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A HEAT TRANSFER TEXTBOOK - THIRD EDITION Episode 1 Part 6

Tham khảo tài liệu 'a heat transfer textbook - third edition episode 1 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 114 Heat exchanger design 3.2 Figure 3.11 A typical case of a heat exchanger in which U varies dramatically. The second limitation our use of a constant value of U is more serious. The value of U must be negligibly dependent on T to complete the integration of eqn. 3.7 . Even if U fn T the changing flow configuration and the variation of temperature can still give rise to serious variations of U within a given heat exchanger. Figure 3.11 shows a typical situation in which the variation of U within a heat exchanger might be great. In this case the mechanism of heat exchange on the water side is completely altered when the liquid is finally boiled away. If U were uniform in each portion of the heat exchanger then we could treat it as two different exchangers in series. However the more common difficulty that we face is that of designing heat exchangers in which U varies continuously with position within it. This problem is most severe in large industrial shell-and-tube config-urations1 see e.g. Fig. 3.5 or Fig. 3.12 and less serious in compact heat exchangers with less surface area. If U depends on the location analyses such as we have just completed eqn. 3.1 to eqn. 3.13 must be done using an average U defined as JA UdA A. 1Actual heat exchangers can have areas well in excess of 10 000 m2. Large power plant condensers and other large exchangers are often remarkably big pieces of equipment. Figure 3.12 The heat exchange surface for a steam generator. This PFT-type integral-furnace boiler with a surface area of 4560 m2 is not particularly large. About 88 of the area is in the furnace tubing and 12 is in the boiler Photograph courtesy of Babcock and Wilcox Co. 115 116 Heat exchanger design 3.2 LMTD correction factor F. Suppose that we have a heat exchanger in which U can reasonably be taken constant but one that involves such configurational complications as multiple passes and or cross-flow. In such cases it is necessary to rederive the appropriate mean temperature .