Question 9.CSGP.133: A refrigerator uses carbon dioxide that is brought from 1 MP......
A refrigerator uses carbon dioxide that is brought from 1 MPa, -20°C to 6 MPa using 2 kW power input to the compressor with a flow rate of 0.02 kg/s. Find the compressor exit temperature and its isentropic efficiency.
C.V. Actual Compressor, assume adiabatic and neglect kinetic energies.
Energy Eq.6.13: – w _{ C }= h _2- h _1=\frac{\dot{ W }}{\dot{ m }}=\frac{2\, kW }{0.02 \,kg / s }=100 \,kJ / kg
Entropy Eq.9.9: s _2= s _1+ s _{ gen }
States: 1: B.3.2 h _1=342.31 \,kJ / kg , \quad s _1=1.4655 \,kJ / kg – K
2: B.3.2 h _2= h _1- w _{ C }=442.31 kJ / kg \Rightarrow T _2= 1 1 7 . 7 ^ { \circ } { C }
Ideal compressor. We find the exit state from (P,s).
State 2s: P _2, s _{2 s }= s _1=1.4655 \,kJ / kg – K \Rightarrow h _{2 s }=437.55 \,kJ / kg
– w _{ Cs }= h _{2 s }- h _1=437.55-342.31=95.24 \,kJ / kg
\eta_{ C }=- w _{ Cs } /- w _{ C }=\frac{95.24}{100}= 0 . 9 5 2