Vacuum principle

The belt filter also works on the vacuum principle, but this time two drums rotate (Figure 4.8). The cake is thus held in a horizontal position during filtration thereby potentially increasing its stability. 

Figure 2.15 Sketch of a commercially available atomizer that operates on the vacuum principle. (Printed with permission from Lippincott Williams Wilkins. Copyright 1974. Dittert, LW.)...

Be aware that evacuating the air from the chamber of a vacuum steam sterilizer prior to sterilization of contaminated material can create a potential hazard by releasing infectious material to the atmosphere (30). Such sterilizers, which operate on the vacuum principle, should not be used for disinfection of potentially infectious waste unless the vacuum cycle can be disabled or turned off. ... 

The course of a surface reaction can in principle be followed directly with the use of various surface spectroscopic techniques plus equipment allowing the rapid transfer of the surface from reaction to high-vacuum conditions see Campbell [232]. More often, however, the experimental observables are the changes with time of the concentrations of reactants and products in the gas phase. The rate law in terms of surface concentrations might be called the true rate law and the one analogous to that for a homogeneous system. What is observed, however, is an apparent rate law giving the dependence of the rate on the various gas pressures. The true and the apparent rate laws can be related if one assumes that adsorption equilibrium is rapid compared to the surface reaction. 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

N is the number of point charges within the molecule and Sq is the dielectric permittivity of the vacuum. This form is used especially in force fields like AMBER and CHARMM for proteins. As already mentioned, Coulombic 1,4-non-bonded interactions interfere with 1,4-torsional potentials and are therefore scaled (e.g., by 1 1.2 in AMBER). Please be aware that Coulombic interactions, unlike the bonded contributions to the PEF presented above, are not limited to a single molecule. If the system under consideration contains more than one molecule (like a peptide in a box of water), non-bonded interactions have to be calculated between the molecules, too. This principle also holds for the non-bonded van der Waals interactions, which are discussed in Section 7.2.3.6. 

The problems already mentioned at the solvent/vacuum boundary, which always exists regardless of the size of the box of water molecules, led to the definition of so-called periodic boundaries. They can be compared with the unit cell definition of a crystalline system. The unit cell also forms an "endless system without boundaries" when repeated in the three directions of space. Unfortunately, when simulating hquids the situation is not as simple as for a regular crystal, because molecules can diffuse and are in principle able to leave the unit cell. 

The Z-spray source utilizes exactly these same principles, except that the trajectory taken by the ions before entering the analyzer region is not a straight line but is approximately Z-shaped. This trajectory deflects many neutral molecules so that they diffuse away toward the vacuum pumps. 

A variation of the same principle is the DDS-vacuum pressure filter which has a number of small disks mounted on a shaft which rotates discontinuously. The cake is formed on both sides of the disks when they are at the bottom position, dipped into the slurry. When the disks come out of the slurry and reach the top position, hydrauhcaHy driven pistons squee2e the cake and the extra Hquid then drains from both sides of the cake. The cake is removed by blowback with compressed air. 

The so-called hyperbar vacuum filtration is a combination of vacuum and pressure filtration in a pull—push arrangement, whereby a vacuum pump of a fan generates vacuum downstream of the filter medium, while a compressor maintains higher-than-atmospheric pressure upstream. If, for example, the vacuum produced is 80 kPa, ie, absolute pressure of 20 kPa, and the absolute pressure before the filter is 150 kPa, the total pressure drop of 130 kPa is created across the filter medium. This is a new idea in principle but in practice requires three primary movers a Hquid pump to pump in the suspension, a vacuum pump to produce the vacuum, and a compressor to supply the compressed air. The cost of having to provide, install, and maintain one additional primary mover has deterred the development of hyperbar vacuum filtration only Andrit2 in Austria offers a system commercially. 

Fig. 3. Schematic representation of the principle of design and operation of a consumble-electrode furnace for melting steels in a vacuum (1).

Heating and Cooling. Heat must be appHed to form the molten zones, and this heat much be removed from the adjacent sohd material (4,70). In principle, any heat source can be used, including direct flames. However, the most common method is to place electrical resistance heaters around the container. In air, nichrome wine is useflil to ca 1000°C, Kanthal to ca 1300°C, and platinum-rhodium alloys to ca 1700°C. In an inert atmosphere or vacuum, molybdenum, tungsten, and graphite can be used to well over 2000°C. 

The endless-belt percolator (Wakeman, loc. cit.) is similar in principle, but the successive feed, solvent spray, drainage, and dumping stations are hnearly rather than circulany disposed. Examples are the de Smet belt extractor (uncompartmented) and the Lurgi frame belt (compartmented), the latter being a kind of linear equivalent of the Rotocel. Horizontal-belt vacuum filters, which resemble endless-belt extractors, are sometimes used for leaching. 

Historically, EELS is one of the oldest spectroscopic techniques based ancillary to the transmission electron microscope. In the early 1940s the principle of atomic level excitation for light element detection capability was demonstrated by using EELS to measure C, N, and O. Unfortunately, at that time the instruments were limited by detection capabilities (film) and extremely poor vacuum levels, which caused severe contamination of the specimens. Twenty-five years later the experimental technique was revived with the advent of modern instrumentation. The basis for quantification and its development as an analytical tool followed in the mid 1970s. Recent reviews can be found in the works by Joy, Maher and Silcox " Colliex and the excellent books by Raether and Egerton. ... 

The main reason for extruding polystyrene is to prepare high-impact polystyrene sheet. Such sheet can be formed without difficulty by vacuum forming techniques. In principle the process consists of clamping the sheet above the mould, heating it so that it softens and becomes rubbery and then applying a vacuum to draw out the air between the mould and the sheet so that the sheet takes up the contours of the mould. 

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271]. 

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