Downcomer clearance

Referring to Fig. 3.5, note that the downcomer B is flooding. The cause is loss of the downcomer seal. The height of the outlet weir is below the bottom edge of the downcomer from the tray above. This permits vapor to flow up downcomer B. The upflowing vapor displaces the downflowing liquid. That is, the vapor pushes the liquid up onto the tray above—which is a cause of flooding. On the other hand. Fig. 3.6 

On the other hand, all trays in a tower below downcomer B will lose liquid levels and dry out when flooding starts in downcomer B. Thus, the following rules apply  

To guarantee a proper downcomer seal, the bottom edge of a downcomer should be about 0.5 in below the top edge of the outlet weir. This dimension should be carefully checked by process personnel when a tower is opened for inspection. It is quite easy for sloppy tray installation to distort this critical factor. 

As the liquid level on a tray increases, the height of liquid in the downcomer feeding this tray will increase by the same amount. Again, excessive downcomer liquid or froth levels result in flooding and loss of tray efficiency. 

The weir height on many trays is adjustable. We usually adjust the weir height to between 2 and 3 in. This produces a reasonable depth of liquid on the tray, to promote good vapor-liquid contact. 

The crest height is similar to the height of water overflowing a dam. It is calculated from 

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It is important to set close tolerances on the weir height, downcomer clearance, and plate flatness, to ensure an even flow of liquid across the plate. The tolerances specified will depend on the dimensions of the plate but will typically be about 3 mm. 

Figure 1.6 Flooding caused by inadequate downcomer clearance.

If the downcomer clearance—which means the distance between the bottom edge of the downcomer and the tray below—is too great, the downcomer becomes unsealed. Vapor flows up the downcomer, and the trays above flood. 

If the downcomer clearance is too small, then liquid backs up in the downcomer, and the trays above flood. To calculate the height of liquid in the downcomer, due to liquid flowing through the downcomer clearance ... 

The term A should be taken as the most restrictive area for liquid flow in the downcomer outlet. Usually, this is the area under the downcomer apron (i.e., the downcomer clearance times the length of the segmental downcomer), but not always. For instance, if an inlet weir is used and the area between the segmental downcomer and the inlet weir is smaller than the area under the downcomer apron, the smaller area should be used. 

Downcomer clearance, in 4.0 Alpha or K Liquid fraction light key 4.050 0.10000... 

Downcomer clearance. A high clearance increases downcomer capacity at the expense of increasing the tendency of the downcomer to pass vapor. A common design practice is to set the clearance to 0.25 to 0.5 in. (6-13 mm) less than the weir height. 

The other two parameters, small clearance under the downcomer and small downcomer top area, have little effect on entrainment flooding, as they are associated with the downcomer only. Downcomer clearance affects downcomer backup, but not downcomer liquid velocity, while downcomer area affects the velocity, but has little effect on downcomer backup. 

The froth height in the center downcomers in the bottom section is only slightly above 30 percent, and increasing the downcomer clearance will suffice to overcome the problem. However, this is unlikely to suffice for the side downcomers in the bottom section. In this example, idle clearance under the downcomer will be increased to 2.0 in in the center downcomers, and to 2.25 in in the side downcomers. The weir height on idle cenler-to-side flow trays in the bottom section will be lowered to 1.5 in to lower tray pressure drop. 

Although there is no need to change the downcomer clearance in the top section, it is a good idea. The froth in the downcomer bottlenecks the top section, the head loss in the downcomer apron is quite high (1.26 in) and the tray spacing relatively low. The clearance in idle top section will therefore also be increased to 2 in. 

Note that in the top trays the weir height is now equal to the clears ance under the downcomer (both are 2 in). In the bottom section, the outlet weirs are shorter than the clearances under the downcomers. This may raise concerns about having adequate ssal on the trays. However, the practice of using outlet weirs shorter than the downcomer clearances is usually edequate for high liquid loads (1), such... 

Seal check. In Sec. 6.5.7, it was decided to go to clearances under the downcomers that exceed the outlet weir heights. For such designs, it has been recommended (1) that the clear liquid height in the downcomer under turndown conditions exceeds the downcomer clearance by at least 2 in. This will be checked here. 

Downcomer seal at turned down conditions (downcomer backup less downcomer clearance, in clear liquid) 3,8 3.0 3,1... 

Select y4-in (6.4-mm) holes and a 1-in (2.54-mm) high weir, with a %-in downcomer clearance. 

Distillate molar rate or subscript for distillate Downcomer clearance, ins. 

For sloped downcomers adjust W (weir length) to the correct value for the downcomer bottom edge. Downcomer clearance, //dcc, is typically a minimum of lin. (25.4 mm) and is usually 0.5 in. (12.7 mm) less than the outlet weir height to maintain a positive seal on the downcomer. Calculate the velocity under the downcomer ... 

Liquid head loss through downcomer clearance (Equation 14.14),... 

Liquid head loss through downcomer clearance... 

Tray type Weir height Weir length Downcomer clearance Tray spacing Foaming factor Fraction of flood Surface tension Liquid viscosity... 

Sieve trays will be used with 60 cm spacing, 6 cm weir height, 0.6 cm hole diameter, 0.25 cm tray thickness, 5 cm downcomer clearance, and hole area 10% of the total tray area. The foaming factor is 0.80 and the froth density in the downcomer is 0.5. The target fraction of flood velocity is 0.70. 

Downcomer sealing. While pressure drop and fouling considerations set the minimum values for downcomer clearance, sealing considerations set the maximum value for downcomer clearance. If the service... 

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