tc7:
Having been in the same situation in the past, the following is what I have done with successful results:
1) First and foremost, do what pmover and TBP have correctly pointed out: you must thoroughly identify what you have in an engineering manner. You must have a mechanical rating of the unit, such as it's MAWP, total heat transfer area (bare tubes - the fins are ineffective), the number of tube passes, the flow area per pass, the inlet & outlet nozzle sizes & ratings on both shell & tube side. You haven't furnished this information, but you obviously can, and should, get it.
2) Don't worry that much about the thermal rating; you're going to have to accept a conservative design. This is definitely not an "optimization" or academic attempt to calculate the answers down to the decimal place. The practical way it is done in actual industrial applications is that a conservative "U" is established. A value of 200 Btu/hr-ft2-oF is, in my experience, too optimistic. I would rely on a value of 150 as being more realistic for this liquid-liquid, U-tube application.
3) Worry more about being able to handle the required flow rates through the unit with a decent pressure drop on both sides. TBP is leading you in the right direction: you must establish what your flow rates are going to be, based on the heat load. From your very limited description of the equipment, I have doubts that you may be able to push 250-260 gpm through the unit - but, you have to furnish more detail.
4) Your shell side pressure drop can be calculated with the Delaware method. However, your description of the shell side baffles is "baffling". I have never heard of “edge flow” baffles and can't imagine what you mean. Try to use standard, conventional nomenclature as spelled out in TEMA, Kern's "Process Heat Transfer", the GPSA Engineering Data Book, or the HTRI. Otherwise, we are wasting a lot time and writing trying to guess what your nomenclature is about.
5) Be aware that the correct way to describe the U-Tube configuration is to state the number of hairpins. That way, one can safely know that there are a total of 214 total tube hole in your tubesheet. Additionally, we have to assume that there is one tube pass in the bundle - the liquid goes in one end of the hairpin and exits the other end. There could be more passes, but since you don't mention this, there probably isn't.
6) Make sure you correct the LMTD by the appropriate flow arrangement correction factor. In order to have this unit operate in the expected manner, the baffles must have a minimum of clearances and by-passing in the shell side. This is the main culprit in not getting efficient heat transfer. The baffles must be in good condition to allow for effective shell side flow.
7) Pay no heed to the fin effect on your tubes. The fins are more of a hinderance in this application. Fins are ineffectual in liquid service. They are meant for gas service where the film coefficients are much lower.
8) pmover recommends a shell and tube mfg conduct an analysis/thermal rating of the exchanger. I would not recommend this mainly because it doesn't add any value to your application. No exchanger manufacturer will, in my opinion, accept any responsibility or liability for a thermal analysis. The information, if you can get it, will only be worth the paper it's written on. You can do the thermal analysis with the same degree of confidence that they would do it under. The mechanical rating is a totally different animal. You can do a credible and professionally stamped mechanical analysis to the extent that you can furnish alloy, wall thickness, flange ratings, and other physical and measurable data. And you can do it with a conservative approach.
Hope this experience helps.
Art Montemayor
Spring, TX
