Unit IV: Properties of Pure Substances - Problems for Practice with Answer

 Problems for Practice: Properties of Pure Substances - Engineering Thermodynamics

PROBLEMS FOR PRACTICE

1. Calculate the state of a steam using steam tables.

(a) Steam has a pressure of 15 bar and specific volume of 0.12 m3/kg 

(b) Steam has a pressure of 10 bar and temperature of 200°C.

[Ans:- (a) wet and (b) wet.]

2. A 0.25 m3 vessel contains steam of 10 bar at 200°C. Find the pressure if the steam is cooled to dry saturated state at which temperature of 130°C isentropically.

[Ans:- p = 9.4 bar and x = 0.938 dry]

3. Find the volume, enthalpy and internal energy of steam 

(i) 500 kPa and 0.75 dry 

(ii) 0.1 MPa and 425°C.

[Ans:- (i) v = 0.2812 m3/kg, h = 2228.65 kJ/kg, u = 2080.95 kJ/kg, 

(ii) v = 0.3183 m3/kg, h = 3317.85 kJ/kg and u = 2999.47 kJ/kg]

4. Steam at 7 bar and 0.9 dryness fraction expands isothermally to 1.5 bar. Calculate the change in internal energy and change in enthalpy.

[Ans:- Δu = 218.995 kJ/kg and Δh = 247.11 kJ/kg]

5. Find the internal energy and enthalpy of steam when steam conditions are 10 bar (i) 0.7 dry (ii) 325°C.

[Ans:- (i) h = 2172.12 kJ/kg, u = 2036.11 kJ/kg and (ii) h = 3105.3 kJ/kg, u = 2835.1 kJ/kg).

6. Find the enthalpy and internal energy of unit mass of steam at temperature of 200°C when (a) dry and saturated (b) steam delivered at 0.7 MPa and (c) quality of steam is 0.85.

[Ans:- (a) h = 2790.9 kJ/kg, u = 2593.18 kJ/kg, (b) h = 2844.2 kJ/kg, u = 2634.27 kJ/kg and (c) h = 2500.12 kJ/kg, u = 2332.06 kJ/kg]

7. A vessel having a capacity of 0.05 m3 contains a mixture of saturated water and saturated steam at a temperature of 245°C. The mass of the liquid present is 10 kg. Find the following:

(i) Pressure

(ii) Mass

(iii) Specific volume

(iv) Specific humidity

(v) Specific entropy, and

(vi) Specific internal energy.

 [Ans:- p = 36.523 bar, m = 10.69 kg, v = 0.004678 m3/kg, h = 1173.156 kJ/kg, s = 2.964 kJ/kgK and u = 1156.07 kJ/kg]

8. Steam is 10 bar and 0.95 dry from a boiler is sent to superheated to 400°C at constant pressure. Using Mollier diagram, find the heat supplied and change in internal energy. 

[Ans:- Q = 565 kJ/kg and Δu = 465 kJ/kg]

9. In a steam engine, the steam at the beginning of the expansion process is at 7 bar, dryness fraction 0.95 and the expansion follows the pV1.1 = constant to a pressure of 0.34 bar. Calculate the work done per kg of steam during the expansion and heat flow per kg of steam to or from the cylinder walls during the expansion.

10. Steam initially at 0.3 MPa, 300°C is cooled at constant volume. (a) At what temperature I will the steam become saturated vapour? (b) What is the quality at 70°C? and (c) What is the heat transferred per kg of steam in cooling from 300°C to 70°C?

11. Steam initially at 1 MPa and 300°C expands reversibly and adiabatically in a steam turbine to 40°C. Determine the ideal work output of the turbine per kg of steam.

12. Steam at 1 MPa and 300°C and flowing at the rate of 1 kg/s passes into a pipe carrying wet steam at 1 MPa, 0.95 dry and flow rate 3 kg/s after mixing. Determine the condition of steam after mixing.

13. The mixture is now expanded in a frictionless nozzle isentropically to a pressure of 0.4 MPa. Determine the final velocity of the nozzle. Neglect the initial velocity of steam in the pipe line.

14. A portion of steam from a boiler drum at 0.5 MPa is passed through a throttling calorimeter after which the pressure and temperature are found to be 700 kPa and 120°C. 

(a) Find the dryness fraction of steam taken from the boiler.

(b) If the calorimeter must read at least 3°C superheat for acceptable results, what is the minimum dryness fraction that could be measured under the given conditions?