Rapid computation of the exit temperature of hot combustion gases flowing inside chimneys

Abstract

This paper addresses the practical calculation of the thermal performance of industrial chimneys by means of recent results in forced convection heat transfer. Aside from its use in the estimation of static draft, the magnitude of the mean bulk temperature of combustion gases at the chimney exit is a key ingredient for the estimation of levels of air pollution in the vicinity of the stack. In addition, and depending on fuel and process characteristics, the wall temperature should be high enough to avoid acid condensation on the inner lining. The temperature decay of the combustion gases is a result of heat transfer processes involving both internal forced convection and external heat transfer to the surroundings. The coupling of these mechanisms renders highly difficult its accurate simulation. Possibly due to this fact, the analysis of the problem is scarcely treated in the energy- and environmental-related literature. Thermal design and performance estimations of chimneys are therefore based on engineering rules of thumb dictated by experience. The aim of this paper is to introduce rigorous heat transfer results into the subject, but from a framework accessible to design engineers. To this end, a 1-D lumped model of this kind of situations is used [1]. Although largely simplified, the model provided adequate estimates of the mean bulk temperatures when compared with those computed with a 2-D distributed model [2], thus satisfying the requirements of simplicity and accuracy for the present purpose. The paper describes the design criteria for thermal calculation of industrial chimneys, the application of the 1-D lumped model through the use of standard heat transfer correlations, and several examples. The sequence of calculations is easy to accomplish and it is explained in such a way that it can be directly employed by engineers engaged in the thermal design of tall chimneys. Comparison with experimental data measured at the stack of a large coal-fired power station is also discussed.