Problems for Practice

PROBLEMS FOR PRACTICE

1. A mass of 10 kg of air at 400 K is mixed with 20 kg of air at 300 K. Determine the change in entropy of universe.

 [Ans:- [ΔS = -0.238 kJ/K]

2. In a steam generator, water is evaporated at 275°C while the combustion gas (Cp = 1.08 kJ/kgK) is cooled from 1500°C to 350°C. The surroundings are at 30°C. Determine the loss in available energy due to the above heat transfer per kg of water evaporated (Latent heat of vaporization of water at C = 1675 kJ/kg).

[Ans:- A.E = 584.79 kJ]

3. 0.5 kg of air of 400° C is heated reversibly at constant pressure to 2100 K. Find the available and unavailable energies of the heat added. Take To = 30°C and Cp = 1.0047 kJ/kgK.

[Ans:- A.E = 544.45 kJ and U.A.E = 173.18 kJ]

4. 15 kg of water 100°C are mixed with 25 kg of water at 75°C while the temperature of the surroundings is 15°C. Determine the decrease in available energy due to mixing.

[Ans: A.E = 27.28 kJ]

5. Air flows through an adiabatic compressor at 3 kg/s the inlet conditions are 2 bar and 310 K and the exit conditions are 20 bar and 560 K. Compute the net rate of availability transfer and the irreversibility. Take To = 298 K.

[Ans:- A.E = 813.22 kJ and I = 59.47 kW]

6. 2 kg of air at 125°C is heated reversibly at constant volume to 1450°C. Find the available and unavailable energy of the heat added. Take To = 25°C, Cp = 1.005 kJ/kgK and R = 0.287 kJ/kgK.

[Ans:- A.E = 1275.63 kJ and U.A.E = 627.07 kJ]

7. One kg of ice at -10°C is exposed to the atmosphere which is at 25°C. The ice melts and comes into thermal equilibrium with the atmosphere. Determine the a) entropy increase of the universe and b) what is the minimum back into ice at - 10°C? Take Cp of ice 2.093 kJ/kgK and the latent heat of fusion of ice is 335 kJ/kg.

[Ans:- ΔS = -0.238 kJ/K and Wmin = 69.06 kJ]