Design practices

In the previous section, a description of the various types of equipment commonly used for carrying out mixing operations was provided. Emphasis was placed on mechanical agitators since these have the broadest range of application, however there are other important devices worth noting, such as jet, pump, eductor, orifice, and inline mixing devices. Some of these important pieces of equipment are briefly described below. 

Agitator is a term most commonly used as a synonym for the more specific term impeller. 

Baffle generally a flat plate attached to and perpendicular to the wall of a mixing vessel to alter the fluid circulation pattern and prevent excessive swirl of the fluid in the vessel.  

Critical speed the mixer shaft speed which matches the first lateral natural frequency of the shaft and impeller system. Excessive vibrations and shaft deflections are present at this speed. 

Impeller Reynolds Number a dimensionless number used to characterize the flow regime of a mixing system and which is given by the relation Re = pNDV/r where p = fluid density, N = impeller rotational speed, D = impeller diameter, and /r = fluid viscosity. The flow is normally laminar for Re 10, and turbulent for Re 3000. 

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The ultimate responsibility for safety rests with me operating company OSHA 1910.119 is clear on this. Each company is expected to develop (and enforce) its own practices in the design, installation, testing, and maintenance of safety systems. Fortunately, some companies make these documents public. Monsanto s Safety System Design Practices was published in its entirety in the proceedings of the International Symposium and Workshop on Safe Chemical Process Automation, Houston, Texas, September 27-29, 1994 (available from... 

In design practice, a less rigorous parameter, HETP, is used as an index of packing efficiency. Tne HETP is the height of packed bed required to achieve a theoretical stage. The terms Hoe and HETP may be related under certain conditions ... 

Generally, depending upon the type of load, different nianiifaclurcrs may adopt to different design practices, such as high r, and low thermal withstand lime or moderate T, and high thermal withstand time. 

Reliability targets are typically set based on previous product failures or existing design practice (Ditlevsen, 1997) however, from the above arguments, an approach based on FMEA results would be useful in setting reliability targets early in the design process. 

Oakley, M. 1993 Managing Design Practical Issues. In Roy, R. and Wield, D. (eds). Product Design and Technological Innovation, Open University Press. 

Hengstebeck, R. J., Distillation-Principles and Design Practices, Reinhold Publishing, 1961. 

In addition, the process engineer must be aware of good design practices for vacuum jets. 

Noise sources are part of noise loops which are printed circuit board connections between high-frequency current sinks and current sources. Following the PC board design practices in Section 3.14 will help greatly in reducing the radiated RFI. Appreciation of the high-frequency characteristics of the common components and PC boards is needed. 

It should be emphasized at this point that the speed of response is cnti-cal. The pressure transient pressure should not fall to less than 50% of the difference in pressure between the standby pump start pressure and the low oil pressure trip pressure. This is normally achievable with good design practice and the use of a switch and direct wiring. There is some tendency to use a transmitter and control through a remote computer. The latter arrangement is difficult to check on a shop test and normally is too slow to meet the requirement. An accumulator can be added and must be used if the requirement cannot be met. This additional hardware contributes to higher initial cost and possible reliability problems in the future. The direct switch method is therefore highly recommended. 

In fact with a Newtonian liquid y = 4Q/ nR. This latter expression, viz. 4Q/ uR, is obviously much easier to calculate than the true wall shear rate and, since they are uniquely related and the simple expression is just as useful, in design practice it is very common when plotting flow curves to plot against... 

In support of the development of graphite moderated reactors, an enormous amount of research has been conducted on the effects of neutron irradiation and radiolytic oxidation on the structure and properties of graphites. The essential mechanisms of these phenomena are understood and the years of research have translated into engineering codes and design practices for the safe design, construction and operation of gas-cooled reactors. 

Cheremisinoff, N. P., Pressure Safety Design Practices for Refinery and Chemical Operations, Noyes Publications, Westwood, New Jersey, 1998. 

The objective of this chapter is to provide an overview of filtration terminology and basic engineering principles, as well as calculation methods that describe the filtration process in a generalized way. The basis equations describing the generalized process of filtration have been around for nearly 100 years, and with few refinements, continue to be applied to modem design practices. 

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