Fouling Factors

When a heat exchanger is placed in service, the heat transfer surfaces are, presumably, clean. With time, in some services in the power and process industries, the apparatus may undergo a decline in its ability to transfer heat. This is due to the accumulation ofheat insulating substances on either or both ofthe heat transfer surfaces. The Tubular Exchanger Manufacturers’ Association (TEMA) undertook the establishment ofstandards defining design practices not covered by the ASME Code for Unfired Pressure Vessels. Because the ASME code is concerned primarily with safe pressure containment and the means for inspecting for it during construction, the contribution ofTEMA to sound mechanical construction has been substantial.

In addition, TEMA published a table of fouling factors to assist the designer in preventing the fouling ofa single item in a process, including several items ofheat transfer equipment. Resistances were tabulated which were to be added to the film resistances (1/Sihi and 1/hoSo) ofspecific process streams so that the operating period ofeach would be similar and assure some desired period ofcontinuous operation. The tables off ouling factors were intended as a crude guide toward the equalizations of cumulative fouling in all fouling streams in the assembly.

The fouling factors published by TEMA became entrenched in industrial heat exchanger design. Fouling factors, by the TEMA definition, are time dependent. They are not present when the apparatus is placed on stream; yet at some definite time in the future, when the apparatus has lost some of its heat transfer capabilities, the fouling factor is deemed to have arrived. TEMA does not delineate the in-between fouling process, and the fouling factor has shed little light on the nature of fouling. Significant is the fact that an item of equipment that failed to comply with the TEMA notion of a desired period ofcontinuous operation became a fouling problem. Within the scope of the definition of a fouling factor, the only means for ameliorating fouling was to employ larger fouling factors for repetitive services.

The entire concept ofthe fouling factor is somewhat indefinite. It is an unsteadystate effect that is added indiscriminately to steady-state heat transfer resistances. The difference between a clean and a fouled exchanger is that an intolerable portion of the available temperature difference between fluids must be used to overcome fouling. Thus, ifthe outside surface So ofa pipe or tube is the reference and rdo is the fouling or dirt factor.