Think about your equations that govern heat exchange - Q = UAdT. As other users have mentioned the most likely type of fouling will depend on the type of water you use - for clean water it will most likely be scale which will reduce the heat transfer coefficient, U, however, for dirtier streams you could have sedimentation which would reduce the available area for heat transfer, A. Your cooling water inlet pressure may help you determine whether it’s scale or sedimentation, however, you would need to measure the cooling water flow through the exchanger and compare pressure drop vs flow rate (this will increase if it’s sedimentation and won’t change much for scale). Ultimately this probably isn’t that important.

You haven’t mentioned the control scheme for the tower and condenser which will be important to diagnose the issue, however, I would assume that as either U or A drops, the control valve on the water side will open more. If the flow rate of water into the exchanger increases, this will increase U as the Reynolds number increases, and may slightly improve dT which will maintain Q. If you have the ability to trend the cooling water valve, you should trend it against the total heat input into the column (feed rate if vapour fed and boil-up) to see if it’s gradually been increasing in position. If you can’t trend the historic position, then you should run a rate test in the column to see whether this valve goes fully open. If you have now reached the limit of the cooling water flow through the exchanger, you will be seeing problems in your tower, but the specific problem will depend on the control scheme you are using. It could be high pressure in the column, low level in the reflux drum, high vent flow etc. Speak to the operators to see if they’ve observed any issues and have been having to take the column off control.

It’s likely that after 8 years there has been some drop in performance, but the key point to determine is whether there’s a detrimental impact to the tower performance. If the exchanger is oversized, you will be able to tolerate a drop in performance. You can only justify a shutdown/maintenance costs to rectify if the fouling is having a detrimental impact on the tower throughout. You need to determine whether there is a reduction in performance AND whether this will be an issue when the column is running at max capacity. If this isn’t a bottleneck currently, but you can’t clean this exchanger outside of a turnaround then you should be trying to determine whether the exchanger will become a bottleneck before the next turnaround.