Steam power plant basically operates on the Rankine cycle. Coal is burnt in a boiler, which converts water into steam. The steam is expanded in a turbine, which produces mechanical power driving the alternator coupled to the turbine. The steam after expansion in prime mover (turbine) is usually condensed in a condenser to be fed into the boiler again. In practice, however, a large number of modifications and improvements have been made so as to effect economy and improve the thermal efficiency of the plant.
The entire arrangement for sake of simplicity may be divided into four main circuits namely:
- Fuel and ash circuit
- Air and fuel gas circuit
- Feed water and steam circuit and
- Cooling water circuit.
1. Fuel and Ash Circuit
Coal is delivered from the supply points to the storage site by road, rail or water. The coal after necessary landing and treatment (handling) is passed on to the furnaces through the fuel feeding devices. Ash resulting from combustion of coal collects at the back of the boiler and is removed to the ash storage by means of scrap conveyors. Combustion is controlled by controlling the great speed, quantity of coal entering the grate and the damper openings.
2. Air and Fuel Gas Circuit
Air is drawn from the atmosphere by a forced draught fan or induced draught fan through the air pre-heater, in which it is heated by the heat of flue-gases passing to chimney, and then admitted to the furnaces. The flue-gases after passing around boiler tubes and super heater tubes are drawn by the induced draught fan through dust collector (or precipitator) economizer and air pre-heater and finally exhausted to the atmosphere through chimney.
3. Feed Water and Steam Circuit
The condensed water is extracted from the condenser by the condensate pump and then forced to the I p feed water heater; where its temperature is raised by the heat from bled steam. The feed water is now pumped to high pressure water heater, where it gets heated by the heat from bled steam extracted at suitable point of steam turbine. It is then pumped into boiler through economizer, in which it is further heated by the heat of flue gases. In boiler water is converted into high pressure steam which is wet. Wet steam is passed through super-heater, where it is dried and further super heated, and then supplied to the steam turbine through the main valve. After giving out its heat energy to the turbine it is exhausted to the condenser where its latent heat is extracted and steam is converted into feed water. At one or more stages a quantity of steam is bled or withdrawn and steam is converted into feed water. Making up water for boiler is taken through the evaporator, where it is heated by low pressure steam extracted at suitable points of turbine.
4. Cooling Water Circuit
Cooling water is supplied from a natural source of supply such as river, canal, sea or lake or cooling towers through screens to remove the matter that might choke the condenser tubes. It is circulated through the condenser for condensing the steam and finally discharged to the suitable position near the source of supply. During the passage, its temperature rises and in the case of cooling towers the heat must be extracted before the water is again pumped to the condenser. The circulation of cooling water to the condenser helps in maintaining a low pressure in the condenser.
