Temperature rotor inlet

Thus the cooled reversible cycle [CHT]rci with a first rotor inlet temperature, Tj, will have an internal thermal efficiency exactly the same as that of the uncooled cycle [CHTJru with a higher turbine entry temperature 3 = Tr, and the same pressure ratio. There is no penalty on efficiency in cooling the turbine gases at entry but note that the specific work output, w = (wj — wc)/CpT = [(0 /x) — 11(j — 1), is reduced, since 0 < 0. 

Efficiency as a function of combustion temperature or rotor inlet temperature (for single-step cooling)... 

An important point needs to be re-emphasised, that the cooled efficiency (t )ici with combustion temperature (T = T oi) >s the same as the uncooled efficiency (t )iu at the rotor inlet temperature (T = T n)... 

Fig. 4.6. Efficiency plols for irreversible uncooled and single-slep cooled cycles (after Ref.. S ). (a) Efficiency against maximum temperature, (b) Efficiency again.st non-dimensional maximum temperature, (c) Efficiency against combustion temperature (F,) and rotor inlet temperature (F ).

The choice of these values is arbitrary. In practice, the cooling fraction will depend not only on the combustion temperature but also on the compressor delivery temperature (i.e. the pressure ratio), the allowable metal temperature and other factors, as described in Chapter 5. But with ip assumed for the first nozzle guide vane row, together with the extra total pressure loss involved (k = 0.07 in Eq. (4.48)), the rotor inlet temperature may be determined. These assumptions were used as input to the code developed by Young [11] for cycle calculations, which considers the real gas properties. 

The (arbitrary) overall efficiency and specific work quantities obtained from these calculations are illustrated as carpet plots in Fig. 4.11. It is seen that the specific work is reduced by the turbine cooling, which leads to a drop in the rotor inlet temperature and the turbine work output. Again this conclusion is consistent with the preliminary analysis and calculations made earlier in this chapter. 

A final calculation illustrates the earlier di.scussion on the difference between combustion temperature = 7 3 and rotor inlet temperature Tn, = 7 . Fig. 4.12 shows... 

Fig. 4.12. Calculation of efficiency of ICBT] plant uncoolcd CBT uj a.s a function of combustion lemperalurc (7eoi) single-step cooled (CBT)k i as a function of rotor inlet temperature (TVi,). Pres,sure ratio r = 30. t)c — 0.8.

Most modem CCGT plants use open air cooling in the front part of the gas turbine. An exception is the GE MS9001H plant which utilises the existence of the lower steam plant to introduce steam cooling of the gas turbine. This reduces the difference between the combustion temperature T ot and the rotor inlet temperature The effect of this on the overall combined plant efficiency is discussed in Ref. [1] where it is suggested that any advantage is small. 

In second-generation PFBC, a topping combustor is used to raise the turbine rotor inlet temperature to state-of-the-art levels. Pulverized coal is fed to a partial-gasifier unit that operates about 870° to 925°C (1,600° to 1,700°F) to produce a low heating value fuel gas and combustible char. The char is burned in the PFBC. The fuel gas, after filtration, is piped back to the gas turbine, along with the PFBC exhaust. 

Figure 6-1. Schematic plot of ga.s turbine rotor inlet temperature against efficiency (after Holcomb, 1995).

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 injection of coolant air in the turbine rotor or stator causes a slight decrease in turbine efficiency however, the higher turbine inlet temperature usually makes up for the loss of the turbine component efficiency, giving an overall increase in cycle efficiency. Tests by NASA on three different types of cooled stator blades were conducted on a specially built 30-inch turbine cold-air test facility. The outer shell profile of all three blade types was the same, as seen in Figure 9-24. 

Common operating limits for turboexpanders are an enthalpy drop of 40-50 Btu/lb/stage of expansion and a rotor tip speed of 1,000 ft/s or higher. Turboexpanders are available with inlet pressures up to 2,500 psig and inlet temperatures over 1,000°F. Allowable liquid production can be as high as 20% of the discharge weight. 

Changes in differential pressures can be caused by variations in either inlet or discharge conditions (i.e., temperature, volume or pressure). Such changes can cause the rotors to become unstable and change the load zones in the shaft-support bearings. The result is premature wear and/or failure of the bearings. 

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