state is much closer to the ambient dead state, so it lost its ability to expand and do work.