For shell-and-tube heat exchangers, why must the temperature difference at the high-temperature end be no less than 20?℃
Thread Content
While reading the chemical process design manual, I came across that section and have a few questions; I’d appreciate it if you could help clarify them! Question 1: Why should the temperature difference at the high-temperature end not be less than 20°C, and the temperature difference at the low-temperature end not be less than 5°C? Question 2: Why must the temperature difference at the low-temperature end be no less than 20°C when heat exchange occurs between the two process streams?
To prevent the pipes from deforming due to large temperature differences, as excessive deformation can affect the lifespan of the pipe bundle; this problem occurred in the old facility, so more heat exchangers were added later on； However, the temperature difference at the hot side of the coiled tube heat exchanger is relatively large
This is to take cost into consideration; a too small temperature difference results in a large heat exchange area for shell-and-tube heat exchangers, and in such cases a plate heat exchanger can be used
This post was last edited by wiseboy on 2022-10-10 09:49. Such general descriptions are merely principles, not rules. Some of the descriptions are inaccurate, or even incorrect. Don’t think just because it’s written in a book, it’s right. For example, you say it shouldn’t exceed 20°C, but the process requirement is exactly 21°C – should we give up on this project? The factory isn’t being built anymore? ?
The figures given in the book are advisory; it doesn’t mean that things will go wrong if those limits are exceeded. The question now is why the concept of not less than 20°C was proposed – what considerations led to that?
This post was last edited by wiseboy on 2022-10-10 at 12:53. I have been designing heat exchangers for 40 years, and I had no idea about this! Moreover, this description is particularly vague; for example: Isothermal condensation of steam at 148°C. For hot water: Steam, high-temperature end: 148°C (gas) —> 148°C (liquid), temperature difference: 0°C. For cold water, low-temperature end: 20°C —> 80°C, temperature difference: 60°C. This is the most common type of shell-and-tube heat exchanger. Under these operating conditions, the temperature difference at the high-temperature end is 0°C; how do you explain a temperature difference of 20°C? There are so many books; I’m amazed.
This post was last edited by jlj913 on 2022-10-11 00:50. Does the temperature difference at the high-temperature end refer to the difference between the inlet temperature of the hot fluid and the outlet temperature of the cold fluid during counterflow?
The high-temperature end mentioned in the book should refer to the temperature difference between the hot fluid inlet and the cold fluid outlet during counterflow heat transfer, right?
That’s not what it means; if that were the case, it would be even more absurd. But in any case, you always tend to think that what’s in the book is correct.
One needs to understand the word choice. It shouldn’t be, it’s not allowed. They are different concepts. It should not be merely a recommendation against it; in special cases, it can be used if the calculations show it is appropriate. In particular, the economic calculations under high energy prices are different from those in the past, when energy consumption was not taken into account and only the initial investment in equipment was considered. In the context of manual compilation, such recommendation data primarily takes into account: 1. Cost efficiency, in order to avoid excessive heat exchange area. 2. It is to leave a margin for process design. Prevent heat exchange dead zones from occurring in production due to load adjustments.
Leading to another question: are there maximum and minimum requirements for the temperature difference in heat exchange within a heat exchanger?