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Q1: How to calculate LMTD for a SHELL and TUBE heat exchanger if one of the fluids enters as superheated vapour
and after condensation exits the exchanger in form of subcool liquid?? How to calculate effectiveness of the exchanger in such case??
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Lmtd Calculations
Started by Guest_saket_*, May 09 2006 08:10 AM
4 replies to this topic
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#2
djack77494
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Posted 18 May 2006 - 12:33 PM
saket,
I believe that there is limited applicability to the use of typical heat transfer equations for a problem such as yours. An equation like Q = U*A*dT is based on a linear relationship between Q and T. A single phase heat transfer problem would (approximately) satisfy this requirement. Condensing or vaporizing a single component would also satisfy this requirement. Sometimes, even multi-component heat transfer with phase change can be approximated with this equation. However, when you have superheated vapors being cooled to saturation and then condensing or when you have saturated vapors condensing and then subcooling, you would typically have major discontinuities in the Q vs. T curve. This would invalidate the single equation (simplistic) approach to the problem. Perhaps the best approach, if you were doing hand calculations, would be to break the exchanger down into zones - one for desuperheating, one for condensation, one for subcooling - for example. Then, within a zone the simplifying assumptions would be valid. Address the entire exchanger by summing the individual zones.
HTH,
Doug
I believe that there is limited applicability to the use of typical heat transfer equations for a problem such as yours. An equation like Q = U*A*dT is based on a linear relationship between Q and T. A single phase heat transfer problem would (approximately) satisfy this requirement. Condensing or vaporizing a single component would also satisfy this requirement. Sometimes, even multi-component heat transfer with phase change can be approximated with this equation. However, when you have superheated vapors being cooled to saturation and then condensing or when you have saturated vapors condensing and then subcooling, you would typically have major discontinuities in the Q vs. T curve. This would invalidate the single equation (simplistic) approach to the problem. Perhaps the best approach, if you were doing hand calculations, would be to break the exchanger down into zones - one for desuperheating, one for condensation, one for subcooling - for example. Then, within a zone the simplifying assumptions would be valid. Address the entire exchanger by summing the individual zones.
HTH,
Doug
#3
Guest_saket_*
Guest_saket_*
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Posted 19 May 2006 - 12:56 AM
Hi doug,
Thanx for your reply.I came across this zone method but in this case heat transfer area in different zones should be known...isnt it??? So in a single heat exchanger (with known heat transfer area) how can i divide the areas in different zones?
Thanx for your reply.I came across this zone method but in this case heat transfer area in different zones should be known...isnt it??? So in a single heat exchanger (with known heat transfer area) how can i divide the areas in different zones?
#4
djack77494
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Posted 19 May 2006 - 08:47 AM
It's quite possible that you will have to solve using a trial and error technique. If possible, I'd suggest you try to take advantage of all the hard work previously done by others. Do a google search and/or inquire about tools (software any spreadsheets) that could advance your efforts. Sorry for not having more concrete suggestions, but I know I've come across such things.
Good luck,
Doug
Good luck,
Doug
#5
Profe
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Posted 19 May 2006 - 10:56 AM
Hi Saket
About your question: I think that you must consult and study the chapters corresponding to the subject and examples in the bestseller
Donald Q. Kern. Process of Heat Transfer. Mc Graw Hill Book, Co., Inc. 1950.
Chapter 12. Simple Vapour Condensation
Example 12.4. Calculation of a vertical condenser sub cooling.
Example 12.5. Calculation of a horizontal condenser sub cooling.
Chapter 13. Mixed Steam Condensation
Example 13.3. Calculations for a Condenser of Multicomponents.
I have Spanish version 1999 and perhaps the chapters do not agree.
About your question: I think that you must consult and study the chapters corresponding to the subject and examples in the bestseller
Donald Q. Kern. Process of Heat Transfer. Mc Graw Hill Book, Co., Inc. 1950.
Chapter 12. Simple Vapour Condensation
Example 12.4. Calculation of a vertical condenser sub cooling.
Example 12.5. Calculation of a horizontal condenser sub cooling.
Chapter 13. Mixed Steam Condensation
Example 13.3. Calculations for a Condenser of Multicomponents.
I have Spanish version 1999 and perhaps the chapters do not agree.
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