Equipments of Steam Power Plant
A modern steam power station is highly complex and it requires numerous main equipments and auxiliary equipments.The important equipments of steam power station are
1.Steam Generating Equipment
4.Water Treatment Plant
5. Electrical Equipment
1.Steam Generating Equipment
It is an important part of steam power station and it is mainly concerned with the generation of superheated steam.It includes items such as boiler,boiler furnace,super heater,economiser,air pre- heater and other heat reclaiming devices
Boiler is a closed vessel in which heat energy obtained by coal combustion is used to convert water into steam and it could be broadly classified into two
(a)Water tube boilers (b) Fire tube boilers
In a water tube boiler water flows through the tube and the hot gases produced by combustion flows over the tube there by heating it.But in the case of Fire tube boilers the combustion gases and other byproducts of combustion flows through the tube surrounded by water.Water tube boilers are more advantageous than fire tube boilers since it requires less space,smaller size of tubes and drums,high working pressure due to small drum,less chance of explosion etc,So we choose water tube boilers of large capacity in steam power stations.The pictures of boiler control board and water tube boiler is given below respectively.
1.2 Boiler Furnace
Boiler furnace is a chamber where fuel(coal) is burnt in order to produce heat energy.In addition to this it provides support and enclosure to combustion equipment ie burner.The walls of boiler furnace are made of refractive materials such as fire clay,silica,kaolin etc.These materials has got the property to retain size,shape,weight and other physical properties even at high temperatures.Furnace walls could be of three types they are
(a) Plain refractory walls
(b) Hollow refractory walls with an arrangement for air cooling.
(c) Water walls
Picture of a drum level controller is shown below
Plain refractory walls are used in small furnace where the temperature is not high.In large plants the temperature is much high and so the plain refractory furnace may get damaged so we use hollow walls with air circulating arrangement to reduce temperature.The latest technique is the water walls in that plain tubes are arranged side by side on the inner face of the refractory walls and these tubes are connected to the upper and lower headers of the boiler.The boiler water is made to circulate through the tubes which would absorb the heat from the furnace walls and makes it cool.The picture of fiber and shell used as refractory material is shown below.
A superheater is a device which superheats the steam i.e., it raises the temperature of steam above boiling point of water. This increases the overall efficiency of the plant. A superheater consist of a group of tubes made of special alloy steels such as chromium-molybdenum. These tubes are heated by heat of flue gases during their journey from the furnace to the chimney. The steam produced in the boiler is led through the superheater where it is superheated by the heat of flue gases. Superheater are mainly classified into two types according to the system of heat transfer from flue gases to steam viz.
(a) Radiant superheter (b) Convection superheater
The radiant superheater is placed in the furnace between the water walls and receives heat from the burning
fuel through radiation process. It has two main disadvantages. Firstly,due to high furnace temperature, it may get overheated and, therefore, requires a careful design. Secondly the temperature of superheater falls with increase in steam output. Due to this limitations, radiant superheater is not finding favour these days. On the other hand, a convection superheater is placed in the boiler tube bank and receives heat from flue gases entirely through the convection process.It has the advantage that temperature of superheater increases with the increase in steam output. For this reason, this type of super is commonly used these days.
It is device which heats the feed water on its way to boiler by deriving heat form flue gases.This results in raising boiler efficiency, saving in fuel and reduced stresses in the boiler due to higher temperature of feed water. An economiser consists of a large number of closely spaced parallel steel tubes connected by header of drums. The feed water flows through these tubes and the flue gases flow outside. A part of the heat of flue gases is transferred to feed water,thus raising the temperature of the latter.
1.5 Air pre-heater
Superheater and economisers generally cannot fully extract the heat from flue gases.Therefore, pre-heater are employed which recover some of the heat in the escaping gases. The function of an air pre-heater is to extract heat from the flue gases and give it to the air being supplied to furnace for coal combustion. This raises the furnace temperature and increases the thermal efficiency of the plant.Depending upon the method of transfer of heat from flue gases to air, air pre-heaters are divided into the following two classes:
(a) Recuperative type (b) Regenerative type
The recuperative type air heater consists of a group of steel tubes.the flue gases are passed through the tubes while the air flows externally to the tubes. thus heat of flue gases is transferred to air. The regenerative type air pre-heater consists of slowly moving drum made of corrugated metal plates. The flue gases flow continuously on one side of the drum and air on the other side. This action permits the transference of heat of flue gases to the air being supplied to the furnace for coal combustion.
Induced draught fan and chimney is shown below