3.1.1 The steam cycle
A power station operates using a closed steam power cycle, where water undergoes various thermodynamic processes in a cyclic process. Figure 1.21 is a simplified diagram of a modern steam plant, showing most of the essential elements.
One half of the cycle consists of the boiler (or heat source) and its auxiliaries; the other, the turbine cycle, consists of turbine, generator, condenser, feed pump and feedwater heaters.
Consider first the boiler plant involved in the cycle. Feedwater is supplied to the boiler drum, where water is boiled and converted into dry saturated steam. This dry steam is further superheated in the superheater and then fed to the HP cylinder of the turbine. The steam expands in the turbine giving up heat energy, a high proportion of which is transferred into work energy on the turbine shaft. The shaft turns an electrical generator which produces electric power. Steam leaving the HP cylinder returns to the boiler, where it is reheated. The reheated steam is further expanded in the IP and LP cylinders, before passing into the condenser.
In the condenser, which is a large surface-type heat exchanger, the steam is condensed by transferring its latent heat of vaporisation to the cooling water (CW). The main steam, having been condensed in the condenser, is now in a liquid state at a very low pressure and approximately saturation temperature. This water drains from the condenser, where it enters the hotwell. The water in the hotwell is pumped by the condensate extraction pump through the low pressure feedheating system to another pump, the boiler feed pump.
In a modern regenerative cycle, some of the steam passing through the turbine cylinders is bled from a series of extraction belts located after selected moving blade stages and fed to the condensate and feedwater heaters. This steam is used to heat the condensate in the LP heaters and the feedwater in the HP heaters, which are of a surface type.
The boiler feed pump increases the water pressure to a level in excess of the drum pressure, to provide for the pressure loss in the boiler circuit and HP feedheating train. The cycle is now complete.