I typically calculate the Uo and watch that. It's not perfect since changing flow rates or physical property changes will also affect the individual heat transfer coefficients but it's easy.
The LTMD is going to increase as you increase the flow rate through one side (for example) though that doesn't mean the exchanger is fouled.
with pressure information, it would be easier, but I only have local pressure indicators, for temperature I have more information since it s implemented on DCS
Since this would apparently be an intermittent operation, it wouldn't harm the process engineer to visit the site once in a while to take some readings. Do you agree ?
I totaly agree for the engineer monitoring level, but we intend to implement indicators on PI for managers, with alarms levels..., to make them see the $losses