Droplet entrainment

Steam plume opacity and/or droplet entrainment possibly objectionable... 

Drift Loss or Windage Loss the amount of water lost from a tower as fine droplets entrained in the leaving air. For an atmospheric type tower this is usually 0.1-0.2% of the total water circulated. For mechanical draft towers it is usually less. 

Figure 5.3e shows the situation when the air velocity was increased to Ugs = 20 m/s. It is seen from this figure that the liquid bridges in churn flow disappeared and a liquid film formed at the side walls of the channel with a continuous gas core, in which a certain amount of liquid droplets existed. The pressure flucmations in this case became relatively weaker in comparison with the case of the churn flow. The flow pattern displayed in Fig. 5.3f indicates that as the air velocity became high enough, such as Ugs = 85 m/s, the liquid droplets entrained in the gas core disappeared and the flow became a pure annular flow. It is also observed from Fig. 5.3f that the flow fluctuation in this flow regime became weaker than that for the case shown in Fig. 5.3e, where Ugs = 20 m/s. 

With increasing superficial gas velocity the gas core with a thin liquid film was observed. The flow pattern, displayed in Fig. 5.14c (the second, third and fourth channels from the top), indicates that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. 

The diameter of hydrocarbon droplets entrained by the velocity of the water phase... 

Equation (5-68) is also determined empirically as shown in Figure 5.29. It can be seen that the mechanism of liquid droplet entrainment and deposition at CHF in annular flow is qualitatively validated by the data trend plotted in Figure 5.29. 

W-3 CHF correlation. The insight into CHF mechanism obtained from visual observations and from macroscopic analyses of the individual effect of p, G, and X revealed that the local p-G-X effects are coupled in affecting the flow pattern and thence the CHF. The system pressure determines the saturation temperature and its associated thermal properties. Coupled with local enthalpy, it provides the local subcooling for bubble condensation or the latent heat (Hfg) for bubble formation. The saturation properties (viscosity and surface tension) affect the bubble size, bubble buoyancy, and the local void fraction distribution in a flow pattern. The local enthalpy couples with mass flux at a certain pressure determines the void slip ratio and coolant mixing. They, in turn, affect the bubble-layer thickness in a low-enthalpy bubbly flow or the liquid droplet entrainment in a high-enthalpy annular flow. 

Studies of heat and mass transfer to films and film condensation are considered only insofar as the results throw light on the flow behavior a brief review of such studies has been published elsewhere (F6). In addition, annular gas/liquid flow in horizontal ducts will not be considered here, since this is usually complicated by droplet entrainment. 

Adomi et al. (Al), 1961 Experimental studies of upward cocurrent flow of argon and water film in tube 0.987 in. X 3.3 ft. gas velocities up to 86 ft./sec. Data on film thicknesses, entrance characteristics, droplets entrained in gas stream. 

Gill et al. (G3), 1963 Experimental study of upward cocurrent flow of air/water system. Data on pressure drop and film thicknesses (and effects of liquid droplet entrainment) as functions of distance from inlet. Effects of waves on film surface considered. 

Zhivalkin and Volgin (Z4a), 1904 Considers pressure drop in downward cocurrent flow of air and films of water, glycerol solutions, water + surfactant, in tubes of diameter 12.9 mm., lengths 150-830 mm. Air velocities 3-45 m./sec. Conditions for droplet entrainment from film also reported. 

Windage. Circulating water lost from the tower as droplets entrained in exhaust air stream. Typically, D = 0.1 to 0.3% of circulation rate (CR). 

C-factor C The C-factor, defined in Eq. (14-77), is the best gas load term for comparing capacities of systems of different physical properties. It has the same units as velocity (m/s or ft/s) and is directly related to droplet entrainment. As with the E-factor, the user should beware of any data for which the area basis is not clearly specified. 

Pinczewski and Fell [Trans. Inst. Chem Eng., 55, 46 (1977)] show that the velocity at which vapor jets onto the tray sets the droplet size, rather than the superficial tray velocity. The power/mass correlation predicts an average drop size close to that measured by Pinczewski and Fell. Combination of this prediction with the estimated fraction of the droplets entrained gave a relationship for entrainment, Eq. (14-202). The dependence of entrainment with the eighth power of velocity even approximates the observed velocity dependence, as flooding is approached. 

Specifying the need for a tray-type column, the type of tray must be determined. Sieve trays are considered most appropriate for this application. They offer a simple and inexpensive construction with low pressure drop (if the hydraulic design is adequate). Bubble cap and valve-type trays offer advantages in controlling liquid droplet entrainment, but pose significant difficulties for installation of cooling coils. 

Such a uniformization of droplet sizes is a phenomenon of interest for practical application. It suggests that, in gas-liquid processes, the amount of fine droplets entrained by gas flow may be reduced so that the processes may carry on more smoothly. 

Woodward, J. L. and A. Papadourakis. 1991. Modeling of Droplet Entrainment and Evaporation in a Dispersing Jet. Proceedings of the International Conference and Workshop on Modeling and Mitigating the Consequences of Accidental Releases of Hazardous Materials. New York American Institute of Chemical Engineers. 

These conditions are designed to give 50 mg H20(g) per Nm3 of dehydrated gas with low acid droplet entrainment. 

During the operation of the tower, water is lost by evaporation, water droplets entrained in the outgoing air, and in a water purge, called blowdown. To reduce carry-over of water droplets the air flows across drift eliminators. The water droplets impinge on the drift eliminators and then flows down to the bottom of the tower. The droplet water loss is about 0.2% of the incoming water [11]. After leaving the drift eliminators, air flows up and out of the tower. Evaporation of water into air transfers heat from the water to the air. Cooling the water requires about 1.0 % evaporation for every 5.56 C (10.0 "F) drop in the water tempera-ture[l 1]. To reduce scale formation in the tower because of dissolved calcium or... 

Drift is the water that is lost from the tower as fine droplets entrained in the exhaust air. For mechanical draft towers, the value is less than 0.2% of the circulating water, but for natural draft towers the drift ranges between 0.3-1.0%. 

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