Heat exchanger fouling assessment

Afgan, N.H. and Carvalho, M.G., 1994, Heat exchanger fouling assessment by confluence based expert system. Proc. 10th Int. Heat Trans. Conf. 4, 339 -334. 

The first of these two articles discusses assessing the degree of fouling in a heat exchanger, while the second presents a graphical method of monitoring fouling over time. 

It has to be said however, that without other information, these published data are of value in making an assessment of the potential fouling resistance. At the same time data on fouling resistances have to be treated with caution, they can only be regarded as a guide. A further limitation is that these values only apply to shell and tube heat exchangers. Conditions in plate heat exchangers for instance, could be quite different. 

If the fouling problem cannot be relieved by the use of additives it may be necessary to make modifications to the plant. Modification to allow on-line cleaning of a heat exchanger can represent a considerable capital investment. Before capital can be committed in this way, some assessment of the effectiveness of the modification must be made. In some examples of severe fouling problems the decision is straightforward, and a pay back time of less than a year could be anticipated. In other examples the decision is more complex and the financial risks involved in making the modification will have to be addressed. 

The purpose of any fouling model is to assist the designer or indeed the operator of heat exchangers, to make an assessment of the impact of fouling on heat exchanger performance given certain operating conditions. Ideally a mathematical interpretation of Equation 4.6 would provide the basis for such an assessment but the inclusion of an extensive set of conditions into one mathematical model would be at best, difficult and even impossible. 

In assessing the potential fouling of a heat exchanger at the initial design stage, it is necessary to consider influences that might occur external to the heat exchanger. 

There are in general, two ways in which plant data on fouling may be directly obtained. They involve an assessment of heat exchanger performance and the use of a side stream of process fluid. The use of probes may provide an indirect method of fouling evaluation. 

In order to reduce capital cost many plants are installed with the minimum of instrumentation. Under these circumstances it may be difficult or even impossible, to make a proper assessment of the fouling encountered in heat exchangers under operating conffitions. Temperatures and flow rates may have to be inferred with the problems of accuracy that this raises. 

An improvement on the assessment of individual heat exchanger performance is to pass a sidestream through some kind of monitor. In general monitors resemble the test equipment used in the laboratory. The use of monitors allows a more careful control of the fouling conditions to be exercised particularly in terms of temperature, temperature changes and flow rates, using actual plant streams. The data obtained are more reliable than those obtained directly from the operation of full scale plant. 

Often, exchangers do not perform as they should, and their performance deviates from optimum. Sometimes, they do not accomplish what they are capable of, and other times they are asked to perform what they are not capable of. The primary purpose of heat exchanger assessment is to identify the root causes if they are due to poor design, excessive fouling, or mechanical faUiure, and determine the required actions to improve the performance. 

The rating assessment was conducted for heat exchangers in the preheat train and the assessment results indicated that fouling occurred, to a varying degree, to most of the... 

---