Design of Tubular Humidifiers for Evaporative Gas Turbine Cycles

Abstract

In the evaporative gas turbine (EvGT) cycle, also referred to as the humid air turbine (HAT) cycle, the compressed air is humidified with water to increase the mass flow through the expander, resulting in high power output and high exhaust heat recovery potential. This paper presents a design methodology for tubular humidifiers, in which pressurized air is led inside of smooth metallic tubes and is brought into contact with a falling water film. The heat required for humidification is mainly taken from exhaust gas from the gas turbine on the shell side and also by recirculating water through the intercooler and the aftercooler. The most important parameters for designing tubular humidifiers are: heat transfer coefficient on the flue gas side; and mass transfer coefficient for water vapor on the air side. Important design aspects include: proper wetting of the tubes; how to avoid flooding of the tubes; entrainment of water droplets into the air stream; and boiling in the water film. All calculations in this paper are based on an evaporative gas turbine cycle applied for combined heat and power generation with a partial-flow humidification circuit, where a fraction of the compressed air is humidified while a major part is by-passed directly to the recuperator. It is concluded that the required heat exchanging surface can be reduced if humidification is carried out for only a fraction of the air (20–30 percent). Finned tubes are recommended to enhance the heat transfer per unit tube length.