Modification of Rankine cycle aims to improve the efficiency of the cycle, in this case made the extraction of steam from turbine generator to heat feedwater, so the steam boiler can work easily and reduce requirement of fuel. Practically, turbine generator with high initial pressure usually uses number of extraction 5 until 7 extraction. Number of extraction can be added 8 – 9 extraction if turbine generator has parameter of critical hot steam. For steam turbine which has middle pressure, the number of extraction is usually limited only 1 until 4.
One of modification of Rankine cycle can be seen in the figure below:
|Figure 1: Rankine Cycle with 1 Extraction|
Steam heat up from boiler is distributed to turbine generator, after going through several levels of turbine blades, some of steam are extracted to the deaerator, while the remaining steam goes to condenser and to be condensed there. Furthermore, water from condenser is also pumped to deaerator. Steam from turbine will mix with water from condenser in the deaerator. From deaerator process, water will be supplied into steam boiler to be converted into steam then will be distributed again into turbine generator.
The purpose of steam is extracted to deaerator is discharge gases that are not condensed, so the heating process in the boiler will be effective, prevent corrosion and increase efficiency. To simplify thermodynamic cycle analysis, the processes mentioned above is simplified in the form of following diagram:
|Figure 2: Diagram T-S of Rankine Cycle with 1 Extraction|