Pressure drop chimney trays

Pipe downcomers from chimney tray too small for flow. Poor separation. Slightly higher pressure drop. Design error. 

The pressure drop of the vapor flowing through the chimneys. The liquid on the tray has to develop enough liquid head to flow against the higher pressure below the tray. 

The narrow-trough vapor distributor shown in Fig. 7.4 is intended to disperse the vapor evenly across the bottom of the packed bed. The width of the chimney does not exceed 6 in. The older-style chimney trays, which may have had a few large round or square chimneys, reduced the separation efficiency of the packing. To work properly, the vapor distributor has to have a reasonable pressure drop, in comparison to the pressure drop of the packed bed. For example, if the expected pressure drop of a 12-ft packed bed is 10 in of liquid, the pressure drop of the vapor distributor ought to be about 3 to 4 in of liquid. 

The main drawback of chimney trays is that they consume more column height than alternative drawoff devices, resulting in a more expensive arrangement. Chimney trays are also relatively high-pressure-drop devices, which is a major disadvantage in packed columns operating in deep vacuum. 

The number of risers must be large enough to ensure good vapor distribution. This is essential in packed towers, especially when the chimney tray delivers vapor to a short or a low-pressure-drop bed. 

Chimney trays equipped with rectangular risers that stretch from one end of the tray to another often use open-top gutters ("rainwater conduits ) rather than hats (see Fig. 8.66 later). The gutters can be V-shaped or U-shaped, are mounted above the risers, and slope toward the short edges of the rectangles. Compared to hats, these reduce riser pressure drop and eliminate the downward velocity component of vapor leaving the chimneys, but at the expense of allowing some liquid to rain into the risers. This makes them less suitable for total drawoffs. 

The author experienced one troublesome case, which was also reported by Lieberman (237), where liquid overflow through the chimneys caused a severe loss of efficiency in the packed section above. The chimney tray had undersized downpipes that were not liquid-sealed either the undersizing or the lack of seal (or both) could have caused the overflow. Lieberman (237) suggests that the overflow led to entrainment and flooding, hence the loss in efficiency. However, subsequent pressure-drop measurements and other observations provided no supporting evidence for the existence of flooding, and the author believes that vapor maldistribution due to liquid overflow (guideline 14 above) caused the loss in efficiency. 

The chevron collector features a high open area for vapor flow, and is therefore a low-pressure-drop device. Pressure drop is usually less than 0.1 in of water (74). This type of collector is most suitable for vacuum applications. This collector also acts as a vapor distributor accordingly, the distance required between the top of this collector and the support plate above is less than 12 in (74). The short vertical distance requirement of this device compared to the chimney tray often makes it less expensive than the chimney tray. 

In bubble cap trays weeping of liquid is prevented by design. A chimney with a cap covers the openings in the plate. Bubble caps typically have a diameter of 50-80 mm. The relative free area is about 10-20%. The pressure drop of bubble cap trays is rather high. 

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