8 replies to this topic

[mhamdi khaled]

Hello,

I am working on preheat exchanger train in topping unit.
The S&T HE with the following parameters:
Shell side:
Atmospheric residue: (fouling resistance: 0,001 m2.C.h/kcal)
Tube side:
Crude oil: (fouling resistance: 0,0004 m2.C.h/kcal)

The EPC company gives us a velocity in shell side equal to 0.24 m/s which looks strange and too much low value.
Any one please give us a standard can guide me in this parameters.

Thanks,

[Pilesar]

I think you are correct to pursue more information. The engineer designer at your EPC company would likely be very glad to explain how the design was determined and describe the reasons that compromises were made. You might increase shell side velocity at the expense of pressure drop by using two exchangers in series. There may be advantages to swapping tube side and shell side services to put the heavier fouling stream inside the tubes. The EPC should have considered the options and may be very proud of a good design and waiting for someone to be interested so they can show off how smart they are. 

[breizh]

hi ,

Very weird ...You definitely need to investigate and question this design.

I've attached a document to explain how to mitigate fooling for similar products to yours. Velocity is key to achieve good  heat transfer.

Attached a link with typical fouling coefficient (pay attention to units system)

https://www.hcheattr...g_factors2.html

 

Good luck

Breizh

[SilverShaded]

I would also question why atmospheric residue is on the shell side, it will likely have a higher fouling tendency than the crude.

[mhamdi khaled]

I think you are correct to pursue more information. The engineer designer at your EPC company would likely be very glad to explain how the design was determined and describe the reasons that compromises were made. You might increase shell side velocity at the expense of pressure drop by using two exchangers in series. There may be advantages to swapping tube side and shell side services to put the heavier fouling stream inside the tubes. The EPC should have considered the options and may be very proud of a good design and waiting for someone to be interested so they can show off how smart they are. 

Dear Pilesar,

 

In fact, this design is: three S&T HE in series just before the atmospheric fired heater.

to check the sizing given by the EPC company, i did the same sizing with same results but the problem is we have fixed a standard value of tube length equal to 4880 mm of all exchangers of TOPPING. so thats why the design here is limited and we can not increase the volocity inside the shell (the baffles spacing is 197 mm; baffles cut is 10%; shell ID=1092 mm so  we can't put lower value of baffles spacing or baffles cut).

trying reverse the locations of fluid and puting the residue in the tube didin't make a good result because the ovrall coefficient decease from 156 kcal/hm2°C to 140.4 kcal/hm2°C (but with a good values of velocity tubes: 0.92 m/s and shell: 1.28m/s) so the actual area is less than the required area then we should put a fourth exchanger in series.

in this case i am looking for a standard or refrence to guide me to decide: to keep the residue in shell side with 0.24 m/s and three exchanger in series or swich the locations of fluid and add a 4th exchanger in series with 0.92 m/s of velocity.

Edited by mhamdi khaled, 06 April 2023 - 07:09 AM.

[mhamdi khaled]

hi ,

Very weird ...You definitely need to investigate and question this design.

I've attached a document to explain how to mitigate fooling for similar products to yours. Velocity is key to achieve good  heat transfer.

Attached a link with typical fouling coefficient (pay attention to units system)

https://www.hcheattr...g_factors2.html

 

Good luck

Breizh

Dear Breizh,

 

thanks for sharing.

[mhamdi khaled]

I would also question why atmospheric residue is on the shell side, it will likely have a higher fouling tendency than the crude.

Dear Silver,

 

making the residue in tube side gives a lower value of overall coefficient U and the actual area will be less than the required.

[SilverShaded]

 

I would also question why atmospheric residue is on the shell side, it will likely have a higher fouling tendency than the crude.

Dear Silver,

 

making the residue in tube side gives a lower value of overall coefficient U and the actual area will be less than the required.

 

Maybe you should consider a different design.

[Pilesar]

Heat exchanger design is a compromise. Sometimes the constraints are too tight for a real solution. Is this design the EPC responsibility? Can you send response to the EPC to force a do-over? A new design for an exchanger in the preheat train may affect the performance of the others since the entire train is part of the system. It will be difficult to redesign just one exchanger without reviewing the others. Enhanced surface tubes, tube inserts, tube diameter, number of tubes are all adjustable by the designer. An inexperienced exchanger designer will not know the possible options to consider. Even an experienced designer may have experience with just a few options and not consider ones they have never used before. I think a collaboration between you and the EPC may yield better results. At least a discussion with the exchanger designer would give you insights into what they perceived as the difficulties.