r/ChemicalEngineering • u/mrnormality • 1d ago
O&G Tracking U of Heat Exchangers
Hello,
Is there really a significance for tracking Overall Heat Transfer Coefficient of heat exchangers over time? Monitoring the heat exchanger outlet temperature or simple lculation for the heat duty would already be a good indicator for the fouling that happens within the exchanger. If this is the case, there is no point in calculating U and tracking it, right?
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u/friskerson 1d ago edited 1d ago
I haven’t done it, but I would agree nominally… though I would consider a second thermocouple on the inlet since Newton’s law of heating/cooling is about differential temperature. Not sure if your system inlet has any notable temperature swings (seasonal, or fresh water supply related) that would muddy the single data point.
But once you have both T’s, you can estimate U with good enough accuracy (for government work) with a simple calculated column of data if you have the coefficient and assume constant. Or if you wanted more accuracy you could run the actual integration over the temperature range for each data point for the desired medium (calculation intensive).
Another strategy that the boneheads at my refinery job didn’t think of until a $25mm total replacement on the main battery is to demineralize/deionize/treat the inlet water which can help to reduce fouling over time, if it’s a water-to-medium exchanger.
Is this class work or field work?
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u/unmistakableregret 1d ago
Technically yes I guess, but U gives you a better feel for what's going on with because it's per area. You could then compare to other exchangers of different size.
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u/Ember_42 1d ago
If you have controls on the heat exchanger, the controls will keep the duty / controlled output temp flat. Until they don't. A calculated U based on actual in / out temps and at least one flow is useful (but be careful with bypass controls, you need the actual output, not the mixed temp to be valid).
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u/ogag79 O&G Industry, Simulation 1d ago
Measuring U, I can see it can help in detecting fouling early on but practically you either do something if either (a) you can't reach your desired process temp even with maximum utility flow or (b) you already have high DP across your HEx, if it's equipped with DP indicator/transmitter.
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u/AlarmedForm630 1d ago
The advantage of tracking the overall heat transfer coefficient U is that you can compare to the nominal value given by the manufacturer. Also, if you track it over time, it will be easier to see the fouling rate and also it will show the effectiveness of a cleaning.
During my master thesis at a pulp and paper mill, that was one of the main indicator to track the performance of the black liquor evaporators.
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u/Kelvininin 1d ago
I use to start and service seeded slurry MVR evaporators. Tracking the heat transfer coefficient was how we monitored the health of the system. Declining heat transfer coefficient meant the unit was scaling.
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u/EmergencyAnything715 1d ago
Measuring U value is an appropriate way of normalizing exchanger performance for different flow conditions, especially if there is variable flow/temperatures on both sides of the exchanger.
Using outlet temperature is one way to monitor for fouling, but its not a great way to view this. Let's say you have low rates during the winter and dont see much of a change in outlet temp. It May be fouled up going into summer, but you didn't realize it because of the conditions the exchanger is operating in.