Determine whether or not it is possible to compress air adia

Determine whether or not it is possible to compress air adiabatically from 70 kPa and 310 K to 140 kPa and 400 K. Assume specific heats change with temperature. What is the entropy change during this process? What is the efficiency of compressor?

Solution

at 70 kPa 310 K   enthalpy = 311 kJ/kg entropy = 7.01 kJ/kg-K Cp = 1010 J/kg K

at 140 kPa 400 K enthalpy = 401 kJ/kg entropy = 7.06 kJ/kg-K Cp = 1010 J/kg-K

Enthalpy change = 401 - 311 = 90 kJ/kg

Cp2 * T2 - Cp1 * T1   = 1010 * 400 - 1010 * 310 = 90.9 kJ/kg

As enthalpy change is not equal to change in specific heats it is not possible to compress air adiabatically.

Entropy change   = s2 - s1 = 7.06 - 7.01 = 0.05 kJ/kg-K