A parametric study of deaerator spray nozzle for an improvement of deaeration efficiency

Abstract

Boilers are used in many fields, and their operation efficiency has become more important as energy conversion has gained attention. There are many ways to improve the efficiency of boilers, but one of the most important aspects is deaeration. Deaeration is the removal of dissolved air, such as oxygen (O2) or carbon dioxide (CO2), from feedwater, which is the water supplied to the boiler. When the feedwater has dissolved oxygen and carbon dioxide, this causes corrosion in the boiler and significantly shortens the lifetime of the boiler. To prevent corrosion in the boiler and ensure its normal operation, a deaerator is used. The deaerator injects water spray through nozzle holes in a high-pressure and temperature atmosphere to remove dissolved O2 and CO2 (Henry’s law). The efficiency of deaeration is affected by the atomization of water droplets because the larger the surface area, the more interaction there is between the air and the atmosphere. The sprayed water from the nozzle hole (the first droplets) heading to the wall surrounding the nozzle splashes the wall and breaks up into a number of small droplets (the second droplets). After the splash, the second droplets are spattered randomly near the wall (spatial mass distribution). Some of them are not spattered, but flow through the wall. At the end of the wall, the droplets fall not like a column, but a film. The microscopic spray characteristics, such as the first and second droplet diameters, are compared with a variety of nozzle-hole diameters (1.0, 2.0, 3.0), angles (0, 1.0, and 2.0), and nozzle-to-wall distances (1.0, 1.6, 2.0). As a result, the first droplet diameter increased with the increase of the nozzle-hole diameter, but was not affected by the nozzle-hole angle or the nozzle-to-wall distance. None of the parameters affected the second droplet diameter, which was a steady Sauter Mean Diameter (SMD) of around 0.25. The mass flow rate fractions between the total flow rate and the second droplet flow rate played an important role in the deaeration efficiency, and optimized results were shown with the nozzle diameter of 1.0, nozzle angle of 1.0, and nozzle-to-wall distance of 2.0. In terms of spatial mass distribution, the second droplets from the 2.0 and 3.0 nozzles were well distributed due to their momentum.