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

What is it about?

In this research article, experimental study was carried out to obtain the heat transfer characteristics be- tween 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 mix- ing 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.

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Why is it important?

A few empirical correlations with satis- factory good fit computational results obtained from a mechanistic model were developed. The obtained correlation functions are applicable to a wide range of Sauter mean particle diameters, similar fluidization conditions and also staggered tube bundles. These empirical correlations can be useful for engineering design or heat transfer characteristics in fluidized bed heat exchangers with horizontal tube bundle.

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