5 replies to this topic

[ramlalithravi]

I would like to know what is the rectification method to solve temperature cross in HYSYS. Which parameter to be adjusted and in such a way to avoid temperature cross.

Which is also associated with "FT Correction factor is Low". I presume FT is resistance co-efficient due to fouling of the material that should be accounted while calculating overall heat transfer co-efficient. So that Low FT factor will results in more Overall heat transfer co-efficient and ultimately leads to temeperature cross. If i am wrong, pls correct me and give ur ideas.

regards,
ramesh

[Amit1]

Ramesh

FT factor is not related to fouling in exchanger. It indicates how far off you are from perfect counter-current flow. The equation, Q=UA (LMTD) assumes perfect countercurrent flow. But that is not actually the case with shell & tube heat exchangers. Hence we use FT factor, which is the LMTD correction factor. It depends upon temperatures,number of shells in series, no of passes etc. Obviously too low FT is not preferred as it will lead to very large heat transferred area.

Coming to your second question, when hysys detects a temperature cross, you can remove the cross by changing outlet temp specification that you have given. If you don't want to change your temp specification, play with number of passes and number of shells in series to remove cross.
Actually the best way to simulate an exchanger is to specify minimum approach or HOCO, HICO, HOCI approach ( depending upon no. of passes on tube & shell side), which will ensure that you don't come across a temperature cross.

Hope this helps

Amit

[AlertO]

hi ramesh

like Amit said, FT doesn't relate to the fouling on exchanger. it's a function of fluid temperature and the number of tube and shell passes.
in case of you, temperature cross presenting and FT is too low, you may try to select the exchanger exactly performing the counter flow exchanger. i.e. 1 shell pass & 1 tube pass.
this can increase the Ft factor and your exchanger will perform at the better effectiveness.

good luck

[Michael Zhang]

The quick way is to define it 1 shell pass & 1 tube pass and make the volumes both shell and tube are zero.

[deltaChe]

Usually we would like that mass velocity of fluid flowing through the exchanger increases, so that the
film coefficients increase improving heat transfer and reducing the physical size and cost of the exchanger. But, increasing the fluid mass velocity will also increase the pressure drop across the unit.

So, it's beter to adjust the pressure drop close to allowalbe pressure drop limit to get a optimal design.

[sheiko]

Ramesh,
I recommend you to read the following discussion:
http://www.cheresour...heat-exchanger/