In order to improve the boiler efficiency, latent heat recovery from the flue gas is very important concept. Three kinds of countercurrent cross-flow heat exchangers, which consist of bare tubes, spirally finned tubes of fin pitch 5 and 10mm, were designed and used for the experiment. The heat exchanger of the bare tubes consists of a staggered bank of 5-4 rows and 50 stages. The length, outer and inner diameter of the bare tube is 482, 27.2 and 23.2mm, respectively. The heat exchangers of finned tubes consist of staggered banks of 3-2 rows, 34 stages for the fin pitch 10mm and 20 stages for the pitch 5mm. The length, outer and inner diameter of the base tube welded with the fins is 482, 34 and 28.8mm, respectively. The thickness and height of the plate fin are 1 and 12mm, respectively. The parametric study varying the flue gas flow rate, feed water temperature and flow rate was conducted. The temperature distributions of water and flue gas in the heat exchanger were measured with sheath K-type thermocouples of 1.6 mm in diameter. The pressure loss and the total amount of condensate generated in the heat exchanger were also measured.

Based on the previous basic studies, a prediction method for the heat exchanger was proposed. In the prediction, the flue gas was treated as a mixture of CO2, CO, O2, N2 and H2O, and the one-dimensional heat and mass balance calculation along the flow direction of flue gas was conducted. The heat and mass transfer on tubes was evaluated with a simple analogy correlation. For the finned tubes, the fin efficiency at the condensing region was calculated with a semi-empirical correlation obtained in the previous basic study. The effect of condensate film on the tubes was considered to be negligibly small for the heat transfer and pressure loss calculation. The experimental results for the temperature distributions of water and flue gas in the test heat exchangers with bare and finned tubes agreed well with the prediction.

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