Inlet Cooling Techniques
Evaporative Cooling of the Turbine. Traditional evaporative coolers that use media for evaporation of the water have been widely used in the gasturbine industry over theyears， especially in hot climates with low humidityareas. The low capitalcost， installation， and operating costs make it attract-ive for many turbine-operating scenarios. Evaporation coolers consist of water being sprayed overthe media blocks， which are made of fibrous corrugated material.The airflow through these mediablocks， evaporates thewater， as waterevaporates， it consumes about 1059 BT. (1117 kJ) (latent heat of vapori.ation) at 60 oF (15 oC). This results in the reduction of the air temperature entering the compressor from that of the ambient air temperature. This technique is very effective in low humidity regions.
The work required to drive the turbine compressor 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 con-stant and therefore by increasing the massflow， 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. lt 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 oF (10 oC)， if the outside temperature is around 90 oF (32 oC). The cost of an evaporative cooling system runs around .50jkw.
Directinlet fogging， is a type of evaporativecoolingmethod， where de-minerali.ed water is converted into a fog by means of high-pressure no..les operating at 1000-3000 psi. (67-200 Bar) This fog then provides cooling
Water
Pump
Evaporative
Cooling
Air
Figure 2-.5. $chematic of evaporative cooling in a gas turbine.
when it evaporates in the air inlet duct of the gas turbine. The air can attain100% relative humidity at the compressor inlet， and thereby gives the lowest temperature possible without refrigeration (the web bulb temperature). Direct high-pressure inlet fogging can also be used to create a compressorintercooling effect by allowing excess fog into the compressor， thus boosting the power output further.
Refrigerated Inlets for the Gas Turbines. The refrigerated inlets are more effective than the previous evaporative cooling systems as they can lower the temperatures by about 45-55 oF (25-30 oC). Two techniques for refrigerating the inlet of a gas turbine are vapor compression (mechanical refrigeration) and absorption refrigeration.
MechanicalRe介rigeration.lnamechanicalrefrigerationsystem，therefriger-ant vapor is compressed by means of a centrifugal， screw， or reciprocating compressor. Figure 2-36 is a schematic of a mechanical refrigeration intake for a gas turbine. The psychometric chart included shows that refrigeration pro-vides considerable cooling and is very well suited for hot humid climates.
Figure 2-.6. Mechanical refrigerated inlet system used to cool the inlet air of the gas turbine.
Centrifugal compressors are typically used for large systems in excess of1，000 tons (12.4 x 106 BT./13.082 x 106 kJ) and would be driven by an electric motor. Mechanical refrigeration has significantly high auxiliary power consumption for the compressor driver and pumps required for thecooling water circuit. After compression， the vapor passes through a con-denser where it gets condensed. The condensed vapor is then expanded in an expansion valve and provides a cooling effect. The evaporator chills cooling water that is circulated to the gas turbine inlet chilling coils in the air stream. Chlorofluorocarbon (CFC) based chillers are now available and can provide a large tonnage for a relatively smaller plot space and can provide cooler temperature than the lithium-bromide (Li-Br) absorption based cooling systems. The drawbacks of mechanical chillers are high capital and oper-ation and maintenance(O&M)cost， high power consumption， and poor part load performance.
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