2 replies to this topic

[Chem01]

Dears, 

We've two Ammonia shell and tube condensers in parallel on both tube and shell side. Ammonia vapours are coming from a compressor discharge on shell side and cooled in the condensers via sea water (SW) on tube side. Both exchangers were cleaned in March-2015 and now I decided to find the fouling factors. Surprised to see the fouling factor is decreasing with time?

All data is result of Aspen simulation, the design case closely match with data sheet. Pl. assist as how the fouling factor can decrease and where is the mistake.

Sea water flow is estimated to match heat transfer on the shell side.

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[MTumack]

Have you seen any changes to your process? Perhaps filters removing debris and Micro-organisms?

 

Could be you have significantly cleaned up the service this exchanger sees, and have therefor increased its efficiency.

 

One trend I see based on that data, is that your Fouling Resistance would appear to scale linearly with your LMTD. Perhaps your Exchanger is installed with Hot fluid travelling upwards, and cold fluid travelling downwards opposite to the fluids tendency to convect naturally due to the heats and changing densities, and your fouling is staying the same.

 

Do you have more data with this?

 

[Excel suggests:

Fouling Resistence = 0.0000395645(LMTD) - 0.0002210234

R²=0.9914]

 

EDIT: I think this is fallacious logic on my part; a higher LMTD would not change the natural circulation of the fluid, merely the difference in T1 vs T2 for that side. My apologies. Still an interesting lead imo. Seems counter-intuitive.

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Edited by MTumack, 20 January 2016 - 11:02 AM.

[Chem01]

Thanks for the reply, 

Process is the same, there are sieves at the sea water intake and these are there since long. Sea water is directly coming with no treatment except some hypochlorite dosing to avoid bacterial growth.

Hot Ammonia vapours are entering from top and Ammonia condenses and taken out from bottom. Sea water is like perpendicular to the ammonia flow with single pass on tube side.

Your observation is interesting as LMTD is linear with fouling, but is it just coincident..?

If i use Fouling resistance = 1/Ud-1/Uc, it is 0.000251, 0.001739, 0.000714,0.000677 respectively for each case. These values are twice the values given above as calculated by Aspen, why this is happening...another issue. Late at night here, revert tomorrow with further thought process.