Heat exchangers desirable characteristics

Adaptive controllers can be usefully applied because most processes are nonlinear (Section 7.16) and common controller design criteria (Section 7.12) are based on linear models. Due to process non-linearities, the controller parameters required to give the desired response of the controlled variable change as the process steady state alters. Furthermore, the characteristics of many processes vary with time, e.g. due to catalyst decay, fouling of heat exchangers, etc. This leads to a deterioration in the performance of controllers designed upon a linear basis. 

In the calculations for the design of a heat exchanger kA has to be found. Either the temperature changes in both fluids or the two values for the heat capacity flow and the temperature change in one of the fluids must be known, in order to determine kA. An operating characteristic which can be explicitly solved for Nx or is desired ... 

For a constant amount of heat transfer, a degraded heat transfer characteristic requires higher fuel temperature, which is not desirable. Therefore, desirable heat transfer properties in the selection of reactor materials, especially those used as core cladding and heat exchanger tubes, are a major consideration. 

Design equations for process equipment rarely have an accuracy of better than 10 percent, and often are much worse. This means that accurate determination of capacities and operating characteristics must rely on tests of the completed plant. Since heat exchangers sometimes foul, pumps and valves wear, and columns occasionally plug, it is desirable to run tests periodically. Typical plant instrumentation is seldom adequate for such testing. For example, rarely does one find a measurement of cooling-water flow rate, let alone inlet and exit cooling-water temperatures. But tests on a water-cooled condenser are useless without this information. 

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