By needle valve

The next phase is the elution phase, where the flow enters the channel from the inlet end and exits, both through the membrane (cross-flow) and through the channel outlet end (outlet flow). The balance between the cross-flow rate Vc and the outlet flowrate Vout according to Eq. (69) can be adjusted by needle valves. Vout is measured by a flowmeter, the value of Vc is either measured directly by a... 

Clean air is introduced into the FLEC with (PTFE) tubing from an air supply unit as described by, e.g., Wolkoff et al. (1995). The air flow into the cell is controlled by needle valves. The outlet is a union cross with an overflow tube to avoid false air intake during... 

Figure 4.20 Sketch of a split-splitless injector for GC. The fractions of the inlet gas flow that exit through the septum purge outlet and the split outlet are controlled by needle valves.

A very convenient method for removal of final solvent traces is the concentration of fhe lasf 0.5 mL of a solution by evaporation with a gentle stream of nitrogen gas while the sample is warmed in a sand bath. This process is usually done at a hood station where several Pasteur pipets can be attached to a manifold leading to a source of dry nitrogen gas. Gas flow to the individual pipets is controlled by needle valves. Always test the gas flow with a blank vial of solvent. 

Ellis, Forrest and Howe [163] made modifications and improvements to the original design for their specific application. All flow controls were done by needle valves and all flow measurements by rotameters. They used a helium flowrate of 50 mL min- and nitrogen flowrates of 3, 5 and 10 mL min-i. 

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

Figure 10.4 Schematic representation of the multidimensional GC-IRMS system developed by Nitz et al. (27) PRl and PR2, pressure regulators SV1-SV4, solenoid valves NV— and NV-I-, needle valves FID1-FID3, flame-ionization detectors. Reprinted from Journal of High Resolution Chromatography, 15, S. Nitz et al, Multidimensional gas cliro-matography-isotope ratio mass specti ometiy, (MDGC-IRMS). Pait A system description and teclinical requirements , pp. 387-391, 1992, with permission from Wiley-VCFI.

A flow sheet of the process, as given by Ur banski (Ref 75, his Fig 53), is shown below According to Urbanski To start the nitration, current from the switch (18) is applied to the electromagnet (6), which closes the air inlet to the injector. Mixed acid is admitted by opening the valve under the acid rotameter (4). The injector now comes into operation. The manometer (13) must show full vacuum. The needle valve (7) is then opened and the vacuum adjusted to about 300mm Hg. The glycerine-glycol mixture is sucked in thru the rotameter (3) to the injector from (2)... 

Regular washer-type faucets are hard to control for low water flow, such as required by condensers. For these purposes, needle valves are recommended. They give excellent control, and flow rate does not change as long as the pressure remains constant. The inexpensive plug-type valves are rarely adequate for laboratory water service. 

Figure 6 shows typical results obtained with the plug-flow quartz reactor containing 0.5 g of Sr(lwt%)/La203 catalyst operated in the continuous flow recycle mode. The inlet CH partial pressure was 20 kPa (20% CH in He) at inlet flowrates of 7.1 and 14.3 cm STP/min. A 20% O2 in He mixture was supplied directly, at a flowrate Fog, in the recycle loop via a needle valve placed after the reactor (Fig. 1). The methane conversion was controlled by adjusting Fog, which was kept at appropriately low levels so that the oxygen conversion...

All three types of valves described above are on-off type devices (binary devices). A basic requirement in the design of these valves is that they offer minimum resistance to flow when open. For many types of calculations it is often justifiable to neglect pressure losses through such devices. But such a simplification cannot be applied to flow control valves. With these devices the control of flow is accomplished either by a constriction or by a diversion. In either case an additional resistance to the flow is introduced. Globe, angle, cross, and needle valves are typical devices of this type. Specific pressure drop correlations should be developed for such devices and used whenever possible. One such correlation used by Stoner (S5) takes the form... 

The core is put into a Hassler sleeve inside a core holder where overburden pressure is applied outside of the sleeve by using water. Gas is not used, since it has the potential to diffuse through the rubber sleeve. The overburdened core holder is connected to the Isco pump, from which 1% acidified brine is pumped into the core. The needle valve that restricts flow from the output end of the core is opened slightly to allow the escape of air and air-saturated brine. The needle valve which restricts flow from the input end of the core is opened fully. Complete saturation is attained after several pore volumes of brine have been pumped through the core under pressure, and this fully saturated core is used throughout the experiment. 

Burdick and Jackson). All solutions were photolyzed to less than 5% conversion in a standard 3 ml capacity, 1-cm path length quartz cell. Samples were irradiated with a 450-Watt medium pressure, Hanovla mercury lamp focused through an appropriate band-pass filter (280 nm or 254 nm) onto the 1-cm quartz cell with the requisite solution. Test solutions could be purged with either helium or oxygen using a needle valve assembly attached to the tapered quartz cell neck. The loss of carbamate due to photolysis and the amounts of known photoproducts were determined quantitatively by GC using eicosane as an internal standard. The columns were 6 stainless steel containing Carbowax 20M on chromosorb G. 

It is more convenient to use the enthalpy of the cold gas evaporating from the LHe in a continuous gas flow cryostat (Fig. 5.4). The flux and hence the temperature of the experiment can be regulated by a heater or a needle valve. 

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