Exchanger Fouling Resistance Decreasing With Time?
Exchanger Fouling Resistance Decreasing With Time?
(OP)
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.
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.





RE: Exchanger Fouling Resistance Decreasing With Time?
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"
RE: Exchanger Fouling Resistance Decreasing With Time?
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.
RE: Exchanger Fouling Resistance Decreasing With Time?
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.
RE: Exchanger Fouling Resistance Decreasing With Time?
RE: Exchanger Fouling Resistance Decreasing With Time?
RE: Exchanger Fouling Resistance Decreasing With Time?
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: Exchanger Fouling Resistance Decreasing With Time?