In boiler combustors and incinerators, the heat exchangers have been being used to recover the waste heat from the flue gases. The flue gases basically consist of the noncondensable gases with a certain amount of the water vapor. While the gas flows across the horizontal tube bank cooled by the cooling water, the heat transfer is enhanced by the vapor condensation. To predict the heat exchanger performance accurately, the effect of the noncondensable gases on the condensation over a horizontal tube should be known. However there are few review works reported on the condensation heat transfer with high concentration of the noncondensables. Therefore, in the present study, reviewed were the correlations for the condensation heat transfer, but with high concentration of the noncondensables. Then the best-estimate correlation was selected. To validate the correlation, it was tested by estimating the heat-recovery rate with a pilot-scale system.
Three correlations for the flat plates and six correlations for the horizontal tubes were considered. For the flat plate condensation, among these three correlations, Asano’s correlation best represents the experimental results in the most range of the noncondensable-gas concentration. For the horizontal tube condensation, most of the correlations cover the mass-fraction range of the noncondensable gas below 0.3. Among them, Fujii’s correlation well represents the experimental results over the wide range of the mass-fraction of noncondensables, even down below 0.3 and up above 0.85, and is tentatively recommended as the best-estimate correlation for the condensation of the vapor/gas mixture outside the horizontal tubes.
A simulation program using the Fujii’s correlation was constructed to analyze 1-D simplified condensing flow across the tube bank. Higher efficiency was anticipated by reducing the flue gas temperature down below the dew point where the water vapor in the flue gas is condensed at the surface of the heat exchanger; that is, the heat transfer by the latent heat is added to that by the sensible heat. This implies that there is an optimum operating condition to maximize the heat recovery from the flue gas. The temperature rises of the flue gas and the cooling water between the inlet and the outlet of the tube bank were compared with the experimental data reported previously. The predicted results agree well with the experimental data. Using this program, the parametric studies have been conducted by changing the operating conditions, such as the velocities and temperatures of the vapor/gas mixture and the cooling water, the number of the rows, and the conductivity of the wall material.
