Minimum velocity

Fluid Temperature, °C Minimum velocity, m/s Pumping rate factor Pressure drop factor Outside tubes Inside tubes... 

The minimum velocity requited to maintain fully developed turbulent flow, assumed to occur at Reynolds number (R ) of 8000, is inside a 16-mm inner diameter tube. The physical property contribution to the heat-transfer coefficient inside and outside the tubes are based on the following correlations (39) ... 

Fluidized Beds When gas or liquid flows upward through a vertically unconstrained bed of particles, there is a minimum fluid velocity at which the particles will begin to move. Above this minimum velocity, the bed is said to be fluidized. Fluidized beds are widely used, in part because of their excellent mixing and heat and mass transfer characteristics. See Sec. 17 of this Handbook for detailed information. 

In the case of particulate fouling, one of the more common types, insuring a sufficient flow velocity and minimizing areas of lower velocities and stagnant flows to help keep particles in suspension is the most common means of deahng with the problem. For water, the recommended tubeside minimum velocity is about 0.9 to 1.0 m/s. This may not always be possible for moderate to high-viscosity fluids where the resulting pressure drop can be prohibitive. 

PR valve risers in flammable service should also be sized such that exit velocities are at least 30 m/s under all foreseable contingencies (except fire) which would cause the valve to release. On the basis of experimental work and plant experience, this minimum velocity, in conjunction with the riser elevation requirements, has been shown to ensure effective dispersion. Entrainment of air and dilution result in a limited flammable zone, with a negligible probability of this zone reaching any equipment which could constimte an ignition source. 

The average face velocity at all openings into a gas cabinet should be at least 1 m s with a minimum velocity at any point of 0.76 m s h These... 

Class I The Class I BSC provides personnel and environmental protection, but no product protection. It is similar in air movement to a chemical fume cupboard, but has a HEPA filter (see Chapter 9) in the exhaust system to protect the environment (Fig. 10.94). In the Class 1 BSC, unfiltered room air is drawn across the work surface. Personnel protection is provided by this inward air velocity as long as a minimum velocity of 0.37 m s" is maintained through the front opening (see the discussion on fume cupboards in Section 10.2.3.3). In many cases Class I BSCs are used specifically to enclose equipment. 

Cjcncraily, as the gas velocity during the drying cycle decreases, the abiliiy of the desiccant to dehydrate the gas increases. At lower actual veiociilfs, drier effluent gases will be obtained. Consequently, it would seem desirable to operate at minimum velocities to fuUy use the desiccant. 

Minimum velocity through holes at weep point, ft/sec... 

The major limitation of velocity transducers is their sensitivity to mechanical and thermal damage. Normal use can cause a loss of calibration and, therefore, a strict recalibration program is required to prevent data errors. At a minimum, velocity transducers should be re-calibrated every six months. Even with periodic re-calibration, however, velocity transducers are prone to provide distorted data due to loss of calibration. 

The critical section of the circuit (Figure 5.3) is where there is no liquid refrigerant left to help move the oil, i.e. the evaporator outlet and the suction pipe back to the compressor. Entrainment velocities of 5-7 m/ s are required to ensure that oil droplets will be carried back by the dry refrigerant gas to the compressor. The principle of continuous fluid velocity means that the evaporator will be in a continuous circuit. This does not imply that it has to be one pipe, since many pipes may be arranged in parallel to get the required heat transfer surface, providing the minimum velocity criteria are met. 

Some small cooling circuits have reversing refrigerant flow (i.e. cooling/ heat pump) and may work at reduced gas flow for capacity control. Under such conditions it may not be possible to maintain the minimum velocity to carry oil back to the compressor, and it will settle in the circuit. Arrangements must be made to increase or reverse the gas flow periodically to move this oil. 

Even when a hood has an integral fan with guaranteed performance, the installer will check the face velocity. For some types of work, laws may require minimum velocities. A highly visible draft gauge should be installed on a hood to inform personnel about its operation. This is especially important where remote or very quiet fans are used. 

The effect of gas velocity on the bed pressure drop (-APbed) with a uniform distributor (Fopen = 1.68 %) in the beds with decreasing and increasing Ug is shown in Fig. 5. As can be seen, -APbed maintains almost a constant value rmtil the minimum velocity of full fluidization (Unur) and then it decreases with decreasing Ug. As shown, Umfd is the maximum velocity of full defluidization, Umpf is the minimum velocity of partial fluidization, and Umu is the minimum velocity of full fluidization [6]. 

Figure 3.9. Molecules only react ifthey have sufficient energy to overcome the energy barrier AE. The height of the barrier can be transformed into a minimum velocity Umir,-...

The second experiment covers the velocity range between the minimum measurable velocity and the minimum velocity detected by the first experiment. It is distorted because of back-folding of high velocities into the spectrum, but it shows the small velocities properly. All voxels from the first map that show zero velocity can now be compared with the corresponding (undistorted) voxels of the second map. If those show a finite velocity, the value can be transferred to the first map. 

The minimum air velocity is set by the particle size. A flow rate of 1,000 lb / hr ft2 is adequate for 420-micron particles. This will be used.34 The minimum velocity is used since it gives the smallest dryer. 

The minimum velocity is the air velocity that must be maintained to prevent the solids from dropping out onto the sides of the pipe. 

The transition to annular flow is defined as the minimum velocity of the gaseous phase needed to drag the droplets with the maximum admissible dimensions under specific conditions, curve E,... 

When evaluating a material for the purpose of establishing dense-phase and long-distance suitability, it is important to undertake all the necessary tests (e.g., particle sizing, particle and bulk densities, fluidization and deaeration). Also, if possible, it is useful to compare such results with those obtained on previously conveyed similar materials (e.g., fly ash). However, it should be noted that such an evaluation only is a qualitative one and it is not possible to predict say, minimum air flows or pipeline pressure drop based on such data (i.e., pilot-scale tests normally are required to confirm minimum velocities, friction factors, etc., especially over long distances and for large-diameter pipes). 

Calculate the minimum velocity at which spherical particles of density 1600 kg/m3 and of diameter 1.5 mm will be fluidised by water in a tube of diameter 10 mm on the assumption that the Carman-Kozeny equation is applicable. Discuss the uncertainties in this calculation. Repeat the calculation using the Ergun equation and explain the differences in the results obtained. 

Relative to specimens examined under the microscope, the a or fast axis corresponds to the direction of the minimum refractive index, the minimum dielectric constant, and the maximum velocity. The y or slow axis corresponds to the maximum refractive index, the maximum dielectric constant, and the minimum velocity. Occasionally, a (3 axis is recognized with intermediate properties between a and y. When working with elongated bireffingent structures, birefringence usually is taken as positive when the y axis is parallel to the longitudinal axis. 

Empirical equations are also available to calculate the pressure drop for slurries flowing through pipelines [Condolios and Chapus (1963b)]. Durand and Condolios (1955) found the following equation to fit the experimental data for sand-water mixtures flowing above the minimum velocity in horizontal pipes ... 

Therefore, the minimum velocity, when z = 4m, is 1.38m/s and the corresponding value of the Reynolds number is... 

Cochet (C3), in measurements with particles in the size range of 0.1-1 micron, found a minimum mobility and migration velocity at a particle diameter of micron. The following are some of his results for the minimum velocity... 

At values of a Froude group of less than unity, particulate fluidisation normally occurs and, at higher values, aggregative fluidisation takes place. Much lower values of the Froude number are encountered with liquids because the minimum velocity required to produce fluidisation is less. A theoretical justification for using the Froude group as a means of distinguishing between particulate and aggregative fluidisation has been provided by Jackson and Murray . 

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