This is the oldest type of tray which act as equilibrium tray plate in the column called as bubble cap tray. It’s nothing but the perforated tray consisting riser above the holes. It consists special passage to flow vapours above through the liquid holding above the plates. Mass transfer occurs exactly above the holes which improves the efficiency of the operation. One or more down comer area can be provided on each tray for betterment of the process.
Bubble cap trays are most efficient for the low vapour and liquid pressure in the column. Weir height helps to trap the liquid and vapours on the trays. Bubble cap trays can be a best choice in the high turndown ratio.
Only disadvantages of the bubble cap trays are it creates higher pressure drop across vapour side.
Where these trays are costlier as well.
Sieve Tray-
This is a simply a plain plate with the perforations. Punched holes helps to the effective mass
transfer throughout the operation. While its direction of punches which is downward enhances the efficiency and performance of the trays. Normal size of the holes are ranges from 0.18 to 1 inch in diameter where standard size will be 0.5 inch. Number of holes or punches and its respective sizes are depending upon the vapour flow rate in tower.
Compared to valve tray, sieve trays have higher entrainment at same fluid velocity. Sieve trays are inexpensive as they are easy to fabricate and simple in structure.
Only disadvantage of the sieve trays is minimum hole velocity. It should be maintained to manage the liquid head above the plates. It offers generally lower turndown ratio.
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