Network design

If a company grows through a merger or acquisition, network re-design addresses rationalization of the supply base, elimination of redundancies, and use 

The supply network may use mathematical and simulation techniques to support optimal decisions. Typical decisions include the optimal number of suppliers to use whether to source globally and, if so, where and how to manage various risks. 

In general, a supply network can be defined as comprising n i =, n) value-adding tiers with = 1, m,) options to choose from, at tier i, so that the number of possible design alternatives for the supply network would be JJ i some of 

Beginning with fundamentals of fluid dynamics, correlations for the pressure loss in channel elements are presented, which are concatenated to fluidic networks to distribute fluid homogeneously over a certain area. Computational fluid dynamic (CFD) simulations of single elements are exploited for analytical pressure loss correlations. These are employed in lumped element modeling of networks and manifolds, which are too complex for direct simulations. Design strategies and methods are presented for charmel networks, manifolds for parallel channels on a plate and headers for stacked-plate devices. 

The flow in microchannels is often dominated by viscous forces leading to straight laminar flow. Here, single-phase flow is treated without any influence of surface or interface forces. The flow regime in channels is characterized by the Reynolds number. Re, which is the ratio of the momentum force and viscous force  

Micro Process Engineering, Vol.l Fundamentals, Operations and Catalysts Edited by V. Hessel, A. Renken, J.C. Schouten, and J.-I. Yoshida Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3.527-31SS0-S 

The balance equations for mass, momentum and energy describe the entire flow situation. The continuity assumption of smooth fluid properties and no-shp flow conditions at the wall hold for most cases in microprocess engineering, hence the change in density p with time is correlated with the velocity vector w as 

The momentum equation is expressed by the Navier-Stokes equation in vector form  

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Details of how this design was developed in Fig. 6.9 are included in Chap. 16. For now, simply take note that the targets set by the composite curves are achievable in design, providing that the pinch is recognized, there is no transfer of heat ac ss it, and no inappropriate use of utilities occurs. However, insight into the pinch is needed to analyze some of the important decisions still to be made before network design is addressed. 

Let us take each of these components in turn and explore whether they can be accounted for from the material and energy balance without having to perform heat exchanger network design. 

In general, the final network design should be achieved in the minimum number of units to keep down the capital cost (although this is not the only consideration to keep down the capital cost). To minimize the number of imits in Eq. (7.1), L should be zero and C should be a maximum. Assuming L to be zero in the final design is a reasonable assumption. However, what should be assumed about C Consider the network in Fig. 7.16, which has two components. For there to be two components, the heat duties for streams A and B must exactly balance the duties for streams E and F. Also, the heat duties for streams C and D must exactly balance the duties for streams G and H. Such balemces are likely to be unusual and not easy to predict. The safest assumption for C thus appears to be that there will be one component only, i.e., C = 1. This leads to an important special case when the network has a single component and is loop-free. In this case, ... 

Figure 16.17 Network design for the process from Fig. 6.2 using two steam levels.

The considerations addressed so far in network design have been restricted to those of energy performance and number of units. In addition, the problems have all been straightforward to design for... 

Consider Fig. 16.24a, which shows the network design from Fig. 16.7 but with a loop highlighted. Heat can be shifted around loops. Figure 16.24a shows the effect of shifting heat duty U around the loop. In this loop, heat duty U is simply moved from unit E to unit B. 

Heat Exchanger Network Design Based on the Optimization of a Reducible Structure... 

The approach to heat exchanger network design discussed so far is based on the creation of an irreducible structure. No redundant features were included. Of course, when the network is optimized, some of the features might be removed by the optimization. The scope for the optimization to remove features results from the assumptions made during the creation of the initial structure. However, no attempt was made to deliberately include redundant features. 

A good initialization for heat exchanger network design is to assume that no individual exchanger should have a temperature difference smaller than AT n. Having decided that no exchanger should have a temperature difference smaller than two rules were deduced in... 

For more complex network designs, especially those involving many constraints, mixed equipment specifications, etc., design methods based on the optimization of a reducible structure can be used. 

Increased heat utilization does not always mean a trade-off, and many studies have shown a reduction in energy consumption as well as capital cost which in itself is a remarkable recommendation for a systematic approach to network design. 

A low temperature of approach for the network reduces utihties but raises heat-transfer area requirements. Research has shown that for most of the pubhshed problems, utility costs are normally more important than annualized capital costs. For this reason, AI is chosen eady in the network design as part of the first tier of the solution. The temperature of approach, AI, for the network is not necessarily the same as the minimum temperature of approach, AT that should be used for individual exchangers. This difference is significant for industrial problems in which multiple shells may be necessary to exchange the heat requited for a given match (5). The economic choice for AT depends on whether the process environment is heater- or refrigeration-dependent and on the shape of the composite curves, ie, whether approximately parallel or severely pinched. In cmde-oil units, the range of AI is usually 10—20°C. By definition, AT A AT. The best relative value of these temperature differences depends on the particular problem under study. 

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