Chapter 4 Heat Exchanger fouling

The theory associated with the transport of particles towards and onto surfaces is ejrtensive and complex. SufiBcient theory however will be presented in this chapter to provide a basic understanding of the principles as they affect heat exchanger fouling. Practical aspects will be considered towards the end of the chapter. 

There are three chapters in this volume of Advances in Chemical Engineering. The chapter, Analysis and Synthesis of Resilient Heat Exchanger Networks by Colberg and Morari of Caltech, is concerned with the development of new generic tools. It provides an overview on the macroscale design of systems that are resilient and flexible to uncertainties and variations in system variables, such as flow rates and fouling of heat exchanger surfaces. 

As alluded to in Chapter 8, the ideal biomass feedstock for thermal conversion, whether it be combustion, gasification, or a combination of both, is one that contains low or zero levels of elements such as nitrogen, sulfur, or chlorine, which can form undesirable pollutants and acids that cause corrosion, and no mineral elements that can form inorganic ash and particulates. Ash formation, especially from alkali metals such as potassium and sodium, can lead to fouling of heat exchange surfaces and erosion of turbine blades, in the case of power production systems that use gas turbines, and cause efficiency losses and plant upsets. In addition to undesirable emissions that form acids (SOx), sulfur can... 

As we learned with the examples of the present chapter, we need to know the overall coefficient of heat transfer for the (thermal) design or performance of a heat exchanger. Since this coefficient usually depends on unknown exit temperatures, we are faced with a trial-and-error procedure. For a first trial, an order-of-magnitude value of the heat transfer coefficient is usually satisfactory. Table 7.2, taken from Ref. 7, gives the value of the overall coefficient for a number of frequently encountered cases. This table, which includes the effect of a fouling factor, is more conservative than Table 1,2. 

The chemical reaction either forms the deposit directly on the heat transfer surface or is involved in forming deposit precursors, which subsequently result in the formation of deposits. Precursors may be formed in the bulk liquid, in the boundary layers near the heat exchanger surfaces or directly on the surface. The precursor may be soluble in the bulk fluid and only give rise to deposition when it is carried by difihision or eddy transport to the wall region. It is entirely possible that if the precursor precipitates or reacts to cause a solid to be formed remote from the wall, then the deposition process will be particulate as described in Chapter 7. If there is a reaction with the wall itself then the mechanism could be regarded as corrosion fouling. 

Residence time in the laminar sub-layer will be a consideration in the design of heat exchangers to handle heat sensitive materials such as food products or petroleum fractions. The effects of temperature may be to decompose the material and produce fouling. Steps will be necessary to reduce or eliminate the problem (see Section 13.3 or Chapter 15). 

Where "on-line" cleaning is considered to be necessary (see Chapter 15) either at installation or at a later date when the extent of a particular fouling problem has been established, the heat exchanger will of course, need to be designed with this in mind. Although the capital investment in such a system will be higher than the conventional method of changing velocity, its use could make a substantial reduction in operational and maintenance costs. 

Changing the pH will affect the corrosion potential of the system (see Chapter 10). As a result unless the material of construction is suitable, particulate deposition in the heat exchanger may be replaced by corrosion fouling. 

Dispersants may also be used in connection with fouling problems other than simple particulate deposition. For instance in corrosion prevention they may be used to prevent solids deposition and subsequent formation of oxygen concentration cells (see Chapter 10). In the contamination of surfaces by microorganisms (see Chapter 12) dispersants (often called biodispersants) are used to prevent or restrict, the approach of micro-organisms towards heat exchanger surfaces. 

Bott, T.R., 1991, Mitigation of fouling in design (and operation), in Foumeny, E.A. and Heggs, P.J., eds. Heat Exchange Engineering, Vol. II, Ellis Horwood, New York, Chapter 7. 

Concerns about fouling are high on the agenda of anyone dealing with plant that may comprise small channels - such as compact heat exchangers (CHEs, see Chapter 4)... 

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