Turbine inlet 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. 

Francis) Lee Smith, Ph D., M.Eng. Principal, Wilcrest Consulting Associates, Houston, Texas Member, American Institute of Chemical Engineers, Society of American Value Engineers, Water Environment Federation, Air and Waste Management Association (Energy Recovery, Economizers, Turbine Inlet Cooling)... 

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). 

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. 

Refrigerated inlet cooling is much more effective in humid areas and can add about 12.8% to the power output of the simple cycle gas turbine. The... 

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. 

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. 

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. 

Natural gas expander cycles can be utilized at places with a high-pressure main gas pipeline and a low-pressure distribution line. The natural gas is expanded in a turbine, thereby cooling the exit gas. Dependent on the inlet conditions, the exit temperature will be so low that a part of the gas is condensed, typically around 12 to 30 %. The liquid is separated from the gas flow in a liquid-vapour separator as shown in Figure 11. 

---