2 replies to this topic

[jprocess]

The best definition from fouling factor concept that i ever found is by Mr.Montemayor:
A margine that we apply in design of shell and tube heat exchangers and this inclusin buy time for us after the time that ecxhanger begin to foul in operation.
As you know, the values of fouling are so small and in design formulas its reverse magnitude play role so it can greatly increase the heat transfer area and may cause higher exchanger cost.
Also at the start of plant life up to 2 or 3 months the exchanger is not fouled but this over-sized exchanger may cause operational problems at that period.
So it seems that inclusion of fouling factor is not good and logical for every HE design problem.Am i right?
Is'nt it better to have a periodic program for cleaning of HE instead of inclusion of fouling factors?
Cheers.

[Adriaan]

Fouling depends on materials used, the design used and the products that flow through the heat exchanger (including their velocity). If you have a heat exchanger that will absolutely positively not foul then you would not need a margin, but for all practical intents and purposes you either need a cleaning schedule (be it automatic/periodical or based on a fouling measurement) or over-capacity.

In the end it is a FINANCIAL descision how you (have to) design and operate an installation. Though from a TECHNICAL point of view that can mean some serious inefficiency the financial efficiency can be greater that way and is almost always of greater importance. After all, cleaning a heat exchanger means downtime, for the cleaning itself but also for taking the equipment out of operation and back in operation, and that is in all probability rather more costly than running it at reduced efficiency.

The obvious thing to do is monitor the HE fouling (by comparing its energy transfer to the transfer capacity at startup) and perform maintenance (cleaning) when it is required (either absolutely needed or there is a 'window' in which it can be easily scheduled).

[Zauberberg]

True wisdom is to choose proper design of heat exchanger, where trade-offs are minimal, or most acceptable from operating and economy points of view. This can be accomplished only if designer has rich field experience. This also means that we certainly have great chances to screw up our first equipment designs in professional life. Once when decision is made, a serious and complete performance monitoring should take place - to confirm design premises. It is not easy to become professional like Art Montemayor; but certainly it is not impossible.

When designing equipment for heat transfer, it is not practical to neglect fouling and rely solely on frequent cleaning of HEXs - in most of industrial (fouling) applications, because it cannot be justified by economics. Moreover, fouling has bigger potential at the beginning of operation, until terminal tube-side velocity is reached (remember the pressure drop VS time and U-coefficient VS time diagrams), so it is not wise at all to "exclude" fouling factor in equipment desing stage. In some cases (it has become a standard during last decade or two), exchanger trains are equipped with individual or group bypasses, to provide for cleaning while unit is on the run.