Low-Flow Applications

Measurement and control of low-flow rates are a requirement in such applications as fuel cells, purging, bioreactors, leak testing, and controlling the reference gas flow in chromatographs or in plasma-emission spectrometers. The most traditional and least expensive low-flow sensor is the variable-area flowmeter. It has a high rangeability (10 1) and requires little pressure drop. Due to its relatively low accuracy, it is limited to purge and leak-detection applications. 

In laminar flow elements, the pressure drop and flow are in a linear relationship. The laminar flow element can be used in combination with either a differential-pressure- or a thermal-type flow detector. These flowmeters provide better rangeability at about the same cost as sonic nozzles. 

Thermal flowmeters also can directly detect low-mass flows without any laminar elements. In that case, they are installed directly into the pipeline as either thermal flowmeters or anemometers. They have a 100 1 rangeability, and can be provided with integral controllers. 

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Diaphragm pumps are used for metering small amounts of additive into a fuel or fuel oil. The cost of these pumps is low compared to other positive displacement pumps. These pumps are excellent metering pumps and are primarily designed for low-pressure, low-flow applications. Also, they are not recommended for pumping high-viscosity fluids. 

Design Considerations Gravity decanters normally are specified as horizontal vessels with a length-to-diameter ratio greater than 2 (and often greater than 4) to maximize the phase boundary (cross-sectional area) between the two settled layers. This provides more effective utilization of the vessel volume compared to vertical decanters, although vertical decanters may be more practical for low-flow applications or when space requirements limit the footprint of the vessel. 

Diaphragm pump Precise flow control Only true positive-displacement pump For low flow applications Must manage pulsation... 

Pelton Turbine - A type of impulse hydropower turbine where water passes through nozzles and strikes cups arranged on the periphery of a runner, or wheel, which causes the runner to rotate, producing mechanical energy. The runner is fixed on a shaft, and the rotational motion of the turbine is transmitted by the shaft to a generator. Generally used for high head, low flow applications. 

For low-flow applications (. , j,e exact size distribution of solids is often not known, and... 

Positive-displacement compressors are most often used for low-flow applications and for slightly higher compression ratios than are centrifugal types. Single-stage positive displacement compressors would appear to be better suited to compression evaporation than centrifugal machines because of lower cost and their characteristic fixed capacity. However, they are not available In a wide range of materials of construction and are limited in capacity and efficiency. Consequently, their application is limited. 

Peclet number independent of Reynolds number also means that turbulent diffusion or dispersion is directly proportional to the fluid velocity. In general, reactors that are simple in construction, (tubular reactors and adiabatic reactors) approach their ideal condition much better in commercial size then on laboratory scale. On small scale and corresponding low flows, they are handicapped by significant temperature and concentration gradients that are not even well defined. In contrast, recycle reactors and CSTRs come much closer to their ideal state in laboratory sizes than in large equipment. The energy requirement for recycle reaci ors grows with the square of the volume. This limits increases in size or applicable recycle ratios. 

The Cv Concept. From a quantitative standpoint, steam loss can be estimated through the application of the C, concept. A familiar term in control valve technology, C, expresses the flow capability of a fluid-controlling device—in this case, a steam trap. A large C,. means a high flow rate a low C, means a low flow rate. 

Other Applications Very small, very low-flow, and relatively high-velocity exhaust inlets, similar to LVHV nozzles, have been used successfully to control fumes from electric soldering irons." " Some investigations have been made into small, point-control exhaust ventilation for aerosols generated by high-speed dental tools. However, such low-volume point-control ventilation systems have not seen widespread use. 

Low-velocity ATD An air terminal device which is designed for thermally controlled ventilation, e.g., displacement flow applications. See also Air terminal device. 

The first successful chiral resolutions through enantioselective membranes have been published recently, but few cases are applicable to the preparative scale, mainly due to mechanical and technical limitations. Low flow rates, saturation of the chiral selectors and loss of enantioselectivity with time are some of the common problems encountered and that should be solved in the near future. 

Application all services except extremely coking, polymer formation or other high fouling conditions. Use for extremely low flow conditions where tray must remain wet and maintain a vapor seal. 

These distributors are fabricated of pipe lengths tied to a central distribution header (usually) %vith orifice holes drilled in the bottom of the various pipe laterals off the header. This style of distributor can be fed by pressure or gravity for clean fluids. The gravity feed is considered better for critical distillation application when uniformity of the flow of the drip points (or flow points) through out the cross-section of the tower is extremely important, and is excellent for low flow requirements such as below 10 gpm/ft2 [131]. 

These meters are suitable for low-pressure applications (<75mbar) and low flow rates. Meters rated at up to... 

Glaser and Litt (G4) have proposed, in an extension of the above study, a model for gas-liquid flow through a b d of porous particles. The bed is assumed to consist of two basic structures which influence the fluid flow patterns (1) Void channels external to the packing, with which are associated dead-ended pockets that can hold stagnant pools of liquid and (2) pore channels and pockets, i.e., continuous and dead-ended pockets in the interior of the particles. On this basis, a theoretical model of liquid-phase dispersion in mixed-phase flow is developed. The model uses three bed parameters for the description of axial dispersion (1) Dispersion due to the mixing of streams from various channels of different residence times (2) dispersion from axial diffusion in the void channels and (3) dispersion from diffusion into the pores. The model is not applicable to turbulent flow nor to such low flow rates that molecular diffusion is comparable to Taylor diffusion. The latter region is unlikely to be of practical interest. The model predicts that the reciprocal Peclet number should be directly proportional to nominal liquid velocity, a prediction that has been confirmed by a few determinations of residence-time distribution for a wax desulfurization pilot reactor of 1-in. diameter packed with 10-14 mesh particles. 

Catalysts and their carriers are provided in micro channels by various means and in various geometric forms. In a simple variant, the catalyst itself constitutes the micro-reactor construction material without need for any carrier [2-A], In this case, however, the catalyst surface area equals that of the reactor wall and hence is comparatively low. Accordingly, applications are typically restricted to either fast reactions or processing at low flow rates for slow reactions (to enhance the residence time). 

Liquid transport is achieved by hydrostatic action, pumping or electroosmotic flow (EOF). So far, chip reactors have been employed at low to very low flow rates, e.g. from 1 ml min to 1 pi min. Applications consequently were restricted to the laboratory-scale or even solely to analytics. However, this is not intrinsic. By choosing larger internal dimensions, similar throughputs as for the other classes of liquid or liquid/liquid micro reactors are in principle achievable. 

For the application referred to, the interdgital micro mixers were used on their own, without tubing attached, as reactors. Especially at low flow rates, the internal flow-through chamber acts as delay loop for providing a sufficient residence time. 

In the deformulation of PE/additive systems by mass spectrometry, much less fragmentation was observed with DCI-MS/MS using ammonia as a reagent gas, than with FAB-MS [69]. FAB did not detect all the additives in the extracts. The softness and the lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Applications of hyphenated FAB-MS techniques are described elsewhere low-flow LC-MS (Section 7.3.3.2) and CE-MS (Section 7.3.6.1) for polar nonvolatile organics, and TLC-MS (Section 7.3.5.4). 

Positive displacement, reciprocating, pumps are normally used where a high head is required at a low flow-rate. Holland and Chapman (1966) review the various types of positive displacement pumps available and discuss their applications. 

Single or multifilament wire with a matrix of Cu or CuNi is commonly available in several diameters. Single-filament wire is usually reserved for low current applications. Multifilament wire has a higher critical current, since supercurrents flow only in the surfaces of superconductors, and in multifilamentary wire there is more superconducting surface. 

The successful application of HPLC was made possible largely by (a) the development of pump systems that can provide constant flow rates at high pressure and (b) the identification of suitable pressure-resistant chromatographic media. Traditional soft gel media utilized in low-pressure applications are totally unsuited to high-pressure systems due to their compressibility. 

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