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  • Calculation the temperature at certain length

    Discussion in 'Calculations' started by Wmech, Apr 26, 2013.

    1. Michael Ross

      Michael Ross Well-Known Member

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      It is good to try some analysis - at least to get a good review of the theories involved.

      Unfortunately, analysis is not always very accurate with heat exchangers (HX). If you were trying to size, for instance a brazed flat plate cross flow liquid heat exchanger (a pretty simple system to analyze relatively speaking) for a particular job, talking to suppliers, you would learn that they do not have curves for many applications, because the curves are nearly all empirical. Someone hired them to prove the function of the HX before they bought one (or many), that is the only reason why the curve exists. They do the testing because analysis is inaccurate.

      HX manufacturers won't comprehensively test their product line, because to do it well, it would take an incredible amount of resource. I was looking for just this sort of HX a few years ago, and I was surprised to learn they had analysis software they were using and writing, but it wasn't easy for them and they really didn't like to base much on it - the analysis results did not correlate well to the real world. This is coming from people who really know what they are doing.

      Let this be a warning to you, not to commit much capital to a design based solely on analysis - do some prototyping. This is the common sense part.

      Your system is going to be more uncertain than a brazed flat plate HX. How you duct the air, how the fan design works (a science and art unto itself), how the mounted environment changes the behavior of the system - compared to lab or prototypical use, how well you really understand the boundary conditions of the system. Do you really have a good model for the system around the HX at least for the more extreme conditions?

      Common sense will be revealed in how you look at the overall picture. You want to try and bracket the situation. What are the extremes of inputs and environment that will affect the HX. You can save the details in between the extremes for later when trying to optimize. But a first you don't want to waste effort on a system design that is just inappropriate. You look at similar applications already in existence as a frame of reference. If your system seems radically different from existing systems, you need to start asking questions about why.

      With HX you want to pay particular attention to approach temperatures. This is where the two flows have a small delta T. The ability of the HX is greatly diminished near the approach temperatures. You need to make sure you cover this situation. Your liquid flow needs to be turbulent or the HX will drop considerably. You need to look at the Moody Chart.

      Common sense comes into the compensation factor you apply - how much you jack up the transfer area (and cost) to make sure you are covered for unforeseen occurrences. THere is no perfect answer, you have to think in terms of risk and benefit.

      Finally, pay very close attention to the mounting of the HX. You might get the HX part perfect and it falls apart in the field. This is where being observant, surveying existing apps, and talking to manufacturers is very helpful. Follow best practices. Do not discount simple prototype failures as trivial. Be observant, take good notes, and don't forget what you saw. Don't be among the many who omit this step.
       
      Last edited: May 1, 2013
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    3. thermal

      thermal Member

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      Michael Ross, bonsoir

      I sort of like your approach. Calculation conditions, testing conditions and actual conditions are not the same. In particular, the velocity profile in a channel that approaches a heat exchanger is in the real world in most cases far from “ideal”.

      Still, designers need to specify something and because of these uncertainties have an uncertainty value in the top of theirs heads. Just as an all over figure, I would suggest 15%.

      What is your opinion of that value?
       
    4. Wmech

      Wmech Member

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      Hello Michael Ross,

      Thank you for the detail notes on engineering. I truly appreciate the amount of effort and time you have put on this topic.

      I understand that the theories do not always apply to the real life situation. As you have stated in this topic, that's why the prototype is there to analysis and cover as much real life situation as possible. I cannot agree more with you on this one. However, the prototype can be done when there is a budget for it. Even if scaling the size of the prototype, there are still some bias compared with the real design and the prototype.

      Taking as many aspects around the design is really crucial to any engineering. Tons and tons of topics can be covered to apply one small design and there are still not enough accurate results. What I am worried about myself is the uncertainty of using the wrong fundamental engineering theories.

      In fact, there is no design for heat exchanger setup. The real question in this topic is "how comfortable can people be in the container with this set up?" Sometimes a simple question from a customer can be difficult to answer especially a prototype is needed and involved. As you have stated, there are some similar design like this and there are many similar designs with different approaching the design. Because i have seen the similar set up like the car radiator, this is where I think it makes sense to me to go analysis in the way to analysis a car radiator.

      If it was up to me to design the whole system like this, the question will not be as simple as finding the exit temperature of the fan. Since the options and the information are limited, I have to find a way to counter the problem with acceptable answer. Once again, a simple question takes a lot of consideration to answer. Fluid dynamic and thermodynamic, even the material engineering needs to be applied and taken into account.

      What i have learned doesn't even cover 5% of engineering not even 1% of the real life situation. You are a very well-experienced engineer I am sure and I know i still have a lot to learn from an engineer like you. Maybe common sense is a practice of being observant and asks the right question. As far as I can tell, I am asking a lot of question that is probably not related to any of the design or I am probably making the question complicated than it supposes to be.

      With your guidance on this topic, i hope i can keep in mind to be observant and taken into account with all the aspect as possible. It seems to me that you need to born with this sort of "engineering" talent in order to be observant. However, the more I observe, the more questions i will have, the more answers will need to be found. Once again, thank you
       
    5. Michael Ross

      Michael Ross Well-Known Member

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      To thermal,

      I think this is only on a case by case basis. Risk and benefit vary for every project. If materials are expensive, or labor is intense, then it is worthwhile to spend more time on optimization. If the necessary material and labor are cheap then you are foolish not to overdesign a bit, and move on the the next thing. This is the common sense of prioritizing work.
       
      Last edited: May 1, 2013

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