Heat exchangers, fouling rates

Six mechanisms have been identified as being responsible for heat exchanger fouling, but it is seldom that one mechanism is responsible for a particular fouling problem a combination of mechanisms is much more likely, although one mechanism may be dominant. The variables that have the greatest influence on these mechanisms, apart from the nature of the fluids being processed, are temperature and flow rate. 

Temperature control requiring additional heat input is normally controlled by regulating the flow rate of steam to the process heat exchanger. A desuperheater should be installed to prevent steam quality variation from causing heat exchanger fouling due to temperature spikes at constant flow. 

Direct Contact Heat Exchangers. In a direct contact exchanger, two fluid streams come into direct contact, exchange heat and maybe also mass, and then separate. Very high heat-transfer rates, practically no fouling, lower capital costs, and lower approach temperatures are the principal advantages. 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

If an appropriate thermal feedback mechanism is not provided, the reaction occurs at the lower stationary state where the reaction rate may be negligible. The reaction could be extinguished, if the temperature of the feed entering the reactor drops below some critical value due to fouling of the heat exchange surface. 

But suppose we are operating a heat exchanger subject to rapid rates of initial fouling. The start-of-run heat-transfer coefficient U is 120 Btu/[(h)(ft2(°F)]. Four months later, the U value has lined out at 38. The calculated clean tube-side velocity is lV2 ft/s. This is too low, but what can be done ... 

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. 

The practical heat-transfer coefficient is the sum of all the factors that contribute to reduce heat transfer, such as flow rate, cocurrent or countercurrent, type of metal, stagnant fluid film, and any fouling from scale, biofilm, or other deposits. The practical heat-transfer coefficient ((/practical) is, in reality, the thermal conductance of the heat exchanger. The higher the value, the more easily heat is transferred from the process fluid to the cooling water. Thermal conductance is the reciprocal of resistance (/ ), to heat flow ... 

Corrosion measurement is also undertaken through the use of various designs of corrosion/fouling monitors that simulate heat exchangers. These monitors vary from simple units to sophisticated items of equipment with computerized data management systems. Most monitors are really designed for fouling measurement, and their ability to provide useful corrosion rate data is often limited. 

An often-used method for the limitation of the heat release rate is an interlock of the feed with the temperature of the reaction mass. This method consists of halting the feed when the temperature reaches a predefined limit. This feed control strategy keeps the reactor temperature under control even in the case of poor dynamic behavior of the reactor temperature control system, should the heat exchange coefficient be lowered (e.g. fouling crusts) or feed rate too high. 

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