General numerical method for hydraulic and thermal modelling of the steam superheaters

Abstract

Two methods for modelling high-temperature tubular cross-current heat exchangers are presented. Superheated steam flows inside the tubes, and flue gas flows outside perpendicular to the axis of the tubes. The methods proposed are based on the conservation equations of momentum and energy. In both methods, the heat exchanger is divided into finite volumes. The fluid temperatures at the outlet of the finite volume are calculated using closed-form explicit formulas. This makes the computer calculation time for the entire superheater very short. The pressure distribution in the superheater is determined from the numerical solution of the momentum conservation equation. A power correlation was determined to calculate the limiting Reynolds number above which the friction factordepends only on the relative roughness of the tube surface. Convective and radiative heat transfer were taken into account in the calculation of the heat flow rate on the gas side. The results of the calculation of a two-pass co-current heat exchanger were compared to assess the accuracy of the two methods, finding that the two methods give very similar results. The strength of method I is that the steam and flue gas temperatures inside a given finite volume can be determined. A model of a two-pass live steam superheater in a supercritical pressure boiler is presented. The calculated average steam and flue gas temperatures downstream of the superheater show good agreement with the corresponding temperatures determined using the heat exchanger's effectiveness and the measured results.