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The Design and Performance Analysis of Reflux and Vent Condensers

机译:回流冷凝器的设计与性能分析

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摘要

Reflux and vent condensers are vertical separators where film condensation occurs. A vapour mixture is supplied at the bottom of the tubes and encounters vertical cold surfaces. A falling film forms and exits from the bottom of the tubes, flowing counter-current to the vapour, but co-current to the coolant on the shell side. Vapour velocities at or above the critical cause flooding, that is, some or all the liquid exits from the top of the tubes rather than from the bottom. The calculation of this 'flooding velocity' is critical for the design procedure, since it imposes a limit on the condensation rate in a given condenser and constitutes the main disadvantage of these condensers. Most of the correlations to predict the flooding velocity come from isothermal experiments data and are not necessarily representative of vent condensation. The current design methods require minimum safety factors of 30% or more. The oversized condensers incur high capital and operational costs and could lead to unsatisfactory operation and even flooding at the design operational conditions. Experiments consisting of steam/air condensation at sub-atmospheric pressures were performed in a 3 m long, double-pipe vertical condenser with an internal diameter of 0.028 m. The experiments provide data of steady-state operation and flooding under condensation conditions. The former allows the evaluation of the design methodologies whilst the latter permits the evaluation of the flooding correlations. The correlation by English et al. is the most successful in predicting the flooding velocity in the vent condensation of steam/air. The deviation from the experimental flooding point varies with the vapour velocity. The results can be used to calculate more adequate safety factors, up to 50% lower than the currently recommended in the open literature. Two design methods are evaluated, namely the film and the equihbrium methods. For this purpose, a software tool was developed using VB.NET. After comparing the predictions of both methods, it becomes apparent that the film method should always be preferred when enough data are available. Previously unpublished recommendations for the computation of the methods are also reported. The deviation of the predicted heat-transfer area from the experimental one is shown to be a function of the Lewis number, where the best predictions are at Le = 0.68. At lower Lewis numbers both methods over-predict the area. This fact is commonly ignored when choosing a safety factor to enlarge the designed area, and in some cases it could be more appropriate to, in fact, decrease it.
机译:回流冷凝器和排气冷凝器是发生薄膜冷凝的垂直分离器。蒸气混合物在管的底部供应,并遇到垂直的冷表面。降膜形成并从管的底部流出,与蒸气逆流,但与壳侧的冷却剂并流。蒸汽速度等于或大于导致溢流的关键原因,即部分或全部液体从管子的顶部而不是底部排出。该“溢流速度”的计算对于设计程序至关重要,因为它限制了给定冷凝器中的冷凝速率,并且构成了这些冷凝器的主要缺点。预测淹没速度的大多数相关性均来自等温实验数据,并不一定代表排放口凝结。当前的设计方法要求最小安全系数为30%或更高。过大的冷凝器会导致高昂的投资成本和运营成本,并可能导致运行不令人满意,甚至在设计运行条件下发生洪水。在内径为0.028 m的3 m长的双管立式冷凝器中,进行了低于大气压的蒸汽/空气冷凝实验。实验提供了凝结条件下的稳态运行和驱油数据。前者允许评估设计方法,而后者则允许评估洪水相关性。由英语等相关。是最成功的预测蒸汽/空气出口凝结中淹没速度的方法。与实验溢流点的偏差随蒸汽速度而变化。结果可用于计算更适当的安全系数,比公开文献中当前推荐的安全系数低50%。评估了两种设计方法,即薄膜法和平衡法。为此,使用VB.NET开发了一种软件工具。比较两种方法的预测后,很明显,当有足够的数据可用时,应始终首选胶片法。还报告了以前未发表的关于计算方法的建议。预测的传热面积与实验值的偏差显示为路易斯数的函数,其中最佳预测为Le = 0.68。在较低的Lewis数下,两种方法都过度预测了面积。当选择安全系数来扩大设计面积时​​,通常会忽略此事实,并且在某些情况下,实际上减小它可能更合适。

著录项

  • 作者单位

    The University of Manchester (United Kingdom).;

  • 授予单位 The University of Manchester (United Kingdom).;
  • 学科 Physical chemistry.
  • 学位 Ph.D.
  • 年度 2008
  • 页码 401 p.
  • 总页数 401
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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