Excess surface pressure

When the roof-to-shell junction is designed so that failure because of excessive surface pressure occurs at the junction rather than the roof or shell, the junction is called frangible. A frangible joint design equation can be derived by substituting Eq. 13.10 into Eq. 13.13, which gives... 

This effect assumes importance only at very small radii, but it has some applications in the treatment of nucleation theory where the excess surface energy of small clusters is involved (see Section IX-2). An intrinsic difficulty with equations such as 111-20 is that the treatment, if not modelistic and hence partly empirical, assumes a continuous medium, yet the effect does not become important until curvature comparable to molecular dimensions is reached. Fisher and Israelachvili [24] measured the force due to the Laplace pressure for a pendular ring of liquid between crossed mica cylinders and concluded that for several organic liquids the effective surface tension remained unchanged... 

Roller conveyors are quite frequently powered, the simplest method being use of a pressure belt in contact with the lower surface of the rolls. A special ripple belt with raised pads is capable of starting up the load but does not build up excessive blocked pressure if the line fills up. Other similar drives are available, with varying degrees of control over the applied power. Most expensive of the powered roller units are those in which each roll is equipped with V-belt or chain drives. Pusher bars suspended from overhead chain conveyors may also be used to move containers along a roller hne. 

Pressure can also be controlled by variable heat transfer coefficient in the condenser. In this type of control, the condenser must have excess surface. This excess surface becomes part of the control system. One example of this is a total condenser with the accumulator running full and the level up in the condenser. If the pressure is too high, the level is lowered to provide additional cooling, and vice versa. This works on the principle of a slow moving liquid film having poorer heat transfer than a condensing vapor film. Sometimes it is necessary to put a partially flooded condenser at a steep angle rather than horizontal for proper control response. 

Air/water vapor mixture, chart, 364,365 Air/water vapor, 359 Capacity at ejector suction, 369 Capacity for process vapor, 362 Evacuation time, 371, 380 Load for steam surface condenser, 367 Non-condensables, 362, 363 Size selection, 371 Steam pressure factor, 373 Steam requirements, 372 Steain/air mixture temperature, 361 Total weight saturated mixture, 362 Capacity, 358 Discharge, pressure, 358 Effect of excess steam pressure, 358 Effects of back pressure, 359 Effects of wet steam, 356 Inter-and-after condenser, 351 Load variation, 370 Materials of construction, 347 Molecular weight entrainment, chart, 360 Performance, 358, 370, 375 Relative comparison, 357... 

Example.—Find the excess of pressure inside a soap-bubble 1 mm. in diameter over the atmospheric pressure. (Surface tension of water =81 ergs per sq. cm.) [16 X 10 5 atm.]... 

Loss of resin from the softener due to poor regeneration procedures or excessive water pressure. The resin may either be lost down the drain or it may enter the FW system, whereupon it melts or disintegrates and causes fouling of waterside surfaces. 

One solution that was considered by Rayleigh (Lamb, 1945) for the determination of bubble collapse time, tm, used the model of a bubble with initial size Rm, suddenly subjected to a constant excess liquid pressure pL. Neglecting the surface tension and the gas pressure in the bubble, Eq. (2-29) may be rearranged to... 

Pulmonary surfactant decreases surface tension of alveolar fluid. Reduced surface tension leads to a decrease in the collapsing pressure of the alveoli, an increase in pulmonary compliance (less elastic recoil), and a decrease in the work required to inflate the lungs with each breath. Also, pulmonary surfactant promotes the stability of the alveoli. Because the surface tension is reduced, the tendency for small alveoli to empty into larger ones is decreased (see Figure 17.2, panel b). Finally, surfactant inhibits the transudation cf fluid out of the pulmonary capillaries into the alveoli. Excessive surface tension would tend to reduce the hydrostatic pressure in the tissue outside the capillaries. As a result, capillary filtration would be promoted. The movement of water out of the capillaries may result in interstitial edema formation and excess fluid in the alveoli. 

Once it has been established that the components of a binary monolayer are to some degree miscible, the energetics of their interaction may be calculated directly from the 11/A isotherms of the mixture and its individual components. As proposed by Goodrich (1957), this technique employs the differences in the work of compression of the binary film and the work required to compress each of the films of the pure components to the same surface pressure. The result is the total free energy of mixing as expressed by the sum of the excess and ideal free energies of mixing in (14), where Nt... 

Since it has been shown that nonideal mixing occurs in the 2.5-15.0 dyn cm 1 range, the excess free energies of interaction were calculated for compressions of each pure component and their mixtures to each of these surface pressures. In addition, these surface pressures are below the ESPs and/or monolayer stability limits so that dynamic processes arising from reorganization, relaxation, or film loss do not contribute significantly to the work of compression. 

Fig. 48 Excess free energies of mixing as a function of varying film composition at surface pressures of 2.5,5.0,10.0, and 15.0 dyn cm "1 for mixtures of ( )- and meso- (a) C-12 6,6 (b) C-15 6,6 (c) C-18 6,6 (d) C-15 9,9 ketodiacids at 25°C. Reprinted with permission from Arnett et al, 1988b. Copyright 1988 American Chemical Society.

The classic studies of Saunders( 17) demonstrated that in the presence of excess surfactant methyl cellulose (MC) would desorb from monodispersed polystyrene latices. MC is one of the most surface active water-soluble polymers (W-SPs) and it will readily dominate the surface pressure 7T (7T = cre - cr t where cr is the surface tension of water and is the surface tension of the aqueous polymer solution) of the aqueous solution. For example, hydroxyethyl cellulose (HEC) lowers the surface tension of water much less than MC or HPMC, and when the combination of HEC and MC or HPMC in water is studied, there is no notable influence of HEC on the surface pressure (Figure 2). 

A bubble of air in a liquid is, as we know, spherical, and it is obvious that this spherical shape can only be maintained if the pressure on the inside is greater than that outside. Let P be the excess of pressure inside per unit surface, and a the radius of the sphere the pressure tending to force the two hemispheres apart is then evidently P x area of largest circle, i.e., P naa. This pressure is balanced by the pull arising from surface tension, which acts round the circumference of the same circle, and is, accordingly, 2ira[Pg.17]

Gibbs adsorption equation phys chem A formula for a system involving a solvent and a solute, according to which there Is an excess surface concentration of solute if the solute decreases the surface tension, and a deficient surface concentration of solute if the solute increases the surface tension. gibz ad sorp shan i.kwa-zhon Gibbs adsorption isotherm physchem An equation for the surface pressure of surface [< ... 

Fig. 5.4 Dimensionless excess modified pressure at surface of sphere. Numerical results of Woo (W9). (Note different scale for Re = 1.0 curve.)...

External pressure implies that the pressure on the outside of the tank or vessel is greater than that in its interior. For atmospheric tanks, the development of an interior vacuum results in external pressure. External pressure can be extremely damaging to tanks because the surface area of tanks is usually large, generating bigb forces. The result of excessive external pressure is a buckling of the shell walls or total collapse. In some cases wind velocities during hurricanes have been sufficient to knock down and collapse tanks. 

Groves (1978) provided an intuitive explanation based on a mechanical model in which water penetrates into the oil/surfactant system, forming liquid crystals but, more to the point, considerably expanding the interface. This is the reason why it is necessary to postulate that water is inconsiderable excess. The surface expands so that instead of a negative interfacial tension what we have is a positive surface pressure. At this point it is not unreasonable to visualize the surface expanding and stranding as postulated in the Gopal model. 

The surface pressure—area and surface dipole moment-area data for both components and a 1 to 1 molar ratio of all but the last mixture were obtained for at least two temperatures in the range 0° to 40 °C. From such data excess thermodynamic quantities were calculated. For reasons... 

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