Turbine Inlet Air Cooling

Turbine inlet cooling (TIC) can increase gas turbine (GT) power output on a hot day by 10 to 30 percent, while improving (reducing) the turbine heat rate (kj/ kWe) by as much as 5 percent. By increasing the air compressor inlet air density, turbine inlet air cooling is the most cost-effective method for increasing turbine gross power output, for fixed-altitude GTs. 

Process Optimization Improve process or equipment performance by optimizing the operating conditions Combustion optimization Gas turbine inlet air cooling Electrical equipment optimization (VSD) Compressed air optimization CbQ -fuel combustion... 

Gas turbine inlet air cooling to increase density of air to improve the overall efficiency of the combustion cycle as manifested by improvement in the heat rate. It is also noteworthy that eooling will serve to pre-filter and reduce velocity and hence the pressure drop across the inlet air filters providing additional performance improvement. 

FIG. 24-65 Mechanical refrigeration TIC systems utilizing chilled water for cooling turbine inlet air, and cooling towers to reject the waste heat into the environment, account for the majority of refrigeration TIC systems sold. 

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 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 cooling of the inlet air using an evaporative cycle, the simplest of the cycles, and which can be put into operation with the least outlay in capital is not very useful in operation in high humidity areas. The system would cost between 300,000- 500,000 per turbine thus amounting to a cost of 135 per KW. 

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. 

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. 

Where hot ambient temperatures are expected, overall turbine efficiency and horsepower output can be increased by installing an evaporative cooler in the inlet. Inlet air flows through a spray of cold water. The temperature of the water and the cooling effect caused by the inlet air evaporating some of the water cools the inlet air. In desert areas where the inlet air is dry and thus able to evaporate more water before becoming saturated with water vapor, this process is particularly effective at increasing turbine efficiency. 

FIG. 24-64 Effect of inlet air ambient temperature on the power output of a typical GT. If ambient air at 95°F (35°C) were cooled to 50°F (10°C), the gross GT power output would be increased by approximately 22 percent, and the gross heat rate improved by 3.7 percent. Operated at its ISO conditions [15°C ( 59°F) at sea level], GT rated performance is 100 percent. (Turbine Air Systems www.tas.com.)... 

An ideal Brayton refrigeration system uses air as a refrigerant. The pressure and temperature of air at compressor inlet are 14.7 psia and 100°F. The pressure and temperature of air at the turbine inlet are 60 psia and 260°F. The mass rate of air flow is 0.031bm/sec. Determine (a) the cooling load, (b) the compressor power required, (c) the turbine power produced, and (d) the cycle COP. 

The combustor inlet air pressure was fixed at 10 atm and the temperature at 530 K. The fuel temperature at injection was maintained between 340 and 360 K. The burner was operated at an overall fuel-air mixture ratio of 0.014, corresponding to normal power (40,000 hp) operation of the FT4 gas turbine. Traversing stainless steel, water-cooled probes were... 

The lean operating boundary shown in Figure 1 (to its interception of the lean limit line) represents the line of constant turbine inlet gas temperature (1407 K or 2073°F) typically required for cruise power. This boundary line shows that there is a minimum allowable premixed equivalence ratio that satisfies power requirements. But, since air-film cooling and perhaps secondary air injection for temperature pattern factor adjustment (at the turbine inlet) will be required in an engine combustor, the useful lean boundary will lie possibly 20-30% to the right of that shown. Cooling requirements should be much reduced from current practice because of the ultralean (cooler) burning zone. 

The main components of any gas turbine unit are the air compressor, the combustion chamber, and the turbine. Compressed air is fed to the combustion chamber, where it is mixed with the fuel. The complete combustion of the fuel generates heat, which is taken up by the combustion gas. The hot gases from the combustion chamber are cooled by mixing them with bypass air in order to reach a suitable turbine inlet temperature, after which they expand through the turbine and to the atmosphere. Some of the bypass air may be used to cool the turbine blades. The work produced in the turbine is used to power the compressor and to generate mechanical power. The mechanical power is used to produce electricity in a generator. 

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