Of linear expansion

Many solids show marked swelling as a result of the uptake of a gas or a liquid. In certain cases involving the adsorption of a vapor by a porous solid, a linear relationship exists between the percentage of linear expansion of Ae solid and the film pressure of the adsorbed material [134, 135]. 

The swelling of the adsorbent can be directly demonstrated as in the experiments of Fig. 4.27 where the solid was a compact made from coal powder and the adsorbate was n-butane. (Closely similar results were obtained with ethyl chloride.) Simultaneous measurements of linear expansion, amount adsorbed and electrical conductivity were made, and as is seen the three resultant isotherms are very similar the hysteresis in adsorption in Fig. 4.27(a), is associated with a corresponding hysteresis in swelling in (h) and in electrical conductivity in (c). The decrease in conductivity in (c) clearly points to an irreversible opening-up of interparticulate junctions this would produce narrow gaps which would function as constrictions in micropores and would thus lead to adsorption hysteresis (cf. Section 4.S). 

Thermal Stresses. When the wak of a cylindrical pressure vessel is subjected to a temperature gradient, every part expands in accordance with the thermal coefficient of linear expansion of the steel. Those parts of the cylinder at a lower temperature resist the expansion of those parts at a higher temperature, so setting up thermal stresses. To estimate the transient thermal stresses which arise during start-up or shutdown of continuous processes or as a result of process intermptions, it is necessary to know the temperature across the wak thickness as a function of radius and time. Techniques for evaluating transient thermal stresses are available (59) but here only steady-state thermal stresses are considered. The steady-state thermal stresses in the radial, tangential, and axial directions at a point sufficiently far away from the ends of the cylinder for there to be no end effects are as fokows ... 

The coefficient of linear expansion of these alloys in the temperature range of 21 to 100°C (70 to 212°F) is 12.2 X lO C (6.8 X 10"V°F), which is slightly above that of cast iron (National Bureau of Standards). Since these loys have practically no elasticity, it is necessary to use expansion joints in relatively short pipe hnes. Connections for flanged pipe, fittings, valves, and pumps are made to 125-lb American Standarci drilling. 

Any bi-niclal combination, having large differences in their coefficients of linear expansion, such as a bimetal of brass and steel is used for sueh applications. One end of a strip is fixed and the other is left free for natural movement. When heated, brass expands more than the steel and bends towards the steel as shown, giving the desired movement to actuate a tripping lever. 

Bimetallie elements are widely used in instruments sueh as thermostats to sense or eon-trol temperatures. There are several bimetallie element types available, sueh as straight strips, eoils and dises, but all rely on the same working prineiple. In its most basie form, the bimetallie strip eomprises of two dissimilar metal strips bonded together, usually of the same surfaee area, but not neeessarily of the same thiekness thermostat. The eom-posite metal strip is elamped at one end to aet as a eantilever beam, and is horizontal at a partieular temperature. When the temperature is inereased, the strip defleets in the direetion of the metal with the least eoeffieient of linear expansion. Its working prineiple relies on the faet that the metals will expand at different rates as the strip is heated. The purpose of this defleetion is to typieally eause the strip to make eontaet with a switeh or eomplete an eleetrie eireuit at a partieular setpoint temperature above the ambient. 

Deflection temperature under load (1.8 MPa) Coefficient of linear expansion (-30°C to +30°C Specific heat 20-300°C >300°C... 

Metal A lomic number Atomic weight Lattice structure Density at 20°C (g/em ) Melting point (°C) Thermal conductivity at 0-l00°C (W/m°C) Specific heat at 0°C (J/kg C) Coefficient of linear expansion at 20-iOO°C X 70 Thermal neutron cross-section (barns) (10-- m ) Resistivity at 0°C (fiil em) Temperature coefficient of resistance o-ioo°c X 10 ... 

Coefficient of linear expansion Unimportant Differences between substrate and coating layer lead to craze and peel... 

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