Heat Exchangers fouling factors

Heat-exchanger fouling, or scale build-up, can be a major factor limiting the kind and concentration of solids in feed that can be processed. The plugging problem can be dealt with by periodically flushing the exchanger with warm water or diluted nitric acid, as in conventional processes. 

A re dew of thermal and hydrodynamic aspects of heat-exchanger fouling appears in Refs. Minimization of fouling through operational and design factors is addressed in... 

Heat Recovery Equipment. Factors that limit heat recovery appHcations are corrosion, fouling, safety, and cost of heat-exchange surface. Most heat interchange utilizes sheU and tube-type units because of the mgged constmction, ease of mechanical cleaning, and ease of fabrication in a variety of materials. However, there is a rich assortment of other heat exchangers. Examples found in chemical plants in special appHcations include the foUowing. 

Often, a reasonable and convenient way to understand the heat transfer process in a heat exchanger unit is to break down the types of heat transfer that must occur such as, vapor subcooling to dew point, condensation, and liquid subcooling. Each of these demands heat transfer of a different type, using different AT values, film coefficients, and fouling factors. This is illustrated in Figure 10-36. It is possible to properly determine a weighted overall temperature... 

Figure 10-144. Approximate relationship of the overall coefficient fouled, and the fouling factor of inside tubes for predicting the economical use of finned tubes in shell and tube units. (Used by permission Williams, R. B., and Katz, D. L. Performance of Finned Tubes and Shell and Tube Heat Exchangers, 1951. University of Michigan. Note For reference only, 1950 costs.)...

The added capability of calculating unknown values based on measured inputs will greatly enhance the system capabilities. For example, the neither fouling factor nor efficiency of a heat exchanger can be directly measured. A predictive maintenance system that can automatically calculate these values based on the measured flow, pressure and temperature data would enable the program to automatically trend, log and alarm deviations in these unknown, critical parameters. 

Table 12.9. Fouling factors (coefficients), typical values for plate heat exchangers...

Factors involved in heat transfer, such as surface-to-volume ratio, agitation characteristics, mixing efficiency, fouling of heat transfer surfaces, scale of operations, and the resulting heat exchanged depend on the system under consideration (e.g., liquid-liquid transfer, liquid-gas transfer, free convection, or forced convection). Standard chemical engineering texts and reference books contain detailed discussions on heat transfer in process equipment. Only a brief summary follows ... 

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