Temperature applications

MI Cables (mineral insulated cables. Fig. 10-181) are the electric heat tracers of choice for high-temperature applications. High-temperature applications are generally considered to maintain temperatures above 250°F or exposure temperatures above 420°F where self-regulating heaters cannot be used. MI cable consists of one or two heating wires, magnesium oxide insulation (from whence it gets its... 

Ceramics Ceramic microfilters for commercial applications are almost always employed as tube-side feed multitube monoliths. They are also available as flat sheet, single tubes, discs, and other forms primarily suited to lab use. They are used for a few high-temperature applications, in contact with solvents, and particularly at very high pH. 

Refractory Hrick Nonmetallic refracioiy materials are widely used in high-temperature applications in which the serwice permits the appropriate type of construciion. The more important classes are described in the following paragraphs. 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

Temperature is the hardest parameter to control in a fractionation system. It exhibits high process and measurement lag. Temperature can also be ambivalent as a measure of composition. Pressure changes are reflected quickly up and down the column. Temperature changes are not. It is typical to provide three-mode controllers for all temperature applications. 

Heat treatment by quenching and tempering improves the low temperature ductility of steels such as 0.5 Cr, 0.5% Mo, 1% Ni Type V. For lower-temperature application (below -196°C), up to 9% nickel is used as the sole alloying element. 

Nickel alloys have two main properties good resistance to corrosion and high-temperature strength. There are alloys for medium-and low-temperature applications and for high-temperature conditions in which creep resistance is of main importance [24]. 

As a slight departure from the present classification scheme, oxide-based cermets can be either oxide particles in a metal matrix or metal particles in an oxide matrix. Such cermets are used in tool making and high-temperature applications where erosion resistance is needed. 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

Those basic matrix selection factors are used as bases for comparing the four principal types of matrix materials, namely polymers, metals, carbons, and ceramics, listed in Table 7-1. Obviously, no single matrix material is best for all selection factors. However, if high temperatures and other extreme environmental conditions are not an issue, polymer-matrix materials are the most suitable constituents, and that is why so many current applications involve polymer matrices. In fact, those applications are the easiest and most straightforward for composite materials. Ceramic-matrix or carbon-matrix materials must be used in high-temperature applications or under severe environmental conditions. Metal-matrix materials are generally more suitable than polymers for moderately high-temperature applications or for modest environmental conditions other than elevated temperature. 

Tables 21.3 and 21.4 show the results of our evaluation on a column set that we felt performed very well. These tables address criteria 1 through 5 described previously. We judged the values listed to be very acceptable for high temperature GPC applications. For room temperature applications, where a smaller particle size column could be used, better values would be expected.

Another area of success has been in applied materials research. Because of the integral nature of materials to advances in energy production and consumption, the laboratories have developed a number of toughened ceramics. When used as a replacement for steel, they will improve the energy performance characteristics of high-temperature applications for components of combined-cycle power plants and vehicle engines. 

Newer fabrics, not in common use but in development, test, and field trials, are described for higher temperature applications by Reference [50]. Application to 400°F—2100°F are potentially available using ceramic fibers Nextel 312 , laminated membrane of expanded PTFE on a substrate, polyimid fiber P-84, Ryton polyphenylene sulfide, and woven fiberglass. The heat and acid resistance of these new materials... 

Recommended for high-pressure (>600 psi), high-temperature applications. Tube shape allows extreme temperature differences (AT>250°F) across the bundle. Often used as integral column bottom reboiler and as tank suction heater to preheat product before pumping. Tube side cannot be made single-pass. 

R-508B) CFC-13 for R-503 and CFC-13 in very low-temperature applications (less than —40° F)... 

Propane, propylene, and ethylene are used in large refrigeration tonnage and very low temperature applications. 

Used by permission Rehrig, P. Hydrocarbon Processing V. 60, No. 10, p. 137, 1981. Gulf Publishing Company, Houston, Texas. All rights reserved. Table 12-8D Impeller Material for Low-Temperature Applications ... 

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