Heat transfer characteristic in an external heat exchanger with horizontal tube bundle

Abstract

In this research article, experimental study was carried out to obtain the heat transfer characteristics between a submerged horizontal tube bundles and a fluidized bed in a large-scale circulating fluidized bed (CFB) boiler with an external heat exchanger (EHE). Authors used operating conditions of fluidized bed heat exchanger to predict the average heat transfer coefficient (HTC) making use of an existing calculation method based on the packed renewal theory. The operational parameters in the tube EHE were measured during performance tests at variable load conditions. The heat transfer characteristics are considered in terms of heat transfer mechanisms such as emulsion phase convection, gas convection and also thermal radiation. The obtained heat transfer data exhibits a maximum value with variation mean bed particle size irrespective of pressure. The results showed that the average HTC increases with a decrease of the Sauter mean particle diameter and with the increase of the fluidizing number as a result of good mixing dynamics in emulsion phase (i.e. emulsion wall contact time, bubble fraction in the bed). Based on the heat transfer data, empirical correlations are proposed for predicting a heat transfer coefficient from fluidized bed to horizontal tube bundle. The mechanistic heat transfer model predicted the average HTC in sufficient good agreement with accessible literature data. The predictions were as accurate refer to experimental data at the examined conditions.