Heat Exchangers with Temperature Pinch

[DK44]

In a particular Cooling application of DM Water & Sea Water, the temperature profiles with pinch are:
Cold Fluid (Sea Water) in / out = 40 C / 50 C
Hot Fluid (DM Water) in / out = 50 C / 40 C

1. Does a Shell & Tube Heat Exchanger of TEMA type E/ H shells suitable
2. Does a Plate Heat Exchanger suitable.
3. Are Plate Heat Exchangers suitable for Temperature Meet, Cross and Pinch.

 

[georgeverghese]

1) The key configuration requirement is the no of tube side passes for this application. You can only use one tube side pass for this case. For type E shell side pass, use only one tube side pass in counter current configuration. You can wire up as many shells as you need (if one shell does not suffice), provided they are all wired up in series on both shellside and tubeside, and the flow arrangement is countercurrent for all shells.
For type H shell, it is a little more complicated. Compute the F factor - see Fig 11-4 on page 11-6 of Perry Chem Engg Handbook in the chapter on Heat Transfer Equipment Design. R = 1.0, S = 1.0. See fig h - the F factor for R = 1.0 and S = 1.0 is less than 0.2. So you cannot make this work with H type shell, even if you have a single tube side pass ,since F < 0.2 - you need a min value of F =0.75 to 0.8 to make it work.

2)A plate HE can be used for this application provided it is 100 % countercurrent flow

3)Am familiar with these terms, but to my knowledge, these terms are too loosely used by process engineers, so I'd rather not comment. But I can say that no HE, shell and tube or plate type, can get you around a internal temp pinch. Whether or not this application involves a temp cross depends on the configuration. For a pure countercurrent configuration, there is no cross. If you were to use 2 or more tube side passes or a type H shell or similar, there would be an unworkable internal temp cross (or pinch).

 

[DK44]

Thank you George for your good remarks.
1. Yes. While Temp Cross can be handled in STHE with Counter Current Sigle pass, Pinch (with given temp Profile) could be problem even you use multiple shells in series. How to resolve the usage of proper type of Shell?
2. Is the above referred profile realistic in the industry.
3. What about X-Shell suitability?
4. Is temperature pinch not a problem with PHE ?. Can it handle ?

 

[Christine74]

Nothing will work as this violates basic thermodynamics.

It's true that countercurrent flow allows for some temperature cross, but the countercurrent arrangement does not produce heat transfer where there is no temperature difference.


-Christine

 

[georgeverghese]

Assume you keep the DM temps fixed - 50 to 40degC

In reality, in a full countercurrent setup with shell and tube, the seawater temp may be going from 37 to say 48degC or so at best. So this gives you an overall LMTD = 2.5degC. Correction factor F = 1.0 for full countercurrent flow.

In an air separations plant with all welded plate HX in full countercurrent flow, the hot air inlet to cold O2 or cold N2 exit temp approach is as close at 1.5-2degC.

Quoting from Perry 7edn on this topic of F for LMTD, on page 11-5

"If only one tube pass per shell is required, the piping
can and should be arranged to provide pure countercurrent flow,
in which case the LMTD is used with no correction."

@christine, the "no temp difference " at no temp difference location " you refer to is valid for 1 shell pass, 2 or more tube passes case. There is no cross in full countercurrent flow. Pls see fig 11-1(a) in Perry 7th edn.

 

[georgeverghese]

Error in my post dated 14Feb
" For type H shell, it is a little more complicated. Compute the F factor - see Fig 11-4 on page 11-6 of Perry Chem Engg Handbook in the chapter on Heat Transfer Equipment Design. R = 1.0, S = 1.0. See fig h f "

For full countercurrent flow for applications such as this requiring close temperature approaches on both sides, available in corrosion resistant materials for up to 20barg design pressure, another alternate choice to the plate heat exchanger is the spiral heat exchanger - compact, cleanable, and high overall htc also - see Perry also for this

 

[georgeverghese]

I've misinterpreted the application of diagram f and h in Fig 11-4 in Perry 7th edn. The schematic attached to each of these figures is poorly explained in the footnotes. A better schematic and narrative for these predominantly crossflow configurations is to be found in DQ Kern Fig 16.17 on page 549 - chapter on Extended Surfaces.

To be brief, there are no LMTD correction factor charts for TEMA H, G , J and X shells, for one tubeside pass / multiple tube side passes that I know of - DQ Kern says these charts are yet to be derived as of 1945 or so. May be some kind soul one can show us these graphs, if they have been developed by now. Even in the recent 7th edn of Perry, such graphs are not shown.

Nevertheless, for applications such as these, pure countercurrent exchangers are the solution.