77 Air is heated from 70 oF to 225 oF while its volume remai

7.7: Air is heated from 70 oF to 225 oF while its volume remains constant. What is the change in the specific entropy (in Btu/lbm-R) of the air?

solution:

specific entropy means entropy change per unit mass

here temprature in kelvin are for air Cv=.718 kj/kg k

T1=70 F=21.11+273=294.11 k

T2=225 F=107.22+273=380.22 k

for specific entropy change at constant volume is given as

s2-s1/m=Cv*ln(T2/T1)=.718*ln(380.22/294.11)=.1843 kj/kg k=.044038 Btu/lbm-R