Control velocity

Flow rate sufficient to provide a working in-draft velocity through all openings (i.e., control velocity). 

Lastly, for a recommended control velocity of 0.5-1.0 m s through the total area of the openings, A, the flow rate is given by... 

The original design was based on a control velocity recommended by dust control design manuals. The original design of 28 m s exhaust flow induced an inward velocity of about 0.5 m s" through the enclosure entrance and trolley shots. This was not sufficient to overcome plume trajectories aimed outward, or to overcome the effect of moderate wind levels. 

A design based on the enclosure open-area control velocity does not consider all the other variables listed as affecting dust control. Calculation procedures to predict many of the other variables would be very complicated if not impossible to perform. Physical modeling of the problem and solution w as therefore used as the basic design tool. 

Air terminal units (ATUs), item at the end of a duct run to control velocity, pressure, flow rate, and/or temperature,... 

Velocity effects can be achieved either by having the test-piece move through a presumably stationary liquid or by having a moving liquid come into contact with a stationary test-piece. Occasionally tests may involve both types of exposure. Details of test procedures are given in NACE TM 0270-70 Method of Conducting Controlled Velocity Laboratory Corrosion Tests. 

Method of conducting controlled velocity laboratory corrosion tests Dynamic corrosion testing of metals in high temperature water... 

In an actual Mdssbauer transmission experiment, the radioactive source is periodically moved with controlled velocities, +u toward and —d away from the absorber (cf. Fig. 2.6). The motion modulates the energy of the y-photons arriving at the absorber because of the Doppler effect Ey = Eq + d/c). Alternatively, the sample may be moved with the source remaining fixed. The transmitted y-rays are detected with a y-counter and recorded as a function of the Doppler velocity, which yields the Mdssbauer spectrum, r(u). The amount of resonant nuclear y-absorption is determined by the overlap of the shifted emission line and the absorption line, such that greater overlap yields less transmission maximum resonance occurs at complete overlap of emission and absorption lines. 

Figure 3-6 Standard utility laboratory hood. Airflow patterns and control velocity are dependent on sash height. Source N. Irving Sax, Dangerous Properties of Industrial Materials, 4th ed. (New York Van Nostrand Reinhold, 1975), p. 74. Reprinted by permission of John Wiley Sons, Inc.

Consider the simple box-type enclosed hood shown in Figure 3-8. The design strategy is to provide a fixed velocity of air at the opening of the hood. This face or control velocity (referring to the face of the hood) ensures that contaminants do not exit from the hood. 

The required control velocity depends on the toxicity of the material, the depth of the hood, and the evolution rate of the contaminant. Shallower hoods need higher control velocities to prevent contaminants from exiting the front. However, experience has shown that higher velocities can lead to the formation of a turbulent eddy from the bottom of the sash backflow of contaminated air is possible. For general operation a control velocity between 80 and 120 feet per minute (fpm) is suggested. 

For a hood with an open area of 50 ft2, using Equation 3-25 and assuming a required control velocity of 100 fpm, we get... 

A laboratory hood has an opening 4 ft in length by 3 ft in height. The hood depth is 18 in. This hood will be used for an operation involving trichloroethylene (TCE) (TLV-TWA 50 ppm). The TCE will be used in liquid form at room temperature. Determine an appropriate control velocity for this hood, and calculate the total air flow rate. 

It is desired to operate the hood of Problem 3-29 so that the vapor concentration in the hood plenum is below the lower explosion limit of 12.5% by volume. Estimate the minimum control velocity required to achieve this objective. The amount of TCE evaporated within the hood is 5.3 lb per hour. The molecular weight of TCE is 131.4. The temperature is 70°F and the pressure is 1 atm. 

Material may be separated by means of an elutriator, which consists of a vertical tube up which fluid is passed at a controlled velocity. The particles are introduced, often through... 

An improvement of this method involves the injection of a sharp band of gas onto the column and elution at a controlled velocity. When the band is about halfway down the column, the flow is stopped and band spreading occurs by diffusion only. 

Voltammetry experiments are not often performed in flow cells for analytical purposes. One reason for this is the special problem of ohmic potential losses (iR drops) at an electrode in a confined stream. Another reason is the problem of precisely pumping solution at a carefully controlled velocity. In general, rotating electrodes are more easily controlled and do not involve serious plumbing problems. On the other hand, flow cells operated at a fixed potential (i.e., at one point along the steady-state voltammetric curve) are eminently useful for electrosynthesis, chromatographic detection, and automated analysis systems. These features will be described in later chapters. 

The transient gas-particle dynamics of the earlier prototypes were found to deliver microparticles with a range of velocities and a nonuniform spatial distribution. For targeted delivery, however, especially in the area of gene and peptide delivery, the system should deliver particles with a narrow and controllable velocity range and a uniform spatial distribution. This was achieved with a certain embodiment called the contoured shock tube configured to achieve uniform particle impact conditions by entraining particles within a quasi-steady gas flow (Kendall et al. 2002). 

Figure 3 Rates of formation of 4-OH-triazolam from triazolam (250 pM) by human liver microsomes in vitro. Each point is the mean ( SE) of four microsomal preparations. Reaction velocities when preparations were preincubated with the macrolide agents are expressed as a percentage of the control velocity with no inhibitor present (inhibitor = 0). Mean IC50 were TAO, 3.3 pM erythromycin, 27.3 pM clarithromycin, 25.2 pM azithromycin, >250 pM. Abbreviations IC50, 50% inhibitory concentrations TAO, troleandomycin. Source Adapted, in part, from Ref. 77.

Guidelines for conducting controlled velocity laboratory corrosion experiments. NACE Standard TM-0270-72. 

Coupled controlled velocity, magnetic resonance imaging (MRI)/rheology measurements of thixotropic and yielding colloidal suspensions further demonstrate the importance of paired measurements [63], Shear rate profiles obtained in laminar tube flow for both Newtonian and non-Newtonian fluids from MRI... 

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