Tunnel cap trays

On the other hand, bubble caps (or even the more ancient tunnel cap trays) are different, in that they do not depend on the vapor flow to retain the liquid level on the tray deck. More on this later. For now, just recall that we are dealing only with perforated tray decks. 

There really is no proper answer to this question. It is quite likely that the archaic, massively thick, bolted-up, cast-iron bubble-cap or tunnel-cap tray was the best tray ever built. However, compared to a modern valve tray, bubble-cap trays... 

Tunnel-cap tray Perforated Perro-Ring Plates Braided metal... 

Reduce the number of wash trays in the vacuum tower. In one installation, two bubble-cap trays replaced four tunnel-cap trays. The flash-zone pressure was therefore reduced. The net effect was beneficial in that the lower pressure drop was more useful than the loss in fractionation represented by the two trays removed. 

Many modifications of the three types of contactors just discussed have been developed in an effort to reduce costs, reduce pressure drop, equalize vapor flow through each contactor, increase plate efficiencies, or, in general, improve the operating performance of the tower. An example of this for modification of bubble-cap towers is the old Uniflux tray originally developed by Socony-Vacuum, which consisted of a series of interlocking S-shaped sections which were assembled in the form of tunnel caps with slot outlets on one side only. Segmental downcomers, similar to the downcomers in conventional bubble-cap columns, were provided. The vapors issued from the Uniflux caps in... 

Fig. 8. Distillation tray incorporating several hundred shallow, tunnel-type caps...

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