Question 9.CSGP.137: Carbon dioxide, CO2, enters an adiabatic compressor at 100 k......
Carbon dioxide, CO_2, enters an adiabatic compressor at 100 kPa, 300 K, and exits at 1000 kPa, 520 K. Find the compressor efficiency and the entropy generation for the process.
C.V. Ideal compressor. We will assume constant heat capacity.
Energy Eq.6.13: w _{ c }= h _1- h _2 \text {, }
Entropy Eq.9.8: s _2= s _1: T _{2 s }= T _1\left\lgroup\frac{ P _2}{ P _1} \right\rgroup^{\frac{ k -1}{ k }}=300\left\lgroup\frac{1000}{100} \right\rgroup^{0.2242}=502.7 \,K
w _{ cs }= C _{ p }\left( T _1- T _{2 s }\right)=0.842(300-502.7)=-170.67 \,kJ / kg
C.V. Actual compressor
Use Eq.8.16 for the change in entropy
Constant heat capacity is not the best approximation. It would be more accurate to use Table A.8. Entropy change in Eq.8.19 and Table A.8:
s _{ T 2}^{ o }= s _{ T 1}^{ o }+ R \ln \left( P _2 / P _1\right)=4.8631+0.1889 \ln (1000 / 100)=5.29806
Interpolate in A.8 ⇒ T _{2 s }=481 \,K , \quad h _{2 s }=382.807 \,kJ / kg \Rightarrow
