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Choosing Shell And Tube Heat Exchanger Type

shell tube heat exchanger t.e.m.a shell and tube heat exchanger heat transfer

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#1 chemeng_student_stream

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Posted 24 September 2016 - 12:23 PM

Hello, I am new to this site, I was planning on designing a STHE for heating up two process streams which are both a mixture of light hydrocarbons,  water, noble gases.  hot streams is at 20 atm and cold stream at 21 atm with mean temperature gradient is around 25 K with  temperatures roughly in the range 500 K to 330 K in the hot stream and 300 K to 460 K in the cold stream. I was thinking using an U tube (A/BEU) HX  because of thermal expansions allowance and consideration of benzene being toxic which doesn't allow the use of other types except AET and BET. I choose U tube because of thermal expansion consideration instead of AET or BET. Since mean temperature gradient is around 25 K so  its low but the operating temperatures are high so I choose U-tube

 

I understand that  individual replacement of tubes are difficult due to tube nesting and the consideration that u-tube bundles are harder to clean mechanically. I am just not sure what the industrial practice is when it comes to cleaning toxic hydrocarbons are, is it mechanical or chemical or since they are toxic when fouling occurs is the bundles not cleaned at all. Instead is it replaced?. Also about fouling, I am a bit confused how much of fouling is caused by light hydrocarbons, is it band enough to cause weekly cleaning or annual cleaning? 

 

 



#2 Bobby Strain

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Posted 24 September 2016 - 02:47 PM

Your choice is fine for academic purposes.

 

Bobby



#3 Dipankarc84

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Posted 25 September 2016 - 02:48 AM

your questions are too broad in nature. Fouling in itself is a full fledged research and is very specific to service types. "Toxic", light hydrocarbons, water, "noble gases" are too generic to comment on. You have to tell us what exactly is your service, what are the flow rates, the phases (liquid/ vapor/ 2 phase), tube side and shell side velocities, pressure drops etc. that you have come up with. 



#4 Pilesar

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Posted 26 September 2016 - 07:00 PM

  U-tubes have several economic advantages. Thermal expansion properties mean no expansion joint is required for U-tubes. Your choice looks okay from your description, but I think it will be tricky making it transfer the heat you want. The temperatures are such that you probably need countercurrent flow. The U-tube will only be countercurrent in one direction inside the tube, then cocurrent in the other direction unless you also route the shell side flow to be countercurrent only. By the time you go to all that trouble, you will wish you had just put in a single pass exchanger with an expansion joint. If your flow rates are small enough you may consider using hairpins or another countercurrent type.

  There are many techniques for manual cleaning. Those specializing in that service find a way to clean just about anything inside a U-tube. If the cleaners have the right equipment, cleaning should be no problem.

  Most chemicals are toxic in some way. Don't drink them or breathe them if they are toxic. Pressure vessels have connections for venting and draining before they are opened all the way for cleaning. These can be used to evacuate the bulk of the fluid from the exchanger prior to removing the vessel head. The cleaning people will wear personnel protection suitable for the job including breathing apparatus if necessary.

Benzene is an environmental regulation issue and maintenance should be included in the government permits.

  Find a chart showing typical fouling resistance for different fluids. They are fairly common and are a useful reference. Exchangers foul, but they are sized so that they do not need to be cleaned too frequently. During design, they are configured so that they can transfer the required heat in a fouled condition. This means that they are over-designed for the 'clean' case. When you size your exchanger, use the appropriate fouling factors to make sure that your process will operate for the expected conditions. Leaving zero as a fouling factor is a mistake you should avoid. With the right fouling factors, size the equipment and trust your calcs.



#5 breizh

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Posted 26 September 2016 - 07:45 PM

Hi ,

On top of what has been explained , you will find some guidelines in the documents attached.

 

Hope this helps

Breizh



#6 chemeng_student_stream

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Posted 27 September 2016 - 06:38 PM

hello guys thanks for your replies, i am really confused because my flow rate is really small at 2800 kg s-1 with a fluid density around 35 kg/m^3 0.022 m3/s.  This means I get really low velocities and thus reynolds number and poor her transfer coefficients. I was wondering if it was possible to have a U-tube heat exchanger with 8 tube passes or 12 or 16 - you just need to split the flow into bits and thus can get far greater velocities. OR should I just use a different type of heat exchanger which can do such high tube passes since I am not sure if you can configure U-tube heat exchangers to be like that...  since as someone mentioned U-tubes are countercurrent only one way :)


Edited by chemeng_student_stream, 28 September 2016 - 06:20 AM.


#7 Bobby Strain

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Posted 27 September 2016 - 08:09 PM

Something is wrong with your numbers.

 

Bobby



#8 chemeng_student_stream

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Posted 28 September 2016 - 05:33 AM

sorry i meant 2800 kg/h oops :(


Edited by chemeng_student_stream, 28 September 2016 - 05:35 AM.


#9 srfish

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Posted 28 September 2016 - 09:37 AM

It is possible to have 8 to 16 tube passes in a shell-tube heat exchanger. It depends on the size of the shell. For example, if the shell is pipe size and greater than 10 inch in size, the  maximum  number of tube passes is 12.






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