In many streams, fouling is an unavoidable by-product of the heat-transfer process. Fouling deposits can assume numerous types such as crystallization, sedimentation, corrosion, and polymerization. Systematic research on fouling is relatively recent and extremely limited (1-3) and almost exclusively concentrated on water—the most common fluid with fouling tendencies. The few research data that exist usually are proprietary or obtained from qualitative observations in plants. However, it is generally recognized that the prime variables affecting fouling buildup are flow velocity, surface temperature, and surface material. In the case of water, the water quality and treatment must also be considered.

The importance of fouling on the design of heat exchangers can be seen from the rate equation

where U is the overall heat transfer coefficient, Btu/h • ft2 • °F, hu h2 is the film coefficients of the two heat-transfer ring fluids, Btu/h • ft2 • °F, and Rf is the fouling resistance, h • ft2 • °F/Btu. It is obvious from equation 1 that the higher the film coefficients, the greater effect the fouling resistance will have on the overall coefficient and therefore on the size of the exchanger.

In tubular heat exchangers, water-side heat-transfer coefficients in the order of magnitude of 1000 Btu/h • ft2

• °F are quite common. In plate exchangers, the coefficients are substantially higher, typically around 2000 Btu/h • ft2

• °F. Assuming that both types of equipment operate with water—water systems, overall clean coefficients of500 and 1000 Btu/h • ft2 • °F respectively are obtained. Using a typical fouling resistance of 0.001 h • ft2 • °F/Btu (equal to a coefficient of 1000 Btu/h • ft2 • °F), the inclusion of the fouling will cause the tubular exchanger size to increase by a factor of 4, while for the plate exchanger, the corresponding factor is 7.

This example clearly demonstrates the crucial importance of fouling, especially in plate exchangers. Yet, fouling resistances that are unrealistically high often are specified and invariably have been based on experiences derived from tubular equipment. The common source of water fouling resistances in TEMA which recommends Rf values spanning a tremendous range between 0.0015 and 0.005 h

Flow velocity, as mentioned earlier, is a crucial operating parameter that influences the fouling behavior. For flow inside the tubes, the definition of flow velocity and the velocity profile is straightforward. But in plate exchangers, flow velocity is characterized by constant fluctuations as the fluid passes over the corrugations. It is postulated that this induces turbulence that is superimposed on the flow velocity as a factor that diminishes fouling tendencies. This has been observed qualitatively in practical applications and confirmed by unpublished APV research.

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