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Title: The effect of downward vapour velocity and inundation on the condensation rates on horizontal tubes and tube banks
Author: Nobbs, D. W.
ISNI:       0000 0004 2714 0177
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 1975
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The rate of steam condensation on a horizontal tube is well accounted for by Nusselt's simple theory, provided that the velocity of the steam across the tube is low, the tube is not inundated by condensate falling on it from above, and the steam is free from non-condensable gases. Of these effects the first tends to enhance the rate of condensation, and the latter two tend to reduce it. Although Nusselt developed a theory to account for inundation, and to account for vapour velocity on a flat plate, the assumptions made are not satisfied in practice, and measurements do not support Nusselt's predictions. Although there exists some theoretical and experimental work on the various effects mentioned, the ranges of velocity and inundation rate covered are inadequate to establish the validity of any theory; moreover, the important aspect of simultaneous interaction of various effects has received little attention. The present work is concerned entirely with furthering our understanding of the effects of velocity and inundation, separately and acting together, but in the absence of non-condensable gases. Two tube bank arrangements were used, both with downward vapour flow; namely square and equilateral-triangle arrangements of tubes having a diameter of 19.05 mm and a pitch-diameter ratio of 1.25. Although all the work was carried out with steam at pressures slightly above atmospheric, by using appropriate ranges of steam velocity and raised cooling water temperatures, conditions analogous in certain respects to typical vacuum condensers were covered. Useful experimental results are presented, and it has also been possible to develop an adequate theory to account for velocity effects in the absence of inundation both for a single tube and for banks of tubes. Qualitatively the results can be summarised as follows: (i) Velocity enhances condensation rates, the effect being most marked when cooling water-steam temperature differences, and therefore heat fluxes, are high. (ii) Beyond a certain velocity, improvement in condensation rate becomes very small, the limiting velocity depending on the heat flux which governs the vapour flow separation. (iii) The usual method of design based on mean tube surface temperature is found to be unsatisfactory, and an improved procedure is proposed. (iv) Inundation almost always reduces condensation rates, but less so than predicted by Nusselt's theory; under certain conditions'it may indeed increase condensation rates
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available