Computer-aided-design

The term computer-aided design (CAD) is widely used within engineering but is often interpretted in different ways. It should not be 

The use of computer programs or packages to carry out material and energy balances is now commonplace, and probably one of the first encounters the student has with CAD. Unless the equipment or process is fairly simple, hand calculations become difficult and time consuming due to the iterative nature of the method. The design project may be the 

The computer program for the material balance contains several parts. First, a description ofeach item of equipment in terms of the input and output flows and the stream conditions. Quite complicated mathematical models may be required in order to relate the input and output conditions (i.e. performance) of complex units. It is necessary to specify the order in which the equipment models will be solved, simple equipment such as mixers are dealt with initially. This is followed by the actual solution of the equations. The ordering may result in each equation having only one unknown and iteration becomes unnecessary. It may be necessary to solve sets of linear equations, or if the equations are non-linear a suitable algorithm applying some form of numerical iteration is required. 

All design studies require physical property data and the efficient use of CAD methods means that this data must be available in a convenient form for computer use. Data in the form of graphs or tables is of limited use, although some data banks do contain only measured data. A more useful form of data storage is to correlate the available data and then store the calculated parameters required for use in appropriate empirical equations (stating the limits over which the correlations apply). Some data banks contain only the calculated parameters whereas others also store the available raw data. The following are examples of some common physical property data banks  

To gain the maximum benefit from the use of a flowsheet program, the operator/designer must be adequately trained. A suitable program will have 20-30 standard units available, numerous equation-solving procedures, control facilities and probably optimization facilities. The unit-equipment subroutine must adequately represent the process equipment, recycle streams need to be specified, and suitable solution convergence is required. For the effective use of CAD packages, it 

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Kennard R W and L A Stone 1969. Computer Aided Design of Experiments. Technometrics 11 137 148. 

Industrial scale polymer forming operations are usually based on the combination of various types of individual processes. Therefore in the computer-aided design of these operations a section-by-section approach can be adopted, in which each section of a larger process is modelled separately. An important requirement in this approach is the imposition of realistic boundary conditions at the limits of the sub-sections of a complicated process. The division of a complex operation into simpler sections should therefore be based on a systematic procedure that can provide the necessary boundary conditions at the limits of its sub-processes. A rational method for the identification of the subprocesses of common types of polymer forming operations is described by Tadmor and Gogos (1979). 

CAD software packages [COMPUTER-AIDED DESIGN AND MANUFACTURING(CAD/CAM)] (Vol7) -use of plasma technology [PLASMA TECFINOLOGY] (Vol 19)... 

Developments. A variety of process modifications aimed at improving surface finish or weld line integrity have been described. They include gas assisted, co-injection, fusible core, multiple Hve feed, and push—pull injection mol ding (46,47). An important development includes computer-aided design (CAD) methods, wherein a proposed mold design is simulated by a computer and the melt flow through it is analy2ed (48). 

N. S. Rao, Computer Aided Design of ElasticatingScrews,T. 2csis s. PubHshers, Munich, Germany, 1986. 

T. Dahl and L. Nelik, Computer Aided Design of a Centrifugal Pump Impeller, ASME 85-WA/EE-lO, Washington, D.C., 1985. 

Y. C. Pao, Elements of Computer-Aided Design and Manufacturings] ohxi Wdey Sons, Inc., New York, 1984. 

Nonimpact Printing. Interest is growing ia the use of nonimpact styles because of the quickness of color changeover and the abiUty to iaterface these machines to computer-aided design systems. Two basic types exist drop on demand and constant drop techniques. 

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