The Design Method

Figure 16.5a shows the matches around the pinch from Fig. 16.4a with their duties maximized to tick-oflF streams. It should be emphasized that the tick-off heuristic is only a heuristic and can occasionally penalize the design. Methods will be developed later which allow such penalties to be identified as the design proceeds. 

The design method used so far, the pinch design method, creates an... 

Melt Index or Melt Viscosity. Melt index describes the flow behavior of a polymer at a specific temperature under specific pressure. If the melt index is low, its melt viscosity or melt flow resistance is high the latter is a term that denotes the resistance of molten polymer to flow when making film, pipe, or containers. ASTM D1238 is the designated method for this test. 

Transfer Coefficient. The design method described depends for its utiHty on the avadabiHty of mass- and heat-transfer coefficients. Typically, ky-a and /i -a are needed. These must be obtained from the standard correlations for mass and heat transfer, from data reported in the Hterature (23—30),... 

Typical overall heat-transfer coefficients are given in Tables 11-3 through 11-8. Values from these tables may be used for preliminaiy estimating purposes. They should not be used in place of the design methods described elsewhere in this section, although they may serve as a useful check on the results obtained by those design methods. 

Algebraic Design Method for Dilute Systems The design method described above can be performed algebraically by employing the following modified version of the Kremser formula ... 

The design method requires the ciosed-ioop poies to be piotted in the. v-piane as K is varied from zero to infinity, and then a vaiue of K seiected to provide the necessary transient response as required by the performance specification. The ioci aiways commence at open-ioop poies (denoted by x) and terminate at open-ioop zeros (denoted by o) when they exist. 

For most traditional materials, the objective of the design method is to determine stress values which will not cause fracture. However, for plastics it is more likely that excessive deformation will be the limiting factor in the selection of working stresses. Therefore this chapter looks specifically at the deformation behaviour of plastics and fracture will be treated separately in the next chapter. 

Plastics also have the ability to recover when the applied stress is removed and to a first approximation this can often be considered as a reversal of creep. This was illustrated in Fig. 1.8 and will be studied again in Section 2.7. At present it is proposed to consider the design methods for plastics subjected to steady forces. 

The design methods are normally a combination of one or more of the above methods. 

The need to satisfy these interrelated and often contradictory factors makes reactor design a complex and difficult task. However, in many instances one of the factors predominates, hence determining the choice of reactor type and the design method. 

The design method presented in this article is best summarized by a stepwise procedure ... 

The design method of Colburn and Hougen has withstood many examinations and is considered the best for any problem of this type. However, it is somewhat long and tedious and several approximation methods have been proposed... 

The design method is illustrated in Example 10-22 and uses Figures 10-121, 10-122, 10-123, and 10-124. 

For fixed offshore platform units, the design method should follow the strength level analysis guidelines in API RP 2A. The drilling and well servicing units should be able to resist the deck movement, i.e., the response of the deck... 

All of the chapters have been carefully reviewed and older (noi necessarily obsolete) material removed and replaced by newer design techniques. It is important to appreciate that not all of the material has been replaced because much of the so-called older material is still the best there is today, and still yields good designs. Additional charts and tables have been included to aid in the design methods or explaining the design techniques. 

The design methods de.scribed above rely on correlations of the overall reactor average quantities obtained from experimental tanks of different scales. The most important deficiency of these methods is that local effects are not taken into consideration, while these might be responsible for the overall reactor performance. Accordingly, if none of the above scale-up criteria is found satisfactory (see e.g. data of Middleton et ai, 1986) a more fundamental approach must be applied, although not necessarily as complex as the one presented in Section 5.4.S.2. Such an approach was presented by Paul et al. (1971) who found that the yield of the desired intermediate in a system of consecutive reactions (iodination of L-tjrosine) correlates reasonably with fluctuations of the velocity, So, these fluctuations could be chosen as a criterion for scale-up of the reactor. The average value for u in the upper part of the tank was evaluated from ... 

The reliability of the design methods if there is some uncertainty in the techniques to be used, it is clearly a waste of time to search out highly accurate physical property data that will add little or nothing to the reliability of the final design. 

For a comprehensive discussion of the problem of vent system design, and the design methods available, see the papers by Duxbury (1976, 1979). 

The design methods for horizontal separators given below are based on a procedure given by Gerunda (1981). 

This chapter covers the design of separating columns. Though the emphasis is on distillation processes, the basic construction features, and many of the design methods, also apply to other multistage processes such as stripping, absorption and extraction. 

The distillation of binary mixtures is covered thoroughly in Volume 2, Chapter 11, and the discussion in this section is limited to a brief review of the most useful design methods. Though binary systems are usually considered separately, the design methods developed for multicomponent systems (Section 11.6) can obviously also be used for binary systems. With binary mixtures fixing the composition of one component fixes the composition of the other, and iterative procedures are not usually needed to determine the stage and reflux requirements simple graphical methods are normally used. 

An estimate of the overall column efficiency will be needed when the design method used gives an estimate of the number of ideal stages required for the separation. 

Plate design, like most engineering design, is a combination theory and practice. The design methods use semi-empirical correlations derived from fundamental research work combined with practical experience obtained from the operation of commercial columns. Proven layouts are used, and the plate dimensions are kept within the range of values known to give satisfactory performance. 

As a rough guide, the following rules of thumb suggested by Frank (1978) can be used to decide the design method to use for a partial condenser (cooler-condenser) ... 

The design methods given in this section can be used for reboilers and vaporisers. Reboilers are used with distillation columns to vaporise a fraction of the bottom product whereas in a vaporiser essentially all the feed is vaporised. 

This is determined by applying a suitable design stress factor (factor of safety) to the maximum stress that the material could be expected to withstand without failure under standard test conditions. The design stress factor allows for any uncertainty in the design methods, the loading, the quality of the materials, and the workmanship. 

In the British Standard, PD 5500, the nominal design strengths (allowable design stresses), for use with the design methods given, are listed in the standard, for the range... 

The design methods and design curves given in the standards and codes should be used for the detailed design of vessels subject to external pressure. 

The maximum intensity of stress allowed will depend on the particular theory of failure adopted in the design method (see Section 13.3.2). The maximum shear-stress theory is normally used for pressure vessel design. 

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