Downcomer inlet baffle

Arrangement b (often referred to as the false downcomer) is popular. It offers better liquid distribution than the others, does not suffer from hydraulic jump, and provides some flexibility in inlet nozzle orientation. The width of the false downcomer should be the same as the width at the bottom of a downcomer (307). For feed into center or off-center downcomers it is recommended to make the x -f- y dimension 12 in (307). If entrainment due to liquid splashing in the false downcomer is a concern, a horizontal baffle, with dimensions of about 2d by Wrf can be installed directly above the nozzle entry into the false downcomer. This baffle is installed at some clearance above the false downcomer. In most cases, there is no need for this baffle. 

A splash baffle is recommended when liquid flow rate is less than 0.1 gpm per inch of outlet weir (144). One successful application of tailor-designed sieve trays with splash baffles at liquid rates ranging from 0.01 to 0.07 gpm per inch of weir has been described (374). Splash baffles should only be used at low liquid loads, because they restrict the downcomer inlet area and can lead to premature downcomer choke. A calming zone upstream of the baffle is sometimes used to minimize this problem (374). Splash baffles also increase tray pressure drop and froth regime entrainment (31). 

Reverse flow Liquid flowing from the inlet on one side of the tray (around a center baffle) reverses its direction at the other side of the tray, and flows back to the downcomer on the same side of the tray where the inlet is. 

The Fractionation Research Inc. unique design for inlet downcomer baffle and outlet weir (160,164) converted a highly channeled liquid flow into plug flow, Despite that it did not improve efficiency. 

FIGURE 5.7-17 Possible downcomer arrangements (a) vertical beffle, no recess, no inlet weir (this type is quite common) ( >) vertical baffle, no recess, inlet weir (c) vertical baffle, recess, no inlet weir (d) sloped beffle, no recess, no inlet weir and (c) double crossflow, vertical haffles, no recerece, inlet weirs. 

For sloped downcomers, the critical liquid velocity is at the bottom, insofar as final disengagement of vapor is concerned. The total volume of the filled portion or lire downcomer can be used in estimating residence lime, For downcomers with bottom recesses, where the liquid must make an extra mm before entering the tray, the pressure lora under the downcomer may he estimated as twice that calculated from Eq. (5.7-30). This rule of thumb applies also to the case where an inlet weir is used io distribute the liquid after it has flowed under die downcomer baffle. 

It has been recommended (67, 68, 255) to avoid orientation of level-measurement nozzles by angles greater than 90° from a vapor inlet or reboiler return nozzle, and to refrain from positioning these nozzles under the bottom downcomers. If the angle exceeds 90°, a shielding baffle should be provided in front of the measurement nozzle. 

Splash baffles are used in low-liquid-load services. The baffle backs liquid up onto the tray and increases its liquid holdup and froth height (31, 83, 374). The baffle also helps to prevent the tray from drying up and promotes froth regime operation at low liquid loads (374). In small-diameter columns (< 2 ft) it also prevents liquid drops formed at the tray inlet from being flung directly into the downcomer. 

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