Turbine inlet temperature

The efficiency of gas turbines is limited by the maximum allowable turbine inlet temperature (TIT). The TIT may be increased by cooling of the blades and vanes of the high pressure turbine. Cooling channels can be casted into the components or may be drilled afterwards. Non-conventional processes like EDM, ECD or Laser are used for drilling. Radiographic examination of the drilled components is part of the inspection procedure. Traditional X-Ray film technique has been used. The consumable costs, the waste disposal and the limited capacity of the two film units lead to the decision to investigate the alternative of Real-Time X-Ray. 

Improved materials, coatings, and cooling techniques permit newer machines to operate at higher turbine inlet temperatures, yielding both increased output and efficiency. Further efficiency gains result from improved aerodynamics in the hot gas path, compressor, and turbine sections. Use is also made of variable inlet guide vanes (IGV). 

A 165-MW-class gas turbine/generator has been introduced by another manufacturer. This machine, also developed by scaling up a proven design, features a simple-cycle efficiency of 37.5% a turbine inlet temperature of 1235°C a pressure ratio of 30 1, up from 16 1 on the previous generation and an output of 165 MW for gas fuel firing under International Standards Organization (ISO) conditions (101 kPa, 15°C (14.7 psia, 59°F)). A combined-cycle facihty based around this machine could achieve efficiencies up to 58% or a heat rate of about 6209 kj/kWh (5885 Btu/kWh). 

The 212-MW unit features a turbine inlet temperature of 1260°C and a pressure ratio of 13.5 1. The manufacturer has subsequently installed a number of larger, more powerful versions of this unit, which produce up to 226.5 MW. Turbine inlet temperature is 1288°C the pressure ratio is 15 1. Five of these high output machines anchor a 1675-MW facihty in the Netherlands. These machines were developed by geometric scaling from a 168-MW,... 

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 Reheat Cycle The regenerative cycle improves the efficiency of a gas turbine but does not provide any added work per pound of air flow. To achieve this latter goal, the concept of the reheat cycle must be utilized. The reheat cycle utihzed in the 1990s has pressure ratios of as high as 30 1 with turbine inlet temperatures of out 2100° F (1150° C). The reheat is done between the power turbine and the compressor trains. The reheat cycle, as shown in Fig. 29-35, con-... 

FIG. 29-34 Performance map showing the effect of pressure ratio and turbine inlet temperature on a regenerative cycle. 

Figure 20.7 shows that up to 1960 turbine inlet temperatures were virtually the same as the metal temperatures. After 1960 there was a sharp divergence, with inlet temperatures substantially above the temperatures of the blade metal itself - indeed, the gas temperature is greater than the melting point of the blades. Impossible Not at... 

Thus, a eursory inspeetion of the effieieney indieate that the overall effieieney of a eyele ean be improved by inereasing the pressure ratio, or inereasing the turbine inlet temperature, and the work per lb (kg) of air ean be inereased by inereasing the pressure ratio, or inereasing the turbine inlet temperature, or by deereasing the inlet temperature. 

The reheat cycle increases the turbine work, and consequently the net work of the cycle, can be increased without changing the compressor work or the turbine inlet temperature by dividing the turbine expansion into two... 

Analysis of this cycle indicates that an increase in inlet temperature to the turbine causes an increase in the cycle efficiency. The optimum pressure ratio for maximum efficiency varies with the turbine inlet temperature from an optimum of about 15.5 1 at a temperature of 1500°F (816°C) to about 43 1 at a temperature of about 2400 °F (1316 °C). The pressure ratio for maximum work, however, varies from about 11.5 1 to about 35 1 for the same respective temperatures. 

Figure 2-21 show the effect of 5% by weight of steam injection at a turbine inlet temperature of 2400 °F (1316 °C) on the system. With about 5% injection at 2400°F (1316 °C) and a pressure ratio of 17 1, an 8.3% increase in work output is noted with an increase of about 19% in cycle efficiency over that experienced in the simple cycle. The assumption here is that steam is injected at a pressure of about 60 psi (4 Bar) above the air from the compressor discharge and that all the steam is created by heat from the turbine exhaust. Calculations indicate that there is more than enough waste heat to achieve these goals. 

Similar to the regenerative eyele, the evaporative regenerative eyele has higher effieieneies at lower pressure ratios. Figures 2-24 and 2-25 show the performanee of the system at various rates of steam injeetion and turbine inlet temperatures. Similar to the steam injeetion eyele, the steam is injeeted... 

The work required to drive the turbine eompressor is reduced by lowering the compressor inlet temperature thus increasing the output work of the turbine. Figure 2-35 is a schematic of the evaporative gas turbine and its effect on the Brayton cycle. The volumetric flow of most turbines is constant and therefore by increasing the mass flow, power increases in an inverse proportion to the temperature of the inlet air. The psychometric chart shown shows that the cooling is limited especially in high humid conditions. It is a very low cost option and can be installed very easily. This technique does not however increase the efficiency of the turbine. The turbine inlet temperature is lowered by about 18 °F (10 °C), if the outside temperature is around 90 °F (32 °C). The cost of an evaporative cooling system runs around 50/kw. 

The two conditions that vary the most in a turbine are the inlet pressure and temperature. Two diagrams are needed to show their characteristics. Figure 3-12 is a performance map that shows the effect of turbine inlet temperature and pressure, while power is dependent on the efficiency of the unit, the flow rate, and the available energy (turbine inlet temperature). The effect of efficiency with speed is shown in Figure 3-13. Figure 3-13 also shows the difference between an impulse and a 50% reaction turbine. An impulse turbine is a zero-reaction turbine. 

Figure 3-16 shows the effeet of efficiency as a function of the load for both the compressor and turbine. Part-load turbine efficiencies are affected more than compressor efficiencies. The discrepancy results from the compressor operating at a relatively constant inlet temperature, pressure, and pressure ratio, while the turbine inlet temperature is greatly varied (Figure 3-17). 

The turbine effieieney ealeulation is more eomplex. The first part is the ealeulation of the turbine inlet temperature. The ealeulation is based on the following equation ... 

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