Steam Power Station : Part1

A generating station which converts the heat energy of local combustion into an electrical energy is called steam or thermal power station.

In this power station, the steam is produced in the boiler by using the heat of the cool combustion. The steam is then expanded in steam turbine which drives the alternator which converts the mechanical energy of the turbine into an electrical energy. The exhaust steam gets condensed in the condenser and fed back into the boiler again, completing the cycle of the power station. This principle is called Rankine cycle.

The energy conversion involved in steam power station is shown in the Fig. 1.

Fig. 1 Energy conversion

1.1 Factors of Selection of Site

The following factors are to be considered for the selection of site for the steam power station, in order to achieve the economical and successful operation of the plant.

1. Supply of fuel : The main fuel for the steam power plant is coal. Thus the power station should be located near the coal mine so that the fuel supply is continuous and adequate. If the plant is located away from the coal mine then sufficient transportation facility must be available.

2. Availability of Water : For the condenser, huge amount of water is required. Hence site must near be the river so that abundant quantity of cooling water is available.

3. Transportation facilities : For transporting the equipments and the machinery required by a modern steam power plant, he site selected must be easily accessible by rail and road.

4. Cost and type of land : The land must be available at a reasonable price to keep the initial cost low. There must be provision for the extension of the plant. The type of the land must be such that it should be able to withstand the weight of the heavy equipments to be installed.

5. Distance from load centers : To keep the cost of the transmission and transmission losses to minimum, the site must be nearer to the load centers. For d.c. system, transmission loss plays an important role but a.c. power can be transmitted at high voltage with reduced transmission cost. Thus this factor is more important for d.c. supply system.

6. Distance from populated area : The continuous burning of coal at the power station produces smoke, Fumes and ash, which pollutes the surrounding area. Such a pollution due to smoke is dangerous for the people living around. Hence the site of the plant must be at a considerable distance from the populated area.

All these factories affect the selection of site for the steam power station.

1.2 General Arrangement of Steam Power Plant

Though steam power plant simply involves the conversion of heat energy to the mechanical energy, it requires many types of supporting arrangements. The Fig. 2 shows the schematic arrangement of steam power station.

The coal is burnt in a place called grate in a boiler. The flue gases are evolved which heats the water in a boiler. The water is converted to a steam by absorbing heat from the flue gases. This steam is called wet steam as it contains suspended water particles. This steam is passed to the superheater where it is converted to superheated steam from the wet steam. This superheated steam is then expanded in the turbine which rotates the turbine. Thus the heat energy is converted to a mechanical energy. The turbine shaft is coupled to an alternator which converts the mechanical energy into an electrical energy. This is then given to the busbar through a transformer and proper switchgear arrangement.

After expanding in the turbine, the exhaust steam is passed through the condenser. In the condenser, the steam is converted into liquid condensate. Using the condensate extraction pump, the condensate is taken to economizer. The economiser again transfer the heat from flue gases to the condensate and then transfer the heated water to the boiler. Thus the cycle is completed. The exhaust flue gases are released to the atmosphere through the chimney.