Thermodynamics UNIT III – Properties of pure substance - Important Questions and Tutorial problems

Thermodynamics UNIT III – Properties of pure substance - Tutorial problems

Important Two marks Questions

1.         What do you understand the terms super heating and sub cooling?

2.         What do you understand by specific steaming rate? State its unit

3.         Define the Sensible heating.

4.         Define the latent heating.

5.         Define triple point.

6.         Define dryness fraction.

7.         List out the advantage of superheated steam

8.         Define degree of superheat.

9.         Define pure substance and give examples

10.     Why Carnot cycle is not suitable for steam power plant?

11.     What is effect of reheat on cycle efficiency?

12.     What are critical states?

13.     What are saturation states?

14.     Draw the T-s diagram for regenerative cycle.

15.     What do you understand the terms super heating and sub cooling?

16.     What is the qualitative difference between heat and work?

17.     What are the four processes that make up the simple ideal Rankine cycle?

18.     The water is at 2.5 bar and 120C. Find the state of water

Ans. wet steam

19.     Define the saturation states

20.     Define the saturation pressure and temperature

21.     Draw p-T diagram for water and label all salient points.

Tutorial Problems

1. Steam initially at 1.5MPa and 250C expands reversible and adiabatically in a steam turbine to 0.1MPa. Determine the ideal work output of the turbine per kg of steam. Show the process in h-s diagram. Also find change in internal energy.

Ans.  X= 0.89 and change in internal energy = 414.9KJ/kg

2. Steam initially at 2.3 MPa and dry saturated. At the end of certain process it is found that the specific volume is 8.001m³/kg and temperature is 54C. Find the change in internal energy.

Ans. IE= 620kJ/kg; x =0.8

3. Steam initially at 5bar and 275C is cooled at constant volume. A) At what temperature will the steam becomes saturated vapour? b) What is the quality at 50C c) what is the amount of heat transferred per kg of steam while cooling from 275C to 50C?

Ans. a) T = 141 K; x =0.4; amount of heat transfer = 2462 kJ/kg.

4. Steam flows in pipeline at 2.7MPa. After expanding to 0.27MPa in a throttling calorimeter the temperature is found to be 150C. Find the quality of the steam in the pipeline.

Ans. X=0.97;

5. Steam at 65bar and 400C leaves the boiler to enter a steam turbine fitted with throttle governor. At a reduced load as the governor takes action the pressure of steam is reduced to 59 bar by throttling before it is admitted to the turbine. Find the increase in entropy due to irreversibility.

Ans. Change in entropy = 0.0413kJ/kgK

6. A mass of wet steam at a temperature of 165C is expanded at constant quality of 0.8 to pressure of 3bar. It is then heated at constant pressure to a degree of superheat of 66.5C. Find the enthalpy and entropy changes during expansion during heating. Draw h-s diagram.

Ans. Change in enthalpy during expansion = -58kJ/kg; heating = 637kJ/kg.

7. Steam initially at 30bar and 300C expands reversible condition in a steam turbine. The exit steam is at dry saturated condition. Find the ideal work output from the turbine per kg of steam. Also find change in sp. Volume.

Descriptive Questions - 

1.      In steam generator compressed liquid water at 10 Mpa, 30C enters a 30mm diameter tube at the rate of 3 liters/sec. steam at 9 Mpa, 400C exists the tube. Find the rate of heat transfer to the water.                                                                                            (8)    

Hint. Steam generator means which is nothing but a boiler, receives water may be from a pump and it deliver steam. Find the difference in enthalpy and multiply by mass of steam to get total heat added. Mass can be found from specific volume and rate of volume of water entering.

2.      Steam at 20 bar, 360C is expanded in a steam turbine to 0.08 bar. It then enters a condenser, where it is condensed to saturated liquid water. The pump feeds back the water into the boiler.         

a)      Assuming ideal processes find the network and the cycle efficiency per kg of steam.

b)      If the pump and turbine have 80% efficiency find the percentage reduction in the network and cycle efficiency                                                                          (8)

3.      In a Rankine cycle the steam at inlet to turbine is saturated at a pressure of 35 bar and the exhaust pressure is 0.2 bar. The flow rate of steam 9.5kg/s.  Find a) the pump work b) the turbine work c) Ranking efficiency d) condenser heat rejection e) work ratio f) specific steam consumption.                                                                                                              (8)

4.      In a steam turbine power plant the condition of steam at inlet to the steam generator is 20 bar and 300C and the condenser pressure is 0.1bar. Two feed water heaters operate at optimum temperature. Determine a) the quality of steam at turbine exhaust b) net work per kg of steam c) cycle efficiency d) the steam rate. Neglect pump work                    (8)

Hint. Optimum temperature can be found (300-Tsat @0.1bar)/3 intervals

5.      Consider a steam power plant operating on the diesel regenerative Ranking cycle with one open feed water heater. Steam enters the turbine at 15MPa and 600C and is condensed in the condenser at a pressure of 10kPa. Some steam leaves the turbine at a pressure of 1.2 MPa and enters the open feed water heater. Find the fraction of steam extracted from the turbine and the thermal efficiency of the cycle.                           (8)

6.      Ten kg of water at 45C is heated at a constant pressure of 10 bar until it becomes super-heated vapor at 300C. Find the changes in volume, internal energy and entropy.      (8)

7.      Consider a steam power plant that operates on a reheat Rankine cycle and has a net power output of 80MW. Steam enters the high pressure turbine at 10MPa and 500C and the low pressure turbine at 1MPa and 500C. Steam leaves the condenser as a saturated liquid at a pressure of 10 kPa. The isentropic efficiency of the turbine is 80% and that of the pump is 95%. Show the cycle on a T-s diagram with respect to saturation lines and determine,

a)      The quality of steam at turbine exit

b)      The thermal efficiency c) mass flow rate of steam.                                              (16)

8.      Consider a steam power plant operating on the ideal Rankine cycle. Steam enters the turbine at 3 MPa and 623K and is condensed in the condenser at a pressure of 10kPa. Find a) the thermal efficiency of this power plant b) the thermal efficiency if steam is super-heated to 873K instead of 623K. and c) the thermal efficiency if the boiler pressure is raised to 15MPa while the turbine inlet temperature is maintained at 873K.   (16)

Ans: a) 33%   b)37%   c)  43%