Properties Of Pure Substance and Steam Power Cycle | Interview Question

Properties Of Pure Substance and Steam Power Cycle | Interview , viva , oral Question and Answers

1. What do you understand by pure substance?

A pure substance is defined as one that is homogeneous and invariable in chemical composition throughout its mass.

2. Draw a p-T diagram for a pure substance?

3. Is iced water a pure substance? Why?

Yes iced water is a pure substance.

Explanation:
Both ice and liquid water are the same substance. H2O Though ice water is a mixture of both solid and liquid it is a pure substance, based on the molecular structure of its components.

4. Distinguish between flow process and non-flow process.

Flow process : It is one in which fluid enters the system and leaves it after work interaction, which means that such processes occur in the systems having open boundary permitting mass interaction across the system boundary.

Non flow process: It is the one in which there is no mass interaction across the system boundaries during the occurrence of process.

5. Why Rankine cycle is modified?

The work obtained at the end of the expansion is very less. The work is too inadequate to overcome the friction. Therefore the adiabatic expansion is terminated at the point before the end of the expansion in the turbine and pressure decreases suddenly, while the volume remains constant.

6. Why Rankine cycle is modified?

7. Define efficiency ratio.

The ratio of actual cycle efficiency to that of the ideal cycle efficiency is termed as efficiency ratio.

8. Define overall efficiency.

It is the ratio of the mechanical work to the energy supplied in the fuel. It is also defined as the product of combustion efficiency and the cycle efficiency.

9. Define specific steam consumption of an ideal Rankine cycle.

It is defined as the mass flow of steam required per unit power output.

10. Name the different components in steam power plant working on Rankine cycle.

Boiler, Turbine, Cooling Tower or Condenser and Pump.

11. What are the effects of condenser pressure on the Rankine Cycle?

By lowering the condenser pressure, we can increase the cycle efficiency. The main disadvantage is lowering the back pressure in release the wetness of steam. Isentropic compression of a very wet vapour is very difficult.

12. Mention the improvements made to increase the ideal efficiency of Rankine cycle.

1. Lowering the condenser pressure.
2. Superheated steam is supplied to the turbine.
3. Increasing the boiler pressure to certain limit.
4. Implementing reheat and regeneration in the cycle.

13. What is the effect of reheat on (a) the network output, (b) the cycle efficiency and (c) steam rate of a steam power plant?

(a) The network output – increase
(b) The cycle efficiency – increase
(c) Steam rate of a steam power plant – decrease

14. Why reheat cycle is not used for low boiler pressure?

At the low reheat pressure the heat cycle efficiency may be less than the Rankine cycle efficiency. Since the average temperature during heating will then be low.

15. What are the disadvantages of reheating?

Reheating increases the condenser capacity due to increased dryness fraction, increases the cost of the plant due to the reheats and its very long connections.

16. What are the advantages of reheat cycle?

1. It increases the turbine work.
2. It increases the heat supply.
3. It increases the efficiency of the plant.
4. It reduces the wear on the blade because of low moisture content in LP state of the turbine.

17. Define latent heat of evaporation or Enthalpy of evaporation.

The amount of heat added during heating of water up to dry steam from boiling point is known as Latent heat of evaporation or enthalpy of evaporation.

18. Explain the term super-heated steam and super heating.

The dry steam is further heated its temperature raises, this process is called as super heating and the steam obtained is known as superheated steam.

19. Explain heat of super heat or super heat enthalpy.

The heat added to dry steam at 100oC to convert it into super-heated steam at the temperature T sup is called as heat of superheat or super heat enthalpy.

20. Explain the term critical point, critical temperature and critical pressure.

In the T-S diagram the region left of the waterline, the water exists as liquid. In right of the dry streamline, the water exists as a super-heated steam. In between water and dry steam line the water exists as a wet steam. At a particular point, the water is directly converted into dry steam without formation of wet steam. The point is called critical point. The critical temperature is the temperature above which substance cannot exist as a liquid; the critical temperature of water is 374.15oC. The corresponding pressure is called critical pressure.

21. Define dryness fraction (or) what is the quality of steam?

It is defined as the ratio of mass of the dry steam to the mass of the total steam.

22. Define enthalpy of steam.

It is the sum of heat added to water from freezing point to saturation temperature and the heat absorbed during evaporation.

23. How do you determine the state of steam?

If V>vg then super-heated steam, V= vg then dry steam and V< vg then wet steam.

24. Define triple point.

The triple point is merely the point of intersection of sublimation and vaporization curves.

25. How is Triple point represented in the p-V diagram?

26. Define heat of vaporization.

The amount of heat required to convert the liquid water completely into vapour under this condition is called the heat of vaporization.

27. Explain the terms, Degree of super heat, degree of sub-cooling.

The difference between the temperature of the superheated vapour and the saturation temperature at the same pressure. The temperature between the saturation temperature and the temperature in the sub cooled region of liquid.

28. When is reheat recommended in a steam power plant?

The purpose of reheating is to increase the dryness fraction of the steam passing out of the later stages of the turbine.

29. What are the processes that constitute a Rankine cycle?

Process 1–2: Isentropic expansion of the working fluid through the turbine from saturated vapor at state 1 to the condenser pressure.
Process 2–3: Heat transfer from the working fluid as it flows at constant pressure through the condenser with saturated liquid at state 3.
Process 3–4: Isentropic compression in the pump to state 4 in the compressed liquid region.
Process 4–1: Heat transfer to the working fluid as it flows at constant pressure through the boiler to complete the cycle.

30. State the advantages of using superheated steam in turbines.

Superheated steam is a steam at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured. The steam can therefore cool (lose internal energy) by some amount, resulting in a lowering of its temperature without changing state (i.e., condensing) from a gas, to a mixture of saturated vapor and liquid.