What is Fouling and monitoring parameters involved and when to clean Heat Exchangers.

Fouling: The deposition of undesirable material on the surface of tubes/shells on an Exchanger.

From Historian data if you find cascade controller at Heat exchanger flow valve is fully open/Max Limit to maintain the outlet temperature then you should act.
Means temperature is maintaining and flow is increasing continously then we have to pay attention.
Even though if we don't have cascade arrangement there will be atleast flow controller so it is very obvious if temperature reduces we increase the hot fluid / steam flow.
Problem: Heat exchanger Fouling.
Monitoring data :Flow Control valve opening/flow and outlet temperature of exchanger.
In general a cascade range controller will be used for the exchanger to maintain outlet temperatures. Arrangement will be like this.

Why at this condition we have to send for maintainance...  read below.

I was reading an excellent blog...

https://eng-software.com/about-us/press/articles/managing-heat-exchanger-fouling-to-reduce-risky-conditions/
I have extracted some information from that article.
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At a constant flow rate, as fouling occurs and the resistance to heat transfer increases, the heat exchanger’s heat transfer coefficient (U) decreases and will result in:

• a lower heat transfer rate
• a higher outlet temperature on the hot side (and lower dT hot)
• a lower outlet temperature on the cold side (and lower dT cold)

Since the outlet temperatures of the hot or cold side may be critical to the quality control parameters for the overall process, the flow rate must be increased to achieve the desired heat transfer rate and outlet temperature. A higher flow rate reduces theconvection heat transfer coefficient (hf) for the fluid and increases the overall heat transfer coefficient (U)to compensate for the effects of fouling. Very often in these applications, an automatic process control loop is used to measure and control the outlet temperature of one side of the heat exchanger by regulating the hot or cold side flow rate with a control valve (and occasionally a centrifugal pump operated by a variable speed drive).

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