Push-pull—

Osmotic Pressure Controlled Oral Tablets. Alza Corp. has developed a system that is dependent on osmotic pressure developed within a tablet. The core of the tablet is the water-soluble dmg encapsulated in a hydrophobic, semipermeable membrane. Water enters the tablet through the membrane and dissolves the dmg creating a greater osmotic pressure within the tablet. The dmg solution exits at a zero-order rate through a laser drilled hole in the membrane. Should the dmg itself be unable to provide sufficient osmotic pressure to create the necessary pressure gradient, other water-soluble salts or a layer of polymer can be added to the dmg layer. The polymer swells and pushes the dmg solution through the orifice in what is known as a push-pull system (Fig. 3). The exhausted dmg unit then passes out of the body in fecal matter. 

Fig. 3. (a) Cross section of the push-pull oral osmotic system (OROS), which has an inner flexible partition to segregate the osmotic propellant from the dmg compartment, (b) Push-pull OROS in operation with the propellant imbibing water, increasing in volume, and pushing the dmg out of the device... 

Developments. A variety of process modifications aimed at improving surface finish or weld line integrity have been described. They include gas assisted, co-injection, fusible core, multiple Hve feed, and push—pull injection mol ding (46,47). An important development includes computer-aided design (CAD) methods, wherein a proposed mold design is simulated by a computer and the melt flow through it is analy2ed (48). 

Fig. 8. Example of a Push-Pull osmotic pump where (a) represents the pump before operation, and (b), during operation.

Pressure-negative (push-pull) combination system normally used when conveying from several pickup points to several discharge points. 

The eommon topologies whieh are eneompassed under this eategory are the buek, half-forward, push-pull, half and full bridge, with only the traditional voltage-mode eontrol method. Its representative eireuit diagram is given in... 

Airborne contaminant movement in the building depends upon the type of heat and contaminant sources, which can be classified as (1) buoyant (e.g., heat) sources, (2) nonbuoyant (diffusion) sources, and (d) dynamic sources.- With the first type of sources, contaminants move in the space primarily due to the heat energy as buoyant plumes over the heated surfaces. The second type of sources is characterized by cimtaminant diffusion in the room in all directions due to the concentration gradient in all directions (e.g., in the case of emission from painted surfaces). The emission rare in this case is significantly affected by the intensity of the ambient air turbulence and air velocity, dhe third type of sources is characterized by contaminant movement in the space with an air jet (e.g., linear jet over the tank with a push-pull ventilation), or particle flow (e.g., from a grinding wheel). In some cases, the above factors influencing contaminant distribution in the room are combined. 

Push-Pull Ventilation of Open Surface Tanks 944... 

BEOs are most often used for point sources or small line or surface sources. See Chapter 7 for descriptions of sources. BEOs are sometimes used for lines or surfaces when the source is moving along the line or on the surface. This naturally demands the exhaust to move with (or be moved with) the source movements (e.g., during painting or seam welding). They have also been used for side suction from baths and tanks-- and these exhausts are usually called rim exhausts see Rim Exhausts. However, for these sources push-pull systems (Section 10.4.3) are often more efficient. Side hoods can also be used, e.g., when molten metal is poured however, in these cases an enclosed exhaust is more efficient. 

Rim exhausts are suitable for area sources of contaminant. They are limited in the area over which they can draw with adequate velocity. In practice, the slot hood should be within 0.6 m of the far edge of the source. For an open surface tank this means that a slot hood on one long side is necessary for tanks up to 0.6 m in width hoods on both long sides are necessary for tanks up to 1.2 m in width and rim exhaust is not practical for tanks wider than 1.2 m. For those situations, push-pull ventilation or enclosure type hoods are recommended.- ... 

For a push-pull system, the source is usually an open surface tank and the airflow acts as a horizontal curtain above the surface. In this case, the person could be anywhere as long as the system works as intended and the curtain is not broken. The curtain will be broken when parts or material are lifted out of or placed into the bath and the contaminants could be spread either through convection or because the supply air blows against the material or part. 

In these cases, push-pull ventilation offers an appropriate mechanism for reducing the overall flow rate required, compared with side exhaust, by up to 50%, while still maintaining dear overhead access. 

Push-pull ventilation systems for open surface tanks consist of two components the push flow is generated by a jet or series of jets that are blown across the surface of the tank towards an exhaust hood along one side of the tank, which pulls and removes the fluid from the jet containing the contaminant. This is shown schematically in Fig. 10.69. 

This section deals mainly with side push-pull ventilation. Center push-pull ventilation is also sometimes used, where two jets of air are blown from a central pipe towards two parallel exhaust hoods at opposite ends of the tank. Much of what vve say about side push-pull systems is equally valid to center push-pull. 

FIGURE (0.69 Schematic diagram of side push-pull system. 

A number of workers at Pennsylvania State University examined the push-pull system and found good agreement between their numerical and experimental work. The computational algorithm SIMPLER was used to solve the flow in the two-dimensional push-pull system and it was concluded that for a tank 1.8 m long, the push jet must have an initial velocity of 3.8 m s, that the exhaust flow rate per unit width should be 0.495 m s", and that the ratio of the pull to push flow rates, q /qj, must be between 8.8 and 17.8. 

More recently, in the middle 1990s, the UK s Health and Safety Executive (HSE) also reviewed the push-pull system. Hollis and Fletcher offer a comprehensive literature review on push-pull ventilation and note that the main conclusions of previous work on push-pull ventilation of tanks are that the control is primarily supplied by the inlet jet, forming a wall jet along the surface of the tank, and that the main purpose of the exhaust hood is to remove the air and contaminant contained within the push jet. 

Flynn et al." applied a finite element based numerical model to solve the problem of a push-pull flow with cross-drafts and demonstrate that the results show good agreement with experimental data. They note, however, that the numerical method is time consuming and therefore computationally expensive. 

The ACGIH " gives recommendations for the design of a push-pull system, apparently based largely on the work of NIOSH in the 1980s the nozzle should be between 3.2 mm and 6.4 mm and that the so-called momentum factor of the jet, the product of its velocity and flow rate per unit width, U,ij should be between 0.39 and 0.59 m s -. The outlet flow rate may then be calculated using the formula... 

Flow Patterns Induced by a Push-Pull System... 

Collectively, for the sake of brevity, we refer to Eqs. (10.92) to (10.96) as the original Verhoff formulae. A numerical analysis of the wall jet in the push-pull situation suggests that the Verhoff formulae fit the numerical data more closely if the following constants are taken ... 

Ingham. - This gives the required minimum value for the momentum ol the equivalent wall jet we must also recall the relationship shown in Fig. 10.72 to determine the required momentum of the offset jet in the push-pull system. 

The flow ratio method was first suggested for use in designing receptor hoods and then it was suggested for design of push-pull systems. The concept of the method is described as follows. 

Figure 10.87 shows the fundamental operation of the push-pull flow. The suction hood should simultaneously exhaust the pushed air (contaminated supply... 

In designing the push-pull hood, one always applies a safety factor, , resulting in the exhaust flow rate for design, which is expressed as the following ... 

The limit value of the flow ratio, K , is expressed as the following experimental equation for two-dimensional push-pull flows ... 

For practical design, the recommended aspects of the push-pull flow (hood) and the safety factor are as follows ... 

The system with a horizontal jet (Fig. 10.92) is similar to the push-pull system used on open surface tanks (Section 10.4.3). One difference is that in this system the jet functions as an injector, whereas in push-pull systems the main function is as a curtain. 

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