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Hello,
My understanding is that it is not possible to have temperature cross in any heat exchanger. But someone told me if you have mutil-shell and multi-tube passes, it's possible to have temperature cross. Is this true? If so, how temperature cross can be achieved?
Thank you very much for your help.
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Temperature Cross
Started by Guest_Guest_*, Jan 10 2005 10:29 PM
2 replies to this topic
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#2
siretb
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ChE Jedi
Posted 18 January 2005 - 11:55 AM
It's indeed possible. One way to see it is that because some sections of the heat transfer have a fairly high temperature difference, enough heat is transfered so as , in the next section the temperature difference gets inverted. Of course this is impossible with single pass heat exchangers.
The hereunder has been found on the web. The link gives you more information. Hope that it will help
(a temperature cross occurs when the fluid being heated has an outlet temperature that falls between the inlet and outlet temperature of the heating medium
This book has some info
Fundamentals of Heat Exchanger Design
Ramesh K. Shah, Rochester Institute of Technology, Rochester, New York, Formerly at Delphi Harrison Thermal Systems, Lockport, New York
Dusan P. Sekulic, Univ. of Kentucky, Lexington, Kentucky
ISBN: 0-471-32171-0
====>
http://www.cepmagazi.../pdf/040440.pdf
The hereunder has been found on the web. The link gives you more information. Hope that it will help
(a temperature cross occurs when the fluid being heated has an outlet temperature that falls between the inlet and outlet temperature of the heating medium
This book has some info
Fundamentals of Heat Exchanger Design
Ramesh K. Shah, Rochester Institute of Technology, Rochester, New York, Formerly at Delphi Harrison Thermal Systems, Lockport, New York
Dusan P. Sekulic, Univ. of Kentucky, Lexington, Kentucky
ISBN: 0-471-32171-0
====>
http://www.cepmagazi.../pdf/040440.pdf
#3
Guest_Guest_*
Guest_Guest_*
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- guestGuests
Posted 24 January 2005 - 09:10 AM
Yes, this can happen when heat exchangers operate in cross-flow.
Consider a shell and tube with the hot fluid on the shell side, and the cold fluid on the tube side. Imagine that the hot fluid enters from the left and the cold fluid enters from the right.
By the time the tube side fluid has travelled from the right side to the left side of the exchanger, it will still be colder than the hot fluid on the left hand side of the exhanger, but could be warmer than the hot fluid on the right hand side of the exchanger.
Imagine now that the tube side flow now enters into another pass, progressing from the left hand side of the exchanger to the right hand side of the exchanger. This fluid could now potentially 'overtake' the shell side fluid, and arrive on the right hand side of the heat exchanger as the warmest fluid. The temperature difference has now reversed.
What this means in a practical sense is that the overall temperature driving force has been diminished. Since the driving force has been diminished, but the same duty is still required, more area must be added in to compensate.
This kind of problem is a great example of when the 'optimal' heat exchanger solution isn't totally obvious. The driving force for this kind of design may be more commercial or due to some other limiting factor, rather than one of minimising the material used by using the minimum possible heat transfer area. Perhaps having a single pass tube arrangement would require more than one shell? Maybe it would prove cheaper to build a single shell with a larger diameter than to weld shells together? Or perhaps space is limited, and there isn't room to install a very long shell on plant? There is more to designing a heat exchanger than calculating the minimum area required.
Dave
Consider a shell and tube with the hot fluid on the shell side, and the cold fluid on the tube side. Imagine that the hot fluid enters from the left and the cold fluid enters from the right.
By the time the tube side fluid has travelled from the right side to the left side of the exchanger, it will still be colder than the hot fluid on the left hand side of the exhanger, but could be warmer than the hot fluid on the right hand side of the exchanger.
Imagine now that the tube side flow now enters into another pass, progressing from the left hand side of the exchanger to the right hand side of the exchanger. This fluid could now potentially 'overtake' the shell side fluid, and arrive on the right hand side of the heat exchanger as the warmest fluid. The temperature difference has now reversed.
What this means in a practical sense is that the overall temperature driving force has been diminished. Since the driving force has been diminished, but the same duty is still required, more area must be added in to compensate.
This kind of problem is a great example of when the 'optimal' heat exchanger solution isn't totally obvious. The driving force for this kind of design may be more commercial or due to some other limiting factor, rather than one of minimising the material used by using the minimum possible heat transfer area. Perhaps having a single pass tube arrangement would require more than one shell? Maybe it would prove cheaper to build a single shell with a larger diameter than to weld shells together? Or perhaps space is limited, and there isn't room to install a very long shell on plant? There is more to designing a heat exchanger than calculating the minimum area required.
Dave
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