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Unformatted text preview: Chapter 11 Workbook Heat Exchangers Introduction In previous classes such as thermodynamics, fluid mechanics, whenever a process stream was heated or cooled, you rarely worried about how that heating and cooling was accomplished. You simply did a H calculation and went on your merry way. In this chapter, we will take an introductory look at the design and analysis of a very important piece of equipment in the chemical factory and in many other devices you use every day the heat exchanger. In a heat exchanger, a hot stream and cold stream are brought into thermal contact, and heat flows from the hot stream to the cold stream. There are numerous types of heat exchangers, and the design and materials used are predicated on the process specifications and the economics of the process. We will analyze the simple concentric-tube heat exchangers, and develop the energy balance equations that will be used to perform design or performance calculations. We will then consider shell-and-tube heat exchangers, and develop the effectiveness-NTU method, which is especially useful for performance calculations. While this may seem like a lot of new material, in reality you have already covered 95% of the material in previous chapters. Heat exchangers are nothing more than internal convection, conduction through a tube wall, and external convection all of which you have already covered. In this chapter, we just put all of these concepts together. A rigorous study of heat exchangers should include more topics that we dont have time to cover in this course. These topics include detailed design specifications - not just area for heat transfer (which we do in this course), but tube ID, tube wall thickness, tube bundle arrangement, baffle design, choice of materials, etc. In your design class you will also learn how to use Aspen Plus to model heat exchangers, and how to combine heat exchangers with other unit operations in a process. Another extremely important topic is called pinch analysis. Pinch Analysis is the efficient use of process streams to accomplish as much heating and cooling duty as possible, so that external utilities (combustion furnaces, steam, cooling water, refrigerants, etc.) do not have to be used. refrigerants, etc....
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- Spring '11