Heat exchange process

To illustrate proportional plus rate control, we will use the same heat exchanger process that has been analyzed in previous chapters (see Figure 28). For this example, however, the temperature controller used is a proportional plus rate controller. 

When processes are subject only to slow and small perturbations, conventional feedback PID controllers usually are adequate with set points and instrument characteristics fine-tuned in the field. As an example, two modes of control of a heat exchange process are shown in Figure 3.8 where the objective is to maintain constant outlet temperature by exchanging process heat with a heat transfer medium. Part (a) has a feedback controller which goes into action when a deviation from the preset temperature occurs and attempts to restore the set point. Inevitably some oscillation of the outlet temperature will be generated that will persist for some time and may never die down if perturbations of the inlet condition occur often enough. In the operation of the feedforward control of part (b), the flow rate and temperature of the process input are continually signalled to a computer which then finds the flow rate of heat transfer medium required to maintain constant process outlet temperature and adjusts the flow control valve appropriately. Temperature oscillation amplitude and duration will be much less in this mode. 

One rational measure of a heat exchange process is the number of transfer units. In terms of gas temperatures this is defined by... 

Convection is frequently thought of in terms of space heating and industrial heat-exchange processes. It should he pointed out that convection plays a cosmic role (in the sun s photosphere, for example), and a very large role In connection with Ihe atmosphere of the earth and some other planetary bodies. For example, when normal convective transpon is inadequate, temperature inversions occur and create smog hazards over large cities. 

The waste gases, mainly nitrogen and oxides of carbon, escape from the top of the furnace. They are used in a heat exchange process to heat incoming air and so help to reduce the energy costs of the process. Slag is the other waste material. It is used by builders and road makers (Figure 10.14) for foundations. 

The phase transition rate in the crystallization of polymeric materials is of the same order as the rates of the heat exchange processes accompanying crystallization. Consequently, the boundary between phases becomes spatially dispersed. This excludes the possibility of using methods based on the front transition model proposed for metals to calculate residual stresses in plastics.148 It is possible to split the general problem and to find the temperature-conversion field independently. Then, assuming that the evolution of temperature T(x,t) and degree of crystallinity a(x,t) in time t and in space (x is the radius vector of an arbitrary point in a body) is known, we can analyze the mechanical problem.143... 

Experimental measurements yielded only a fivefold extension of the reaction cycle time, a difference largely caused by heat storage effects in the small-scale equipment used, which disproportionately enhanced the cooling effect observed in the inert bed control experiment. Despite this less satisfactory result, the desorptive cooling concept would still seem to offer potential for dramatic improvement in performance for regenerative heat exchange processes. 

The heat is available at 1200 K, but there will be temperature differences in the heat exchanger, so more available work will be lost in the heat exchange process. What can we learn from this example If we examine the Carnot factor, the answer seems to be clear. If we increase the operating temperature of the combustor, we can increase the efficiency and lose less work in the process. For example, if we had chosen an operating temperature of 2000 K, as could be possible in the suspended bed, we would have obtained an efficiency of 0.79, which is quite considerable. However, any gain in efficiency could be offset by the increase in work necessary to pulverize the coal For the sake of simplicity, we have not included these in this analysis. From the point of view of efficiency of combustion,... 

Existing literature on the control of reactor-external heat-exchanger processes is relatively scarce, concerning mostly the implementation of linear (Ali and Alhumaizi 2000, Henderson and Cornejo 1989) and nonlinear (Dadebo et al. 1997) control structures on specific processes. These studies report several control challenges, including difficult tuning of PID and model-based controllers due to the ill-conditioning of the process model. 

Table 7.1. Nominal parameter values for the reactor-heat-exchanger process (adapted from (Marroquin and Luyben 1973))...

Table 7.2. Temperatures, compositions, and flows for the reactor-heat-exchanger process at the operating points considered...

On the basis of the arguments regarding the cause of the non-minimum-phase behavior of the reactor-external-heat-exchanger process, the term... 

Criterion Biot determines the ratio of intensity of external heat exchange processes (numerator) and effective thermal conductivity of a hydride layer (denominator). To carry out frontal chemical reactions of hydrogen sorption -desorption, small numbers Biot (Bi<0.1) are preferable. Number Bi can be decreased by several ways 1) decreasing of the characteristic layer size 2) decreasing of intensity of an external heat transfer (but time of non-stationary processes is growing) 3) increasing of effective hydride bed thermal conductivity. 

Just as the hydrodynamic boundary layer was defined as that region of the flow where viscous forces are felt, a thermal boundary layer may be defined as that region where temperature gradients are present in the flow. These temperature gradients would result from a heat-exchange process between the fluid and the wall. 

This heat-exchange process is represented by the network element shown in Fig. 8-41. The total network for the physical situation of Fig. 8-39 is shown in Fig. 8-42. 

Super-heated water vapor has been widely used in many industrial processes such as heat-exchange process and drying, and has also been used in the activation process for activated carbon production. Recently, the super-heated water vapor has been utilized in food industry for production of instant food and drying of vegetables and tea leafs. The characteristics of the super-heated water vapor [3] are (1) it can heat the materials without oxidation because it does not contain oxygen and carbon dioxide, (2) drying speed becomes much faster than super-heated air due to heat emission of water molecules, and (3) waste gas is easily recovered by condensing. 

The preceding observations relate well with observations in industrial-scale scraped-surface heat exchanger processing lines. 

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