Turbine inlet cooling temperature effects

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 gas turbine is a high-volume air machine. The compressor air power required is usually between 50-70 percent of the total power produced by the turbine. Thus, the ambient temperature affects the output of the gas turbine. On hot days, the gas turbine produces less output than on cold days. In dry climates, the use of evaporative cooling in the gas turbine decreases the effective inlet temperature and increases the power output of the unit. 

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.)... 

The radiator used in the reactor has a drastic effect on the output power of the reactor. For the models used, the temperature of the gas coming out the reactor is held at a fixed temperature. Gas passing through the turbine has a certain temperature drop. The gas then reaches the radiator which further cools the gas. The amount that the gas is cooled in the radiator is highly dependent on the size of the radiator. This becomes extremely important when determining the amount of work the compressor has to do. At lower temperatures and pressures, the work done by the compressor to compress the gas to the reactor inlet conditions is low. As the temperature increases the amount of work increases, decreasing the net power output. 

In conventional subcritical pressure LWRs, such as BWRs or PWRs, the core is effectively cooled by the boiling heat transfer. Therefore, the coolant inlet temperature is set below its saturation temperature and the saturated steam is sent to the turbine. (In BWRs, the core inlet and outlet temperatures are 216 and 286°C, respectively. In PWRs, the inlet and outlet temperatures are 289 and 325°C, respectively.) The boiling phenomenon starts as the coolant becomes heated close to its saturated temperature. The coolant starts its phase change from liquid to gas with large discontinuous property changes. The coolant flow becomes a two-phase flow and the bulk coolant temperature is kept below its saturation temperature. There have been very few reactors that could produce superheated steam one example was the American Boiling Nuclear Super heater Power Station (BONUS) an integral boiler-super heater, which was shut down permanently in 1968 and decommissioned by 1970. 

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