Numerical model of a steam superheater with a complex shape of the tube cross section using Control Volume based Finite Element Method

Abstract

In modern utility boilers, characterized by high values of steam pressure and temperature, the individual superheater stages and also the individual passes are made of different low alloy steel grades. The use of tubes having a complex shape of cross section allows the building of the platen with smooth side surfaces, which are arranged in the upper part of the combustion chamber. This avoids the erosion of the superheater tubes and deposition of slag and ash in the spaces between adjacent tubes. The superheaters of this type are widely used in CFB boilers. To select the appropriate steel for each pass and each stage, the maximum wall temperature of tube need to be determined. Steam and tube wall temperature were computed in the superheater using a method proposed in the paper and compared with the results obtained by CFD simulation using Star-CCM+ software. The method presented in the paper can be easily applied to the modeling of flow and thermal processes in superheaters with complicated flow arrangements which usually are found in steam boilers. The advantage of the proposed method is a short computing time in comparison with detailed and complex 3D models. Using the methodology developed in the article, time needed to obtain final results at steady-state conditions can be lower even dozen times in comparison to CFD calculations. The proposed calculation method can be used in the design of modern subcritical and supercritical boilers, in which the temperature of the live steam is very high. Very careful and precise calculations of steam and tube walls temperature along the steam flow path will avoid the overheating of the tube material.