Gas Turbine Inlet Air Fogging Systems: Application of Psychrometric Principles to Study Cooling and Humidification Processes

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Gas turbine inlet air-cooling utilizing a high pressure fogging system is theoretically based on the thermodynamic principle of adiabatic saturation. The dry bulb temperature of inlet air decreases by losing its sensible heat to the liquid water droplets generated by the fogging system. This sensible heat is enough to provide the latent heat necessary to evaporate and diffuse these droplets into the surrounding air stream. As a result of this heat interchange accomplished through a complex mechanism of heat transfer and fluid dynamics, the temperature and moisture content of inlet air is decreased & increased respectively resulting in an increase in its density. Neglecting the enthalpy of liquid water, which is insignificant compared to the water’s latent heat of vaporization, the total heat, i.e. enthalpy of the air stream remains unchanged. The net heat exchange outside of this control volume is zero because the droplets, which subsequently diffuse into the air stream, gain the sensible heat lost by inlet air. Therefore, the theoretical concept of adiabatic saturation is valid in this case. However, in real systems, the cooling and humidification process is seldom adiabatic. This paper applies psychrometric principles in conducting a detailed mathematical analysis of various cooling and humidification processes accomplished by fogging systems. The measurement of actual moisture content of air upstream and downstream of a fogging system is vital to gas turbine inlet air-cooling. The moisture content of air and its dry bulb temperature can provide accurate information on the system path and how well it approximates the adiabatic saturation process and determining system efficiency. The deviation from the adiabatic saturation process is attributed to several factors, such as external heat gain or loss, water temperature, etc. The measurement of moisture content can enable the fogging system control logic to control the required water flow rate through the system to obtain maximum performance without over spraying excessive water or under cooling inlet air. This paper presents possible scenarios of deviation from an adiabatic process and methods to determine fogging system total effectiveness.