Spray height

For sieve trays, a spray height of 15 inches is obtained when the jetting factor is 6-7. 

For a 15-inch spray height, a tray spacing of at least 21 inches is recommended. 

At lower tray spacing, entrainment flooding may be related to lifting of the froth envelope and to froth rather than spray height. This correlation must not be extended to lower tray spacing. 

A rather similar outfit with three 6-foot sections of boom is used in southern nurseries, with a low-pressure manifold, and using flat-spray pattern, 80 Teejet nozzles No. 8001 or 80015 manufactured by Spraying Systems Co., Bellwood, 111. These are spaced 20 inches apart on the boom at a spraying height of 17 to 19 inches. Pressures used are usually 50 to 60 pounds. 

Figure 1.8 is a realistic picture as to what we would see if our towers were made of glass. In addition to the downcomers and tray decks containing froth or foam, there is a quantity of spray, or entrained liquid, lifted above the froth level on the tray deck. The force that generates this entrainment is the flow of vapor through the tower. The spray height of this entrained liquid is a function of two factors ... 

High vapor velocities, combined with high foam levels, will cause the spray height to hit the underside of the tray above. This causes mixing of the liquid from a lower tray, with the liquid on the upper tray. This backmixing of liquid reduces the separation, or tray efficiency, of a distillation tower. 

The spray height, or entrainment, between the trays, increases... 

When the spray height from the lower trays, impacts the upper trays, the heavier, butane-rich liquid contaminates the lighter liquid on the upper trays, with heavier butane... 

The absolute tower pressure (in psia) increased by 17 percent, and hence the volume (as well as the velocity of vapor through the valve tray caps) declined by 17 percent. The reduced vapor velocity reduced the dry tray pressure drop, thus reducing both the spray height above the tray deck and the liquid backup in the downcomers. 

Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

The term incipient flood is that point in a trayed tower s operation when the spray height of liquid from the tray below begins to impinge on the tray above to the extent that entrainment reduces fractionation efficiency. Incipient flood in a packed column is that point in the column s operation at which liquid hold-up increases to an extent that reduces fractionation efficiency. 

With respect to the wave loads, it is seen that a substantial reduction of the horizontal and uplift forces is achieved, which would result in a reduction of about 50% of the required weight of the superstructure to ensure sliding stability. With respect to the hydraulic performance, it is seen that wave reflection is reduced by about 25%, and as a result of the reduction of wave overtopping the required crest level above still water level is reduced by about 40%. Even without using any splash reducer at the front and back wall of the concrete superstructure, the splash/spray heights are reduced by half. Further details on the results in Table 12.1 are given by Oumeraci and Muttray, Muttray et Oumeraci et Takahashi et Schiittrumpf et Muttray et al. and Oumeraci et al ... 

Therefore, one of the most challenging tasks to cope with salt spray consists of the development of innovative shapes of the structure crest to substantially reduce the splash/spray height induced by the breaking waves at the structure. For this purpose, special tests were performed to analyze the effectiveness of various alternatives to reduce splash/spray height. 

Fig. 12.16. Efficiency of various alternatives for the reduction of splash and spray height.

The spray height of this entrained liquid is a function of two factors ... 

When the vapor flow through a tray increases, the height of froth in the downcomer draining the tray will also increase. This does not affect the foam height on the tray deck until the downcomer fills with foam. Then a further increase in vapor flow causes a noticeable increase in the foam height of the tray deck, which then increases the spray height... 

When the spray height from the tray below hits the tray above, this is called the incipient flood point, or the initiation of jet flooding. 

Note, though, that jet flood may be caused by excessive downcomer backup. It is simple to see in a glass column separating colored water from clear methanol how tray separation efficiency is reduced as soon as the spray height equals the tray spacing. And while this observation of the onset of incipient flood is straightforward in a transparent tower, how do we observe the incipient flooding point in a commercial distillation tower ... 

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