Re-entrainment

H. W. Spencer, 111, and G. B. Nichols, "A Study of Rappiug Re-entrainment ia a Large Pilot Electrostatic Precipitator," Preprint 76-42.5, 68th MPCA. MnnualMeeting Portland, Oregon, June 27—July 1, 1976. 

Replacement in kind - A replacement that satisfies design specifications, resuspension - Re-entrainment of particulate into a wind field for dispersion. 

Re-entrainment The induction of air that has been discharged back into a jet, at a position upstream from the discharge. 

Ni has been found to be about 5 turns of the gas stream in the unit, and is considered somewhat conservative. When re-entrainment takes place, N, may drop to 1.0 or 2.0. The API study presents an excellent survey of cyclone dust collectors [7]. 

The unit shown in Figure 4-49 has been used in many process applications with a variety of modifications [18,19,20]. It is effective in liquid entrainment separation, but is not recommended for solid particles due to the arrangement of the bottom and outlet. The flat bottom plate serves as a protection to the developing liquid surface below. This prevents re-entrainment. In place of the plate a vortex breaker type using vertical cross plates of 4-inch to 12-inch depth also is used, (Also see Reference [58].) The inlet gas connection is placed above the outlet dip pipe by maintaining dimension of only a few inches at point 4. In this type unit some liquid will creep up the walls as the inlet velocity increases. 

If the design of Figure 4-41 is used for liquid-vapor separation at moderately high liquid loads, the liquid sliding down the walls in sheets and ripples has somewhat of a tendency to be torn off from the rotating liquid and become re-entrained in the upward gas mov ement. 

Wetting—of particle to help agglomerate and prevent re-entrainment. 

Once foam or froth in the downcomer backs up to the tray above, it tends to be re-entrained in the overflowing liquid, making it apparently lighter, and accentuating this height of liquid-foam mixture in the downcomer. The downcomer must be adequate to separate and disengage this mixture, allowing clear liquid (fairly free of bubbles) to flow under the downcomer seal. 

Unless beams are present at the entry the stubs should be close to the stmctural ceiling. Beam fairing may be used where the void has to be shallow and this can reduce the risk of re-entrainment due to local high-velocity pressure. Zone barriers are fitted within the void where separate rooms are to be fed. The ceiling need not cover the whole room. 

Since much of the vented material will be liquid, separators such as knockout pots or tangential entry separators can provide disengagement and possible recovery. Figure 5 is a typical vapor-liquid separator design found to be effective for these applications. Inlet design superficial vapor velocity is about 100 ft/sec, with sufficient volume provided to accumulate the entire reactor liquid contents. The lip on the outlet vapor line and the horizontal plate to separate the accumulated liquid are important features to prevent re-entrainment. 

It is clear that both atmospheric and surface dusts are complex materials and not all that easy to describe. A summary is given in Fig. 2 of the sources of atmospheric and surface dusts and their inter-connection. Both natural and anthropogenic sources contribute to both dusts. The inter-connection between the two dusts is wet and dry deposition from the atmosphere to the ground, and the re-entrainment of surface dust through wind and human activity into the atmosphere. Dust is an important global component of our earth, and impinges on the wellbeing of people. 

Inertial sampling methods Impactors impingers 0.2-20 0.2-20 Number of data points to construct distribution, re-entrainment 50, 51... 

Golombek, D. A Cardinali, D. P. (1993). Melatonin accelerates re-entrainment after phase advance of the light-dark cycle in Syrian hamsters. Antagonism by flumazenil. Chronobiol. Int. 10, 435-41. 

The velocity needed to re-entrain deposited particulates (V ) may need to be determined for particular applications. The velocity Vfup is being found (Zenz, 1964 Cabrejos and Klinzing,... 

In specifying settling chamber dimensions, gas flow velocities must be maintained below the re-entrainment velocity (pick-up velocity) of deposited particulate. As a general guideline, linear gas velocities are kept below 10 ft/sec (600 ft/min = 304.8 cm/sec). This is satisfactory for most materials however, some low-density particulates are re-entrained at lower velocities. 

The efficiency of settling chambers increases with residence time of the waste gas in the chamber. Because of this, settling chambers are often operated at the lowest possible gas velocities. In reality, the gas velocity must be low enough to prevent dust from becoming re-entrained, but not so low that the chamber becomes unreasonably large. The size of the unit is generally driven by the desired gas velocity within the unit, which should be less than 3 m/s (10 ft/sec), and preferably less than 0.3 m/s (1 ft/sec). 

In spite of its low vapor pressure, endrin has been found to volatilize significantly (20-30%) from soils within days after application (Nash 1983). In air, endrin will be primarily absorbed to particulates which may be re-entrained to soil or surface water via wet or dry deposition. Laboratory studies have indicated that a predominant mechanism for the transformation and degradation of endrin in air under field conditions is via photochemical reactions and rearrangements to yield primarily endrin ketone, with minor... 

Various modifications may be made to improve the operation of the cyclone separator in special cases. If there is a large proportion of fine material present, a bag filter may be attached to the clean gas outlet. Alternatively, the smaller particles may be removed by means of a spray of water which is injected into the separator. In some cases, the removal of the solid material is facilitated by running a stream of water down the walls and this also reduces the risk of the particles becoming re-entrained in the gas stream. The main difficulty lies in wetting the particles with the liquid. 

This equation shows that for a given collection efficiency, the precipitator size is inversely proportional to particle drift velocity and directly proportional to gas flow rate. Increasing the gas density (migration velocity is a function of gas viscosity) by reducing its temperature or increasing the pressure will reduce the precipitator size. However, theory does not account for gas velocity. This is a variable that influences particle re-entrainment and the drift velocity. This typically requires an ESP design at lower velocities than predicted in theory. 

Evidence for the involvement of ocular photoreception came from the characterization of the norpA mutation in flies, which renders them eyeless and devoid of ocellar function. Flies harbouring this mutation take a longer time to re-entrain their activity rhythms to a shifted LD cycle (Helfrich-Forster et al 2001). However, the observation that they still entrain suggested involvement of a second system in circadian photoperception. 

A second system contributing to light entrainment was elucidated by the identification of cryptochrome, a clock component. Characterization of a fly mutant, showed that they also take a longer time to re-entrain their... 

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