Back-flow effect

FIGURE 4.6. (a) Back-flow effect during the process of the director reorientation, (b) The dependences of the velocity V and director angle 0 on the coordinate z are given at different times ti < t2 < ta- The dotted line shows possible nonmonotonous relaxation of 0 at high voltages ti < ti < t2 < ta-... 

Gas Distrihutor The gas distributor has a considerable effect on proper operation of the flmdized bed. Basically there are two types (1) For use when the inlet gas contains solids and (2) for use when the inlet gas is clean. In the latter case, the distributor is designed to prevent Back flow of sohds during normal operation, and in many cases it is designed to prevent back flow during shutdown. In order to provide distribution, it is necessary to restrict the gas or gas and solids flow so that pressure drops across the restriction amount to from 0.5 kPa (2 in of water) to 20 kPa (3 Ibf/iu ). 

Flammable or toxic vapors can be piped to a flare after separation of liquid is obtained. An important design problem in flare use is the very high vent rate experienced for a relatively short time, if an existing flare is used. Also back-pressure effects on the liquid separator vessel must be considered, especially if choked flow of vapor occurs downstream of the separator. 

A comparison with Burchard s first cumulant calculations shows qualitative agreement, in particular with respect to the position of the minimum. Quantitatively, however, important differences are obvious. Both the sharpness as well as the amplitude of the phenomenon are underestimated. These deviations may originate from an overestimation of the hydrodynamic interaction between segments. Since a star of high f internally compromises a semi-dilute solution, the back-flow field of solvent molecules will be partly screened [40,117]. Thus, the effects of hydrodynamic interaction, which in general eases the renormalization effects owing to S(Q) [152], are expected to be weaker than assumed in the cumulant calculations and thus the minimum should be more pronounced than calculated. Furthermore, since for Gaussian chains the relaxation rate decreases... 

In real tubular (or column) reactors there is, usually, a back-mixing effect which influences the performance of the ideal plug-flow reactor. This axial dispersion is higher for fluidized-bed reactors than for packed-bed reactors, although comparatively lower than for continuous-feed stirred-tank reactors, where the mixing is complete. 

An example of such a product is ioBio , marketed by Baltimore Aircoil Co. (BAC). It is proposed for small cooling systems. This system is, in effect, an in-line dispenser of prilled elemental iodine (U), with replaceable iodine canisters available. The equipment is placed in the makeup waterline to the tower, and in-line options include filters, water meters, and back-flow prevention devices. Claims made are that the biocidal action of the elemental iodine is not affected by changes in temperature, system load, or water chemistry. Also, the system is compatible with virtually all scale and corrosion inhibitors and does not contribute to the corrosion of metal surfaces. 

Beside static mixers, there are practically no alternatives to the "ubiquitous" stirred tank, if one excepts loop reactors (167) and the somewhat special back-flow mixer (168). Imagining entirely new principles for mixing reactants is a challenge for future researchers. First estimations show that an "informed" mixing system, working as a Maxwell demon would be much more effective than our present devices (169). 

The cycle starts with the plastification of the core component in the injection unit. Then the extruder moves to the bottom position, the injection unit moves forward to the extruder nozzle to link the nozzles of the extruder and the injection unit. The extruder starts plastification of the skin component and extrudes the melted skin component into the screw antechamber of the injection unit. Thus the skin and core components are located one after the other in the screw antechamber. After the extruder moved back to the top position, the injection unit moves forward to the mold followed by a conventional filling phase. Due to the fountain flow effect the first injected material forms the skin layer followed by the second component forming the core. Compared to the standard sandwich process the injection phase of the monosandwich process is less complicated as it is identical to the conventional injection molding process. 

A more effective method of protecting against back-flow is the double check valve (Fig. 5). Should upstream pressure drop below a certain level, the two check valves close to protect against backflow. When pressure is restored, the valves open. 

Many authors pointed out that although the axial dispersion coefficient is much higher than the radial dispersion coefficient in a CFB, the back-mixing effect is still negligible because of the large axial convective flow . ... 

The axial velocity vectors increase at the level of 180 mm in the centre as well as at the periphery. There is still a back flow in the radial position of 120 to 220 mm. This indicates a back flow of flue gas with recirculation effects in the combustion process. The recirculation of flue gas is visible on levels up to 240 mm. 

Sinks should be of stainless steel with no overflow, and water must be of at least potable quality. Wherever possible, drains should be avoided. If installed they must be fitted with effective, readily cleanable traps and with air breaks to prevent back-flow. Any floor channels should be open, shallow and cleanable and connected to drains outside the area. They should be monitored microbiologically. 

The effect of D/d was investigated in a further paper [247]. For D/d = 3.8 and 5 this geometric parameter had no effect on Ne up to Q = 0.2, but for D/d = 2.4 the Ne(Q) curves split with increasing Fr values ever more strongly. Judat [248] explained this on the basis of the increasing back flow of gas bubbles in the suction region of the stirrer with decreasing D/d ratio. 

The functional sections in co-rotating twin screw extruders are sequentially arranged and usually have no back couphng upstream due to the partially filled sections. Thus material modifications or flow effects have no influence and are decoupled one another unlike in single-screw extruders, for example. 

The Nafion membranes are produced in this way and with a fabric backing such as PTFE or mixed PTFE - rayon fabrics These supporting materials improve the mechanical strength of the film and keep the dimensional changes in bounds In general, for chlor-alkali electrolysis, the side of the membrane with the highest resistance, selectivity and charge density is preferred toward the cathode side to limit the undesirable effects of the back flow of hydroxide ions into the anode chamber The anolyte side of the membrane polymer is thus less dense, less selective and more conductive than the catholyte side of the separator film ... 

Mini-columns for analyte separation/concentration can also behave as reactors, resembling the packed bed reactor. In this context, organic polymer monoliths, largely used in the medical and biological fields [73], should be highlighted. Monolithic mini-columns consist of continuous beds with macropores and mesopores which are characterised by low back-pressure effects. These columns offer several other advantages [74], as emphasised in Chapter 8. In the context of flow analysis, monolithic mini-columns were implemented in a sequential injection analyser in 2003 [75] and the potential and limitations of the approach, called Sequential Injection Chromatography, were recently reviewed [76]. 

Consideration of relative proton affinities alone is not sufficient to explain the directionality of transport in proton pumps. For efficient proton pumping it is essential that the activated proton (state 2) cannot flow back to group A, which thermodynamically would be dictated by the fact that A has much higher proton affinity than C. To that effect, relative insulation to proton transport in state 2 is required either between B and A, or alternatively, between A and surface I. This requirement is often described in terms of an alternating access model [Jardetzky, 1966] and is now fairly well understood in bacteriorhodopsin [Lanyi, 1999]. Likewise, proton leakage from C to B must be prevented in step 4, where B has a much higher proton affinity than C. In the depicted scheme (Fig.2), the back flow of is hampered by the high proton activity of C relative to O, so that proton relay to B from A is rapid compared to alternative reprotonation from O via C. 

Electroosmosis causes a change in the level of the liquid in communicating vessels, i.e. in the anodic and cathodic parts of a U-shaped tube. This effect, referred to as the electroosmotic rise, turns out to be very strong for example an applied voltage of 100 V may result in a change in liquid levels of up to 20 cm. Electroosmosis and the electroosmotic rise are thus related to the motion of the liquid with respect to the immobilized disperse phase (porous diaphragm). In the case of electroosmotic rise, at equilibrium the electroosmotic transfer of the liquid is compensated by its back flow due to the change in hydrostatic pressures in different arms of the U-shaped tube. 

Nevertheless, machines with concave die rings and internal press rollers do have advantages. For example, if the feed material exhibits a certain elastic behavior, because the forces in the relatively long and slender nip increase slowly, a more complete conversion of temporary elastic into permanent plastic deformation takes place. Fig. 8.41b is another presentation of the forces at work. Feed, ideally deposited in a uniform layer on the die, is pulled into the space (nip) between roller and die and compressed. Friction between roller, die, and material as well as interparticle friction in the mass are responsible for the pull of the feed into the nip and for densification. Smooth surfaces of roller and/or die may result in slip. Axial grooves in the roller, which may also favor build-up of a thin layer of material, and the above mentioned residual layer of densified feed on the die effectively reduce slip. Low interparticle resistance to flow or a distinct plasticity result in a more or less pronounced tendency of the mass to avoid the squeeze" (back-flow), thus reducing densification and potentially choking the machine (see above). 

Sinks and drains should be avoided wherever posable and should be exduded from areas where aseptic operations are earned out Where installed they should be designed, located, and maintained so as to minimize the risks of microbial contamination they should be fitted with effective, easily deanable traps with air breaks to prevent back-flow. Any floor channel should be open and easily deanable and be connected to drains outside the area in a manner that prevents ingress of microbial contaminants. 

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