HX in series (AES vs AFS) - configuration

[02164]

This is related to possible arrangement of exchangers for series configuration, their pros and cons. I posted this in petroleum refining section, but dint get answers.

I am designing a Feed bottom exchanger with cold side approach of 40°F.
Temperature cross exists and hence i opted 2 shells in series .
Initially i designed AES (E shell) and it took a considerable amount of time, since i faced a reverse heat flow in one of the shells. Finally i managed to get rid of it by increasing the tube passes and tuning relevant parameters.

But later , i tried AFS in series and found that there was no problem of reverse heat flow. Furthermore i was able to reduce required heat transfer by about 25% compared to AES in series.

My question :
Most reference show AES in series, and i havent come across AFS in series.
Can somebody advise me if AFS in series is acceptable.
Does this configuration have any kind of drawback, or problems.

Thanks in advance,

[raj_dhamelia]

I think when you have two AFS in series it means having total 4 AES in series and thus if temperature cross exists then with 2AFS=4AES in series the effectiveness goes up and you get more heat transfer for a given installed area. Previously you have taken only two AES in series and hence effectivness was less.
The advantage with using four AES in your case may be that you can have them normally used 4 in series but you can also pipe them to have two parallel banks with two exchangers each in series. This may help in keeping the impact of giving two exchangers for maintenance keeping one bank of two in series.
Problem with AFS is that they are not conventional in a sense that it requires special construction technology and maintenance skills. This is what limits there use. Refer article by R. Mukherjee on the subject.