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METHODS OF IMPROVING PERFORMANCE OF THE GAS TURBINE
INTRODUCTION:
Gas turbines are used in various applications in different regions of the world. Therefore always tend to find ways to improve the overall cycle of the plant. Cooling of air at the intake helps increasing the density of air flowing into the plant and increases the power output. Several attempts have been made by the researches in new cooling techniques mostly concerned with evaporative coolers, chillers etc. There are many types of air cooling techniques like evaporative inlet air cooling, absorption cooling and mechanical compression.
An absorptive cooler is the one in which a low grade energy is used to drive the system and generate the cooling effect. Evaporative cooling is considered to be the cheaper and the simplest one. But it is highly efficient in dry and hot ambient conditions only. Reports have suggested a 10% to 20% rise in power output by direct controlled spray of water. Researchers have also reported that for every 1% rise in power output there must be a fall of 0.6oC in the temperature of the working fluid. In absorption cooling refrigerants such as the ammonia or lithium bromide is used. The rise in thermal efficiency of the cycle is 4% when compared to that of the evaporative cooling which is around 2.2%.
But care should be taken that the cooling effect never takes the working fluid temperature below 280 K which may lead to formation of ice crystals in the compressor blades and decreases the performance of it since the air temperature drops by 6K as it enters the compressor.
DESCRIPTION:
There are many factors which affect the overall performance of the gas turbine namely efficiencies of the turbine and compressor, compressor inlet temperature, pressure ratio, turbine inlet temperature, combustion efficiency, etc. Briefly if the temperature of the ambient air is high then the specific volume is more which leads to higher works of compression resulting in decreasing efficiencies (shown in fig.1).
It is very much known that if the efficiency of the turbine or compressor is increased then the power output and the thermal efficiency of the cycle increases.
The other factor on which the thermal efficiency depends is the pressure ratio. For fixed values of the compressor and turbine efficiencies, the cycle efficiency can be plotted against pressure ratio for various loads as shown. When losses are taken into account efficiency of the cycle depends on the turbine inlet temperature and pressure ratio. The graph clearly shows that it reaches a peak and then decreases at higher loads because of decrease in fuel supply to maintain a fixed turbine inlet temperature resulting from the higher compressor delivery temperature being outweighed by the increased work necessary to drive the compressor.
The other important factor is the turbine inlet temperature. As the graphs clearly show higher the turbine inlet temperature higher is the work output. The highest possible temperature that can be is limited by the materials available of the turbine blades.
In this section we mainly concentrate about decreasing the inlet temperature at the plant intake or at the compressor inlet or both.