Sieve/valve trays, diameter

Fair [183] relates sieve trays and includes valve tray remarks to the extensive work done for bubble cap trays. Figure 8-137 and 8-139 show flooding data for 24-in. spacing of bubble cap trays from [81] and represents data well for 36-in. diameter columns, and is conservative for smaller columns. Fair s work has been corrected to 20 dynes/ cm surface tension by ... 

Comparison of Diameters of Sieve, Valve, and Bubblecap Trays for the Same Service... 

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

Solution Table 14-12 presents measurements by Billet (loc. cit.) for ethyl-benzene-styrene under similar pressure with sieve and valve trays. The column diameter and tray spacing in Billets tests were close to those in Example 9. Since both have single-pass trays, the flow path lengths are similar. The fractional hole area (14 percent in Example 9) is close to that in Table 14-12 (12.3 percent for the tested sieve trays, 14 to 15 percent for standard valve trays). So the values in Table 14-12 should be directly applicable, that is, 70 to 85 percent. So a conservative estimate would be 70 percent. The actual efficiency should be about 5 to 10 percent higher. 

Using low fractional hole areas A fractional hole area reduction to about 5 percent of the bubbling area typically boosts sieve tray turndown to about 3 to 4 1 at the expense of a lower maximum capacity, i.e., of a larger column diameter. This technique is not recommended because traying the column with valve trays is normally a cheaper alternative. 

Liquid channels less when gas emerges sideways from the tray openings. Liquid-channeling on bubble-cap (163) and valve trays (163) is far smaller than on sieve trays. On sieve trays, channeling decreases as hole diameter increases, presumably due to widening of the cone angle of the gas jet (157). 

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