Exchanger Fouling Resistance Decreasing With Time?

[tractable09]

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.

 

[itdepends]

How are you controlling these condensers?

It looks like the cooling water flow is constant- which means you are either restricting the flow of ammonia into the HX (which will change the condensing pressure- and this is what you need to calculate the LMTD).

OR- you're controlling the level of liquid ammonia in the exchanger- which will change the surface area for the purpose of calculating the LMTD.

I can't see how you would have such a constant ammonia outlet temperature without doing one of the above- given the large difference in SW inlet/outlet temp between March and the June/December scenarios.



As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[tractable09]

Thanks for the reply,
Sea water(SW) is the cooling media and there is no control on this side. The flow is not measured, it is delivered by fixed number of pumps and therefore flow rate is kept constant in calculations.
On Ammonia side, the compressor speed changes based on ammonia vapor load. This is the reason you can see a variation in Amm. flow in different months. Compressor discharge pressure do not change much and that's why the outlet Ammonia temperature of condenser stays in a close range. Ammonia is condensing and condensation temperature is fixed by the pressure. In each case Ammonia vapor flow is less then data sheet flow which condenser can easily handle. Further condenser is getting higher sea water then data sheet flow also getting lower sea water temperature inlet compared to design, all this making condenser job easy.
There is no level control on the condenser.

 

[itdepends]

Regarding your heat exchange calcs- have you done them both ways? (using mass flow of seawater and temperature change for case A and mass flow of ammonia plus temperature/phase change for Case B)?

If they don't match it might point to an issue with either a temperature or mass flow measurement.

EDIT- just to throw another thought in there- you're condensing the ammonia right? It looks like the degree of superheat in the ammonia changes between the winter and summer cases. The heat transfer rate for cooling ammonia gas is going to be much lower than the heat transfer rate for condensing gas to liquid.

 

[25362]

A progressively cleaner water?

 

[georgeverghese]

It may not be possible to make realistic estimates of the fouling factors here - what is the effect of one or more minor measurement errors in flows or temps on FF ?

 

[itdepends]

If the data is real then he should be able to distinguish between the cases. The LMTD variation is significant with only a small change in the heat flux.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[25362]

Depending on the plant's location, water quality could suffer in areas experiencing increases in rainfall.