Steam Power Plants Interview Questions and Answers:

1. Why overall efficiency of a thermal power plant is very low ?

Ans. In a steam power station, more than 50% of total heat of combustion is lost as heat rejected to the condenser and this loss is unavoidable as heat energy cannot be converted into mechanical energy without a drop in temperature, and the steam in condenser is at the lowest temperature. This is the reason that the thermal efficiency of a power plant is quite low.

2. How is thermal efficiency of a steam power plant improved ?

Ans. The thermal efficiency of the plant mainly depends upon three factors (i) pressure and (ii) temperature of the steam entering the turbine and (iii) the pressure in the condenser. The thermal efficiency increases with the increase in temperature and pressure of the steam entering the turbine. For this reason high pressures and temperatures are used. The thermal efficiency is effectively increased by decreasing the pressure in the condenser. Pressure in the condenser is kept very low usually 0.04 kg/cm2.

The thermal efficiency also increases by reheating the steam between turbine stages, but is somewhat inconvenient. Bleeding of steam also affects the thermal efficiency.

3. What factors determine the choice of site of a thermal power plant ?

Ans. The factors governing choice of site for a thermal power plant for economical and efficient generation are (i) Availability  of supply of water (ii) Availability of coal (iii) Availability of land at a reasonable cost (iv) Type of land (v) Transportation facilities (vi) Facilities for disposal of ash (vii) Distance from populated area and (viii) Nearness to load centre.

4. Why thermal power stations are always situated by the side of a river or lake ?

Ans. The thermal power stations are always situated by the side of a river or lake so as to meet large quantity water requirement. Water is required in a steam power station (i) to raise the steam in boilers. (ii) for cooling purposes such as in condensers (iii) as a carrying medium such as in disposal of ash and (iv) for drinking purposes.

5. Why is the use of fire tube boilers restricted to low cost, small sire and low pressure plants ?

Ans. Fire tube boilers are compact in size, have low initial cost and have the ability to raise rapidly large quantities of steam per unit area of fire grate but have the following drawbacks.

As water and steam, both are in the same shell, higher pressure of steam are not possible. the maximum pressure which can be had is about 17.5 kg/cm2 and with a capacity of 15,000 kg of steam per hour. For higher pressures or higher rates of evaporation, the shell and fire tube boilers become extremely heavy and unwieldy. In the event of a sudden and major tube failure, steam explosions may be caused in the furnace due to rush of high pressure water into the hot combustion chamber which may generate large quantities of steam in the furnance.

Fire tube boilers use is, therefore, limited to low cost, small size and low pressure (to about 10 kg/cm2) plants.

6. What factors determine the size and type of boiler ?

Ans. The selection of the size and type of boiler depends upon (i) the output required in terms of amount of steam per hour, operating temperature and pressure (ii) availability of fuel and water and (iii) the probable load factor. The other factors which influence the choice of a boiler are availability, initial cost, maintenance costs, labour costs, fuel costs and the space requirement.

Water tube boilers are used where large amounts of steam are to be produced at high temperature and pressure and weight and space considerations are important. To meet a required demand, the choice between two boilers will be based on economic considerations i.e., total annual cost (fixed cost plus running cost). The noteworthy point is that the total cost of the fuel used by the boiler in its lifetime may be 3 to 4 times the initial investment.

7. What is the function of the deaerator in a steam power plant ?

Ans. The function of deaerator is to reduce dissolved oxygen content in the condensate (feedwater) by heating the feed water on the way from low pressure feed water heaters to the high pressure feed water heaters by the heat from bled steam.

8. Discuss the necessity of superheated steam in thermal power plant.

Ans. Superheated steam not only improves the efficiency but also have the following advantages:

  • Reduction in requirement of steam quantity for a given output of energy owing to its high internal energy reduces the turbine site.
  • Superheated steam being dry, turbine blades remain dry so the mechanical resistance to the flow of steam over them is small resulting in high efficiency.
  • No corrosion and pitting at the turbine blades occur owing to dryness of steam.

9. Why are superheaters and reheaters used in steam power plants ?

Ans. Superheaters are used in steam power plants to remove the last traces of moisture from the saturated steam leaving the boiler tubes and also to increase its temperature above the saturation temperature from the heat of combustion gases from the furnace.

Reheaters are used in steam power plants to resuperheat the partly expanded steam from the turbine so that the steam remains dry as far as possible through the last stage of the turbine.

10. Why economiser and air preheaters are used in steam power plants ?

Ans. Economiser and air preheaters are such devices which recover the heat from the flue gases on their way to the chimney and raise the temperature of feed water and air supplied for combustion respectively and thus improve the efficiency of steam plants.

11. How does air preheating save fuel ?

Ans. In preheating of the incoming air the heat recovered from the flue gases leaving the economiser is utilized, which otherwise would have gone waste. This raises the temperature of the furnace gases, improves combustion rates and efficiency, and lowers the stack temperature, thus improving the overall efficiency of the boiler.

12. Why the heating of water is necessary before it is supplied to the boiler ?

Ans. Heating of water, before being supplied to the boiler is necessary because of the following factors:

  • Overall plant efficiency is improved.
  • Thermal stresses due to cold water entering the drum of boiler are avoided.
  • There is an increase in the quantity of steam produced by the boiler.
  • The dissolved oxygen and carbon dioxide which would otherwise cause boiler corrosion are removed in the feed water heaters.
  • Some other impurities carried by steam and condensate, due to corrosion in boiler and condenser, are precipitated outside the boiler.

13. What is the function of the condenser in a steam power plant ?

Ans. The condenser does the job of condensing the steam exhausted from turbine and thus it helps in maintaining low pressure (below atmospheric) at the exhaust, thereby permitting expansion of steam in the turbine to a very low pressure. This improves the plant efficiency.

14. Why is it necessary to maintain high vacuum in a condenser ?

Ans. Maintenance of high vacuum in the condenser is essential for efficient operation. Any leakage of air into the condenser destroys the vacuum and causes (i) an increase in the condenser pressure which limits the useful heat drop in the prime mover and (ii) a lowering of the partial pressure of the steam and of the saturation temperature along with it. This means that the latent heat increases and therefore, more cooling water is required. Also, the under cooling of the condensate is likely to be more severe. This will result in lower efficiency.

15. How does the jet condenser differ from a surface condenser ?

Ans. The essential difference between a jet condenser and a surface condenser is that in the former, the exhaust steam mixes with the cooling water and the temperature of the condensate and the cooling water is the same when leaving the condenser; and the condensate cannot be recovered for use as feed water to the boiler; heat transfer is by direct conduction; in the latter i.e., in surface condenser, the exhaust steam and cooling water do not mix with each other, the water being circulated through a nest of tubes, the heat transfer being by convection. The temperature of the condensate may be higher than the temperature of the cooling water at outlet and the condensate is recovered as feed water to the boiler. Both the cooling water and the condensate are separately withdrawn.

Advantages of jet condensers are low initial cost, low requirements of floor area and cooling water and low maintenance charges.

However, its disadvantages are that the condensate is wasted and high power is required for pumping water.

16. What for evaporators are used in steam power plants ?

Ans. Evaportors are used for supplying pure water as make-up feed water in steam power plants.

17. Why are cooling towers used in thermal power stations ?

Ans. Cooling towers are provided to cool the hot water coming out from the condenser.

18. What is the function of steam nozzle ?

Ans. The function of a steam nozzle is to convert the heat energy of steam into kinetic energy and the chief use of a steam nozzle is to develop a high velocity jet of steam for driving a steam turbine.

19. On what factors does the selection of coal for a steam power plant depend ?

Ans. The selection of coal for a steam power plant depends on calorific value, weatherability, sulphur content, ash content, particle size, grindability index, caking characteristics etc.

20. Why are semi-bituminous coals one of the best coals for power generation ?

Ans. Semi-bituminous coals are the highest grade of bituminous coals having high fixed carbon content and the highest calorific values. These have volatile matter between 14 and 22 per cent and enthalpy of combustion between 27,000 and 35,000 kJ/kg. So semi-bituminous coals are one of the best coals for power generation.

21. Give the necessary requirements for efficient combustion of fuel.

Ans. The necessary requirements for efficient combustion of fuels are as follows.

  • The proper quantity of primary or secondary air required for complete combustion.
  • Adequate stoker or grate area required for burning particular quantity of fuel.
  • Minimum operating and maintenance cost.
  • Attainment of proper designed temperature.
  • There should be no formation of caking during burning of fuel.
  • The system should be easy to handle and dependable.

22. How does overfeed and underfeed stokers differ ?

Ans. Overfeed stoker and underfeed stoker differ in the manner of feeding coal below or above the level at which primary air is admitted in the furnace. In the case of underfeed stokers, fuel is supplied into the furnace below the point of air admission while in the case of overfeed stokers, the coal is supplied into the grate above the point of admission of air.

23. What is tangential firing ?

Ans. In tangential firing, the burners are placed in the corners of the furnace and they send horizontal streams of air and fuel tangent to an imaginary circle in the centre of the furnace.

24. What are the functions of primary air and secondary air in a pulverized coal fired boiler ?

Ans. The function of primary air in a pulverized coal fired boiler is to dry the coal and convey the powdered fuel to the furnace while that of secondary air is to complete the combustion.

25. Why is ‘excess oxygen’ required in steam power generation ?

Ans. The oxygen of heated air in combustion chamber reacts with the carbon of coal and form carbon dioxide during the process of combustion. Carbon dioxide so formed is taken away as draught through chimney. As coal is the basic fuel and for combustion of coal oxygen is necessary hence excess oxygen is required in steam power generation.

26. What do you understand by fluidized bed combustion ?

Ans. Fluidization is a new method of mixing fuel and air for obtaining combustion. A fluidized bed may be defined as the bed of solid particles behaving as a fluid.

27. What for draught system is employed in steam power plants ?

Ans. Draught system is employed to admit an amount of air into the furnace for the combustion of fuel and to cause the flow of the products of combustion from a chimney.

28. How does the induced draught differ from forced draught ?

Ans. In an induced draught system, the blower is installed near the base of the chimney and the burnt gases are sucked out of the boiler, reducing the pressure inside the boiler to less than atmospheric one. This induces fresh air to enter the furnace. In case of a forced draught system the blower is installed near the base of the boiler. In this system of draught the air pressure throughout the system is above the atmospheric pressure and air is forced to flow through the system (furnace, economiser, air preheater and chimney).

29. What is balanced draught ?

Ans. A balanced draught is a combination of induced draught and forced draught. In balanced draught system, the pressure of air below the boiler grate is above atmospheric and that above the grate is below atmospheric. The induced draught blower helps in sucking the products of combustion out of the system where as the forced draught helps in overcoming the resistance inside the boiler, thus enabling supply of sufficient air to the fuel bed for proper and complete combustion.

30. In a coal-fired power station mention three distinct uses of water.

Ans. Three distinct uses of water in a coal-fired thermal power station are (1) for generating steam in the boiler (ii) for condensing steam in the condenser and (iii) for washing the coal so as to reduce the ash content.

31. Why is treatment of feed water essential ?

Ans. Treatment of feed water is essential so as to remove the suspended and soluble solids and gases. These impurities in feed water may cause scale formation, corrosion, carry over and embrittlement in boiler and other apparatus.

32. What is zeolite process and how is it advantageous to other processes of treatment of water ?

Ans. Zeolite treatment is one of the ion exchanging process wherein cations of two or more positive charges are exchanged for sodium or hydrogen and zeolite is either a natural or artificial material that has the property of being able to exchange its base. Natural zeolites have a sodium base that can be exchanged for calcium or magnesium in raw water and the water will be softened. The advantage of this process is that the hardness is disposed off as a solution instead of solid requiring filteration.

33. Why is it necessary to employ non-salient pole field structure in a turbo-alternator ?

Ans. Turbo-alternators are driven by steam turbines which have high efficiency at large speeds, and therefore, synchronous machines driven by steams (i.e., turbo-alternators) are high speed machines. Non-salient pole field structure is used as the salient pole construction is impractical owing to large mechanical forces.

34. What is the largest size of thermal unit being used in India ?

Ans. The largest size of thermal unit being used in India is 500 MW.

35. Why thermal plants are not suitable for supplying fluctuating loads ?

Ans. Thermal power plants are not suitable for supplying fluctuating loads because any change in load demand requires corresponding change in the output energy. In the thermal plants input energy is produced by burning of coal and hence there is always a time lag between the change in energy at the output and input, which is not desirable and hence such power stations are used only as base load plants, for supplying constant power.

36. What is the running speed of a turbo-alternator in a central thermal power station ?

Ans. 3,000 rpm.

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