Effects of fouling on the efficiency of heat exchangers in lignite utility boilers

The work presented in this article is directed towards the understanding of the mechanism of ash deposition on the surfaces of tubes of heat exchangers in lignite utility boilers and the evaluation of the influence of fouling on heat exchanger efficiency. For this purpose a numerical model was developed to predict the deposition of particles onto the heat transfer surfaces. The deposition model is combined with a numerical procedure that solves the two phase flow and temperature field around the tubes of the heat exchanger. Predictions of the flow field in both in-line and staggered tube bundles are validated through comparisons with experimental measurements that are conducted in laboratory model geometries. Experimental techniques include laser-sheet flow visualisation and laser Doppler Anemometry (LDA). Observations in full scale geometries of lignite utility boiler heat exchangers that are in current operation are provided by the industrial partner. The results of the work performed indicate that the maximum deposit height on the surface of the first tube is reached in about two weeks for a staggered tube bundle arrangement. It is the actual spacing of the tube arrangement that plays an important role in reducing the fouling rate while closely spaced tubes in in-line arrangements show signs of bridge formation between subsequent tube rows. (C) European Communities 1997. Published by Elsevier Science Ltd.