Heat exchangers optimum design

In heat-exchanger network designs, there exists an optimum AT, yj the network. E5q>lain carefully... 

Figure 7.9 The Xp parameter avoids steep slopes on the Fp curves, whereas minimum Fp does not. (Reprinted from Ahmad, Linnhoff, and Smith, Cost Optimum Heat Exchanger Networks II. Targets and Design for Detailed Capital Cost Models, Computers Chem, Engg., 7 751, 1990 with permission from Elsevier Science, Ltd.)...

Optimum Pressure Drop. For most heat exchangers there is an optimum pressure drop. This results from the balance of capital costs against the pumping (or compression) costs. A common prejudice is that the power costs are trivial compared to the capital costs. The total cost curve is fairly flat within 50% of the optimum (see Fig. lb), but the incremental costs of power are roughly one third of those for capital on an aimualized basis. This simple relationship can be extremely useful in quick design checks. 

Rebrov, E. V., de Croon, M. H. J. M., ScHOUTEN, J. C., Design of a micro-structured reactor with integrated heat-exchanger for optimum performance of highly exothermic reaction, Catal. Today 69 (2001) 183-192. 

To find the optimum design it will be necessary to cost a number of alternative designs, seeking a compromise between the capital costs, determined by the number and size of the exchangers, and the utility costs, determined by the heat recovery achieved. 

Ahmad S, Linnhoff B and Smith R (1990) Cost Optimum Heat Exchanger Networks II Targets and Design for Detailed Capital Cost Models, Comp Chem Eng, 7 751. 

Grossmann IE and Sargent RWH (1978) Optimum Design of Heat Exchanger Networks, Comp Chem Eng, 2 1. 

If the optimum size heat exchanger for the initial plant is installed, an additional exchanger either in parallel or in series will be required when the plant is expanded. This may be the best option when the heat transfer involves condensation and subcooling. The exchanger can be designed to perform both functions initially, and then when the plant is expanded an aftercooler can be installed and the initial equipment can act only as a condenser. 

Economic analysis of designs at lower natural hypochlorite strengths equally show potential investment benefits. They are, however, much less significant than the batch and high concentration cases described above. While an economic case can be made for retrofitting an in-loop reactor to a system that already has an end-of-pipe treatment system based on payback, it is not always clear that this is a better option than an end-of-pipe hybrid system as described earlier in the chapter. For a particular system the optimum solution is often as much a function of the required expenditure on the heat exchangers as it is the relative cost of the reactor options. 

We follow a three-step procedure First, we must find how equilibrium composition, rate of reaction, and product distribution are affected by changes in operating temperatures and pressures. This will allow us to determine the optimum temperature progression, and it is this that we strive to approximate with a real design. Second, chemical reactions are usually accompanied by heat effects, and we must know how these will change the temperature of the reacting mixture. With this information we are able to propose a number of favorable reactor and heat exchange systems—those which closely approach the optimum. Finally, economic considerations will select one of these favorable systems as the best. 

An economic design of a cooling tower is presented. The analysis is based on consideration of all elements in the cycle, such as turbine and condenser. The optimum conditions are given for heat exchanger and condensing temperatures. 6 refs, cited. (In German). 

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