2 replies to this topic

[edycastillo]

well I want to know what happens with the overall coefficient "U" when you use double segmented baffles against single segmented in the same conditions.

I can see that double segmented baffle reduces the cross flow velocity reducing the pressure drop, but in terms of what happens with "U" i.m a little confused.

I want to know if this is correct. I will expect to reduce Reynolds and with a lower reynold decrease my overall coeficient ("U"). So double segmented baffles will be for designs that you are not allow to have big pressure drops but you have to consider the penalty in the "U" having more area????(Please if you have comments i will aprecciate).

thank you

P.D. Art if you see this post I just want to say hello and that it is great to have people like, you trying to help others to be better.

Eduardo

[Art Montemayor]

Eduardo:

It's a pleasure to welcome you to the Industrial Forum as a recent successful graduate. I hope to hear more from you.

Your question is a basic one and you have practically answered it yourself with your suspicions of a trade-off. I've often found that in the field of industrial heat transfer, a trade-off is often a matter that one has to confront. The old engineering adage says "there are no free rides or free lunches" - and you have found a good example of that. When you reduce the velocity through the shell side of a shell & tube heat exchanger, you effectively have reduced the Reynolds Number and, consequently, affected the value of the resultant convection film coefficient - usually the most important one. As a result of a lower velocity and lower pressure drop, you must pay the price for the "lunch" - which is usually a reduced Overall "U" and, of course, more capital money because now the required exchanger has to be larger in effective heat transfer area.

Note, however, that the above explanation may not be true if you reduce the spacing between the double baffles - or reduce the diameter of the shell and make it longer. So, in order to be able to state that double segmental baffles yield a lower "U" as compared to single segmental baffles you must state all the involved parameters. However, in a general and non-specific case you would normally expect to apply a double segmental baffle (or a "doughnut" baffle) design if you were requiring a reduced shell-side pressure drop.

What you are expecting to happen can be seen to occur when you have access to an excellent heat exchanger design program - like HTRI's. With this program you can change the baffle types, spacings, and pressure drop requirements and immediately spot the differances it makes it the final design.

Like any other heat exchanger design a lot depends on the many factors a designer has to consider as basic data:

*single phase or phase changes?
*allowable pressure drop?
*corrosive/erosive fluids?
*horizontal or vertical orientation?
*removable bundle or fixed?
*temperature cross possible?
*tube-side velocity too low, necessitating multiple tube passes?

But basically you are correct in expecting to pay something extra for obtaining a favorable low pressure drop.

I hope this explanation helps to orient you in your understandings.

Art Montemayor

[mbeychok]

Art:

If I may inject a bit of humor, I have always defined the Second Law of Thermodynamics as being "There ain't any free lunches!".

Regards,