Modeling heat exchanger

Examples of the use of models for the design of large-scale systems include the measurement of pressure drop and heat transfer in model heat exchangers, the mixing and rate of reaction in a bench-top batch reactor and the prediction of pressure drops in pipelines. 

The heat-transfer coefficient is a function of all chain, where heat flow passes, and for one-dimensional (plane model) heat exchange between liquid and hydride bed divided by heat-conducting wall can be expressed by ... 

The reduction in j with time therefore represents an apparent reduction in a (by about 20% from the initial value), but in reality the relatively low value of a associated with gas streams will not be affected greatly by the presence of the deposit unless the deposit becomes thick enough to change substantially the flow pattern across the fins. The presence of the fouling layer however interposes a thermal resistance between the air flow and the metal fins and core tube of the model heat exchanger, thereby reducing the effective heat transfer and giving the impression that the heat transfer coefficient has diminished. 

FIGURE 4 Schematic of ANL test facility for 1-MWt model heat exchangers. [From Lewis, L. G., and Sather, N. F. (1978). OTEC Performance Tests of the Union Carbide Flooded-Bundie Evaporator. Argonne National Lab., Dec. ANL/OTEC-PS1.1... 

Pump and tank model Compressor model Heat exchanger model Cooling tower model Boiler model Furnace model... 

Pump and tank model Compressor model Heat exchanger model Cooling-tower model Steam-generation model Furnace model Distillation model Reaction model Separation model Absorption and stripping model Combination of the preceding models... 

These measurements can be made on actual operating units on line but they may also be done using model heat exchanger units or instrumented pipe loops run in parallel to system flow. Figure 10.12 shows such an instrumented pipe loop test unit with five parallel, instrumented pipe runs. Water flow from the target system is diverted through this unit, so that conditions are representative of the actual operating system [18]. 

Figure 6.30 Corrosion sensing elements in model heat exchanger for multitechnicpie electrochemical monitoring (schematic). Adapted from Winters et alA )...

An increase in the Nnsselt number can be justified since the ORNL prediction was for fully developed laminar flow. Since one of the purposes of offset strip fins is to break up the boundary layer, the modeled heat exchangers will have their entire heat transfer stuface in the entrance effects region of laminar flow... 

After maximizing heat recovery in the heat exchanger network, those heating duties and cooling duties not serviced by heat recovery must be provided by external utilities. The outer-most layer of the onion model is now being addressed, but still dealing with targets. 

FIgur 7.4 If film transfer coefficients difier significantly, then nonvertical h t transfer is necessary to achieve the minimum area. (Reprinted from Linnhoff and Ahmad, Cost Optimum Heat Exchanger Networks I. Minimum Energy and Capital Using Simple Models for Capital Cost," Computers Chem. Engg., 7 729, 1990 with permission from Elsevier Science, Ltd.)... 

Ahmad, S., Linnhoff, B., and Smith, R., Cost Optimum Heat Exchanger Networks II. Targets and Design for Detailed Capital Cost Models, Computers Chem. Eng., 14 751, 1990. 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

The best way to approach the retrofit synthesis of the heat-exchanger network is to model all five tasks simultaneously. A mixed-integer nonlinear programming model is usually formulated to accomplish this goal. 

A novel variation is a cyhndrical model equipped with a tube bundle to resemble a sheU-and-tube heat exchanger with a bloated shell [Chem. Proce.s.s., 20 (Nov. 15, 1968)]. Conical ends provide for redistribution of burden between passes. The improved heat-transfer performance is shown by Fig. 11-61. 

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