Pressure driven devices

Pressure-driven devices (Figure 4.1-6), such as pressure swirl atomizers, use aerodynamic drag at the gas-liquid interface to amplify natural disturbance to pinch droplets off liquid sheets and columns. Finer mists can be generated by adding a centrifugal force to the process that must overcome viscous damping and surface tension. 

Pressure-driven devices include capillary viscometers and slit-die viscometers, in both of which the flow is driven by pressure. In the case of the capillary viscometer the pressure is generated by an upstream piston, and in the case of the slit-die viscometer flow is generated by an extruder. In both cases, measurements of pressure drop and flow rate are used to determine the viscosity. Both techniques have the inherent problem of pressure drop, which may result in phase separation. For this reason, the techniques are suitable for low-pressure measurements, which may mean that the polymer has not reached equilibrated CO2 concentrations. 

The concepts of the cone-plate and biconical rheometers developed in the 1940s (Fig. 13). The cone-plate instrument is due to Freeman and Weissenherg [FIO] and intended for modest-viscosity fluids. It has the basis of his rheogo-niometer which also measured normal stresses. The biconical rheometer was developed in the same period by Piper and Scott [P12] of the BRMRA and was from the beginning intended for rubber. Similar instruments are discussed by Turner and Moore [T12] and Montes et al. [M37, M38]. In the latter instruments, the pressure is controlled by charging the rubber into the rheometer by an attached pressure-driven device. 

In order to produce a robust flow for chromatography on a chip, and which is in the low nanoliter per minute range, several approaches are possible. Flow is either produced by pressure-driven devices (referred to as top-down systems) or by electroosmosis-driven devices (referred to as bottom-up systems). 

Fuel, usually gasoline or diesel fuel, is transferred to the engine from the fuel tank (or tanks) by this pump, which is either a mechanically driven device or, as is now more common, electrically driven. The fuel pump delivers fuel to a carburetor (gasoline) or fuel injection system (diesel and newer gasoline engines), which distribute the fuel under pressure in a spray to the proper cylinder. Many devices that were formerly mechanically driven are now replaced by computer-controlled devices. 

Air-pressure-driven active devices. Air-pressure-driven aerosolization is the concept employed in a number of devices currently in different stages of development with drugs for local or systemic action. These devices rely on a small patient-operated air pump. Air is compressed by mechanical means (piston or bellows) and is released on the external trigger given by the patient s inspiratory cycle. Because of the use of this air pump, these devices have an active aerosolization mechanism and are assumed to be less flow-rate-dependent than passive DPI devices. 

The capillary viscometer. The most common and simplest device for measuring viscosity is the capillary viscometer. Its main component is a straight tube or capillary, and it was first used to measure the viscosity of water by Hagen [28] and Poiseuille [60], A capillary rheometer has a pressure driven flow for which the velocity gradient or strain rate and also the shear rate will be maximum at the wall and zero at the center of the flow, making it a non-homogeneous flow. 

Static mixers. Static mixers or motionless mixers are pressure-driven continuous mixing devices through which the melt is pumped, rotated, and divided, leading to effective mixing without the need for movable parts and mixing heads. One of the most commonly used static mixers is the twisted tape static mixer schematically shown in Fig. 3.25. 

This device consists simply of a T-type junction where two pressure-driven flows are contacted (see Figure 1.169) [48]. After a short passage, such a bi-laminated stream is exposed to injections from both sides by adjacent channels, which yields a cross-flow configuration. From there on, the injected flow passes a long main channel and finally reaches a reservoir. 

In a microstructured reactor plant, in contrast, the flow rate will be dominated by the pressure loss. Typical pressure losses in micro devices are of the order of 1 bar at a flow of 11 h 1 (water) [50,93], If sufficient pump capacity is available, the pressure loss in a micro structured device is limited by the mechanical stability of the reactor housing, which is often made of steel and hence a loss of several bar is certainly acceptable. Even the combination of up to 10 different micro devices only amounts to about 10 bar in this example. The main advantage of a micro structured reactor plant is that the flow rate can be adjusted more freely because the flow is pressure driven and not influenced by a single gravity-driven device as in a miniplant. 

Another possible solution to this problem has been implemented by Attiya et al. [97]. The device contained a large sample introduction channel with a volume flow resistance >105-fold lower than that in the analysis microchannels. This approach enabled interfacing the large sample introduction channel with an external pump (up to 1-mL/min flow rate) for pressure-driven sample delivery without perturbing the solutions and electrokinetic manipulations within the... 

Membrane installations operated in nuclear industry are pressure-driven systems majority of them are reverse osmosis plants. Uncontrolled growth of operation pressure may result in module damage and valves leaks resulted in contamination hazard. The selection of appropriate pumps and security devices can avoid the danger of pressure overgrowth and its detrimental implications. The security valves outlets have to be connected with existing waste distribution systems to direct the eventual leaks to the waste collecting tanks. 

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