February 14, 2018


All the development discussed in above section are implemented in modern steam power plant. This is represented by modified Rankine Cycle illustrated in the fig. below. A power station operates using a closed steam power cycle, where water undergoes various thermodynamic process in a cycle. The figure shows a simplified diagram of a modern steam plant, showing most of the essential elements One half of the cycle, consist of the boiler (steam generator) and its auxiliaries. The other half, the turbine cycle, consist of turbine, generator, condenser, feed pump and feed water heaters. Feed water is supplied to the water wall via boiler drum through economizer, 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 turbine cylinder through main steam line via Emergency Stop Valves and Control Valves. The steam expands in the turbine giving up heat energy, a high proportion of which is converted 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 supplied to the IP cylinder through hot reheat lines via interceptor valves and control valves. After doing the work on IP rotor, the steam exhausted from IP cylinder is directly fed through cross around or cross over pipes to L.P. Cylinder. Finally the steam exhausted by LP cylinder goes to condenser. The shafts of HP-IP-LP and Generator are connected in tandem (series arrangement), so generator receives the mechanical power added by all the three cylinders. In condenser, which is a large surface-type heat exchanger, the steam is condensed by transferring its latent heat of evaporation to the cooling water. The steam having been condensed in the condenser is now in the form of condensate at very low pressure and saturation temperature. This condensate is pumped by condenser extraction pump from condenser hot well and is passed through the low pressure regenerative feed heating system and deaerator (direct contact heat exchanger) to increase the temperature of water and to remove dissolved gasses such as O2 and CO2 to form feed water suitable for boiler. Boiler feed pump sucks feed water from deaerator and pumps to boiler drum via high pressure regenerative feed heating system (HP Heater) and feed regulation station. In modern regenerative cycle, some of the steam passing through the turbine cylinder is bled from a series of extraction points located after selected moving blade stages and fed to the condensate heater (LP) and feed water heaters (HP), which are of surface type heat exchangers. The BFP increases the feed water pressure to a level in excess of the drum pressure, to the pressure loss in the boiler and HP feed heating circuit. The cycle is now complete. The modern cycle has been developed over a period of time and the developments were implemented in steam power plant. The same is represented in modified Rankine Cycle (TS Diagram) illustrated at the end of this section. The net effect of all the improvement was to achieve the higher efficiency of thermal cycle in the range of 45-50%. The development of the cycle is discussed and the efficiency improvement calculations are discussed in detail under section - Stem Cycle Theory and Cycle Constraints. The net effect of all the improvement is to achieve the higher efficiency of thermal cycle in the range of 40%-50%. However the overall efficiency of the unit is the product of efficiencies of the boiler, turbine, generator and thermal efficiency, therefore, will be reduced to 39-41% at rated parameters. Various efficiencies are listed below. a. Boiler Efficiency = 0.8-0.9 b. Thermal Cycle Efficiency = 0.4-0.5 c. Internal Efficiency of Turbine = 0.85-0.94 d. Mechanical Efficiency of Turbine = 0.99-0.995 e. Generator Efficiency = 0.98-0.985 Ŋ Thermal Plant (0.35-0.41) = ŋThermal Cycle X ŋBoiler X ŋTurbine Internal X ŋGen. = 0.88 X 0.47 X 0.95 X 0.99 X 0.98 Thermal Plant Efficiency = 0.3812 38.12%

Leave a Comment

There are no Comments at the moment

leave a comment