Design optimum

Optimum Design of Pumping, Compression, and Vacuum Systems... 

The energy audit has seven components as-it-is balance, field survey, equipment tests, checking against optimum design, idea-generation meeting, evaluation, and foUow-up. 

Checking Against Optimum Design. This attempts to answer the question whether a balance needs to be as it is. The first thing to compare against is the best current practice. Information is available ia the Hterature (13) for large-volume chemicals such as NH, CH OH, urea, and ethylene. The second step is to look for obvious violations of good practice on iadividual pieces of equipment. Examples of violations are stack temperatures > 150° C process streams > 120° C, cooled by air or water process streams > 65° C, heated by steam t/ urbine 65% reflux ratio > 1.15 times minimum and excess air > 10% on clean fuels. 

It should be noted that the fraction of column cross-sectional area available for gas dispersers (perforations, bubble caps) decreases when more than one downcomer is used. Thus, optimum design of the plate involves a balance between hquid-flow accommodation and effective use of cross section for gas flow. 

Exploration for an acceptable or optimum design of a new reaction process may need to consider reactor types, several catalysts, specifications of feed and product, operating conditions, and economic evaluations. Modifications to an existing process hkewise may need to consider many cases. These efforts can oe eased by commercial kinetics services. A typical one can handle up to 20 reactions in CSTRs or... 

Since and l ( are interdependent parameters the optimum design is achieved when /nr at is of the same magnitude as the relay operating current, i.e. I ( —... 

The influence of transport process in two-phase reaction systems depends on flow conditions, which change with the size of the equipment. This is the reason for the historic observation that performance changes as processes are scaled up and therefore scale-up should be done in several steps, each limited to a small increase in size. This is a slow and expensive method and still does not guarantee optimum design. 

The cost effectiveness of an adsorption cycle machine depends both on the COP, which will affect the operating costs and also on its size, which will influence the capital cost. The COP in a particular application will be both a function of the adsorbent properties and of the cycle used. Complex cycles described below can deliver high COP s but require more heat transfer area and are therefore larger, leading to a higher capital cost. There is a compromise between efficiency and complexity which determines the optimum design. 

The obvious question is Ts there an optimum design for the corrugations Unfortunately the answer is No because if one wishes to increase transverse stiffness then the obvious thing to do is to increase D up to the point where buckling problems start to be a concern. Usually this is when D/h = 10, for short-term loading and less than this for long term loading because of the decrease in modulus of viscoelastic materials. 

Figure 7-8 Narrow Optimum Design Figure 7-9 Wide Optimum Design...

Standard shapes for composite stiffeners are not likely to occur for most aerospace applications. There, the value and function of the structure warrant optimizing the stiffener design. In contrast, for more everyday applications such as scaffolding, stairways, and walkways in chemical plants, competitive pressures lead to a situation where compromises in stiffener efficiency are readily accepted (overdesign) in order to achieve lower cost than would be associated with optimum design. 

These comments are directed to the inexperienced designer, and in general, amplify material previously presented. The comments are somewhat random in nature but are nonetheless important considerations in optimum design. 

Economic optimum design usually implies high circulation rates, although not high enough to give mist flow. 

Optimum design is not often achieved in all respects however, the fundamentals and application cost factors of Nakayama are of real value in selecting goals and design features. 

---