Steam Power Plants

Q. Why the thermal efficiency of a steam power plant is quite low?

Ans. In a steam power station, more than 50% of the 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 the condenser is at the lowest temperature. This is the reason that the thermal efficiency of a power plant is quite low.

Q. 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 us in condensers
(iii) as a carrying medium such as in disposal of ash and
(iv) for drinking purposes

Q. 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 (feed water) by heating the feed water on the way from low-pressure feedwater heaters to the high-pressure feed water heaters by the heat from bled steam.

Q. 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 re-superheat the partly expanded steam from the turbine so that the steam remains dry as far as possible through the last stage of the turbine.

Q. 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.

Q. 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.

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

Ans. The condenser does the job of condensing the steam exhausted from the 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.

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

Ans. Evaporators are used for supplying pure water as makeup feed water in steam power plants.

Q. What is the function of the 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.

Q. 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.

Q. 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 to 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 overfeeding stokers, the coal is supplied into the grate above the point of admission of air.

Q. What is tangential firing?

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

Q. 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.

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