Nitrogen enters a steady-state process at T = 150 K and P = 0.1 MPa. A compressor increases the pressure to 0.4 MPa, and is followed by a heat exchanger. A second compressor increases the pressure to 2 MPa and feeds into a second heat exchanger. Each heat exchanger lowers the temperature isobarically to 150 K. If each compressor is adiabatic and has an efficiency of 80 %, determine the TOTAL work added per kilogram of nitrogen and the TOTAL heat removed per kilogram of nitrogen. Use Figure 2-3 for data.

Question

A. +192 kJ/kg; -210 kJ/kg
B. +298 kJ/kg; -4556 kJ/kg
C. +154 kJ/kg; -172 kJ/kg
D. +192 kJ/kg; -172 kJ/kg
E. Insufficient information to solve

Answer 1

A. Correct. You can perform enthalpy change “calculations” on Figure 2-3 except for the efficiency adjustments.
B. Incorrect. You probably misread Figure 2-3. Check the axes again and retry the problem.
C. Incorrect. You have neglected to calculate the actual work needed. This looks like the reversible work.
D. Incorrect. You did not use the efficiency-corrected shaftwork to adjust the outlet enthalpies. See step 4 of Example 5-2.
E. Incorrect. All the information needed is supplied. If you are looking for a heat capacity, use Figure 2-3 instead, it is more accurate.