CHAPTER 4 STEAM TURBINE and CYCLE HEAT BALANCE

Transcription

1 CHAPTER STEAM TURBINE and CYCLE HEAT BALANCE.1. Steam Turbine Principles Steam Turbine Analysis Arrangements of Steam Turbines..... Heat Balance System Performance... 7 Chapter 1

2 .1. Steam Turbine Principles A steam turbine is a device that converts thermal energy of steam into kinetic energy and then into mechanical energy (i.e. mechanical work). Steam turbines are generally classified into two groups: impulse and reaction. Impulse force and reaction force are shown in Fig..1. Figure.1. Principle of Reaction and Impulse Forces Figure.2. Principles of Turbines In impulse turbines steam expands in stationary nozzle to attain a high velocity and then flows over the moving blades, converting some of its kinetic energy into mechanical work. In reaction turbines, steam expands both in stationary nozzle on wheel and mowing blades. In Chapter 2

3 practice, the turbines used in power generation always have both impulse and reaction sections..2. Steam Turbine Analysis Turbines convert enthalpy of the working fluid to mechanical work. A steam turbine has many stages, each of which generally consists of one row of stationary nozzles and one row of moving blades. Each stage is designed to convert a certain amount of thermal energy into mechanical energy (i.e. shaft work). Superheated Steam 1 Blades Stages W out Steam Extraction Lines Figure.3. Turbine schema 2 Exhaust The analysis of turbines starts with the first law. Assuming that the turbine operates in steady state, then the mass flow rate is constant throughout. Also, assuming that the change in enthalpy is much, much greater than the change in kinetic and potential energy, then the first law becomes W ( h ) t, out msteam 1 h2 = & Turbine performance is measured by turbine efficiency. To determine it, the following equation is used Chapter 3

4 η turb = h1 h h h 1 2 2s To determine the isentropic exhaust enthalpy, use the pressure of the real exhaust as one known property, then use the entropy from the inlet steam, as an isentropic process means constant enthalpy. Therefore, with entropy and pressure, you can determine the state and the enthalpy. As it is seen in Figure.3, the turbine has an extraction line after each stage. Thus, the mass flow rate of the steam decreases while it goes from the first stage to the last stage. Therefore, when we calculate the turbine work out, we have to consider each stage as a single turbine. Then one can write the turbine work as ( h h ) W t out = Wstage, i = mi i, in i, &, i i out.3. Arrangements of Steam Turbines Chapter

5 Tandem-Compund 2 flows Tandem-Compund Flows 1 2 Cross-Compiund 2 Flows Cross-Compund Flows Figure.. Turbine Arrangements. (1. High Pressure Turbine, 2. Intermediate Pressure Turbine, 3. Low Pressure Turbine,. Reheater,. enerator.) Chapter

6 .. Heat Balance A Rankine cycle with condensing turbine mainly consists of four components: steam generation units, turbine, condenser, and pump. The actual steam turbine cycle is much more complicated. The general objective of heat and mass balance is to determine the system performance that is measured in terms of turbine Net Heat Rate (NHR) or ross Heat Rate (HR). To investigate the system performance, firstly, we have to look at the pump configuration. The pump (or pumps) is driven in three ways: Turbine-Driven Pump Electrical Motor-Driven Pump Diesel Motor-Driven Pump Turbine-Driven Pump Cycle: HR = (Btu/kWh) enerator Output + Auxiliary Turbine Output NHR = (Btu/kWh) enerator Output Electrical Motor-Driven Pump Cycle: HR = enerator Output NHR = enerator Output - Electrical Power Reqired for pump Diesel Motor-Driven Pump Cycle: HR = NHR = enerator Output Chapter 6

7 .. System Performance The performance of the turbine system is generally expressed in terms of turbine net heat rate. In practice, the factors listed below are related to the steam turbine system performance: Condenser pressure Steam inlet pressure Low-pressure turbine exhaust end. Chapter 7