Pressure drop, trays downcomer apron

Zda clear liquid height for pressure drop through downcomer apron in. of liquid /Zf froth height on tray in. of froth... 

The clear liquid back-up is obtained from a tray-pressure balance and is normally taken to be the sum of the tray-pressure-drop, the clear liquid height on the active area of the tray, and the pressure-drop of liquid flowing under the downcomer apron onto the active area. 

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 downcomer backup flooding. 

Downcomer backup. The factors that resist liquid flow from the downcomer onto the tray below are the froth height on the tray, the pressure drop on the tray, and the friction loss under the downcomer apron. These factors cause liquid to back up in the downcomer. Each of these factors can be expressed in terms of clear liquid heads. A tray pressure balance gives... 

D3. The calculations in Example 10-3 were done for conditions at the top of the column. Physical properties will vary throughout the column, but columns are normally constructed with identical trays, downcomers, weirs, etc., on every stage (this is sinpler and cheaper). For a 12-foot diameter column, calculate entrainment, pressure drops, downcomer residence time, and weeping at the bottom of the column. The results of Problem 10.D2 are required. If the column will not operate, will it work if the gap between the tray and downcomer apron is increased to 1.5 inches ... 

Clearly, downcomer backup can be estimated if we know the clear liquid height H. However, it is not straightforward in predicting H. As consists of three components, namely, clear liquid height h, tray pressure drop /r and downcomer apron pressure drop h, let us look into each individual components one at a time. 

Downcomer flood is controlled by a combination of tray pressure drop, weir loading, and downcomer apron. Remember that it is the liquid froth level instead of clear liquid height that determines the downcomer flood. This fact is clearly stated in equation (12.24). To prevent downcomer flood in the design, downcomer size must be provided generously. From equations (12.40), (12.42), and (12.44), feed rate and reboiling duty are the major operating parameters to avoid downcomer flood. 

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