Figure 1b is a schematic of a turbine with condensation and with one controlled steam bleed for process and domestic heat demands. In these turbines, a portion of steam is bled from intermediate stages to be used by consumers. The remaining portion of the steam passes the subsequent turbine stages and after that passes to the condenser. The bleed pressure is kept steady regardless of the turbine load, a special regulator device being used for this purpose. In the turbine shown in Figure 1c, there are two controlled steam extractions at different pressures.
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Figure 1d is the schematic of a turbine with two pressures. This turbine uses not only fresh steam from the boiler, but also exhaust steam from hammers, presses, pump, air blower and compressor drives.
Steam turbine design is influenced by the turbine capacity, initial steam parameters (sub- and supercritical), its operation conditions within the power generation system (base-load, peak-load, semi-peak load), final steam moisture content, technological characteristics and other factors. Low capacity turbines (up to 50 mW) are as a rule of one-cylinder type.
The disadvantages of high capacity condensing turbines are connected with the limited flow rates of the final stages. To overcome this difficulty, these turbines are constructed with division of the main steam flow (before it enters the final stages) into several parallel flows. Each part of these turbines is designed for the maximum steam flow rate Qm (Figure 2.)
Steam turbines are used as parts of stationary and transport (marine) steam turbine power units. Besides turbines these power units also include boilers (steam-generators), steam condensers and other devices. Steam turbines constructed for combined operation with gas turbine units are also used as parts of combined steam-gas plants (see Gas Turbines) with applications in both stationary and transport (marine) power units.
The real cycle apbb'hta (Figure 4) of the simplest steam turbine unit includes the ap process of increasing pressure of the water in the pump, the pb process of heating water at constant pressure to the boiling temperature, and bb' process of evaporation at constant temperature. The b'h process corresponds to water superheating, the ht process corresponds to the expansion of steam in the turbine. The ta process which is the closing process of the cycle corresponds to heat removal in the condenser.
Reaction ratio at the middle diameter in the high pressure and intermediate pressure cylinders of steam turbines increases with the number of stages from 0.2 to 0.4, and in the low pressure cylinder from 0.3 to 0.7.
A variety of techniques are used for increasing the efficiency of steam-turbine units. One of the methods is increasing initial parameters of the steam. For example, when pressure ph is increased, the saturation temperature increases. The result is an increase of the average temperature at which heat is supplied; thus the thermal efficiency ηt of the ideal cycle increases too. However, in practice an increase of pressure to the value more than 9-10 MPa does not result in the increase of the theoretical work and does not significantly affect the unit efficiency. Also, steam moisture content at the end of the expansion process increases with the increase of pressure and results in greater losses in the course of the steam expansion and also in the turbine blade erosion. Therefore, the general tendency is to limit the moisture content to 13-15 per cent.
Simultaneous increase of the values of ph and Th may considerably increase the steam-turbine unit efficiency. For this purpose many present-day steam-turbine units have intermediate (repeated) superheating of the steam after expansion in the first group of stages. In this case the theoretical work of the turbine, the cycle work and thus the cycle effiency increase, the moisture of the steam at the end of the expansion process decreases, and the amount of heat transfered in the condenser increases. The temperature of superheating as well as the initial temperature is limited by the thermal characteristics of the flow passage metal parts.
The efficiency of steam-turbine units increases when regenerative extraction of steam from the turbine is used. Regenerative extraction is uncontrolled bleed of the steam from the stages with the aim of increasing the feed water temperature in the unit. In this cycle the feed water is heated by the heat released in the process of the steam cooling and condensation.
Condensing steam turbines have an efficiency in the range ηe = 36 to 42%. From this it follows that only a small portion of heat released in the process of fuel combustion is transformed into effective work. Turbine units for power and steam generation have higher overall efficiency. In these units the heat from the fuel is used for power generation and for obtaining heat at some prescribed temperature level. .The theoretical work of the unit with the steam turbine for power and heat generation is less than that of the steam turbine unit with condensing steam turbine. The useful work of the cycle of the steam turbine unit for power and heat generation is also lower than that of the condensing turbine. However, the steam turbine unit for combined power and heat generation makes effective use of the heat of condensation and therefore its overall efficiency is higher than that of a condensing steam turbine unit.
Abstract:In the last few years, waste-energy recovery systems based on the Organic Rankine Cycle (ORC) have gained increased attention in the global energy market as a versatile and sustainable technology for thermo-electric energy conversion from low-to-medium temperature sources, up to 350 C. For a long time, water has been the only working fluid commercially adopted in powerplants: axial and, for smaller machines, radial inflow turbines have been the preferred expanders since their gulp capacity matches the ρ-T curve of water steam. The density of most organic compounds displays extremely large variations during the expansion (and the volume flow rate correspondingly increases along the machine channels), so that Radial Outflow Turbines (ROTs) have been recently considered instead of traditional solutions. This work proposes a two-dimensional inviscid model for the stage optimization of a counter-rotating ROT, known as the Ljungström turbine. The study starts by considering five different working fluids that satisfy both the gulp requirements of the turbine and the hot source characteristics. On the basis of a limited number of geometric assumptions and for a fixed set of operating conditions, different kinematic parameters are optimized to obtain the most efficient cascade configuration. Moreover, as shown in the conclusions, the most efficient blade profile leads to higher friction losses, making further investigation regarding the best configuration necessary.Keywords: turbine CFD; Ljungström turbine; Organic Rankine Cycle
Fluid Mechanics and Thermodynamics of Turbomachinery is the leading turbomachinery book due to its balanced coverage of theory and application. Starting with background principles in fluid mechanics and thermodynamics, the authors go on to discuss axial flow turbines and compressors, centrifugal pumps, fans, and compressors, and radial flow gas turbines, hydraulic turbines, and wind turbines.
Additional types of turbomachine are considered and examples of high-speed characteristics are presented. The important ideas of specific speed and specific diameter emerge from these concepts and their application is illustrated in the Cordier Diagram, which shows how to select the machine that will give the highest efficiency for a given duty. Also, in this chapter the basics of cavitation are examined for pumps and hydraulic turbines. The measurement and understanding of cascade aerodynamics is the basis of modern axial turbomachine design and analysis. In Chapter 3, the subject of cascade aerodynamics is presented in preparation for the following chapters on axial turbines and compressors. This chapter was completely reorganized in the previous edition. In this edition, further emphasis is given to compressible flow and on understanding the physics that constrain the design of turbomachine blades and determine cascade performance. In addition, a completely new section on computational methods for cascade design and analysis has been added, which presents the details of different numerical approaches and their capabilities. Chapters 4 and 5 cover axial turbines and axial compressors, respectively. In Chapter 4, new material has been added to give better coverage of steam turbines. 2ff7e9595c
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