Design situations

There are many different special circumstances that can occur in silos that need special attention. Several are specifically identified in EN 1991-4 (2007), but even these require an extensive description for a full explanation. However, a few are briefiy noted here so that the reader can seek further information where it is needed. 

Internal structures within silos (tubes to assist flow, flow promotion devices such as Chinese hats and cone-within-cone structures, etc.) may be subject to large forces from storedsolids. Some advice on these may be found in theAustralian Standard (AS 37741996). 

Finally, where silos may be subjected to seismic loads, much care is needed. In elevated silos, a huge mass is supported on a relatively soft spring, leading to a low natural frequency which is easily excited by seismic waves. In on-ground silos, vertical compressions and high shear forces develop in the walls due to the horizontal excitation (Rotter Hull 1989), and care must be taken to ensure that the structure is strong enough, but also to ensure adequate connection in the base details. Some information may be found in EN 1998-4 (2006). 

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As indicated above, the stress-strain presentation of the data in isochronous curves is a format which is very familiar to engineers. Hence in design situations it is quite common to use these curves and obtain a secant modulus (see Section 1.4.1, Fig. 1.6) at an appropriate strain. Strictly speaking this will be different to the creep modulus or the relaxation modulus referred to above since the secant modulus relates to a situation where both stress and strain are changing. In practice the values are quite similar and as will be shown in the following sections, the values will coincide at equivalent values of strain and time. That is, a 2% secant modulus taken from a 1 year isochronous curve will be the same as a 1 year relaxation modulus taken from a 2% isometric curve. 

However, for the high strain rates appropriate for the analysis of typical extrusion and injection moulding situations it is often found that the simple Power Law is perfectly adequate. Thus equations (5.22), (5.23) and (5.27) are important for most design situations relating to polymer melt flow. 

A laminate can be subjected to thermal, moisture, and mechanical loads with the objective of surviving those loads. A method of strength analysis is required to determine either (1) the maximum loads a given laminate can withstand or (2) the laminate characteristics necessary to withstand a given load. The maximum loads problem is, of course, an analysis situation, and the laminate characteristics problem is a design situation that will be discussed in Chapter 7. 

This mathematical optimization procedure is a rational process because the slope (or derivative) enables us to know which way to go and how far to go. In contrast, in the search procedure, we just arbitrarily choose some values of x at which to evaluate the function. Those arbitrary choices are much like what people do in most design situations. They are simply searching in a rather crude mannerfotThe solution to the problem, and they will not achieve the solution precisely. With mathematical optimization, our hope is both to speed up that process and to get a more precisely optimum solution. 

An important but time-consuming factor in practically every design situation and in development of flowsheets is the collection and assembly of physical property data for the components of the. system in question. Often it is not sufficient tc obtain single data points from various tables, since many designs cover rather wide ranges of temperature and pressure and the effects of these on the properties must be taken into account. 

The use of these equations requires some assumptions or judgment regarding the degree of opening for fluid flow. Even so, this is better than assuming a wide open or full flow condition, which would result in too low a resistance to flow for the design situation. 

Based on air and perfect gas laws, vessel uninsulated, and vessel will not reach rupture conditions. Review for specific design. situations [33a]... 

In a design situation there will be constraints on the possible values of the objective function, arising from constraints on the variables such as, minimum flow-rates, maximum allowable concentrations, and preferred sizes and standards. 

Typically, in a design situation, the problem will be to determine the number of stages required at a specified reflux ratio and column pressure, for a given feed, and with the product compositions specified in terms of two key components and one product flow-rate. Counting up the number of variables specified it will be seen that the problem is completely defined ... 

A method of thermodynamic analysis for site steam systems will next be developed to allow the thermal loads and levels on a complete site to be studied. For this, a temperature-enthalpy picture for the whole site is needed, analogous to the grand composite curve for an individual process, as developed in Chapter 16. There are two ways in which such curves can be developed. The first relates to a new design situation. 

A new design situation would start from the grand composite curves of each of the processes on the site and would combine them together to obtain a picture of the overall site utility system12. This is illustrated in Figure 23.27, where two processes have their heat sink and heat source profiles from their grand composite curves combined to obtain a site hot composite curve and a site cold composite curve, using the procedure developed for composite curves in Chapter 16. Wherever there is an overlap in temperature between streams, the heat loads... 

When there is only one continuous independent variable, there are some well-developed theories that give the best method for sampling. This is adequately discussed in the literature. Since it is not a typical plant-design situation, it will not be discussed further here. Anyone who is interested should read Douglas Wilde s book Optimum Seeking Methods. 1... 

In most plant design situations of practical interest, however, the several pieces of equipment interact with each other, the output of one unit being the input to another that in turn may recycle part of its output to the inputter. Common examples are an absorber-stripper combination in which the performance of the absorber depends on the quality of the absorbent being returned from the stripper, or a catalytic cracker-catalyst regenerator system whose two parts interact closely. 

The typical reactor design situation is to be given the feed conditions (flowrate, temperature, and composition), the kinetic information, and the desired conversion. The problem is to determine the temperature and the size of the reactor. 

Flow modeling is an excellent tool for enhancing the performance of any process vessel. Applying such a technique to reactors can be especially fruitful, because of reactors central role in chemical processes. In most reactor-design situations, the reactions and the catalysis system are set beforehand. For any given combination of them, reactor performance becomes a complex function of the underlying transport processes. These in turn are governed by the fluid dynamics of the reactor. 

The relations between energy efficiency and capital cost must be evaluated from the analysis of the overall plant system. At some point, improved energy efficiency will require more investment. However, many of the practical processes of today may well be operating quite far from this point. Further process analysis along these lines may well be fruitful, particularly for grass roots design situations. 

Verink s equation for determining the present worth (PW) for different economic design situations using straight-line depreciation were written as ... 

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