Fractional hole area

In Eq. 14-96, L = m liquid downflow/(hr-m weir length) 2LudAf= fractional hole area based on ac tive ( bubbling ) area for instance, Aj =... 

A fractional hole area (or valve slot) greater than 11% of the bubbling area. 

Large fractional hole area, long flow path relative to tray spacing and high liquid flow rate are the key factors leading to the formation or intensification of vapor cross-flow channeling on sieve and valve trays. 

Fractional hole area (actual hole area/bubbling area, Ab)... 

An optimal tray design, one that balances tray and downcomer area so that neither prematurely restricts capacity, and set weir height, weir geometry, clearance under the downcomer, and fractional hole area so as to maximize efficiency and capacity. 

Fractional hole area Af The ratio of hole area to bubbling area (sieve trays) or slot area to bubbling area (valve trays). 

Fractional Hole Area Typical sieve and fixed valve tray hole areas are 8 to 12 percent of the bubbling areas. Smaller fractional hole... 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

York, 1992). Figure 14-29 demonstrates the effect of liquid rate and fractional hole area on CSB. As liquid load increases, CSB first increases, then peaks, and finally declines. Some interpret the peak as the transition from the froth to spray regime [Porter and Jenkins, I. Chem. E. Symp. Ser. 56, Summary Paper, London (1979)]. CSB increases slightly with fractional hole area at lower liquid rates, but there is little effect of fractional hole area on CSB at high liquid rates. CSB,slightly increases as hole diameter is reduced. 

The main factor that affects weeping is the fractional hole area. The larger it is, the smaller the gas pressure drop and the greater the weeping tendency. Larger liquid rates and taller outlet weirs increase... 

The terms in Eqs. (14-123) to (14-126) are in English units and are explained in the Nomenclature. For sieve trays, m= 1.94 and C = 0.79. Note that the constants are a slight revision of those presented in the original paper (C. L. Hsieh, private communication, 1991). Clear liquid height is calculated from Colwell s correlation [Eqs. (14-115) to (14-122)]. The Hsieh and McNulty correlation applies to trays with 9 percent and larger fractional hole area. For trays with smaller hole area, Hsieh and McNulty expect the weeping rate to be smaller than predicted. 

Low dry tray pressure drop. On sieve and fixed valve trays, this means high (>11 percent) fractional hole area. On moving valve trays, this means venturi valves (smooth orifices) or long-legged valves (>15 percent slot area). On all trays, the channeling tendency and severity escalate rapidly as the dry pressure drop diminishes (e.g., as fractional hole area increases). 

VCFC is usually avoided by limiting fractional hole areas, avoiding venturi valves, and using forward-push devices. Resitarits and Pap-pademos [ Paper presented at the AIChE Annual Meeting, Reno, N ev. (November 2001)] cited tray inlet inactivity as a contributor to VCFC, and advocate inlet forward-push devices to counter it. 

Fractional Hole Area Efficiency increases with a reduction in fractional hole area. Yanagi and Sakata [Ind. Eng. Chem. Proc. Des. Dev. 21, 712 (1982)] tests in commercial-scale towers show a 5 to 15 percent increase in tray efficiency when fractional hole area was lowered from 14 to 8 percent (Fig. 14-43). 

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. 

Entrainment flooding is predicted by an updated version of the Souders and Brown correlation. The most popular is Fair s (1961) correlation (Fig. 20), which is suitable for sieve, valve, and bubble-cap trays. Fair s correlation gives the maximum gas velocity as a function of the flow parameter (L/G)V(Pg/Pl), tray spacing, physical properties, and fractional hole area. 

Fractional hole area (sieve trays). Eight to 10% is generally considered optimum. Higher area may enhance capacity at the expense of more weeping at low gas flow rates. 

The hardware design proceeds In two phases primary (basic) and secondary (detailed layout). The primary phase sets column diameter, type of tray, and split of tray area Into bubbling and downcomer areas. This phase also provides a preliminary (and usually close) estimate of tray spacing, number of passes, and other features of tray and downcomer layout such as weir height, fractional hole area, hole diameter, and clearance under the downcomer. These estimates are later firmed up in the secondary phase. 

At high liquid rates (>6 gpm/in of outlet weir), high ratio (>2.5) of flow-path length to tray spacing, and a high fractional hole area (> 11 percent), cross flow of vapor in opposite direction to the liquid can build up froth near tray inlet and center. The froth buildup raises the... 

Figure 6.9 Factors affecting the flood capacity factor. FRI sieve tray test data, DT 4 ft, S = 24 in, hw = 2 in, dH = 0,5 in, straight downcomers, AJAT = 0.13. (a) Effect of liquid rate, Af = 0,08, (i>) Effect of fractional hole area. Cydohexane-Af-heptane, 24 psia.

Fractional hole area CSB increases with fractional hole area (Fig. 6.96). Roughly, when fractional hole area is between 0.05 and 0.08, an increase in fractional hole area of the order of 0.01 will enhance CSB by about 5 percent (1,18,19,26,28,29). When fractional hole area exceeds 0.1, the rate of increase of Csb with hole area is substantially lower (1,18,19,23,28,29). 

Fair s correlation (19, Fig, 6.10). The Fair flood has been the standard of the industry for entrainment flood prediction and was recommended by most designers (5,11,18,30-33). CSB is a function of the flow parameter Fu [Eq, (6.7)], tray spacing, surface tension, and fractional hole area. CSB is based on the net area AN, and is evaluated from Fig. 6.10, The flooding vapor velocity is calculated from... 

Weep prediction, The weep point of valve trays can be calculated from the Bolles extension (71) of Fair s weep point correlation (31). The same correlation (Fig. 6.18) is used, except that the sieve fractional hole area is substituted by the ratio of valve slot area to tray active area. An alternative weep point correlation for valve trays was presented by Klein (73). Hsieh and McNulty (63) extended their sieve tray weep rate correlation (Sec. 6.2.12) to valve trays. The extension is complex, and discussed elsewhere (63). 

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