GATE 2020 ME (ME1) – Question 61
Ideal-gas air enters a perfectly insulated compressor at 310 K. Compressor pressure ratio is 6. Air has constant cₚ = 1005 J/(kg·K) and cₚ/cᵥ = 1.4. Isentropic efficiency is 85%. Find outlet minus inlet specific enthalpy (kJ/kg), nearest integer.
Practise this question in The GATE Grind →
Show answer and explanation
Correct answer: 244 to 246
Explanation
Isentropic outlet temperature is $T_{2s}=310(6)^{(1.4-1)/1.4}$. Compressor efficiency is (h2s−h1)/(h2−h1), so $\Delta h=1.005(310)(6^{2/7}-1)/0.85\approx\mathbf{245}$ kJ/kg. Divide the ideal rise by efficiency, rather than multiply.