Example
A rigid tank contains 5 kg of refrigerant-134a initially at 20?C and 140 kPa. The refrigerant is cooled until its pressure drops to 100 kPa. Determine the entropy change of the refrigerant during this process.
Constant volume process: v1=v2
initial state: P1=140 kPa, T1=20?C, from table A-10
s1=1.0532(kJ/kg K), v1=0.1652(m3/kg)
final state: P2=100 kPa, and v2=v1=0.1652 (m3/kg)
from table A-9 vf=0.0007258(m3/kg)
since vf < v2 < vg it is inside the saturation region
x2=(v2-vf)/vfg=(0.1652-0.0007258)/0.191=0.865
from table A-9, sf = 0.0678, sg = 0.9395
= 0.0678 + 0.865(0.9395-0.0678) = 0.822
DS = m(s2 - s1) = (5)(-0.231) = -1.157 (kJ/kg K)