Flow restrictors

One of the most important safety components on any high pressure compressed gas system is the flow restrictor. There are two t5q)es of flow-restricting devices an excess flow switch and a flow restrictor. The excess flow switch can be either mechanical or electromechanical and has an excess flow sensor which trips a valve, shutting off gas flow when a preset limit is exceeded. The restrictor is a passive device (limiting orifice) which is sized to limit the flow of gas to a predetermined rate. 

The goal when selecting a restrictor is to deliver the amount of gas required for the process, sized in consideration of the lowest pressure at which the cylinder will be used. Concurrently, the restrictor must be of a size that limits the flow at high cylinder pressures to a rate at which exhaust ventilation or other treatment systems can provide adequate dilution in case of failure in a component of the gas system. For some pyrophoric gases, there is evidence that the use of a restrictor can mitigate the hazard associated with unrestrained gas release. 

The physical characteristics that limit gas flow through an orifice are size of the orifice, pressure of the system, molecular weight, density, and 

Flow rates of nitrogen at 2500 psi are 65 000 L/min through a 7.13 mm (0.281-in.) cylinder valve opening 27 000 L/min through a severed 0.89 mm (0.035-in.) wall thickness, 6.35 mm (1/4-in.) tube and 30 L/min through a 0.006-in. limiting orifice. These are instantaneous flow rates which decrease as the cylinder pressure decreases. The decrease in the first two instances is very rapid as the cylinder quickly vents. 

At 150 psi the flow rate through the 0.006-in. orifice is 2.8 L/min nitrogen. This must be taken into consideration when sizing the orifice for process flow. 

These are effective in dealing with the al1-too-human tendency to want to rinse with the cleanest water possible, which is sometimes not necessary in a well-designed rinse system. Placed directly in the inlet to a rinse tank, they restrict the flow to a predetermined level. Once the correct flow is determined, it is a simple matter to keep that flow in place. However, restrictors are non-adjustable, which can be a problem in jobshops with variable work flows. 

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Vapor Pressure. The Shiley Infusaid implantable infusion pump utilizes energy stored in a two-phase fluorinated hydrocarbon fluid. The pump consists of a refillable chamber that holds the dmg and a chamber that holds the fluid. The equiUbrium vapor pressure of the fluid, a constant 60 kPa (450 mm Hg), compresses the bellows, pumping the dmg through a bacterial filter, a capillary flow restrictor, and an infusion cannula to the target body site (56,116). 

The term bladed refers to raised, continuous flow restrictors with a standoff distance from the bit body of more than 1.0 in. In most cases cutters are affixed to the blades so that the cutter arrangement may also be described as bladed. The term ribbed refers to raised continuous flow restrictors with a standoff distance from the bit body of 1.0 in. or less. Cutters are usually affixed to most of the ribs so that the cutter arrangement may also be described as ribbed. The term open face refers to nonrestricted flow arrangements. Open face flow designs generally have a more even distribution of cutters over the bit face than with bladed or ribbed designs. 

Externally pressurized gas journal bearings have the same principle of operation as hydrostatic liquid-lubricated bearings. Any clear gas can be used, but many of the design charts are based on air. There are three forms of external flow restrictors in use with these bearings pocketed (simple) orifice, unpocketed (annular) orifice, and slot. 

The liquid enters the micro channel device via a large bore that is connected to a micro channel plate via a slit (Figure 5.2). The slit acts as a flow restrictor and serves for equipartition of the many parallel streams [1, 3, 4]. The liquid streams are re-collected via another slit at the end of the micro structured plate and leave the device by a bore. The gas enters a large gas chamber, positioned above the micro channel section, via a bore and a diffuser and leaves via the same type of conduit. 

Figure 6.13 Flow restrictors of different design A, linear B, tapered C, integral and D, frit. On the right side is shown a modified high pressure cell for UV detection using open tubular columns.

The instrumental requirements for supercritical fluid extraction are quite simple. A pump is essential to generate the extraction pressure in a themostated extraction vessel. The soluble sample components are then swept from the vessel through a flow restrictor into a collection device that is normally at ambient pressure. The fluid used for supercritical fluid... 

The chromatographic and mass spectrometric choices facing the analyst in coupling SFC and MS successfully, namely injection method column type of flow restrictor and mass spectrometer ionisation method and type of vacuum system, have been described [398]. In SFC-MS coupling, the restrictor plays a major role, as the expansion behaviour to a large extent determines the overall performance of the SFC-MS system and defines the range of applications. 

SCF carrier freezing and blocking the flow restrictor. Some interfaces lack sensitivity [430], SFC-MS lacks information at m/z values below 50, due to the high abundance of the CO2 peak. SFC-MS is not as promising as was originally thought. 

In reduced-flow LC-MS systems, the solvent flow into the spectrometer is reduced to a level where the pumping system can cope. Essentially, three such systems have been developed direct-liquid-introduction (DLI), flowing FAB [531] and electrospray [532]. An alternative approach to belt transport interfacing is to deliver the column eluate directly into the MS source and use Cl techniques. Methods based on this principle are called direct-liquid-injection systems, which are comprised of capillary flow restrictors, diaphragms,... 

Figure 11.19 384-parallel single-bead reactor with independent microreaction chambers and integrated flow restrictors. Typical bead diameter 1 mm. 

Using a 1000 psi flow restrictor coil (length of 0.007" i.d. tubing), again pump the water at ImL/min. After the flow has stabilized, record the high and low pressure points for a period of time (typically, 1 min). The high and low points should vary by less than a specified amount (e.g., <30 psi). 

Off-line SFE is conceptually a simple experiment to perform and requires only relatively basic instrumentation. The instrumental components necessary include a source of fluid, most often CO2 or CO2 with an organic modifier, a means of pressurizing the fluid, an extraction cell, a method of controlling the extraction cell temperature, a device to depressurize the supercritical fluid (flow restrictor), and a device for collecting the extracted analytes. 

Case (h) was subsequently removed by installation of a site-registered flow restrictor as a back-up to the flow controller. 

For mechanical systems (e.g. (a) and (b) above), it is important to ensure that, during maintenance, the safety functions of equipment are recognised. Otherwise, they may be replaced with unsuitable equipment. For instance, it is preferable for flow restrictors to use an orifice welded within a short length of pipe, so that it cannot easily be left out, rather than, an orifice plate that slips between pipe flanges. It is desirable to have some method of clearly indicating that such equipment has a safety critical function. 

Syrup is pumped from a BiB container or syrup reservoir by a pressure activated pump, through another cooling coil in the cooler and on to one of the dispense valves. When the valve is opened to dispense product, pressure in the syrup line drops and the pump is activated until the valve shuts. The ratio of syrup to carbonated water is adjusted by flow restrictors in the feed pipes to the valve. 

The principal design feature that permits economical application of the wet/dry tower is the summer damper component. This is a door-like air flow restrictor that is located in the heated dry air stream between the air-cooled heat exchanger and the fan. Its purpose is to reduce the air flow through the dry stream, while boosting the air flow in the wet stream, thereby enhancing the wet section thermal performance during summer operation. 

Reactant feeds are generated by vaporizing liquid flows from HPLC pumps with manometric pulse dampeners or high-pressure syringe pumps and mixing this vapor with gas components metered through mass flow controllers. This reaction feed is then divided between the 48 channels equally by using flow restrictors such as silica capillaries or micromachined channels (Fig. 3.12). The capillaries feed into the inlet stand-offs of the reactor modules. 

M 20] [P 19] The pTAS module is made for performing colorimetric analyses as typically applied in cuvette tests, e.g. for on-site water analysis [30], A continuous test replaces copious manual pipetting of the sampling volumes. The module consists of a micro flow restrictor, a micro mixer and an optical microcuvette for colorimetric analysis. The sample is injected by a conventional FIA (flow injection analysis) system. By close connection, a dead volume of only 2.2 pi is given. 

Some support structures are also included for detachably retaining the various components of the system. Preferably the support structure can be of the assembly board type , which provides prearranged flow channels and connector ports. The desired components of the system can be fastened into these connectors by pins. The flow control system that makes up the ICS system can include pumps, flow channels, manifolds, flow restrictors, valves, etc. These components are equipped with the necessary fittings that allow them to be sealed with the prearranged or selectively located flow channels or connectors. The flow system can also include detachable mixing devices, e.g., static or ultrasonic, or with a chip-like design. The reaction units, whether chip-like or not, can be of thermal, electrochemical, photochemical or pressure type [84]. 

It should also be mentioned that the parallelization of reactors raises other problems such as the maldistribution of the reactants to the single units. While here usually passive devices such as flow restrictors are effectively applied, more severe problems are encountered if the process temperature should be controlled actively. One solution is to heat the whole micro reactor (assuming isothermal conditions due to large heat transfer coefficients) at a constant temperature controlled by the temperature of the flow at the reactor exit [13]. 

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