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Heat Exchanger Shell Side Velocity?

heat exchanger shell and tube chemical engineering velocity

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#1 jimmyweng

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Posted 21 November 2017 - 12:07 AM

In designing the heat exchangers single pass (2 tube pass), I put superheated steam 200C - saturated steam 100C to heat Water from 25C to 60C. Steam is put in Shell side and cool water in Tube side. The velocity result in the shell side is quite big. Meanwhile in the tube side, it is in an accepted range for velocity of water in tube side. Is big velocity not acceptable in the shell side? Is there standard for steam velocity in shell side like standard water velocity in tube side (1-2.5 m/s)? 

 
Please look at the excel file. Thank you.

 

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#2 Pilesar

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Posted 21 November 2017 - 09:34 AM

   The shell side velocity needs to be below critical velocity. TEMA also has guidelines for maximum recommended rho-v-squared. Your spreadsheet indicates shell side velocity of 1435 m/s with pressure drop of 54000 kPa which might indicate you need additional tweaks to your design. For your spreadsheet, perhaps first look to modify your design to constrain the pressure drop to a reasonable value since you should know the steam inlet pressure and the minimum outlet pressure.

   Are you trying to not condense the steam, but only desuperheat it? I made this same mistake in my college senior project! The engineering answer is 'don't do that, it does not work'. The problem with that is the tube wall temperature will be close to the temperature of the water side and the steam WILL condense when it touches the cold metal. When you use steam for a heating medium in an exchanger, condense it fully! The real-life method to desuperheat steam is to directly inject water into the steam.


Edited by Pilesar, 21 November 2017 - 09:42 AM.


#3 jimmyweng

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Posted 21 November 2017 - 10:28 AM

   The shell side velocity needs to be below critical velocity. TEMA also has guidelines for maximum recommended rho-v-squared. Your spreadsheet indicates shell side velocity of 1435 m/s with pressure drop of 54000 kPa which might indicate you need additional tweaks to your design. For your spreadsheet, perhaps first look to modify your design to constrain the pressure drop to a reasonable value since you should know the steam inlet pressure and the minimum outlet pressure.

   Are you trying to not condense the steam, but only desuperheat it? I made this same mistake in my college senior project! The engineering answer is 'don't do that, it does not work'. The problem with that is the tube wall temperature will be close to the temperature of the water side and the steam WILL condense when it touches the cold metal. When you use steam for a heating medium in an exchanger, condense it fully! The real-life method to desuperheat steam is to directly inject water into the steam.

Dear Pilesar,

 

In my case, if I still use superheated steam and then the output is condensed water, so I should use steam density or water density to calculate the volumetric flow rate? Thank you.



#4 srfish

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Posted 21 November 2017 - 10:34 AM

Large velocities are acceptable in the shell side if they do not cause tube vibration. With a baffle spacing that close, it would be a surprise to me if calculations show tube bundle vibration.



#5 jimmyweng

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Posted 21 November 2017 - 10:51 AM

Large velocities are acceptable in the shell side if they do not cause tube vibration. With a baffle spacing that close, it would be a surprise to me if calculations show tube bundle vibration.

Dear srfish,

 

I made a mistake, I wrote the baffle cut 25% but I use 45% baffle cut to calculate the baffle spacing. So any suggestions?



#6 Pilesar

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Posted 21 November 2017 - 11:38 AM

If you are now planning to condense the steam, you will need a lot less of it! Recalculate your steam flow rate and your shell side velocities will be much lower. You will be considering inlet velocities to the shell and to the bundle, so use the inlet density for these calcs. There is no requirement that the baffle spacing be uniform throughout the bundle. But for your student design, I suggest you select uniform spacing to accommodate the 'worst case' of the inlet flow as it is simpler and 'not wrong'. The pressure drop through the shell side will NOT be linear so you may need to adjust your pressure drop formula to account for the changing internal volumetric flow.






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