6 replies to this topic

[Flyou]

Hi everybody,

 

I am a process design engineer in the refining field at the basic engineering step (licensor).

 

During unit designs, I often specify heat exchanger process datasheets.  Those exchangers will be sized rigorously by detailed engineering at a later step of the project.

 

To establish the process datasheet, I simulate the unit using Pro II and the process/process exchangers are considered either 1 pass shell/2 pass tube (TEMA shell and tube HX, considering an Ft < 1) or 1 pass shell/1pass tube (double pipe HX, considering an F_t = 1 for perfect counter current flow) depending on the size of the exchanger.

 

To determine the sizing case of the exchanger, I first simulate all the cases with a fixed Hot Outlet temperature minus Cold Outlet temperature (T₂ – t₁).  Then I identify the maximum UA case, and I apply this UA to all the cases.  Doing this assumes that the heat transfer coefficient is constant for all the cases - which is not the case in the real life since it depends a lot on the throughput to the exchanger (U = f(Re)).  Please refer to the attached sketch.

 

As a result, for the sizing case, the resultant HOCO (T₂ – t₁) is equal to the HOCO I have simulated first, while all the other cases - and especially the turndown case (50% capacity) - the HOCO is less.

 

If, for the first simulation, I take the HOCO = 0 °C, then for the other cases, the resultant HOCO is negative.  For the turndown case in particular, the resultant HOCO in the simulation could sometimes be as low as -50 °C to satisfy the equation Q = F_t*UA*DT_M.

 

It is nonsense to consider such a low HOCO since in the real life, a temperature cross must be limited to a much higher HOCO than -50 °C.  The wrong assumption in this method must be that UA is considered constant.  For this reason, I have considered until now that the HOCO cannot go lower than -5 °C.

 

I do not have access to HTRI and I do not have much feedback from the field.  Does anybody have experience in this lower throughput case in the field?  What is the lowest HOCO that can be seen on site?  Is this value of -5 °C acceptable?

 

Thanks for your answers.

Edited by Art Montemayor, 10 June 2013 - 06:04 PM.
Composition

[latexman]

HOCO?

[Flyou]

HOCO = Hot Outlet temperature minus Cold outlet temperature

[Art Montemayor]

Flyou:

 

I also have problems trying to interpret your explanation.  Can you consider the following edited version of your post as accurate with respect as to what you have in mind?  If so, I can paste this edited explanation over your original post and it can be the basis of your query.  You can, of course, rely on your own original posting.  I am only trying to avoid confusion and our members getting mixed up as to what you mean to say.  A Sketch or drawing can quickly orient a member to understanding what you are trying to do.  If you agree, feel free to edit, employ, or add to the attached Excel workbook.

 

 

 Heat Exchanger Sizing.xlsx   13.7KB   212 downloads

Edited by Art Montemayor, 10 June 2013 - 06:06 PM.
Deleted recommendation

[srfish]

Flyou,

 

Yes -5 C is acceptable, Even for multiple tube pass heat exchangers. Much higher HOCOs can be used in counter-flow exchangers.

[Zubair Exclaim]

i think you are talking about what we call APPROACH which is a better parameter and we try using minmum 20 ºF approach temperature for sizing of heat exchanger

[Flyou]

To Art:

I totally agree on your version of my post, sorry if i was not that precise. For the english mistakes, i am sorry, english is not my mother tongue. And for the precisions, you are right, i should have explained the terms i used (HOCO for example). I am quite new here, i will try to send a more precise post next time with schemes if needed so as to be understood by everyone.

 

To srfish:

Thanks a lot for your confirmation. I will go on with the HOCO of -5°C for shell and tube exchangers 1-2 (i.e. 1 pass shell, 2 pass tube).

For pure counter-flow, i agree with you, more negative HOCOs can be seen. Theorically, HOCOs could be very high.

 

To Exclamation:

Yes i think we can use the term approach also, i have already heard this term. For the sizing case, i agree with you, you simulate with a certain HOCO which can be 20°F (10°C). But then, when you simulate another case where less area is needed (turndown), the HOCO or approach depends on the area installed for the sizing case and could be negative.