Turbines, thermodynamics

Figure 2.5 Gas turbine thermodynamic cycle. Simple-cycle gas turbine.

Several terms and concepts are commonly used in the discussion of actual steam turbine thermodynamic performance characteristics. 

In apphcation to electric utihty power generation, MHD is combined with steam (qv) power generation, as shown in Figure 2. The MHD generator is used as a topping unit to the steam bottoming plant. From a thermodynamic point of view, the system is a combined cycle. The MHD generator operates in a Brayton cycle, similar to a gas turbine the steam plant operates in a conventional Rankine cycle (11). 

Fig. 9. Brayton cycle, where A = compressor inlet, B = combustor inlet, C = power turbine inlet, and D = exhaust (a) thermodynamic relationships and...

Fig. 12. Combustion turbine engine simple cycle (a) schematic of plant and (b) thermodynamics, where the horizontal lines correspond to the pressure...

Rankine Cycle Thermodynamics. Carnot cycles provide the highest theoretical efficiency possible, but these are entirely gas phase. A drawback to a Carnot cycle is the need for gas compression. Producing efficient, large-volume compressors has been such a problem that combustion turbines and jet engines were not practical until the late 1940s. 

The actual steam rate is obtained by dividing the theoretical steam rate by the turbine efficiency, which includes thermodynamic and mechanical losses. Alternatively, internal efficiency can be used, and mechanical losses applied in a second step. 

When testing to estabhsh the thermodynamic performance of a steam turbine, the ASME Performance Test Code 6 should be followed as closely as possible. The effec t of deviations from code procedure should be carefully evaluated. The flow measurement is particularly critical, and Performance Test Code 19 gives details of flow nozzles and orifices. The test requirements should be carefully studied when the piping is designed to ensure that a meaningful test can be conducted. 

Turbine-Blade Cooling The turbine inlet temperatures of gas turbines have increased considerably over the past years and will continue to do so. This trend has been made possible by advancement in materials and technology, and the use of advanced turbine bladecooling techniques. The olade metal temperature must be kept below 1400° F (760° C) to avoid hot corrosion problems. To achieve this cooling air is bled from the compressor and is directed to the stator, the rotor, and other parts of the turbine rotor and casing to provide adequate cooling. The effect of the coolant on the aerodynamic, and thermodynamics depends on the type of cooling involved, the temperature of the coolant compared to the mainstream temperature, the location and direction of coolant injection, and the amount of coolant. 

The surprise was finally clarified by remembering that this was an air operated plant built in a thermodynamic cycle, (the Brayton or gas turbine cycle) with a 18,000 HP air compressor. This generated 5 MW of salable... 

There is a Second Law thermodynamic advantage in operating an expander at as low a temperamre as possible. In most applications it has been aiTanged to discharge just above tlie dew point of tlie expanded gas. If the cold compressed gas could enter tlie expander at or near its dew point, the expander would then operate condensing and at the lowest possible temperamre. Such condensate has traditionally been troublesome in turbines, but tliis has been solved in modern turboexpanders. 

The thermodynamic analysis presented here is an outline of the air-standard Bray ton cycle and its various modifications. These modifications are evaluated to examine the effects they have on the basic cycle. One of the most important is the augmentation of power in a gas turbine, this is treated in a special section in this chapter. 

Figure 9-6. Schematic of an Impulse turbine showing the variation of the thermodynamic and fluid mechanic properties.

Gehring, S., and Riess, W., Through-Flow Analysis for Cooled Turbines. 3rd Conference on Turbomachinery-Fluid Dynamics and Thermodynamics, London March, 1999. 

Another reason for the increased use of gas turbines as prime movers in the process industry is the high thermodynamic cycle efficiencies and subsequent low operating cost. 

Usually, a gas turbine plant operates on open circuit , with internal combustion (Fig. 1.3). Air and fuel pass across the single control surface into the compressor and combustion chamber, respectively, and the combustion products leave the control surface after expansion through the turbine. The open circuit plant cannot be said to operate on a thermodynamic cycle however, its performance is often assessed by treating it as equivalent to a closed cyclic power plant, but care must be taken in such an approach. 

An important field of study for power plants is that of the combinedplant [ 1 ]. A broad definition of the combined power plant (Fig. 1.5) is one in which a higher (upper or topping) thermodynamic cycle produces power, but part or all of its heat rejection is used in supplying heat to a lower or bottoming cycle. The upper plant is frequently an open circuit gas turbine while the lower plant is a closed circuit steam turbine together they form a combined cycle gas turbine (CCGT) plant. 

The Carnot engine (or cyclic power plant) is a useful hypothetical device in the study of the thermodynamics of gas turbine cycles, for it provides a measure of the best performance that can be achieved under the given boundary conditions of temperature. 

In defining the thermal efficiency of the closed gas turbine cycle, such as the one shown in Fig. 1.2, we employed the first law of thermodynamics (in the form of the steady-flow energy equation round the cycle), which states that the heat supplied is equal to the work output plus the heat rejected, i.e. 

In this chapter we will develop more rigorous approaches to the analysis of gas turbine plants using both the first and second laws of thermodynamics. 

Subsequently, in Chapter 4, we deal with cycles in which the turbines are cooled. The basic thermodynamics of turbine cooling, and its effect on plant efficiency, are considered. In Chapter 5, some detailed calculations of the performance of gas turbines with cooling are presented. 

Subsequently, in Chapter 5, we shall show how the cooling quantities may be determined we give even more practical cycle calculations, with these cooling quantities (ip) being determined practically rather than specified ab initio. But for the discussions in this chapter, in which we assess how important cooling is in modifying the overall thermodynamics of gas turbine cycle analysis, it is assumed that tp is known. 

Open cooling of turbine blade rows—detailed fluid mechanics and thermodynamics... 

In Chapter 5 (and Appendix A), the detailed fluid mechanics and thermodynamics involved in cooling an individual turbine blade row are discussed, enabling if/ to be... 

El-Masri. M.A. (1986b), On thermodynamics of gas turbine cycles Part I second law analysis of combined cycles, ASME J. Engng Power Gas Turbines 107, 880-889. 

To further understand the thermodynamic philosophy of the improvements on the EGT cycle we recall the cycle calculations of Chapter 3 for ordinary dry gas turbine cycles—including the simple cycle, the recuperated cycle and the intercooled and reheated cycles. 

The CHAT cycle may be seen as a low loss evaporative development of the dry intercooled, reheated regenerative cycle [CICBTBTX]. It offers some thermodynamic advantage—increase in turbine work (and heat supplied ) with little or no change in the compressor work, leading to an increased thermal efficiency and specific work output. 

Lloyd. A. (1991), Thermodynamics of chemically recuperated gas turbines. CEE.S Report 256, Centre For Energy and Environmental. Studies, University Archives, Department of Rare Books and Special Collections, Princeton University Library. 

Macchi, E,. Consonni, S,. Lozza. G, and Chiesa, P. (1995), An assessment of the thermodynamic performance of mixed gas-steam cycles. Parts A and B, ASME J. Engng Gas Turbines Power 117, 489-508. 

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