= 0.0756 m/s or about 80 percent of flood. The proposed column is entirely adequate for the service required.

System Limit (Ultimate Capacity) This limit is discussed later under "System Limit."

Downcomer Backup Flooding Aerated liquid backs up in the downcomer because of tray pressure drop, liquid height on the tray, and frictional losses in the downcomer apron (Fig. 14-32). All these increase with increasing liquid rate. Tray pressure drop also increases as the gas rate rises. When the backup of aerated liquid exceeds the tray spacing, liquid accumulates on the tray above, causing down-comer backup flooding.

Area Under Downcomer
FIG. 14-32 Pressure-drop contributions for trays. hd = pressure drop through cap or sieve, equivalent height of tray liquid; hw = height of weir; how = weir crest; hhg = hydraulic gradient; hda = loss under downcomer.

Downcomer backup is calculated from the pressure balance hdc = ht + hw + hoU > + hda + hhg (14-92)

where hdc = clear liquid height in downcomer, mm liquid ht = total pressure drop across the tray, mm liquid hw = height of weir at tray outlet, mm liquid how = height of crest over weir, mm liquid hda = head loss due to liquid flow under downcomer apron, mm liquid hhg = liquid gradient across tray, mm liquid

The heights of head losses in Eq. (14-92) should be in consistent units, e.g., millimeters or inches of liquid under operating conditions on the tray.

As noted, hdc is calculated in terms of equivalent clear liquid. Actually, the liquid in the downcomer is aerated and actual backup is hdc = (14-93)

(dc where (dc is an average relative froth density (ratio of froth density to liquid density) in the downcomer. Design must not permit h'dc to exceed the value of tray spacing plus weir height; otherwise, flooding can be precipitated.

The value of (dc depends upon the tendency for gas and liquid to disengage (froth to collapse) in the downcomer. For cases favoring rapid bubble rise (low gas density, low liquid viscosity, low system foamability) collapse is rapid, and fairly clear liquid fills the bottom of the down-comer (Fig. 14-17). For such cases, it is usual practice to employ a higher value of (dc. For cases favoring slow bubble rise (high gas density, high liquid viscosity, high system foamability), lower values of (dc should be used. As the critical point is approached in high-pressure distillations and absorptions, special precautions with downcomer sizing are mandatory. Table 14-6 lists values of (dc commonly used by the industry.

Downcomer Choke Flooding This is also called downcomer entrance flood or downcomer velocity flood. A downcomer must be sufficiently large to transport all the liquid downflow. Excessive friction losses in the downcomer entrance, and/or excessive flow rate of gas venting from the downcomer in counterflow, will impede liquid downflow, initiating liquid accumulation (termed downcomer choke flooding) on the tray above. The prime design parameter is the downcomer top area. Further down the downcomer, gas disengages from the liquid and the volumes of aerated liquid downflow and vented gas

TABLE 14-6 Criteria for Downcomer Aeration Factors

Foaming tendency

Bolles' criterion

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