ME2100 Applied Thermal Engineering
Tutorial 2 - Exergy
1. A piston–cylinder device initially contains 2 L of air at 100 kPa and 25°C. Air is now
compressed to a final state of 600 kPa and 150°C. The useful work input is 1.2 kJ.
Assuming the surroundings are at 100 kPa and 25°C, determine (a) the exergy of
the air at the initial and the final states, (b) the minimum work that must be supplied
to accomplish this compression process, and (c) the second-law efficiency of this
process. [(a) 0, 0.171 kJ, (b) 0.171 kJ, (c) 14.3 percent]
2. A piston–cylinder device contains 5 kg of refrigerant-134a at 0.7 MPa and 60°C. The
refrigerant is now cooled at constant pressure until it exists as a liquid at 25°C. If the
surroundings are at 100 kPa and 25°C, determine (a) the exergy of the refrigerant at
the initial and the final states and (b) the exergy destroyed during this process. [(a)
124.4 kJ, 212.165 kJ, (b) 14.467 kJ]
3. A rigid tank is divided into two equal parts by a partition. One part of the tank
contains 1.5 kg of compressed liquid water at 300 kPa and 60°C and the other side
is evacuated. Now the partition is removed, and the water expands to fill the entire
tank. If the final pressure in the tank is 15 kPa, determine the exergy destroyed
during this process. Assume the surroundings to be at 25°C and 100 kPa. [3.75 kJ]
4. An iron block (sp. heat capacity 0.45 kJ/kg.K) of unknown mass at 85°C is dropped
into an insulated tank that contains 100 L of water (sp. heat capacity 4.18 kJ/kg.K)
at 20°C. At the same time, a paddle wheel driven by a 200-W motor is activated to
stir the water. It is observed that thermal equilibrium is established after 20 min with
a final temperature of 24°C. Assuming the surroundings to be at 20°C, determine
(a) the mass of the iron block and (b) the exergy destroyed during this process. [(a)
52.8 kg, (b) 382.3 kJ]
5. Air is compressed steadily by an 8 kW compressor from 100 kPa and 17°C to 600
kPa and 167°C at a rate of 2.1 kg/min. Neglecting the changes in kinetic and
potential energies, determine the second law efficiency of the compressor. Assume
the surroundings to be at 17°C. [87.26]
6. 1 kg of water at 0.2 MPa is initially enclosed within a volume of 0.10 m 3 in a vertical
piston-cylinder arrangement. The piston initially rests on stops and will move when
the pressure is 1.0 MPa. During a certain process, heat is added to the water from a
source at 400°C until the temperature reaches 400°C. Determine the final state of
the water and the work done. Assuming the surroundings to be at 25°C and 100
kPa, determine (a) the work done, (b) the heat transfer, (c) the exergy destroyed,
and (d) the second-law efficiency of this process. [207 kJ, 2430.6 kJ, 504.48 kJ,
26.96 %]
7. Refrigerant-134a at 1 MPa and 100°C is throttled to a pressure of 0.8 MPa.
Determine the reversible work and exergy destroyed during this throttling process.
Assume the surroundings to be at 30°C. [4.85 kJ/kg, 4.85 kJ/kg]
8. Steam enters an adiabatic turbine at 6 MPa, 600°C, and 80 m/s and leaves at 50
kPa, 100°C, and 140 m/s. If the power output of the turbine is 5 MW, determine (a)
the reversible power output and (b) the second-law efficiency of the turbine.
Assume the surroundings to be at 25°C. [(a) 5.81 MW, (b) 86 percent]
9. Steam enters a turbine at 10 MPa, 600°C, and 60 m/s and leaves at 20 kPa and 130
m/s with a moisture content of 5 percent. The turbine is not adequately insulated
and it estimated that heat is lost from the turbine at a rate of 150 kW. The power
output of the turbine is 2.5 MW. Assuming the surroundings to be at 25°C,
determine (a) the reversible power output of the turbine, (b) the exergy destroyed
within the turbine, and (c) the second-law efficiency of the turbine. (d) Also, estimate
the possible increase in the power output of the turbine if the turbine were perfectly
insulated. [(a) 3106.56 kW, (b) 606.56 kW, (c) 80.5%]
10. Steam enters an adiabatic nozzle at 8 MPa and 500°C with a velocity of 70 m/s and
exits at 4 MPa and 400°C. Assuming the surroundings to be at 25°C, determine (a)
the exit velocity of the steam, (b) the isentropic efficiency, and (c) the second law
efficiency. [(a) 610.7 m/s, (b) 87%, (c) 93%]
11. Liquid water at 15°C is heated in an insulated chamber by mixing it with saturated
steam. Liquid water enters the chamber at the steam pressure at a rate of 4.6 kg/s
and the saturated steam enters at a rate of 0.23 kg/s. The mixture leaves the mixing
chamber as a liquid at 45°C. If the surroundings are at 100 kPa, 15°C, determine (a)
the temperature of saturated steam entering the chamber, (b) the exergy
destruction during this mixing process, and (c) the second-law efficiency of the
mixing chamber. [(a) 113°C, (b) 113.575 kW, (c) 0.207]
12. An insulated vertical piston–cylinder device initially contains 15 kg of water, 9 kg of
which is in the vapor phase. The mass of the piston is such that it maintains a
constant pressure of 200 kPa inside the cylinder. Now steam at 1 MPa and 400°C is
allowed to enter the cylinder from a supply line until all the liquid in the cylinder is
vaporized. Assuming the surroundings to be at 25°C and 100 kPa, determine (a) the
amount of steam that has entered and (b) the exergy destroyed during this process.
[(a) 23.73 kg, (b) 7.61 MJ]